Chemistry of Multitudinous Active Sites for Oxygen Reduction

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Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal-Nitrogen-Carbon Electrocatalysts Kateryna Artyushkova, Alexey Serov, Santiago Rojas-Carbonell, and Plamen Atanassov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07653 • Publication Date (Web): 29 Oct 2015 Downloaded from http://pubs.acs.org on November 4, 2015

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The Journal of Physical Chemistry

Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition MetalNitrogen-Carbon Electrocatalysts Kateryna Artyushkova*, Alexey Serov, Santiago Rojas-Carbonell, Plamen Atanassov Chemical & Biological Engineering Department University of New Mexico, Albuquerque, NM 87131, USA KEYWORDS: electrocatalysis, oxygen reduction, non-PGM, XPS, structure-to-property ABSTRACT.

Development and optimization of non-platinum group metal (non-PGM) electrocatalysts for oxygen reduction reaction (ORR), consisting of transition metal–nitrogen–carbon (M-N-C) framework is hindered by the partial understanding of the reaction mechanisms and precise chemistry of the active site or sites. In this study, we have analyzed more than 45 M-N-C electrocatalysts synthesized from three different families of precursors, such as polymerbased, macrocycles, and small organic molecules. Catalysts were electrochemically tested and analyzed structurally using exactly the same protocol for deriving structure-to-property relationships. We have identified possible active sites participating in different ORR pathways: (1) metal-free electrocatalysts support partial reduction of O2 to H2O2; (2) pyrrolic 1 ACS Paragon Plus Environment


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nitrogen acts as a site for partial O2 reduction to H2O2; (3) pyridinic nitrogen displays catalytic activity in reducing H2O2 to H2O; (4) Fe coordinated to N (Fe-Nx) serves as an active site for 4e- direct reduction of O2 to H2O. The ratio of the amount of pyridinic and Fe-Nx to the amount of pyrrolic nitrogen serves as a rational design metrics of M-N-C electrocatalytic activity in oxygen reduction reaction occurring through the preferred 4e- reduction to H2O.

Introduction Development of Fuel Cell (FC) technology has an impact on the future of automobile transportation, distributed power generation, portable electronics and communications. Among the several Fuel Cell technologies, one is based on the implementation of Polymer Electrolyte Membranes (PEMFCs). The practical applications of PEMFC depend on the utilization of platinum metal electrocatalysts (usually supported on a highly dispersed carbon material). Significant research effort has been directed towards replacement of the platinum by alternative platinum-free (which may contain Ru or Pd)1-5 or entirely non-platinum group metals (non-PGMs) materials mainly consisting of metal–nitrogen–carbon (M-N-C) network. Heteroatomic polymeric precursors6-11; metal macrocyclic compounds12-21; the metal salt and nitrogen/carbon organic molecules22-25 have been reported as a source for the pyrolytic formation of M-N-C materials. Development and optimization of non-PGM electrocatalysts consisting of an M-N-C framework are hindered by the complex nature of the materials, a partial understanding of the reaction mechanisms and precise chemistry of the active site or sites. Reaction pathways. Possible reaction pathways must be discussed as these have direct implications for designing the most active material. The most efficient ORR mechanism that will lead to the highest efficiency in power generation with reduction of O2 to H2O via 4esingle site mechanism. Alternatively, reduction of O2 may happen via 2 step 2e- reactions 2 ACS Paragon Plus Environment

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(2x2), in which oxygen is first reduced to H2O2, followed by further reduction to H2O. Those two consecutive steps may occur on the same (single type) active site or two different active sites (dual site mechanism) as shown in Figure 1.19 Such heterogeneous multicomponent materials as M-N-C contain multiple types of surface chemical moieties, which may act as: (1) Site 1 for O2H2O2 - first 2e- step reduction (S1) (2) Site 2 for H2O2H2O - second 2e- step reduction (S2) (3) Site ½ for two steps of first and second 2e- reduction (S*) (4) Site 3 for single step 4e- O2 H2O reduction (S) and (5) be catalytically inactive and serve only as a conductive open framework.

