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Kinetics of Oxygen Electroreduction on Me-N-C (Me = Fe, Co, Cu) Catalysts in Acidic Medium. Insights on the Effect of the Transition Metal Luigi Osmieri, Alessandro H. A. Monteverde Videla, Pilar Ocón, and Stefania Specchia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02455 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

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Kinetics of Oxygen Electroreduction on Me-N-C (Me = Fe, Co, Cu)

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Catalysts in Acidic Medium. Insights on the Effect of the Transition

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Metal.

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Luigi Osmieri a,b *, Alessandro H. A. Monteverde Videla a, Pilar Ocón b, Stefania Specchia a *

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a

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24, 10129, Torino, Italy.

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b

Politecnico di Torino, Dipartimento di Scienza Applicata e Tecnologia, Corso Duca degli Abruzzi

Universidad Autónoma de Madrid, Departamento de Química Física Aplicada, C/Francisco

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Tomás y Valiente 7, 28049, Madrid, Spain.

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* Corresponding Authors:

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[email protected], [email protected]

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[email protected]

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Abstract

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The influence of three different transition metals (Me = Fe, Co, Cu) on the oxygen reduction

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reaction (ORR) kinetics in acidic medium of Me-N-C catalysts synthesized using Me(II)-

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phthalocyanine as precursors is investigated in this work. Through a detailed electrochemical

19

characterization using cyclic voltammetry and rotating ring-disk electrode, several kinetics

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parameters such as Tafel slope, reaction order for oxygen and proton, apparent activation energy,

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selectivity towards hydrogen peroxide production, and kinetics of reduction of adsorbed oxygen

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were determined. The behavior of these three catalysts is analyzed in detail. A comparison between

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each other of the catalysts, and with a Pt-based catalyst is done. The results obtained provide clear

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evidence of the important role played by each transition metal in the formation of more or less

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effective active sites. The ORR kinetics behavior can be well interpreted according to the 1 ACS Paragon Plus Environment


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occurrence of a redox-mediated coverage of the active sites at low overpotentials (close to the ORR

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onset), which has influence on the Tafel slope, as well as on the oxygen adsorption and activation

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energy of the process. The results clearly show that, among the other transition metals considered,

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Fe is the best performing one in carrying out the ORR.

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1. Introduction

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To make the commercialization of proton exchange membrane fuel cells (PEMFC) viable in the

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next future, a fundamental point will be to lower as much as possible their platinum content.1 In

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facts, Pt is used as the preferred catalyst in these devices at both anode and cathode, due to its

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enhanced activity towards both hydrogen oxidation reaction (HOR) and oxygen reduction reaction

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(ORR),2 but it suffers from two main problems: high cost and scarcity.3 The kinetics of ORR

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occurring at the cathode is several orders of magnitude slower compared to the anodic HOR, and

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consequently most of the Pt is located at the cathode of PEMFC rather than at the anode.4 In this

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regard, many research efforts have been pursued in the last decades to develop Pt-free ORR

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catalysts, using several types of precursor materials.5 Since from the discovery in the early ’60 that

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macrocyclic molecules containing coordinated transition metals (mainly Fe and Co) are active

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towards ORR,6 and then with the discovery that the activity and the stability can be considerably

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improved after pyrolyzing the precursors, several types of Me-N-C materials have been synthesized

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and tested as ORR catalysts.7–9 In recent years, the electroactivity of some of these materials, which

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was originally considerably lower compared to Pt-based catalysts in acidic medium, has also shown

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remarkable and encouraging improvements.10–14

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The ORR kinetics in acidic conditions on Pt-based catalysts under different forms, i.e. single crystal

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Pt electrodes,15 polycrystalline Pt,16 and Pt catalysts supported on various carbon-based materials

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such as Vulcan XC72R,17 have been extensively studied in the past decades. These studies led to the

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determination among other things, of the apparent activation energy, reaction order for O2,

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predominant reaction mechanism, O2 adsorption mechanisms and rate determining steps of the 2 ACS Paragon Plus Environment

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ORR.18–21 On the other hand, the literature is scarce about analogous studies conducted on Pt-free

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ORR catalysts, and in particular for Me-N-C catalysts. An exhaustive study was conducted by

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Chlistunoff22 on a Fe-N-C catalyst synthesized using polyaniline as N precursor. Jaouen et al.

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performed an activation energy calculation on a Fe-N-C catalyst for ORR.23 However, no

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comparative studies have been carried out so far investigating the effects of different transition

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metals such as Fe, Co, and Cu on the behavior of these Me-N-C catalysts on such kinetic

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parameters like activation energy and reaction order. The role of the transition metal present in the

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precursor(s) used for the synthesis of pyrolyzed NNM catalysts for ORR has been discussed in the

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literature from the point of view of the ORR activity and selectivity induced in the final catalysts in

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some other works.24–28

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In spite of the debate about the fact that the transition metal is really part of the ORR active site, or

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it just serves for its formation during the pyrolysis is still going on,29 it is commonly assumed that

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this transition metal center coordinated with N atoms is the ORR active site in unpyrolyzed

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macrocyclic molecules like porphyrins, corroles and phthalocyanines.30–34 These molecular

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structures are commonly found in living beings (i.e. in hemoglobin and cytochrome-C-oxidase),

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which use molecular dioxygen to carry out cellular respiration. These molecules are proteins, which

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have the “ability” to bind with O2, and transport it from one site to another through the body, or to

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act as proper catalysts for the 4-electrons O2 reduction to H2O. In nature, the metal centers involved,

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for example in the transport of O2 in blood, mono- or di-oxygenation involving oxygen atom

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incorporation, and substrate oxidation (dehydrogenation or removal of electrons), are Fe and

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Cu.35,36 Interestingly, Co ion is not present in such proteins, and the substitution of Fe with Co in

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human adult hemoglobin (HbA) has shown a reduction in oxygen affinity by over a factor of 10, an

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oxygen affinity too low for use as a blood substitute.37

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In this work, our purpose is to investigate in detail the behavior of a series of Fe-N-C, Co-N-C, and

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Cu-N-C catalysts towards ORR in acidic medium in a three-electrode cell configuration. This study

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provides a deeper insight to better understand the influence of the different transition metals (Fe, 3 ACS Paragon Plus Environment


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Co, and Cu) on the ORR kinetics in acidic medium, in particular regarding the apparent activation

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energy, reaction orders for O2 and H+, behavior towards the reduction of the adsorbed O2 via cyclic

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voltammetry, and main reaction pathway. This would enable to compare the behavior of these Me-

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N-C catalysts among each other, and with Pt-based catalysts, which have been more extensively

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described in the literature.

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Our findings reveal good agreement with the proposed theory of potential-dependent ORR active

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sites coverage and consequent blocking effect, which is in strict relation to the red-ox potential of

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the metal-based active sites, and at the origin of the “asymmetric volcano plot” for these Me-N-C

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catalysts, as recently proposed in the literature.38–40 This study provides new insights supported by

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several experimental pieces of evidence about the effect of using Fe, Co, or Cu as the transition

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metal precursor, and their influence on the ORR performance of the final catalyst.

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2. Experimental Section

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2.1. Catalysts synthesis.

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The catalysts were synthesized as described in our previous work.41 Briefly, Me(II)-phthalocyanine

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(Me = Fe ,Co, Cu) were dissolved in a proper solvent and impregnated on SBA-15 ordered

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mesoporous silica used as a sacrificial template. After complete solvent evaporation, a pyrolysis

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under inert atmosphere (flowing N2) was carried out in a tubular quartz furnace for 1 h at 800 °C.

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Then the SBA-15 silica was removed by washing in 5 wt. % HF solution.

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2.2. Ink formulation for RDE tests.

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For RDE tests of Me-N-C catalysts, the ink was prepared by dispersing a given mass of catalyst

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(mcat, typically around 10 mg) in a solution obtained mixing known volumes of isopropanol,

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deionized water, and 5 wt. % Nafion® ionomer hydro-alcoholic solution. An ink formulation is

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characterized by its mass ratio of Nafion ionomer to catalyst, or Nafion-to-catalyst-ratio (NCR).

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The volumes (in µL) of Nafion solution, deionized water and isopropanol to use to prepare the ink

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are calculated as follows: =

∙ ∙ 0.05

= 15 ∙ = 50 ∙ − − 3

Where mcat is expressed in mg and ρnaf is the density of the Nafion 5 wt. % solution expressed in g

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mL−1. With this formulation, the catalyst density in the ink is 0.02 mg µL−1. The ink is kept under

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sonication (130W, Soltec 2200M3S sonicator) for 30 min to achieve a good dispersion. To have the

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desired catalyst loading, a proportional volume of ink is pipetted on the RDE electrode surface.

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For comparison, a commercial Pt-based catalyst (20 wt. % Pt on Vulcan XC-72 QuinTech,

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Germany) was also tested. For this catalyst, the ink was prepared by dispersing 10 mg of catalyst,

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33 µL of 5 wt. % Nafion® solution, 734 µL of isopropanol, and 20 µL of deionized water. This led

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to having 2.5 µg of Pt per µL of ink. After 30 minutes under sonication, a proportional volume of

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ink was pipetted to have a Pt loading on the electrode of 38 µg cm−2.

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2.3. Electrochemical tests.

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The electrochemical tests were conducted in a conventional three-electrodes electrochemical cell

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configuration, using a rotating disk electrode equipment (RRDE-3A ALS, Japan) and a multi-

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potentiostat (Bio-Logic SP-150, France). The cell was equipped with a glassy carbon disk (4 mm

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diameter) – Pt ring (7 and 5 mm outer and inner diameter, respectively) working electrode, a Pt

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helical wire counter electrode, and a saturated calomel reference electrode (SCE). For Me-N-C

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catalysts, the electrolyte was an aqueous solution of H2SO4 (in different concentrations depending

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on the type of test), while for Pt/C catalyst was 0.1 M HClO4 solution was used (to avoid the

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detrimental effect of SO42− and HSO4− ions adsorption on Pt, being HClO4 a non-specifically

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adsorbing electrolyte for Pt42). The electrolyte was saturated with pure N2, pure O2 or with mixtures 5 ACS Paragon Plus Environment


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of both gases in different proportions, depending on the type of test, by direct gas bubbling into the

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solution. For N2–O2 mixtures, the gas flows were controlled using two mass flowmeters

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(Bronkhorst ELFLOW series, Netherlands), maintaining constant the total flow rate at 150 NmL

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min−1. For RRDE measurements, the ring potential was kept at 1.2 V vs. RHE. At this potential, the

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H2O2 oxidation reaction is under diffusion control.42 The RRDE tests were conducted using a bi-

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potentiostat (CH Instruments Mod. CH760E USA). The ring background current measured at

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potentials higher than the reaction onset was subtracted from the ring signal throughout the whole

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scanned potential range.43

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Before starting the tests, 50 cyclic voltammetry (CV) cycles at 100 mV s–1 scan rate were

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performed in the potential window 0.0–1.2 V vs. RHE in N2 saturated electrolyte, to obtain a clean

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and stable working electrode surface.44

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For the ORR activity measurements, staircase voltammetries (SV) were recorded from 1.2 to 0.0 V

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vs RHE in O2-saturated electrolyte with a potential step of 0.01 V and a holding time at each

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potential of 30 s. In this way, the background capacitive current had passed, and a steady-state value

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of the faradaic current was measured, enabling to obtain steady-state polarization curves.45 The

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RDE rotation speed was set at 900 rpm.

