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Effect of pH on the Activity of Platinum Group Metal-free Catalysts in Oxygen Reduction Reaction Santiago Rojas-Carbonell, Kateryna Artyushkova, Alexey Serov, Carlo Santoro, Ivana Matanovic, and Plamen Atanassov ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03991 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 1, 2018
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ACS Catalysis
Effect of pH on the Activity of Platinum Group Metal-free Catalysts in Oxygen Reduction Reaction Santiago Rojas-Carbonell, Kateryna Artyushkova, Alexey Serov, Carlo Santoro, Ivana Matanovic and Plamen Atanassov* Department of Chemical & Biological Engineering, Center for Micro-Engineered Materials (CMEM), University of New Mexico, Advanced Materials Lab, 1001 University Blvd. SE, Albuquerque, NM 87131 (USA) ABSTRACT: The impact of the electrolyte’s pH on the catalytic activity of platinum group metal-free (PGMfree) catalysts towards the oxygen reduction reaction (ORR) was studied. The results indicate that the ORR mechanism is determined by the affinity of protons and hydroxyls towards multiple functional groups present on the surface of the PGM-free catalyst. It was shown that the ORR is limited by the proton-coupled electron transfer at pHs below 10.5. At higher pHs (>10.5), the reaction occurs in the outer Helmholtz plane (OHP), favoring hydrogen peroxide production. Using a novel approach, the changes in the surface chemistry of PGMfree catalyst in a full pH range were studied by X-ray photoelectron spectroscopy (XPS). The variations in the surface concentration of nitrogen and carbon species are correlated with the electron transfer process and overall kinetics. This study establishes the critical role of the multitude of surface functional groups, presented as moieties or defects in the carbonaceous “backbone” of the catalyst, in mechanism of oxygen reduction reaction. Understanding the pH-dependent mechanism of ORR provides the basis for rational design of PGM-free catalysts for operation across pH ranges or at a specific pH of interest. This investigation also provides the guidelines for developing and selecting ionomers used as “locally-confined electrolytes,” by taking into account affinities and possible interactions of specific functional groups of the PGM-free catalysts with protons or hydroxyls facilitating the overall ORR kinetics.
KEYWORDS: Oxygen Reduction Reaction, PGM-free catalysts, pH-dependent activity, XPS, mechanism
INTRODUCTION. Fuel cells transform the chemical energy contained in the fuel into useful electricity. Commonly, the fuel cells utilize oxygen as an oxidant due to its high redox potential and its availability in the atmosphere at practically no additional cost.1 Oxygen reduction reaction (ORR) is one of the most studied electrochemical processes due to its importance in living organisms as well as in electrolytic and power generation technologies. ORR is of particular importance to fuel cell technology, where it is used across a wide range of temperatures and at extreme pH regions: acidic2,3 and alkaline2,4. New biological fuel cells and bio-electrochemical reactors are emerging as a technology that utilizes ORR in a broad range of pH of various enzymes, supramolecular assemblies of complex biocatalysts, and microorganisms and their communities (biofilms).5,6,7 In acidic media, Pt and other platinum-group-metals (PGMs) catalysts exhibit high efficiency in the reaction of oxygen reduction.8–10 In alkaline media, Pt has excellent electrocatalytic activity towards ORR but it very sensitive to poisoning by anions, and consequently, prone to losing its activity.11 Also, Pt cannot be utilized in conditions at which pollutants are present (e.g., microbial fuel cells) due to fast ACS Paragon Plus Environment
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poisoning.12–14 High cost of PGMs and poisoning issues led to the extensive research in the attempt to identify readily available low-cost alternative materials with the performances comparable to that of platinum.15 From the alternative PGM-free catalysts, two groups of materials stand out as suitable for applications across the range of pH: i) metal-free carbon-nitrogen systems N-C16,17 and ii) M-N-C type of materials where PGM-free transition metal is incorporated into carbon-nitrogen matrix (M=Fe, Co, Ni etc).2,18–20 Carbonaceous materials have several characteristics that make them promising alternatives to PGM catalysts, such as high surface area, high chemical stability, high electrical conductivity, low-cost, and commercial availability. Moreover, carbonaceous materials are resistant to the influence of pollutants. 21,22
The catalytic activity of these materials towards ORR, however, is quite low in acidic
alkaline media,
16,17
16,17
and
while in neutral media the output can be comparable to PGM-free M-N-C