Figure 1. Possible reaction pathways and active sites of ORR. a) dual site 2x2 e-, b) single site 2x2 e- and c) single site direct 4 e- mechanism. The most active ORR electrocatalysts must contain the largest amount of species S and S* and smallest amounts of species S1 with an adequate amount of species S2 to reduce hydrogen peroxide to water. Active sites hypothesis. The relationship between activity and the amount and types of species (surface moieties) present in M-N-C materials is usually established through 3 ACS Paragon Plus Environment


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spectroscopic correlations with such analytical methods as XPS, XANES, XPES, TOF-SIMS, Mossbauer spectroscopy and others.6, 21, 26-33 XPS is the workhorse of surface spectroscopic techniques for the analysis of catalyst structure due to its ability to determine surface oxidation states and chemical environment with a resolution that allows discrimination of chemical species and identification of the surface moieties.27, 34-39 The makeup and structure of the active site/sites of the non-PGM ORR electrocatalysts, including geometry (coordination) and chemistry (composition and oxidation state), remain contentious even after 50 years of research. There is an emerging agreement that iron and nitrogen functionalities, displayed on the surface if the carbonaceous substrate/support, govern ORR activity. This is often combined with a broadly accepted hypothesis that microporous surface area plays a critical role forming the active site.29 Candidate structures participating in ORR include multitudes of nitrogen defect motifs in the carbon matrix of different degrees of graphitization, with metal incorporated as metal nano-particles, corresponding (native) oxides and/or as atomically dispersed, oxidized metal species, linked (coordinated) to nitrogen defects in carbonaceous matrix in a variety of possible configurations. The types of nitrogen and iron-nitrogen functionalities that can be present in these materials are displayed in Figure 2. These consist of in-plane defects such as graphitic nitrogen and Fe-coordinated to three or four nitrogen atoms and a plurality of possible edge sites such as pyridinic, pyrrolic, quaternary and Fe-N2/Fe-Nx sites.

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Figure 2. Nitrogen and Fe-Nx functionalities existing in MNC materials. There are two main hypotheses: one claiming that nitrogen functionalities on/in carbonbased support are directly responsible for their ORR activity

40-43

; and the second suggesting

that nitrogen groups serve as the coordinating environment for metal ions, serving as reactive centers for the ORR.

17, 20, 42, 44-45

Theoretical calculations have confirmed that incorporation

of nitrogen atoms in the carbon matrix enhances its electronic properties atoms are not catalytically active towards a direct reduction of oxygen.

46

, but nitrogen

47-48

Mechanistic

studies of ORR on nitrogen-doped CNTs showed that only the 1st step of oxygen reduction reaction O2H2O2 is occurring without metal.

49

Importance of metal for complete oxygen

reduction by bifunctional mechanism elucidated in detail.19, 27, 38 In the recent work, Uzkan et.al have attempted to distinguish between metal-containing catalysts FeNC in which metal center is preserved and “metal-free” CNx catalysts obtained by acid-washing of FeNC to leach away any exposed metal, in which remaining metal is encased in carbon and does not 5 ACS Paragon Plus Environment


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participate in ORR.50 These two materials were evaluated in terms of their possible active sites and degradation mechanisms, using activity and extended durability tests in a fuel cell and half-cell. CNx was less active than FeNC but more stable. CNx underwent slower degradation in fuel cell stability tests and attained stability faster while FeNC showed continued activity loss. This work has shown that iron plays an active role in catalyzing ORR for FeNC, but iron encased in carbon, as was present in “metal-free” CNx material obtained by acid washing, did not manifest itself with higher ORR activity. CNx catalysts exhibited a higher pyridinic-nitrogen content. The acid-washed FeNC catalysts exhibited higher pyridinic-nitrogen content than unwashed FeNC, which was explained by authors by iron centers leaching out from Nx-Fe centers, leaving behind exposed edge-nitrogen sites. This explanation contradicts previous observations that acid leaching washes away metallic particle but does not affect metal-coordinated to nitrogen. 19, 21, 38, 51 From reported studies, several hypotheses of possible active sites have been suggested as summarized below: 1. There appears to be a growing consensus that the catalytic sites can be attributed to Fe-Nx centers. Based on both XPS and

57

Fe Mossbauer spectroscopy, a direct

correlation between Fe-Nx center and the kinetic current density with respect to ORR has been established.9,

14, 17, 27, 29, 38, 52-53

In-situ X-ray spectroscopy and

electrochemical testing identified dual-site 2 x2 mechanism in acidic media with the Fe-N4 site serving as the primary oxygen adsorption site and coexisting metallic iron nanoparticles, surrounded by the native FexOy oxide, acting as the secondary active sites.54 In another study, thorough characterization using X-ray photoelectron, Mössbauer and in situ X-ray absorption spectroscopies demonstrates the successful formation of FeNxCy moieties that are active towards the oxygen reduction reaction. 55