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For the adsorbed oxygen electroreduction tests, CV cycles at different scan rates (within the range

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5– 500 mV s−1) were recorded first under N2-saturated electrolyte and then in O2-saturated

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electrolyte, in the potential window from 1.2 to 0.0 V vs RHE. For the CV experiments in the

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presence of O2, at least two consecutive full CV cycles were recorded, to enable the subtraction of

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the second cycle from the first one, as done for the calculation of the differential voltammograms

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(to eliminate the contribution of the reduction of diffused O2). In addition, between one CV

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experiment at a certain scan rate and the subsequent one, to allow O2 to adsorb onto the catalyst

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surface completely, O2 was left bubbling into the electrolyte for at least 15 minutes. This time was

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verified to be enough to assure a saturation of the catalyst surface with O2 (no difference in the

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ORR current peaks was observed with higher saturation times). 6 ACS Paragon Plus Environment

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For the Koutecky-Levich experiment, linear sweep voltammetries (LSV) were recorded at different

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rotation speeds (200–500–900–1600–2500–3600 rpm) and 5 mV s–1 scan rate.

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For activation energy calculation experiments, LSV were recorded at 5 mV s–1 scan rate and with a

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RDE rotation speed of 900 rpm at different temperatures (10–15–20–25–32–40–50–60 °C) by

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placing the cell in a thermostatic bath, which temperature was regulated and controlled by a water

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heating-cooling system (Mod. CRIOTERM 190, I.S.CO. s.r.l., Italy). These measurements were

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carried out in isothermal conditions, being the reference and working electrode both placed inside

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the thermostatic cell.

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For the Rotating Ring-Disk Electrode (RRDE) test, the ring potential was fixed at 1.2 V vs RHE,

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the disk potential was scanned at 5 mV s−1, and the electrode rotation speed was set to 900 rpm.

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At the end of each test, an electrochemical impedance spectroscopy measurement was done at the

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open circuit voltage (OCV), with a wave amplitude of 10 mV and frequencies in the range of 10

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kHz–1 Hz. The high-frequency resistance value was used to subtract the ohmic drop contribution

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from SV and LSV curves.42,46

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Hereafter, the electrode potentials were corrected and referred to the reversible hydrogen electrode

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(RHE), and the current densities were normalized to the geometric area of the glassy carbon disk

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electrode.

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3. Results and Discussion

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3.1. Preliminary electrode optimization: catalyst loading and Nafion-to-catalyst ratio.

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A preliminary test on the preparation of the working electrode for RDE tests was performed using

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the Fe-N-C catalyst (which is the most active catalyst, as shown afterward). The first part of the

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optimization consisted in testing different catalyst loadings deposited on the glassy carbon surface

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of the RDE. The chosen amounts were determined from the ink density, that is, the catalyst mass

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content per volume of ink (mg mL−1). According to the ink formulation described in Section 2.3, 7 ACS Paragon Plus Environment


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five different electrodes were prepared, by pipetting 1–2–3–4–5 µL of ink on the RDE. These ink

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volumes result in 0.16–0.32–0.48–0.64–0.80 mg of catalyst per cm2 geometric electrode area,

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respectively. Then, polarization curves were recorded in the O2-saturated electrolyte with a rotation

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speed of 900 rpm. As shown in Figure 1a, better results were obtained increasing the catalyst

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loading from 0.16 to 0.64 mg cm−2. No improvement was obtained with a further increase to 0.80

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mg cm−2. The iL values also tend to increase as the catalyst loading increase.

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To eliminate the effects of the mass transport limitations, the kinetic currents were calculated by

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Equation (1):47 ∙

= −

9

(1)

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Where: ik is mass transport-corrected current density, i is the measured current density, and iL is the

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limiting current density measured in the plateau region of the polarization curve at high

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overpotential. A correction for the ohmic drop was also done, based on the resistance values

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obtained by the EIS measurement as described in Section 2.3. The as-calculated ik values can be

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transformed in specific mass current densities (im, A g−1), simply dividing by the catalyst loading on

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the electrode (mg cm−2). Figure 1b shows the as calculated potential vs. logarithm of specific mass

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current density plot.

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Figure 1. (a) SV of Fe-N-C catalyst in O2-saturated 0.5 M H2SO4 at 900 rpm with different catalyst loadings and NCR = 0.2. (b) Tafel plot obtained from (a) after mass-transport correction and normalization per mass of catalyst on the electrode.

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However, the differences found in the ORR measurements with different catalyst loadings (Figure

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1a) were reduced after the mass transport correction and the current density normalization per mass

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of catalyst (Figure 1b). The results with 0.64 mg cm−2 catalyst loading are still slightly better than

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the others, especially at higher overpotentials. Based on these results, if not differently stated, we

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decided to fix the catalyst loading used hereafter for the RDE experiments to 0.64 mg cm−2.

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In the second part of the preliminary optimization study, the effect of NCR was investigated,

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keeping fixed at 0.64 mg cm−2 the catalyst loading. Different inks were prepared as described in 9 ACS Paragon Plus Environment


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Section 2.2, with four different NCR: 0.10–0.20–0.33–0.50. As shown in Figure 2a, in spite of the

2

constant catalyst loading, the capacitive currents vary changing the NCR, having a maximum for

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NCR = 0.2. The influence of Nafion on the capacitive current could be linked to pseudo-capacitance

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phenomena, and to modifications of the contact interface between the catalyst surface and the

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electrolyte.23,48–50 However, the Nafion ionomer not only provides proton conductivity but, more

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importantly in the RDE system, also acts as a binder for the catalytic particles, causing

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modifications of the percolating network for electrons and protons within the catalyst layer.23

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Therefore, the decrease in the capacitive current density with the increase of NCR can be explained

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by the formation of a thick Nafion film, which encapsulates some of the catalytic particles, causing

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a loss of electrical contact with the electrolyte. Similar results were also found in the literature.49,51

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1 2 3


Figure 2. (a) CV of Fe-N-C catalyst with an ink prepared with different NCR recorded in N2saturated 0.5 M H2SO4 solutions at 10 mV s−1 with a catalyst loading of 0.64 mg cm−2. (b) SV of 11 ACS Paragon Plus Environment


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Fe-N-C catalyst with an ink prepared with different NCR in O2-saturated 0.5 M H2SO4 at 900 rpm. (c) Tafel plot obtained from polarization curves in (b) after mass-transport correction.

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The ORR tests in Figure 2b show that different NCR cause also variations in the ORR activity of

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the Fe-N-C catalyst. The best performance was obtained with NCR = 0.20. The mass-transport

6

corrected kinetic current density was also calculated using Equation (1) as described before. Figure

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2c shows that the differences among the different NCR are less evident in the kinetically controlled

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region (low overpotentials), but they become more evident in the mixed kinetic-diffusion controlled

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region and in the diffusion-limited current density region at higher overpotentials. By increasing the

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NCR to 0.33 and 0.50, the diffusion-limited current density slightly decreases. This effect could be

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explained by the formation of a thicker Nafion film around catalytic particles, which could limit the

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transport of oxygen to the catalyst active sites, leading to a decrease of the limiting current.52 Also

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with a lower NCR of 0.10, the performance was worse. This fact could be related to the binding-

14

effect between Nafion and the catalyst particles in the catalyst layer. In the literature, the effect of

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NCR on ink formulation has been studied, with the NCR mass ratio varying generally from 0.10 to

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1.50,23,51,53–55 but also with considerably lower values (between 0.014 and 0.087).49 The optimum

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NCR for ORR activity depends on the nature of the different catalysts characteristics (i.e. active

18

sites, surface area, pore size distribution, hydrophobicity), which play a crucial role in the final

19

activity. In particular, as demonstrated in recent works,48,49 Nafion could form self-assemblies onto

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the surface of a catalyst depending on the hydrophobicity/hydrophilicity of its carbonaceous

21

surface. This points out the importance of performing such this type of NCR optimization before

22

testing a new electrocatalyst for PEMFC applications.

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Since the highest activity for this Fe-N-C catalyst was obtained with an NCR = 0.2, this NCR value

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was used for all the tests in the present work, considering that the other Me-N-C catalysts have

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similar morphological and superficial composition characteristics.41

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3.2. ORR activity. 12 ACS Paragon Plus Environment

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Figure 3a shows the ORR steady-state polarization curves of the Me-N-C catalysts synthesized

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using different Me(II)-phthalocyanine precursors (Me = Fe, Co, Cu, Zn) and for the catalyst H-N-C,

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prepared using the unmetallized phthalocyanine (H-Pc), as described in our previous work.41

4

5 6 7 8 9 10 11

Figure 3. (a) SV of the Me-N-C catalysts (loading 0.64 mg cm−2) measured in O2-saturated 0.5 M H2SO4 at 900 rpm. The SV of a commercial Pt/C catalyst measured in 0.1 M HClO4 in both cathodic and anodic scan directions is also shown for comparison. (b) Plot of the linear zones of the potential vs logarithm of the mass-transport corrected current densities from (a).



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Tafel slopes were calculated after correction for mass-transport limitation, starting from the data in

2

Figure 3a. Cathodic transfer coefficients and exchange current densities were also calculated, as

3

summarized in Table 1. The cathodic transfer coefficient αc is defined by Equation (2):56,57

= −

4

" $%|' | !

"(

(2)

5

where: E is the applied potential, R is the gas constant, T is the absolute temperature, F is the

6

Faraday constant and ic is the cathodic current density. Since the symbol ln|ic| implies that the

7

argument of the logarithm is of dimension one, the ic value is ideally divided by the corresponding

8

unit, e.g., ln(|ic| / mA cm–2). Therefore, in practice, αc is defined as the reciprocal of the

9

corresponding Tafel slope, –dE/dln|ic|, made dimensionless by the multiplying factor RT/F.56,57

10

Other characteristic parameters which define the performance of an ORR catalyst are the onset

11

potential (Eon) and the half-wave potential (E1/2). In this paper, Eon has been defined as the potential

12

required to generate a current density of 0.1 mA cm−2 in a steady-state RDE experiment. This

13

definition is arbitrary, and we assumed it following other literature works, in order to minimize the

14

effect of residual currents in the staircase voltammetry RDE measurement.12 Thus, it has to be

15

intended as an experimental “screening factor” that can be used for example to discriminate whether

16

an ORR electrocatalyst shows a potentially good activity to deserve to be tested in a PEMFC

17

device. E1/2 is the potential required to have half the maximum current density in the polarization

18

curve.