materials.22–24 Therefore, presently carbonaceous materials are mainly used as catalyst supports rather than as catalysts in both acidic and alkaline media.25–28 In PGM-free M-N-C catalyst, earth-abundant transition metals M such as Mn, Fe, Co or Ni are atomically dispersed within the carbon-nitrogen rich matrix, designated as N-C.29–31 These are usually synthesized from metal, nitrogen, and carbon-containing precursors through high-temperature treatment (pyrolysis).32–39 M-N-C catalysts have demonstrated good performances in acidic media,40–42 outstanding and unprecedented results in neutral media,30,43–45 and comparable and even superior to Pt performance in alkaline media.46–49 As mentioned before, three types of fuel cells based on different pH, are proton exchange membrane fuel cells (PEMFCs; pH~1), microbial fuel cells (MFCs; pH~7), and alkaline membrane fuel cells (AMFCs; pH~13). The mechanism of ORR at different pH media differ.50 For example, in an acidic media; protons are combined with oxygen at the cathode in several alternative pathways. ORR can follow a 2e- transfer mechanism with the production of H2O2, a 4e- transfer mechanism with H2O as a final product (full reaction as ½O2 + 2H+ + 2e- H2O
51
) and a sequential 2x2e- mechanism in which
hydrogen peroxide is being reduced by the same or different catalytic site with the production of water.52 H2O2 produced during the first step of ORR can also be converted to water through chemical decomposition.53 The role of different nitrogen and metal surface moieties in these three types of mechanisms in acidic media was reported recently.52 In alkaline media, OH- plays a crucial role in the ORR mechanism being its primary product. The reaction can occur via a 2e- mechanism forming HO2and OH- or a direct 4e- mechanism with the generation of OH- 51 (full reaction as O2+ H2O+ 2e- 4OH). A sequential 2x2e- mechanism can also take place.2,4
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Another difference between ORR in different pHs is based on inner-sphere and outer-sphere reactions. In alkaline media, surface hydroxyl groups promote surface-independent outer-sphere electron transfer,54 while in acidic media immediate involvement of active sites such as metal coordinated to nitrogen in inner-sphere electron transfer is of direct relevance.55,56 The mechanism of ORR in neutral media is far less studied and, probably, the most challenging due to the limited availability of both H+ and OH-. Recently, Malko et al. have conducted a pH study on the FeN-C catalyst and it was shown that the reaction follows acidic-type mechanism until pH of 11, after which the mechanism switches to the alkaline-type.57 It is known that M-N-C catalysts consist of different types of nitrogen (e.g., pyridinic, pyrrolic, graphitic, metal-nitrogen directly coordinated, etc.) as well as different types of metal-contained (atomically dispersed and zero-valent metal-rich phases) moieties that contribute differently to the electron transfer mechanism.52,58 Moreover, the type and abundance of surface oxygenated species, such as -OH, -OC, =O, and the degree of protonation of nitrogen may also be affected by changes in proton concentration.59–65 The influence of the changes in the catalysts surface chemistry caused by different pHs on the ORR performance has not yet been spectroscopically identified. The effect of the chemistry and morphology of the PGM-free catalysts have been previously studied for extreme pHs, mainly for pHs arround 1, 7 and 14.31,57,66–69. To the best of our knowledge, a comprehensive study that explains the connection between the change in the electrolyte’s pH, it’s effect on interfacial chemistry spectroscopically measured, and the M-N-C catalytic activity has never been presented. In this study, Fe-N-C catalyst was synthesized using the sacrificial support method (SSM)70 utilizing a charge-transfer organic salt, nicarbazin, as the organic precursor containing both N and C atoms.71 The dependence of surface chemistry of the catalyst on pH was studied by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements using Rotating Ring Disk Electrode (RRDE) technique were performed in 18 different electrolytes with pH values covering the entire range from pH 1.1 to pH 13.5. For the first time, an unambiguous relationship between electrolyte pH, surface chemistry, and catalyst ORR performance has been established. The results of this work may impact not only traditional lowtemperature fuel cell technologies, such as polymer electrolyte membrane fuel cells (PEMFC) and alkaline membrane fuel cells (AMFC) and corresponding electrolyzer technologies, but also a broad spectrum of newly introduced bio-electrochemical devices such as biofuel cells, microbial fuel cells, and microbial bioreactors.
EXPERIMENTAL CATALYST SYNTHESIS ACS Paragon Plus Environment
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The PGM-free catalyst used in this study is iron-nicarbazin derived and prepared following the sacrificial support method based on the procedure described before72 and used in practical applications, including automotive PEMFC and AMFC. The details of synthesis are included in the Supplementary Information section.