19 20

Table 1. ORR parameters calculated from the data in Figure 3 for all the Me-N-C catalysts and the commercial Pt/C catalyst. Low η region Catalyst Fe-N-C Co-N-C Cu-N-C Zn-N-C H-N-C Pt cath Pt anod

Eon [V vs RHE] 0.83 0.79 0.68 0.50 0.50 0.87 0.93

E1/2 [V vs RHE] 0.72 0.71 0.50 0.33 0.32 0.76 0.82

Tafel slope [mV dec−1] 64.0 53.8 60.5 62.6

αc 0.924 1.099 0.977 0.944

High η region i0 [mA cm−2] 5.14 · 10−8 1.00 · 10−9 1.41 · 10−7 2.30 · 10−6

Tafel slope [mV dec−1] 140.8 143.3 123.2 131.8 140.1 202.2 125.7

αc 0.420 0.413 0.480 0.449 0.422 0.292 0.470

i0 [mA cm−2] 8.67 · 10−4 7.15 · 10−4 3.66 · 10−6 3.26 · 10−7 7.45 · 10−7 3.63 · 10−2 3.93 · 10−3

21 14 ACS Paragon Plus Environment

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1

Analyzing the data shown in Table 1, Fe-N-C is the most active catalyst. It exhibits 100 mV

2

negative shift in both Eon and E1/2 in comparison with Pt/C (considering the anodic scan direction).

3

Moreover, its diffusion limiting current almost corresponds to the limiting current observed for

4

Pt/C. Co-N-C has a slightly lower activity, and a lower diffusion limiting current, with a not well-

5

developed plateau region. Cu-N-C is considerably less active than the two previous catalysts,

6

having about 250 mV lower Eon and 320 mV lower E1/2 than the Pt/C. Zn-N-C and H-N-C have a

7

very poor activity in acid medium, both showing 430 mV lower Eon and about 500 mV lower E1/2

8

compared to Pt/C.

9

Remarkably, that Zn-N-C and H-N-C exhibit almost the same ORR activity, being their polarization

10

curves practically superimposed (see Figure 3a-b). This suggests that even if the Zn presence leads

11

to considerable modifications in the final catalyst chemical-physical properties (e.g. surface area

12

and pore size distribution, total amount of N incorporated),41 this will have practically no influence

13

on the ORR activity in acidic medium. Similar results were found for tests in alkaline medium in

14

our previous work.41 This finding confirms the important role played by the most effective

15

transition metals (Fe and Co) in the formation of the ORR active sites during the heat treatment. Cu

16

also has certain effectiveness in the formation of active sites, but considerably lower than Co and

17

Fe.

18

In the Tafel plots of both Fe-N-C and Co-N-C two different Tafel slope zones have been identified

19

(see Figure 3b), a first one at low overpotentials (low η), approximately between 0.85 and 0.75 V

20

vs. RHE, and a second one at high overpotentials (high η), approximately between 0.75 and 0.60 V

21

vs. RHE. At low η the Tafel slope is lower, being approximately of 60 mV dec−1. At high η the

22

slope increases, and the linear trend is less evident, suggesting that a change in the reaction

23

mechanism is occurring.22 A linearization was also made in this zone, in spite of the lower

24

coefficient of determination (R2), and the calculated Tafel slopes are around 140 mV dec−1. An

25

analogous behavior was found by many other authors for similar Me-N-C catalysts.22,23,26,58,59

26

Considering the exchange current densities in the low η region, Fe-N-C has an almost double i0 15 ACS Paragon Plus Environment


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Page 16 of 75

1

value than Co-N-C, whereas Pt/C shows an almost one and two orders of magnitude higher i0 value

2

compared to Fe-N-C in the cathodic and anodic scan direction, respectively.44,60 In the high η

3

region, the i0 values for Fe-N-C and Co-N-C are closer, being again almost one and two orders of

4

magnitude lower than for Pt/C catalyst in the cathodic and anodic scan direction, respectively.

5

Cu-N-C catalyst does not exhibit the double slope behavior, and a single linear region with a slope

6

value of about 123 mV dec−1 is observed along all the potential range from 0.68 to 0.55 V vs. RHE.

7

The i0 of Cu-N-C in the high η region is almost two orders of magnitude lower compared to Fe-N-C

8

and Co-N-C.

9

Zn-N-C and H-N-C catalysts also show a single-slope behavior in the potential range 0.5–0.3 V vs.

10

RHE, with a Tafel slope of 132 and 140 mV dec−1, respectively, and i0 values are one order of

11

magnitude lower compared to Cu-N-C. Since the ORR activity of Zn-N-C and H-N-C in acidic

12

medium is very poor and thus they are not interesting for potential application as cathodic catalysts

13

for PEMFC, hereafter we will not consider these two catalysts for the investigation of the other

14

kinetic aspects of the ORR.

15

To investigate more in detail the effect of the catalyst loading on the ORR kinetics, we recorded the

16

polarization curve in RDE at 900 rpm rotation speed with two different ink quantities deposited on

17

the electrode for Fe-N-C, Co-N-C, and Cu-N-C catalysts. Figure 4 shows the Tafel plots of the

18

respective SV polarization curves in Figure 3a for all of the catalysts with 0.64 mg cm−2 loading,

19

and in Figure 1a for Fe-N-C with 0.16 mg cm−2 loading. The SV polarization curves for Co-N-C

20

and Cu-N-C with 0.32 mg cm−2 loading are not reported here.

21


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1 2 3 4 5 6


Figure 4. Mass transport corrected Tafel plots for Fe-N-C, Co-N-C, and Cu-N-C catalysts: (a) Current density referred to the geometric area of the electrode; (b) Current density referred to the mass of catalyst deposited on the electrode.

7

For all of the catalysts, when the current densities are referred to the geometric area of the electrode

8

(Figure 4a), the apparent ORR kinetics appears to be enhanced with a higher catalyst loading.

9

However, when the current densities are normalized per unit mass of catalyst deposited on the

10

electrode surface (Figure 4b), the plots with low catalyst loading undergo a vertical shift, making

11

them almost overlap the plots with high catalyst loading in the whole potential range considered.

12

Therefore, for all the catalysts, we obtained a good agreement between mass specific kinetic current

13

densities (A g−1) after the mass-transport correction for both high and low catalyst loadings. In 17 ACS Paragon Plus Environment


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Page 18 of 75

1

particular, for Fe-N-C and Co-N-C, this indicates that the curvature of the E vs. log|i| plots (with a

2

consequent changing in the Tafel slope), does not originate from incomplete catalyst utilization in

3

the catalytic layer61 or uncompensated resistance,46 but it is the consequence of changes in the

4

intrinsic ORR kinetics, which is potential-dependent.22

5

The behavior of the two most active catalysts (Fe-N-C and Co-N-C) in terms of observation of two

6

different Tafel slope regions at different overpotentials, can be compared to the behavior of Pt-

7

based catalysts, as also reported in our previous work.60 In particular, Pt-based catalysts also show a

8

change in the Tafel slope from ~60 mV dec−1 to ~120 mV dec−1 with the increase of η. For Pt-based

9

catalysts, this fact is attributed to a potential-dependent coverage and consequent blocking of ORR

10

active Pt surface by chemisorbed oxygen-species (i.e. thin Pt oxide layer). This occurrence causes a

11

change in the O2 adsorption conditions with the potential.39,62 In particular, at low η, where the Pt

12

surface is highly covered by oxide species, the O2 adsorption will be similar to the situation

13

described by a Temkin isotherm. At higher η instead, the Pt surface is practically free from oxide

14

coverage, and thus the O2 adsorption will take place in conditions similar to what described by a

15

Langmuir isotherm (that is, virtually no interaction between adsorbed molecules). However, this

16

does not involve a change in the rate determining step of the reaction, that remains the first electron

17

transfer.62

18 19

3.3. Electroreduction of adsorbed oxygen.

20

3.3.1. First method

21

The first analytical method we used to study the behavior of our catalysts towards the reduction of

22

the adsorbed O2 via CV, consists in subtracting the CV cycle recorded in N2-saturated electrolyte

23

from the first CV cycle recorded in O2-saturated electrolyte. Thus, in the residual current of the

24

differential curve (that is, the blue CV curve in Figure 5), the pure capacitive (electric double layer)

25

and pseudo-capacitive (surface functionalization red-ox processes) effects63 have been eliminated.

26

Thus, the differential curve contains the contribution of the reduction of both the oxygen adsorbed 18 ACS Paragon Plus Environment

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1

on the catalyst surface, and the oxygen diffusing from the bulk of the solution. At lower scan rates,

2

the contribution of the diffused oxygen is likely to be higher, because the lower scan rate provides

3

more time to the oxygen diffusing from the bulk of the electrolyte to reach the catalyst surface.

4

Figure 5 shows the results obtained for Fe-N-C, Co-N-C and Cu-N-C catalysts. Comparing them,

5

we can see that the intensity of the current density peak, Ip, in the differential CV is considerably

6

higher for Fe-N-C and Cu-N-C than for Co-N-C, as evidenced by the CV plots recorded at 10 mV

7

s−1. These differential CV show only one direct reduction peak, without the presence of any reverse

8

anodic peak, suggesting that the process is irreversible.64 In accordance with the theory of the

9

potential sweep techniques, Ip varies linearly with the square root of the scan rate, ν1/2, in a process

10

totally governed by diffusion.22,64 On the other hand, for a diffusionless process, totally ascribed to

11

reactions of species adsorbed on the electrode surface, the variation of Ip with ν1/2 is quadratic.22,65

12

For Fe-N-C the Ip values in the scan rate range considered (5–500 mV s−1) vary approximately

13

between 2.4 and 16.6 mA cm−2, as evident in the Ip vs. ν1/2 plot in Figure 5a, which shows an

14

almost quadratic trend (with a coefficient of determination, R2 = 0.968). However, the curve trend

15

could also be fitted with a straight line (but with a slightly lower R2 = 0.946). This suggests that the

16

ORR phenomena occurring have both the contribution of diffusion and surface adsorption.

17

For totally irreversible processes, it is possible to determine the cathodic transfer coefficient, αc,

18

defined as in Equation (2), by using the slope of the plot of the peak potential, Ep, of a CV vs. the

19

logarithm of the scan rate (ν), using Equation (3) for a diffusionless process, and Equation (4) for a

20

diffusion-controlled process:57

21

= − +$,-.∙! (3)

22

= − +$,-.∙)! (4)

23

These equations are less accurate when reactants and products are specifically adsorbed and the

24

degree of surface coverage approaches unity.57

).*

).*



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Page 20 of 75

1

However, looking at the Ep vs. log(ν) plot in Figure 5a, it seems that the slope is changing with the

2

potential, thus no linear trend can be found. As previously discussed, we can hypothesize ORR to

3

be a diffusionless process, and thus use Equation (3) to calculate αc. If we consider the slope as a

4

derivative, and we divide the plot into three different zones (corresponding to low, intermediate,

5

and high ORR overpotentials), we can find a quite good linear trend, with different slopes, and

6

consequently different αc. This lets to deduce that the reaction mechanism on Fe-N-C catalyst is

7

intrinsically changing with overpotential, similarly to what previously found for RDE tests

8

described in Section 3.2, and by Chlistunoff.22 Table 2 summarizes these values.

9 10

Table 2. Parameters for the calculation of αc derived from data in Figure 5.

E zone Fe-N-C Low η Intermediate η High η Co-N-C – Cu-N-C Intermediate η High η

E range (V vs. RHE) 0.70 – 0.60 0.60 – 0.45 0.40 – 0.10 – 0.50 – 0.30 0.30 – 0.00

Slope –0.116 –0.248 –0.641 –0.148 –0.201 –0.445

R2 0.982 0.994 0.938 0.957 0.988 0.985

αc 0.51 0.24 0.09 0.20 0.29 0.13

11 12

As discussed before, the Ip values of Co-N-C in the ν range considered (10–500 mV s−1) are

13

considerably lower compared to Fe-N-C, varying approximately between 0.7 and 5.5 mA cm−2. If

14

we consider the Ip vs. ν1/2 plot in Figure 5b, we observe that it shows an almost linear trend

15

(although showing a low R2 = 0.88, due to the poorly defined current peaks observed for this

16

catalyst), suggesting that the ORR is mainly governed by diffusion contribution. This fact well

17

agrees with the lower Ip found for Co-N-C. Concerning the Ep vs. log(ν) plot in Figure 5b, in the

18

scan rate range 10–200 mV s−1, where the ORR is almost controlled by diffusion, we got an

19

approximately linear trend (with R2 = 0.957). This corresponds to αc = 0.20, which similar to the

20

one found for Fe-N-C in the intermediate η region.

21

For Cu-N-C the Ip vs. ν1/2 plot in Figure 5c is much better fitted with a parabolic curve (R2 = 0.999),

22

than with a straight line (R2 = 0.946), suggesting that the process could be well approximated by the 20 ACS Paragon Plus Environment

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1

reduction of the O2 adsorbed on the catalyst surface. The difference between the behavior of Cu-N-

2

C compared to Fe-N-C catalyst can be ascribed to differences on the catalysts surface features,

3

which could influence the amount of surface-confined O2 within the catalytic layer. This can be

4

caused for example by a different Nafion agglomerates distribution, leading to a difference in the

5

relative contribution of the reduction of adsorbed and diffused O2 on within the two catalyst layers

6

of Fe-N-C and Cu-N-C. The Ip values of Cu-N-C in the ν range considered (10–350 mV s−1) are

7

comparable to the values found for Fe-N-C, varying approximately between 1.5 and 26.2 mA cm−2.

8

From the Ep vs. log(ν) plot in Figure 5c, considering that in this case we are under conditions of a

9

diffusionless process (with predominant reduction of adsorbed O2), the slope is changing with the

10

potential, and a single linear trend cannot be found, similarly to Fe-N-C. However, by dividing the

11

plot into two zones, a better linear trend with different slopes was obtained, suggesting a consequent

12

change of αc and reaction mechanism, like in the case of Fe-N-C.

13

The different behavior of Co-N-C compared to Fe-N-C and Cu-N-C, could suggest that O2 adsorbs

14

much more weakly on Co-based active sites than on Fe- and Cu-based sites. These results and

15

considerations open an insight on the role that the different transition metals play in ORR, and on

16

their effectiveness during the pyrolysis in the formation of good ORR active sites.

17

In addition, these findings could indicate that the Fe-based and the Cu-based active sites exhibit a

18

similar behavior toward the adsorption of O2 and the reduction of the adsorbed O2, except that Fe-

19

sites perform their role at more positive potentials, resulting in a better ORR catalytic performance

20

for PEMFC applications. All these results are in accordance with the theory of redox-mediated

21

formation of the active sites proposed in the literature.38–40,66 Thus, the redox potential (Eredox) of Fe-

22

N-C active sites is more close to the top of the “volcano plot”. For Cu-N-C, Eredox could be located

23

more negatively on the left side of the asymmetric “volcano plot”. The active sites of Co-N-C show

24

higher ORR Eon compared to Cu-N-C (almost comparable to Fe-N-C), but their ability to adsorb

25

and subsequently reduce O2 is remarkably lower. These results could be a further confirmation of



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Page 22 of 75

1

the fact that the active sites formation Eredox potential for Co-N-C is located in more positively than

2

Fe-N-C, on the right side of the asymmetric “volcano plot”.

3

4 5 6 7 8 9 10

Figure 5. CV recorded at 10 mV s−1 in electrolyte saturated with O2 (black curve) and N2 (red curve), and their subtraction (blue curve), peak current density of the subtracted CV in function of the square root of scan sate, and peak potential of the subtracted CV in function of the logarithm of scan rate. (a) Fe-N-C; (b) Co-N-C; (c) Cu-N-C.

11 12

3.3.2. Second method

13

The second analytical method consisted in the determination of the differential CV by subtracting

14

the second CV cycle from the first CV cycle recorded in the O2-saturated electrolyte. Even though a

15

complete elimination is never possible, this would reduce as much as possible the contribution of 22 ACS Paragon Plus Environment

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1

the reduction of the diffused O2 to the peak current density.22 Figure 6a-b-c shows the differential

2

CV peaks obtained for Fe-N-C, Co-N-C, and Cu-N-C catalysts.

3

For Fe-N-C, almost symmetrical full differential CV peaks were obtained (see Figure 6a) in all the ν

4

range considered (10–350 mV s−1), with Ip values between 0.7 and 13 mA cm−2. Consequently, the

5

calculation of the full width at half maximum (FWHM) was possible for all of the peaks recorded at

6

different ν. In addition, the FWHM values, ∆E1/2, were plotted vs. the ν1/2, and a linear trend was

7

obtained. This αc value close to unity is similar to the value found in RDE experiment in Section 3.2

8

in the low overpotential region (see Table 1). The third plot in Figure 6a represents the variation of

9

αc (calculated as indicated by Laviron in the case of diffusionless electrochemical systems:65 αc =

10

62.5 mV / ∆E1/2 (mV)) with the peak potential Ep, which in turns varies with ν. The results show

11

that there is a decrease of αc with decreasing Ep, suggesting a potential-dependent change in the

12

reaction mechanism, as found in Section 3.3.1, as well as in Section 3.2. The αc values range

13

approximately between 0.6 and 0.2. In the RDE experiments (Section 3.2, Table 1) the αc value in

14

the high η region (between 0.7 and 0.6 V vs RHE) was 0.42, which very well matches with the αc

15

values in this potential range in Figure 6a.

16

For Co-N-C the situation is considerably different. Observing the differential CV in Figure 6b, a

17

peak trend is only slightly evident for all the scan rates considered, and the peaks show very low Ip

18

values compared to both Fe-N-C and Cu-N-C. Moreover, the peaks do not follow the trend of

19

increasing Ip and ∆E1/2 with the increase of the scan rate. On the contrary, for ν values between 50

20

and 200 mV s−1, Ip seems to decrease with the increase of ν. This result appears to be a further

21

confirmation of the finding of Section 3.3.1, about the fact that the reduction of adsorbed O2 is

22

much less important on Co-N-C catalyst, being the diffusion contribution dominant, in comparison

23

with Fe-N-C and Cu-N-C. As a consequence, for Co-N-C it was not possible to find a linear

24

correlation in the plot of ∆E1/2 vs. ν1/2, and consequently neither to calculate the αc extrapolated at

25

scan rate = 0, nor to find a trend of variation for the αc with Ep was possible for this catalyst (see

26

Figure 6b-c). In Figure 6c, the calculated αc values for Co-N-C oscillate between 1.04 and 0.73. 23 ACS Paragon Plus Environment


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Page 24 of 75

1

However, the reliability of these values is too poor, due to the very low contribution of the adsorbed

2

O2 reduction on this catalyst. Nevertheless, comparing these results with the αc value obtained for

3

Co-N-C in RDE experiment (αc = 1.01, see Table 1) we again obtain a good agreement.

4

For Cu-N-C the situation is more similar to Fe-N-C, with the difference that the ORR starts at

5

considerably lower potentials. Almost symmetrical differential CV peaks are evident in Figure 6c,

6

with Ip values between 0.6 and 12 mA cm−2. However, at high scan rates (150 and 200 mV s−1) the

7

peaks appear cut, because their Ep is too low and close to the lower limit of the CV potential

8

window. Consequently, for the calculation of the ∆E1/2 we considered only the scan rate values

9

between 10 and 100 mV s−1. The ∆E1/2 vs. ν1/2 plot in Figure 6b shows a linear trend, which enables

10

the calculation of the αc value extrapolated at ν = 0. This value is equal to 0.48, which is almost

11

one-half of the value found for Fe-N-C, and perfectly matches the αc value found for Cu-N-C

12

catalyst in RDE polarization curve in Section 3.2 (Table 1). Concerning the αc vs. Ep plot in Figure

13

6c, similarly to Fe-N-C, there is a decrease of αc with decreasing Ep, varying approximately

14

between 0.40 and 0.25.

15


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1 2 3 4 5 6 7


Figure 6. Differential CV recorded in O2-saturated 0.5 M H2SO4 at different scan rates; FWHM of the differential CV peaks vs. the square root of the scan rate; cathodic transfer coefficient for the reduction of adsorbed O2 calculated from the FWHM of the differential CV peaks widths for (a) FeN-C, (b) Co-N-C, and (c) Cu-N-C catalysts.

8

As in Section 3.3.1, also these results indicate that Fe-N-C is the catalyst that better performs

9

towards adsorbed oxygen reduction, approaching more the top of the asymmetric “volcano plot”

10

described in the literature regarding O2 binding energy on its active sites, and metal-based active

11

sites Eredox.

12

affinity on the surface of this catalyst. Also for Cu-N-C the results confirm the finding of the

13

previous section.

38,66

The results obtained for Co-N-C are a further evidence of the poor O2 adsorption

14 15

3.4. Reaction order for O2 25 ACS Paragon Plus Environment


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Page 26 of 75

1

For the determination of the reaction order for O2, staircase voltammetry experiments were

2

performed in 0.5 M H2SO4 solution saturated with O2–N2 mixtures in four different proportions, to

3

have O2 at different partial pressures. Figure 7 shows the experimental results, where the O2 partial

4

pressures used in each experiment are also reported.

5

For all the catalysts, the limiting current density decreases as the O2 partial pressure in the gas flow

6

bubbling into the electrolyte solution decreases. A similar trend of results was obtained by Paulus et

7

al. for a Pt/C catalyst.42


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1 2 3 4 5 6 7


Figure 7. Tafel plot in the kinetic zone of the polarization curve recorded in 0.5 M H2SO4 at 900 rpm with different O2 partial pressures at 25°C, and double-logarithmic plots of mass transport corrected current density at different potentials in function of O2 concentration for (a) Fe-N-C, (b) Co-N-C, and (c) Cu-N-C catalysts.