ELECTROCHEMICAL MEASUREMENTS The rotating ring disk electrode (RRDE) technique was used to assess the electrochemical performance of the PGM-free catalyst. An optimized loading (Figure S1) of 175 µg cm-2 of catalyst was obtained by depositing ink, made from a suspension of 5 mg of catalyst in 850 µL of isopropanol and 150 µL of Nafion (0.5wt% in isopropanol), onto the mirror polished glassy carbon disk. This loading ensured a complete coverage of the glassy carbon. The electrolytes used were prepared by selecting the appropriate buffer solution that would provide buffering capacity for the required pH range. The pH values were measured using an Orion Star A111 pH meter (Thermo Scientific). The buffer solution used are shown in Table 1. The RRDE cyclic voltammetry (CV) was carried out using WEB30-Pine bipotentiostat and a Pine Instruments Rotator (Pine Instruments, Raleigh, NC) as described in detail in Supplementary Information. The potential for the ring was selected to be within the region where the water is thermodynamically stable and in the region where the peroxide is electrochemically oxidized as shown in Pourbaix diagram in Figure S2. Table 1. Buffers used as electrolytes for each pH value pH
Constituents
pH
Constituents
1.1
Phosphoric acid
6.1
Potassium diphosphate, potassium dihidrogenphosphate
1.3
Sulfuric acid
7.2
Potassium diphosphate, potassium dihydrogenphosphate
1.6
Perchloric acid
8.4
Potassium diphosphate, potassium dihidrogenphosphate
2.4
Phosphoric acid, potassium dihydrogen phosphate
9.6
Sodium bicarbonate and sodium carbonate
2.8
Citric acid-potassium dihydrogen phosphate
9.8
Boric acid, potassium hydroxide
3.6
Malic acid, potassium hydroxide
10.6
Sodium bicarbonate and sodium carbonate
4.6
Malic acid, potassium hydroxide
11.2
Boric acid, potassium hydroxide
5.2
Acetic acid, potassium acetate
12.5
Boric acid, potassium hydroxide
5.5
Potassium diphosphate, potassium dihidrogenphosphate
13.5
Potassium hydroxide
CHEMICAL CHARACTERIZATION
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The surface chemistry of PGM-free catalyst was analyzed by XPS Kratos Ultra DLD Spectrometer as has been described before.52 A subset of the electrolytes was used to make suspension of catalysts to study the chemistry of catalyst resulting from interacting with different buffer solutions. This was attained by adding 100 µL of the electrolyte to 1 mg of catalyst, which was then left to interact for two days. The XPS spectra from the catalyst and dried suspension of catalysts with electrolytes were obtained. Three areas per sample were analyzed. High-resolution N 1s, C 1s, and O 1s spectra were processed using CasaXPS software.
RESULTS: SURFACE CHEMISTRY High-resolution XPS experiments were performed to understand the surface chemical composition of the catalyst. Figures S3 and S4 show high-resolution N 1s and C 1s spectra obtained and fitted with individual peaks according to the previously established curve fit.45,73,74 There are multiple types of nitrogen in this catalyst, such as pyridinic, nitrogen coordinated to iron Nx-Fe,
pyrrolic or
hydrogenated, graphitic, quaternary and multiple types of nitrogen coordinated with oxygen. Protonated nitrogens such as protonated pyridine are also detected in pristine catalyst powder. Carbon consists of graphitic carbon, aliphatic carbon, carbon coordinated to nitrogen and different types of surface carbonoxygen species such as phenolic (-C-OH), lactone and pyrone(-C=O) and carboxylic (-COOH). There are several important considerations for understanding the influence of the electrolyte pH on the oxygen reduction reaction. The first is the prevalence and concentration of either protons or hydroxyl ions in the electrolyte, which determines the inner vs. outer-sphere electron transfer mechanism of the reaction. A clear distinction between these mechanisms is expected at high and very low pHs while in the middle range75 between 5 and 9, this separation is unclear and both mechanisms can be occurring during ORR. Excess of either hydronium ions or hydroxyls can also result in their specific binding to the moieties at the surface of the catalyst, therefore, modifying possible active sites. The second consideration is protonation/deprotonation of nitrogen and carbon-oxygen groups that are present on the surface of the catalyst layer. Several groups59–65,75–82 have investigated the acid dissociation constants (pKa) displayed by carbon-based materials that contain nitrogen and oxygen species. The values of pKa associated with different oxygen and nitrogen-containing moieties that can lose or accept proton are summarized in Table 2. At pH values higher than the pKa, the nitrogen and oxygen species loses its protons and becomes either anionic or neutral. Oppositely, at pH values lower than the pKa, that chemical species will capture protons becoming neutral or cationic. Based on these pKa values, it can be predicted the pH ranges at which surface carbon and nitrogen moieties will change their state affecting, therefore, the affinity of oxygen towards binding and the type of possible oxygen
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reduction steps in which these species participate. Figure 1 graphically represents the expected effect the pH has on the chemistry of functional groups of the PGM-free catalyst. Table 2. Acid dissociation constants for the functional groups of PGM-free catalysts. pKa
Group
Reference
-29.5
Graphitic Nitrogen
75
4.5
Carboxylic
64,77–79
6.5
Pyridinic
59,80,82
8.5
Lactonic-Pyrone
63,65,81
10
Phenolic
62,65
17.5
Pyrrolic
75
Figure 1. Schematic of the suggested surface chemistry for the PGM-free catalyst at corresponding pH range, as described in the text: (a) highly acidic (pH