8

The O2 solubility in moderately concentrated H2SO4 aqueous solutions follows the Henry’s law,67,68

9

thus it decreases linearly with the O2 partial pressure.

10

The reaction order for oxygen at constant electrode potential can be defined by Equation (5): 27 ACS Paragon Plus Environment


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0$,1 |2 |

1

= /

2

Where: RO2 is the reaction order for oxygen, im is the mass-transport corrected current density, C is

3

the O2 concentration in the electrolyte, E is the electrode potential, T is the temperature.

4

According to Equation (5), the reaction order can be calculated from the slope of a double-

5

logarithmic plot of im vs. C*, where C* is defined as the O2 concentration normalized to the O2

6

saturation concentration in the electrolyte for pure O2 at 1 bar, C0 (being C0 = 1.05 mM in 0.5 M

7

H2SO4 aqueous solution). As shown in the double logarithmic plots in Figure 7, and summarized in

8

Table 3, for all the three catalysts RO2 in the kinetic control potential range is not constant. In

9

particular, for potentials closer to the onset potential (Eon), RO2 is about 0.5. Then, as η increases,

10

RO2 increases as well. Considering the most active catalysts, that is Fe-N-C, the reaction order is

11

~0.5 close to Eon, and increases up to ~0.85 at 0.7 V. For Co-N-C RO2 is also ~0.5 close to Eon, but

12

in the same potential range its increase is more marked, reaching ~1 at 0.7 V. Cu-N-C catalyst

13

behaves similarly to Fe-N-C, in a 100 mV more negative potential range. Some studies about the

14

reaction order for O2 have been done for Pt-based catalysts in RDE and PEMFC in the literature,

15

some of them showing values close to unity in all the potential ranges considered,19,42,69 some others

16

showing values lower than unity, (between 0.7 and 0.85).70–72

0$,13

4

Page 28 of 75

(, ,

(5)

17 18 19

Table 3. Reaction orders for O2 calculated at different potentials in the kinetic control zone for FeN-C, Co-N-C and Cu-N-C catalysts. Catalyst E vs RHE [V] Fe-N-C Co-N-C Cu-N-C 0.80 0.54 0.48 0.77 0.65 0.76 0.75 0.69 0.84 0.72 0.76 0.92 0.70 0.84 0.98 0.46 0.67 0.58 0.65 0.66 0.62 0.75 0.60 0.80

20 28 ACS Paragon Plus Environment

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1

Unfortunately, in the literature only few studies report the reaction order for O2 for similar types of

2

Me-N-C catalysts in acidic conditions. Gojkovic et al. reported a reaction order for O2 equal to 1,

3

but these authors measured it using a different method, and at lower potentials compared to our

4

measurements.62 Chlistunoff found a reaction order of 1 at all the considered potentials for a

5

pyrolyzed Fe-Polyaniline catalyst which activity was tested in oxygen and air-saturated acidic

6

electrolyte.22

7

The RO2 values that we obtained, showing an increase from ≈ 0.5 at potentials close to Eon, to values

8

approaching 1 at higher η, in our opinion could be an indirect confirmation of the redox-mediated

9

formation of the active sites occurring on our catalysts with the increase of η. This reasoning could

10

be envisioned as an “incomplete catalyst utilization” occurring at low η, where the active sites are

11

going to be formed due to the redox-mediated mechanism, and the active sites coverage is

12

decreasing from θ ≈ 1 to θ ≈ 0. For the above-mentioned Pt catalysts showing RO2 < 1, an analogous

13

phenomenon could take place, with the surface coverage with oxidized species, which causes the Pt

14

surface to not be fully available to carry out the ORR.

15

3.5. Reaction order for H+

16

H+ ion, in addition to O2, is the other reactant of the ORR in acidic conditions. For the determination

17

of the reaction order for H+, ORR activity experiments have to be performed at different H+

18

concentrations, that is, at different electrolyte solution pH. This is in contrast to the fact that the

19

determination of the order of an electrochemical reaction for one of its reactants has to be

20

performed at a constant electrode potential. In fact, if we use the RHE as a potential reference scale,

21

we have to consider that this reference varies with the electrolyte solution pH of about 60 mV per

22

pH unit.

23

However, the reaction order for H+ (RH+) can be approximately expressed by Equation (6):22

24 25

;$,1
8

@A>B

D F

I@

E A>B + /).*GH 4 :I$,1= ? >8

(6)



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1 2

where: i is the current density, CH+ is the molar concentration of H+ ion, ERHE is the potential

3

measured vs. RHE, αc is the cathodic transfer coefficient, R is the gas constant and T is the absolute

4

temperature. The SV with three different H2SO4 concentrations (0.10–0.25–0.50 M, which

5

respectively pH values are 1–0.6–0.3) were measured and the results are shown in Figure 8.

6 7


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1 2 3 4

Figure 8. SV recorded at 900 rpm in O2-saturated H2SO4 solutions at different pH, and the respective Tafel plots after mass-transport correction for (a) Fe-N-C, (b) Co-N-C, and (c) Cu-N-C catalysts.

5

For Fe-N-C and Co-N-C, in the low η range (between 0.8 and 0.7 V vs. RHE) the Tafel plot is

6

almost linear (see Figure 8a-b) and the slope is equal to ~ 60 mV dec−1. In addition, the plots at

7

different pH values are almost completely overlapping in this potential range. Thus, at a certain

8

potential in this range, we have:

9

:;$,1= ?

;$,1
8

@A>B

≅ 0 (7)

10 11

D F

J

J

E /).*GH 4 = HKL.$ M$,-. ≅ NO PQ I.RST

(8)

12 I@

13

A>B :I$,1= ? ≅ 60 mV dec J

14

Substituting expressions (7), (8), and (9) into Equation (6), the reaction order for H+ is

15

approximately 1. In the same way, considering approximately no variations of the plots with pH, the

16

reaction order for H+ suffers a decrease as the cathodic overpotential increase, due to the changes in

17

the Tafel slope (see Table 1). As for the reaction order for O2, also in this case the literature studies

18

are scarce. Again, Chlistunoff performed a similar study for a pyrolyzed Fe-Polyaniline catalyst,

19

which exhibited a similar behavior.22 For Cu-N-C, the same considerations can be made. However,

20

this catalyst exhibits a constant Tafel slope of about 120 mV dec−1 in the potential range between

21

0.60 and 0.40 V vs RHE (see Figure 8c). Thus, in this potential range, the reaction order for H+

22

should be lower than unity.

23

Regarding the determination of RH+ in these experimental conditions, a consideration deserves to be

24

made: the variation of H2SO4 concentration causes a non-negligible variation of O2 solubility.68,73

25

This affects the conditions of validity of determination of the reaction order for H+, that is, the

26

concentration of one of the other reactants taking part in the reaction (in this case O2) is not

>8

(9)



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1

constant. In fact, for a rigorous determination of the reaction order for H+, only the H+ concentration

2

must vary, while all the other parameters (i.e., temperature, pressure, potential, O2 concentration)

3

must be constant. Even in the small pH range considered in our experiments (from 1.0 to 0.3, that

4

is, for an H2SO4 concentration varying from 0.1 to 0.5 M) the variation of O2 concentration in the

5

electrolyte solution due to the increase of H2SO4 concentration is already considerable. In fact,

6

looking at the polarization curves in Figure 8 in the high η region, where the contribution of the

7

mass transport in higher, the diffusion limiting currents values recorded at lower pH are lower. This

8

is due to the decrease of O2 solubility with decreasing pH. A correction for O2 solubility in

9

solutions with different pH should be operated to eliminate these discrepancies. For this reason, we

10

limited our experiments to this small pH range, assuming valid our calculation of the reaction order

11

for H+ only in the low η region (kinetic limitation).

12 13

3.6. Activation Energy calculation for ORR

14

The activation energy (Ea) was evaluated at different fixed potentials using the Arrhenius Equation

15

in the temperature range 10–60 °C. In performing the RDE experiments at different temperatures,

16

two different effects must be taken into account. The first is the ORR kinetic increase with

17

increasing temperature, and the second is the decrease in O2 concentration with increasing

18

temperature. In particular, the O2 concentration in the electrolyte solution varies from 1.38 mM at

19

20 °C to 0.61 mM at 60 °C.74 Thus, to take into account this effect, the current density measured at

20

different temperatures has been corrected according to the Equation (10):23

21

22

JP_

i∗P = iP ∙ exp : =

`

O.ab

?

(10)

23 24

Where: im is the measured mass-transport corrected current density, CO2 is the actual O2

25

concentration in the liquid electrolyte at the temperature at which the measurement was done, and 32 ACS Paragon Plus Environment

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1

im* is the corresponding current density for an oxygen concentration of 1 mM. Then, the Arrhenius

2

plot (logarithm of current density vs. the inverse of the absolute temperature) can be drawn, and Ea

3

can be calculated from the linearization of the Arrhenius law:

4 @

g i∗P cTe = i∗P cTfe ∙ exp /).*GH 4

5

(11)

6 7

where T is the temperature at which the measure was done, and im*(T∞) is a constant value. The

8

slope of the Arrhenius plots is equal to –Ea / (2.3 R).

9

Figure 9 shows the Tafel plots at different temperatures with the above mentioned corrections, and

10

the respective Arrhenius plots, for Fe-N-C, Co-N-C and Cu-N-C catalysts.

11

The Arrhenius plots were calculated at different potentials in the regions under kinetic and mixed

12

kinetic-diffusion control for the three different catalysts. Table 4 summarizes the corresponding Ea

13

values, which are comparable with the values reported in the literature for ORR for Pt-based

14

catalysts, which vary between 27 and 11 kJ mol−1 at potentials in the range 0.70–0.95 V.18,20,21,23,42

15

Unfortunately, as for the reaction orders, also for Ea there is a lack in the literature for this type of

16

Me-N-C electrocatalysts. Jaouen et al.23 obtained values around 9 kJ mol−1 at 0.9 V vs RHE for a

17

series of pyrolyzed Fe-N-C catalysts synthesized using carbon black, iron acetate, and

18

phenanthroline.



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Page 34 of 75

1 2 3 4 5 6

Figure 9. Effect of the temperature on ORR activity in RDE experiments in O2-saturated 0.5 M H2SO4 at 900 rpm. Tafel plot after correction for mass-transport and O2 solubility at different temperatures, and the respective Arrhenius plot at different potentials for (a) Fe-N-C, (b) Co-N-C, and (c) Cu-N-C catalysts.

7

For Fe-N-C and Co-N-C catalysts a decrease of Ea with the increase of η is observed. In particular,

8

in the low η region, where the Tafel slope is of about 60 mV dec−1, the Ea has values close to 20 kJ

9

mol−1. Then, moving to higher η, where the Tafel slope has almost a double value, the Ea values

10

tend to decrease, approaching 14–16 kJ mol−1 at 0.65 V vs. RHE and even lower values of 12–13 kJ 34 ACS Paragon Plus Environment

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1

mol−1 at 0.6 V vs. RHE. A similar trend of decreasing Ea with increasing overpotential

2

(simultaneously with the changing in the Tafel slope) was found in the literature for Pt/C, Pt-Ni/C

3

and Pt-Co/C catalysts in acidic conditions.18 Unlike the two previous catalysts, for Cu-N-C the

4

activation energy was calculated in the potential range between 0.60 and 0.45 V vs. RHE, having

5

this catalyst a 150 mV lower Eon. The Ea of Cu-N-C, similarly to the Tafel slope (see Figure 3b),

6

does not significantly change with η. In fact, for Cu-N-C catalysts, Ea oscillates between 21.3 and

7

23.6 kJ mol−1 at all the potential considered.

8

Thus, Fe-N-C and Co-N-C catalysts show the expected trend of Ea decrease with the increase of η.

9

On the other hand, the Cu-N-C catalyst shows an almost constant Ea value with η. This anomalous

10

behavior is not trivial to be interpreted at this point, and a deeper study, which is out of the scope of

11

this work, would be necessary to better understand the reason. A possible reason for this behavior is

12

a potential dependent variation of the pre-exponential factor of the Arrhenius equation (e.g.

13

including the entropic term), which could compensate the corresponding decrease of the apparent

14

activation energy with η. A similar behavior of constant activation enthalpies with the increase of η

15

was reported by Paulus et al. for Pt and Pt-alloys catalysts.21

16

Paulus et al.42 determined the activation energy for a Pt catalyst supported on carbon in the low

17

overpotential region using the Arrhenius method in both H2SO4 and HClO4 electrolytes, obtaining

18

values of 26–28 kJ mol−1. Similar values were obtained for low index Pt single crystal surfaces and

19

polycrystalline Pt surfaces.15,16,18,21 Neyerlin et al. reported a series of activation energy values for

20

Pt/C catalysts measured in a fuel cell. These values are in the range between 42 and 96 kJ mol−1.19

21

The reason of these significantly higher Ea in comparison with the values found in three-electrode

22

cell configuration with a liquid electrolyte, are related to the higher complexity of the fuel cell

23

system, where phenomena like water flooding effects in the cathode catalyst layer may occur.42,75

24

However, it must be considered that the calculation of Ea at the same η (as done in this work), is

25

only an estimation of the Ea value. Thus, Ea is an “apparent” activation energy value.21 In the case

26

of Pt in fact, the im* in Equations (10) and (11) is strongly dependent on the quantity of oxides on 35 ACS Paragon Plus Environment


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Page 36 of 75

1

the Pt surface which, in turn, is temperature-dependent, even though the oxide coverage is

2

considered as a pre-exponential term in Equation (11).21

3 4 5

Table 4. Activation energies calculated at different potentials for Fe-N-C, Co-N-C and Cu-N-C catalysts (from Figure 9). Activation Energy [kJ mol–1] E vs RHE [V] Fe-N-C Co-N-C Cu-N-C – 0.75 20.91 22.82 – 0.70 19.57 18.50 – 0.65 16.34 14.56 0.60 13.93 12.67 21.71 – 0.55 – 23.61 – – 0.50 23.60 – – 0.45 21.33

6 7

Trying to make an analogy between Pt-based catalysts and our most active catalysts (i.e., Fe-N-C

8

and Co-N-C), we can speculate that the reaction mechanism could be the same: the first step is the

9

adsorption of O2 on the catalyst active site (O2 O2*), followed by the first electron transfer,

10

which is the rate determining step: O2* O2*−. Surface coverage effects have been proposed as the

11

reason for changes in ORR kinetic behavior on Pt surfaces, where changes in the surface coverage

12

of adsorbed oxygen-containing groups occur.76–78 These oxygen-containing groups, such as −OH

13

species, can originate from both H2O and O2,78 and can control the availability of the O2 adsorption

14

sites.21 A clear overlap between the onset potential of ORR and the removal of these oxygen-

15

containing species from the Pt surface is observed,78 considering the CV of Pt in de-aerated

16

solutions and the ORR experiments in RDE. In this region, the Tafel slope change also occurs.

17

Trying to make a parallelism between Pt-based catalysts and our NNM catalysts (e.g. Fe-N-C), the

18

presence of a redox peak in the CV, i.e., related to the Fe2+/Fe3+ couple,22,79 could be compared to

19

the Pt-oxides reduction peak in the CV of Pt surface. The onset of ORR on Pt surface is associated

20

with the appearance of the Pt-oxides reduction peak in the CV recorded in the absence of O2, as

21

discussed in the literature.78 Thus, the onset of the ORR on our NNM catalysts could be associated 36 ACS Paragon Plus Environment

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1

with the potential at which the redox peak starts to appear.22,79,80 However, in our catalysts, this

2

redox peak is not so clearly evident (see Figure 2a) as in other NNM catalyst reported in the

3

literature, or as the Pt-oxides reduction peak in the CV of Pt-based catalysts. In particular, in our

4

catalysts, the presence of these red-ox peaks related to transition metal-based red-ox centers, could

5

be “hidden” by the much high capacitive currents (e.g., in comparison with Vulcan which is the

6

usual carbon support for Pt-based catalysts),79 which are due to the high specific surface area and

7

surface functionalization (pseudo-capacitive effects due to the presence of oxygenated functional

8

groups like quinone/hydroquinone on the surface of our catalysts).50,63,80,81 The presence of a broad

9

peak in the potential range between 0.8 and 0.4 V vs RHE is detected in our catalysts (see Figure

10

2a and Figure 5). We could state this is a way to justify the analogies between the behavior of these

11

NNM catalysts and the Pt-based ones.

12 13

3.7. Koutecky-Levich analysis.

14

In order to have some indication about the ORR reaction pathway, i.e., if O2 is reduced directly to

15

H2O via a direct 4 e− mechanism, or via a two-steps 2 e− + 2 e− mechanism with the formation of a

16

peroxide intermediate, or only partially to H2O2 via a 2 e− mechanism, a series of electrochemical

17

tests were performed. The first test was the Koutecky–Levich (K–L) analysis. Figure 10 shows the

18

results of the LSV recorded in the O2 saturated electrolyte at different RDE rotation speed for Fe-N-

19

C, Co-N-C, and Cu-N-C catalysts. For all the three catalysts the limiting current densities are lower

20

than those predicted by the Levich equation.26 The variation of the current density with the rotation

21

speed is a direct means to investigate if the reaction is under mass-transport control of reactants

22

diffusing from the bulk solution or under kinetic control.26 As expected, for all of the catalysts there

23

is a noticeable increase in the diffusion-limited current density with the RDE speed. However, no

24

one of the catalysts exhibits a perfect “plateau” in the diffusion-limited region at high η, and this is

25

more evident for Co-N-C catalyst.

A linear trend with an almost parallel slope at different



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Page 38 of 75

1

potentials is observed in the K–L plots for all of the catalysts, enabling the calculation of the

2

number of electrons involved in the ORR, as summarized in Table 5.

3

Another informative aspect of the K–L plot is the intercept of the straight line, which should in

4

principle be 0 if the current is fully controlled by O2 diffusion in solution. This is not the case for of

5

our Me-N-C catalysts, which all exhibit a non-zero intercept (see K–L plots in Figure 10),

6

suggesting the presence of an additional transport limitation, which could be related to the presence

7

of a Nafion film between the solution and the electrode surface. This phenomenon has been

8

reported in the literature for both Pt-based42 and Pt-free catalysts.82 The presence of this Nafion film

9

in our experiments is likely, since a relatively high amount of Nafion was used to prepare the

10

catalyst ink (NCR = 0.2). The related additional transport resistance value can be calculated from

11

the intercept of the K–L plots. However, these values are considerably lower than transport

12

resistance caused by the diffusion in the solution, and can be neglected in the calculation of the

13

mass transport corrected current densities (the calculation is not reported here).

14

Analyzing the results, it must be considered that the K–L theory was developed for a smooth and

15

thin electrode surface.79 However, when the catalyst loading on the electrode is on the order of

16

hundred µg cm−2 (as typically used in the literature for NNM catalysts, and also in this work), we

17

cannot affirm that the catalyst film deposited on the electrode is so thin to be under the hypothesis

18

of the K–L theory.22,83 Among others, a recent publication points out how the K–L method is not

19

suitable to determine number of electrons involved in ORR. In facts, ORR is not a single-step one-

20

way reaction, and it is not always first order with respect to oxygen concentration (as moreover

21

found in our results, see Section 3.4).84 Thus, the ORR does not fulfill the assumptions of the K–L

22

method. In the same work of it is demonstrated that the number of electrons calculated using K–L

23

method is always significantly different from the one calculated using RRDE.84

24


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1

2 3 4 5 6 7 8

Figure 10. LSV recorded at 5 mV s−1 scan rate and different rotation speeds in O2-saturated 0.5 M H2SO4 after background capacitive current subtraction, and the corresponding K−L plots at different potentials in the diffusion-limited region for (a) Fe-N-C, (b) Co-N-C, and (c) Cu-N-C catalysts.

9

Considering the results in Table 5 as a qualitative comparison among our catalysts, Fe-N-C is the

10

catalyst with the better performance in terms of selectivity towards a complete 4 e− ORR. Co-N-C

11

catalyst has a considerably lower selectivity, despite having an Eon very close to the Fe-N-C one. A 39 ACS Paragon Plus Environment


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Page 40 of 75

1

higher number of electrons was obtained for Cu-N-C compared to Co-N-C, but the former has a

2

remarkably lower Eon than the latter.

3 4 5 6

Table 5. Total number of electrons involved in the ORR for Fe-N-C, Co-N-C and Cu-N-C catalysts resulting from the K-L analysis in Figure 10. Catalyst Fe-N-C Co-N-C Cu-N-C

0.05 V vs. RHE 3.73 2.71 3.09

# of e− 0.1 V vs. RHE 3.70 2.75 3.05

0.2 V vs. RHE 3.64 2.87 2.97

7 8 9

3.8. Hydrogen peroxide reduction.

10

To deeper investigate which is the predominant pathway for ORR, the catalysts’ activity toward the

11

H2O2 reduction reaction (HPRR) in the absence of O2 was assessed. To this purpose, LSV were

12

measured in N2-saturated solution after the addition of H2O2 in a concentration of 1 mM, which is

13

very close to the concentration of O2 in saturated 0.5 M H2SO4.60,73 Moreover, this concentration is

14

about the maximum H2O2 concentration that can be found during ORR experiments in RDE.85 As

15

shown in Figure 11a, in spite of almost the same concentration of H2O2 and O2 in the solution, the

16

currents due to ORR are almost 3 times higher for Fe-N-C and Cu-N-C and almost 2 times higher

17

for Co-N-C in comparison to those due to HPRR. Moreover, the increase of HPRR current with

18

cathodic potential is almost linear, and the maximum reduction current is smaller than the diffusion

19

limited currents expected for 1 mM H2O2 from the Levich equation. Similar results have been

20

reported in the literature for heat-treated Fe-N/C catalysts.22,26,82 This suggests that HPRR is under

21

kinetic control within all the scanned potential range. The kinetics of HPRR is sluggish compared to

22

ORR, since it is never fast enough to completely reduce all the available flux of H2O2, even at high

23

η. It must be considered that part of the reduction current measured during the experiment carried

24

out in the presence of 1 mM H2O2 could also be originated from H2O2 chemical 40 ACS Paragon Plus Environment

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1

disproportionation.26,86 In facts, the H2O2 generated during ORR on a catalytic site can be reduced

2

to H2O on the same site or on another site either through electroreduction or through

3

disproportionation.26 In the latter case, the O2 originating from the disproportionation reaction could

4

be electrochemically reduced to H2O2 once again. If the latter case occurs, it will result in an

5

apparent 4 electrons reduction of O2 even if the electroreduction of H2O2 to H2O never occurs, or it

6

occurs in minimum part, as demonstrated for our catalysts (see results in Fig. 11a). In the past,

7

Jaouen and Dodelet investigated the relative importance of H2O2 disproportionation on ORR

8

pathway and they found it to be minor.26 On the other hand, the work of Masa et al. shows that the

9

H2O2 disproportionation occurs fast on a Fe/N/C catalyst in alkaline conditions, and cannot be

10

neglected.87 In this work the measurement of the rate of chemical disproportionation of H2O2 was

11

not performed because, even if occurring, it will result in an apparent 4 electrons ORR pathway, as

12

it can be indirectly inferred from the results in Fig 11 a-b.



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Page 42 of 75

1 2 3 4 5 6

Figure 11. (a) LSV at 1600 rpm in N2-saturated 0.5 M H2SO4 + 1 mM H2O2 and in O2-saturated 0.5 M H2SO4 for Fe-N-C, Co-N-C and Cu-N-C catalysts. (b) H2O2 molar generation calculated from RRDE test performed in 0.5 M H2SO4 for Fe-N-C, Co-N-C, and Cu-N-C and in 0.1 M HClO4 for the Pt/C catalyst.

7

3.9. RRDE test.

8

As a further means to investigate the ORR pathway, RRDE test was performed. Equation (12) was

9

used to calculate the percentage of H2O2 produced during the ORR:

10 11

% H) O) = 100 ∙

)kl /n q

ko p/ rl 4

(12)

12


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1

where Id is the current at the disk, Ir is the current at the ring and N is the ring collection efficiency

2

(44%).27 Figure 11b shows the results, evidencing that Co-N-C is the catalyst producing the highest

3

H2O2 amount, which reaches a maximum value of about 15 % at 0.4 V vs. RHE at 900 rpm. Cu-N-

4

C, leads to the formation of a considerably lower amount of H2O2, with a maximum of 3.5 molar %

5

at 0.5 V vs RHE at 900 rpm, notwithstanding the lower electroactivity in terms of onset and half-

6

wave potentials. Fe-N-C is the catalyst that shows the lowest H2O2 production (between 0.5 and 2

7

% at 900 rpm), confirming its better performance not only in terms of activity but also in terms of

8

selectivity. Similar H2O2 production values were measured by other groups for similar Fe-N/C

9

pyrolyzed catalysts in acidic conditions.22,26,79 A commercial Pt/C catalyst was also analyzed for

10

comparison. For this catalyst, the H2O2 generation at lower η is considerably lower than for the Me-

11

N-C catalysts, especially in the anodic potential scan direction. In the low η region (approximately

12

between 0.85 and 0.60 V vs. RHE) the H2O2 generation is higher in the cathodic scan direction, due

13

to the presence of oxides on the Pt surface at these potentials, which could partially hinder the

14

complete 4 e− O2 reduction. In addition, a significant increase in the ring current for Pt/C catalyst

15

was detected approaching the hydrogen underpotential deposition region. This behavior is typical of

16

Pt in acidic conditions.42,46,79

17

The H2O2 amount detected by RRDE technique are considerably lower than the values calculated

18

by the K–L analysis in Section 3.7. These discrepancies are a further confirmation that the

19

conditions of validity of the K–L model are not verified in these ORR measurements, as pointed out

20

in Section 3.7.

21

From a rigorous point of view, the RRDE experiments must be carried out at different rotation

22

speeds, to investigate the effect of the rotation speed on the H2O2 production.84,88 Thus, the results

23

in Figure 11b should be considered more from a qualitative point of view, in terms of comparison

24

between the different behavior induced by the different transition metals (Fe, Co, Cu) in the three

25

different catalysts with respect to the ORR selectivity, rather than as an extremely rigorous

26

quantitative determination of H2O2 production. The results in Figure 11b show an evident 43 ACS Paragon Plus Environment


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qualitative behavior in agreement with the proposed theory of the “asymmetric volcano plot” and

2

the redox-mediated formation of the active sites, with other experimental and theoretical studies

3

reported in the literature, and with the other results shown in this work.

4

By concluding, from the results of the tests described in Sections 3.7, 3.8, and 3.9, it can be deduced

5

that all the three Me-N-C catalysts are able to reduce O2 (to H2O and/or H2O2), but much less to

6

reduce H2O2 to H2O, as discussed in our previous work for Fe-N-C catalyst under acidic

7

conditions.60 However, on Co-N-C catalyst, the partial 2e− O2 reduction to H2O2 proceeds in a

8

greater extent than on Fe-N-C and Cu-N-C. This behavior can be related to the findings of Section

9

3.2. In particular, a weaker adsorption of O2 on Co-based active sites can explain both the lower

10

peak current densities observed in the tests of Section 3.2, as well as a higher detection of H2O2 in

11

RRDE experiments. O2 is adsorbed weakly on Co-based sites, resulting in an easier desorption after

12

the partial 2 e- reduction, before the O–O bond breaking occurs.25,89,90

13 14

3.10. Resume of experimental evidence and discussion about the ORR kinetics.

15

Analyzing and trying to interpret the results of Sections 3.3.1 and 3.3.2, we can consider some

16

experimental evidence reported in other literature studies about biological molecules where

17

adsorption, transport and reduction of O2 on Fe-, Co-, and Cu-macrocyclic units is involved.

18

As a background and confirmation of our experimental results, the work of Chen et al.25 about

19

experimental and theoretical (DFT) studies on Fe-phthalocyanine and Co-phthalocyanine based

20

catalysts for ORR, shows that the O2 adsorption energy on the metallized center is related to the

21

catalytic activity. In particular, the Fe-based catalyst showed almost three times higher O2

22

adsorption energy (in absolute value) compared to the Co-based one, showing at the same time 100

23

mV more positive Eon and E1/2, thus suggesting that the O2 adsorption energy is a key parameter for

24

enhancing the ORR kinetics. In addition, they found that the selectivity towards 4 e− reduction is

25

associated with the adsorption energy and the adsorption configuration of the peroxide intermediate

26

on the catalytic site. In particular, the O–O bond cleavage during the adsorption is more favored on 44 ACS Paragon Plus Environment

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1

the Fe-based sites, which also show higher H2O2 adsorption energy, and led to the formation of a

2

lower amount of peroxide during ORR. On the contrary, on the Co-based sites, the lower H2O2

3

adsorption energy hampers the O–O bond breaking, increasing the relative significance of the 2 e−

4

pathway producing H2O2 on this catalyst. All these considerations are confirmed by our

5

experimental results (see Section 3.9).

6

Molecular dynamics simulations conducted on Co-substituted proteins responsible for O2

7

adsorption and transport in living beings (e.g. hemoglobin and myoglobin),90 which contain Fe-

8

based centers for the oxygen bonding, have shown that the oxygen ligand rotational motion around

9

the metal-oxygen bond is considerably faster than for -Co−O2 than for Fe−O2. Consequently, the

10

Fe-complex shows more localized ligand sites, while for Co-complex several configurations are

11

possible with a higher mobility of -Co−O2 compared to Fe−O2, supporting the previous results.

12

Even though we cannot demonstrate the existence of such these porphyrin- and phthalocyanine-like

13

structures in our catalysts after the pyrolysis, based on some literature studies, the formation of Me-

14

Nx clusters embedded in graphene, and even more connecting the edges of different graphene

15

segments (with the presence of Fe–N4 active sites bridged between graphitic micropores), was

16

found to be energetically favorable.91,92 This is in agreement with experimental evidence reported in

17

the literature,93,94 as well as in this work, considering the high microporosity of these catalysts.41

18

The ORR catalytic activity is related to the binding energy of O2 on the catalyst surfaces, thus, a

19

good ORR catalyst must have an optimal (neither too strong nor too weak) binding energy of O2 on

20

its surface.38,91,95,96 In particular, an optimal Me-N-C catalyst for ORR should bind the O2 molecule

21

to the transition metal-based active site neither too strongly nor too weakly. In this regard, the series

22

of 3d-type transition metals (from Cr to Zn) is expected to exhibit a volcano shape as a function of

23

the O2 adsorption energy.

24

The dependence of the rate of an electrocatalytic reaction on electrode potential is set by two

25

complementary effects of the potential: (i) the potential-dependent formation of the active sites via a



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redox-mediated mechanism, and (ii) the lowering of the Ea of the processes (involving electron

2

transfer) occurring on the active sites.39

3

The turnover frequency (TOF), which is related (inversely proportional) to Ea, and Eredox of the

4

active center, are in turns strictly related to each other, being determined by the intrinsic nature of

5

the active sites. If the active sites possess a high Eredox, their site-blocking effect is lowered, but at

6

the same time, they will have a low O2 binding energy, which causes a low TOF (high Ea barrier to

7

be overcome). As described by J. Li et al.,38,66 considering both Ea and Eredox, the optimal Eredox

8

value should be between 300 and 400 mV lower than the ORR thermodynamic potential (E0 =

9

+1.23 V vs. SHE).

10

According to the kinetic equation describing the relation between current (i) and electrode applied

11

potential (E):38,39

12

cse ∝

13

1−u =

14

assuming that αc = 0.5, a Tafel slope of 120 mV dec−1 must be observed for ORR. Nevertheless,

15

both Pt-based catalyst and several highly active Me-N-C catalysts reported in the literature, show

16

curvature in the Tafel slope, which is potential-dependent. In particular, as for our Fe-N-C (and

17

partially also for Co-N-C) catalyst, the Tafel slope in the low η region is ~ 60 mV dec−1, increasing

18

to ~ 120 mV dec−1 at high η. This behavior is well explained by the theory of the redox surface

19

mediated generation of active sites at low η.38–40

20

Here we will try to resume all the experimental evidence found in this work, and to propose an

21

explanation of the behavior of the three different metals (Fe, Co, Cu) according to the site-blocking

22

effects and the redox potential mediated theory of the active sites formation.38

23

Fe-N-C

24

The behavior of Fe-N-C catalyst is analogous to the behavior of Pt/C catalysts: it exhibits a double

25

Tafel slope behavior with a 60 mV dec−1 slope at low η, becoming gradually 120 mV dec−1 at

J

Jp|

∙ c1 − ue ∙ vwx /− =

J

~S Jp}

(y

4 ∙ vwx /−

(( z {

4 (13)

(14)


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1

higher η. This behavior is in accordance to the theory of the site-blocking of a redox-mediated

2

mechanism of formation of the active sites. Moreover, according to what proposed in the

3

literature,38 the Eredox (and consequently the O2 adsorption energy) of Fe-N-C active sites should be

4

located more in proximity of the top of the asymmetric “volcano plot” compared to Co-N-C (likely

5

located on the right side branch) and Cu-N-C (likely located on the left side branch).

6

Regarding the apparent activation energy, Ea, which is related to the TOF according to what stated

7

by Mukerjee and co-workers, the value is higher at high E, and it decreases with the decrease of E.

8

The RO2 at potentials close to Eon (low η) is < 1, increasing from about 0.5 moving towards 1 with

9

the decrease of E. The calculation of the RH+, even if less accurate, shows a decrease with the

10

potential.

11

According to Equation (14), at potentials E >> Eredox the coverage (θ) approaches unity, and thus no

12

active sites are free for the ORR, and the ORR faradaic current is null. At E RO2 of Fe-N-C at low overpotentials (higher

6

amount of “free” active sites already available, that is, lower θ); Ea of Co-N-C > Ea of Fe-N-C at

7

high overpotentials (lower O2 binding energy); steeper decrease of Ea with potential for Co-N-C

8

compared to Fe-N-C; higher H2O2 production (related to the lower site-O2 binding energy, which

9

results in a faster desorption of the peroxide intermediate in the bulk of the electrolyte before the O–

10

O bond cleavage; lower ORR peak currents in the differential CV for the reduction of adsorbed O2

11

(confirming that O2 is only weakly adsorbed on the catalyst surface, being adsorption much

12

hindered than in Fe-N-C).

13

By concluding, for Co-N-C, all these experimental evidence suggest that the Eredox is located in the

14

right side branch of the asymmetric volcano plot postulated by Mukerjee et al., and the Ea of the

15

rate determining step of the faradaic ORR is higher than for Fe-N-C. In addition, the steeper change

16

in the Tafel slope at high η for Co-N-C seems to be more driven by the achievement of the

17

maximum number of active sites available on the catalyst surface,40 being this the factor limiting the

18

current density, more than the kinetics of the ORR rate determining step.

19

Cu-N-C

20

Compared to Fe-N-C and Co-N-C its Eon is remarkably lower, and the most evident difference that

21

it shows compared to the former catalysts is the single Tafel slope of 120 mV dec−1. It also shows

22

other differences, having an almost constant Ea at different potentials. The analogy is the increase of

23

the RO2 with the increase of η in the kinetic control potential range. Analogously to Fe-N-C catalyst,

24

Cu-N-C exhibits well developed adsorbed ORR current peaks (located at lower E). The reason why

25

a single 120 mV dec−1 Tafel slope is observed for Cu-N-C could be attributed to the fact that for its

26

active sites the Eredox potential is situated on the left-side branch of the asymmetric “volcano plot”, 49 ACS Paragon Plus Environment


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1

corresponding to a high O2 adsorption energy, which somehow hampers the ORR at low η. The

2

reason why the 60 mV dec−1 Nernstian Tafel slope is not observed for this catalyst could be

3

attributed to the fact that the site blocking species are released at a potential higher than the Eon of

4

the ORR. Thus, due to the too high “site−O2” binding energy, the ORR occurs at lower potentials,

5

when all the available active sites are “free” from blocking species (θ ≈ 0). The ORR activity of the

6

Cu-N-C catalyst is too low, in consequence of its low Eon and too strong O2 binding energy on the

7

active sites.96 These considerations also explain the almost constant value of Ea (no ORR limited by

8

the “Nernstian” generation of active sites), and possibly also the RO2 lower than 1 (between 0.5 and

9

0.8) in the kinetic zone of the polarization curve.

10 11

4. Conclusions

12

A set of electrochemical tests in the three-electrodes cell has been proposed to investigate the

13

kinetics of the ORR in acidic medium on a series of Me-N-C (Me = Fe, Co, Cu) via CV and RDE-

14

RRDE. These tests enabled to determine the following parameters: Tafel slope, reduction of

15

adsorbed O2, reaction orders for O2 and H+, apparent activation energy, and H2O2 selectivity. The

16

results obtained for the three different catalysts evidenced a clear influence of the transition metal in

17

the ORR activity and kinetics.

18

Fe-N-C was found to be the most performing catalyst, showing a potential dependent double Tafel

19

slope behavior, according to the surface redox-mediated coverage of active sites. Compared to the

20

other two catalysts, its Ea and Eredox should be located more in the proximity to the top of the

21

asymmetric “volcano plot”, as proposed in the literature. Indeed, its behavior is more similar to the

22

one of a Pt-based catalyst.

23

Co-N-C also shows an apparently potential dependent double Tafel slope behavior, but it performs

24

slightly worse compared to Fe-N-C in terms of Eon. Its adsorbed O2 reduction Ip were found to be

25

considerably lower, and it produces much more H2O2 during ORR. These results indicate that its O2

26

adsorption energy is weaker, suggesting that the Eredox of its active sites should be located more 50 ACS Paragon Plus Environment

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1

positively compared to Fe-N-C, causing an enhancement in the Ea of the ORR. Co-N-C should be

2

located more on the right-side branch of the asymmetric “volcano-plot” compared to Fe-N-C.

3

Cu-N-C shows a considerably lower Eon, a single Tafel slope behavior, and high adsorbed O2 Ip

4

similarly to Fe-N-C, but more displaced to negative potentials. This behavior suggests that the Eredox

5

for the active sites of Cu-N-C should be more negative, causing an enhancement of Ea due to the

6

stronger O2 adsorption energy. Thus, Cu-N-C cannot be considered a suitable ORR catalyst, since it

7

is located too negatively in the left-side branch of the asymmetric “volcano plot”.

8 9

Acknowledgments

10

Funding by the Italian Ministry of Education, University and Research [PRIN NAMEDPEM,

11

“Advanced nanocomposite membranes and innovative electrocatalysts for durable polymer

12

electrolyte membrane fuel cells”, grant n. 2010CYTWAW] is gratefully acknowledged. Funding

13

from the Spanish Ministry of Economy project ENE2016-77055-C3-1-R and from the Madrid

14

Regional Research Council (CAM) project S2013/MAE-2882 (RESTOENE-2) are acknowledged

15

as well.

16 17

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Figure 1. (a) SV of Fe-N-C catalyst in O2-saturated 0.5 M H2SO4 at 900 rpm with different catalyst loadings and NCR = 0.2. (b) Tafel plot obtained from (a) after mass-transport correction and normalization per mass of catalyst on the electrode. 105x152mm (300 x 300 DPI)

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Figure 2. (a) CV of Fe-N-C catalyst with an ink prepared with different NCR recorded in N2-saturated 0.5 M H2SO4 solutions at 10 mV s−1 with a catalyst loading of 0.64 mg cm−2. (b) SV of Fe-N-C catalyst with an ink prepared with different NCR in O2-saturated 0.5 M H2SO4 at 900 rpm. (c) Tafel plot obtained from polarization curves in (b) after mass-transport correction. 105x233mm (300 x 300 DPI)



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Figure 3. (a) SV of the Me-N-C catalysts (loading 0.64 mg cm−2) measured in O2-saturated 0.5 M H2SO4 at 900 rpm. The SV of a commercial Pt/C catalyst measured in 0.1 M HClO4 in both cathodic and anodic scan directions is also shown for comparison. (b) Plot of the linear zones of the potential vs logarithm of the mass-transport corrected current densities from (a). 105x153mm (300 x 300 DPI)


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Figure 4. Mass transport corrected Tafel plots for Fe-N-C, Co-N-C, and Cu-N-C catalysts: (a) Current density referred to the geometric area of the electrode; (b) Current density referred to the mass of catalyst deposited on the electrode. 104x149mm (300 x 300 DPI)



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Figure 5. CV recorded at 10 mV s−1 in electrolyte saturated with O2 (black curve) and N2 (red curve), and their subtraction (blue curve), peak current density of the subtracted CV in function of the square root of scan sate, and peak potential of the subtracted CV in function of the logarithm of scan rate. (a) Fe-N-C; (b) Co-N-C; (c) Cu-N-C. 210x164mm (300 x 300 DPI)


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Figure 6. Differential CV recorded in O2-saturated 0.5 M H2SO4 at different scan rates; FWHM of the differential CV peaks vs. the square root of the scan rate; cathodic transfer coefficient for the reduction of adsorbed O2 calculated from the FWHM of the differential CV peaks widths for (a) Fe-N-C, (b) Co-N-C, and (c) Cu-N-C catalysts. 210x166mm (300 x 300 DPI)



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Figure 7. Tafel plot in the kinetic zone of the polarization curve recorded in 0.5 M H2SO4 at 900 rpm with different O2 partial pressures at 25°C, and double-logarithmic plots of mass transport corrected current density at different potentials in function of O2 concentration for (a) Fe-N-C, (b) Co-N-C, and (c) Cu-N-C catalysts. 210x226mm (300 x 300 DPI)


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Figure 8. SV recorded at 900 rpm in O2-saturated H2SO4 solutions at different pH, and the respective Tafel plots after mass-transport correction for (a) Fe-N-C, (b) Co-N-C, and (c) Cu-N-C catalysts. 210x230mm (300 x 300 DPI)



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Figure 9. Effect of the temperature on ORR activity in RDE experiments in O2-saturated 0.5 M H2SO4 at 900 rpm. Tafel plot after correction for mass-transport and O2 solubility at different temperatures, and the respective Arrhenius plot at different potentials for (a) Fe-N-C, (b) Co-N-C, and (c) Cu-N-C catalysts. 210x224mm (300 x 300 DPI)


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Figure 10. LSV recorded at 5 mV s−1 scan rate and different rotation speeds in O2-saturated 0.5 M H2SO4 after background capacitive current subtraction, and the corresponding K−L plots at different potentials in the diffusion-limited region for (a) Fe-N-C, (b) Co-N-C, and (c) Cu-N-C catalysts. 209x227mm (300 x 300 DPI)



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Figure 11. (a) LSV at 1600 rpm in N2-saturated 0.5 M H2SO4 + 1 mM H2O2 and in O2-saturated 0.5 M H2SO4 for Fe-N-C, Co-N-C and Cu-N-C catalysts. (b) H2O2 molar generation calculated from RRDE test performed in 0.5 M H2SO4 for Fe-N-C, Co-N-C, and Cu-N-C and in 0.1 M HClO4 for the Pt/C catalyst. 105x152mm (300 x 300 DPI)


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(Me = Fe, Co, Cu) Catalysts in Acidic Medium - ACS Publications

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