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2. Pyridinic nitrogen (Npyridinic) also plays a more pivotal role than any other kinds of nitrogen-containing species.25,

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Matter et al. ascribed the high activity of the

catalysts containing a more pyridinic nitrogen to a higher proportion of graphite edge planes exposed.57 It was suggested that the pyridinic nitrogen could be considered to be a marker for edge plane exposure since this type of moiety is predominantly found on the edge of the carbon layer.57 A higher ratio of Npyridinic to Nquaternary was reported to correlate with higher catalytic activity in ORR. 58 3. There are several reports in the literature claiming pyrrolic N (Npyrrolic) as an active site. 48, 59-61 Kurugnot gave evidence that this type of moieties is essential for oxygen adsorption and reduction to water via 4e- mechanism.62 4. In other reports, graphitic

31

and quaternary nitrogen species

63-64

were positively

correlated with ORR activity. Rueff et al. studied the mechanism on metal-free nitrogen-doped graphene-based catalysts and came to conclusions that pyridinic nitrogen improves offset potential and converts mechanism from 2e- to 4e- while activity depends on the amount of graphitic nitrogen.43 Limitations of reported findings. In all of studies used to derive the hypothesis of an active site or sites, their chemical structure is suggested through correlations between activity as measured either as half-wave potential, E1/2, or current at a fixed voltage, i.e. I at 0.8 V, and concentration of certain species as determined by spectroscopic method such as XPS. In these structure-to-performance studies, the type of precursor and synthetic approach is fixed while parameters such as temperature and duration of pyrolysis, ratio of precursors, loadings of precursors are varied. The cross-comparison between different reports discussed above 19, 27, 29, 31, 38, 43, 48-49, 56-64

14, 17,

is complicated because different electrochemical testing protocols, as

well as, different metrics of activity are used. The direct comparison between spectroscopic identifications is also complicated because the different acquisition and data processing 7 ACS Paragon Plus Environment


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parameters are used. For identifications derived from XPS data, for example, different parameters of data acquisition and analysis, such as X-ray sources, spectral resolution, calibration approaches and parameters used in decomposing convoluted spectra into individual components, are used. What is even more significant is that when XPS data interpretation is done inconsistently, the conclusions on the direct relationship between the types of chemical species, as determined from XPS, and possible active site may be erroneous. The experimental N 1s spectra are typically resolved into several intensity distributions by fitting Gaussian line-shapes. It is essential to use adequate parameters during this resolution. Above all, the width of the peaks has to be fixed to the same value to compare spectra from one sample to another. Making conclusions on the decrease and increase or disappearance and appearance of peaks and, therefore, species, when the widths of the peaks used in the resolution are not only different within the same spectrum but also varies from spectra to spectra (sample to sample) is incorrect.33 We have recently published a minireview, which discussed the requirements for appropriately drawn conclusions on structures derived from high-resolution XPS spectra for bio-nanocomposites. The only difference between bio-nanocomposites and M-N-C electrocatalysts is that the latter do not suffer from time and X-Ray sensitivity. All other requirements, such as the use of standards for understanding appropriate width of peaks used in the resolution, proper charge calibration using Au powder rather than the internal carbon and cross-correlation between spectra for identification of species, must be met.65 Besides cross-laboratory study published in 200928, there is no systematic study comparing performance and structure of electrocatalysts obtained from different precursors at a variety of compositions that are tested electrochemically using the same fixed protocol and analyzed spectroscopically using the same acquisition and data processing approach.


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Systematic structure-to-performance study. Due to the heterogeneous nature of the materials the understanding of the interplay between chemistry, activity and mechanism of ORR is still missing. The lack of this knowledge hinders the rational design for performance optimization. In this report, we present the data from more than 40 electrocatalysts containing metal and 8 without metal from three families of precursors, such as polymer-based, macrocycles and small organic molecules. The precursors used are shown in Table 1. For each of the precursors, several electrocatalysts were synthesized using the same sacrificial support method introduced by Serov 5, 8, 24-26, 66-72, at different temperatures of pyrolysis, ratios between the precursor and iron salt and two ways of leaching the support, KOH or HF. Each of the electrocatalysts was electrochemically tested using exactly the same RDE protocol and analyzed structurally by XPS for deriving structure-to-property relationships for the identification of active sites. Table 1. Precursor types used as nitrogen and carbon source

Name

Class

C/N ratio

Formula

Carbendazim

small organic molecule

9/3

C9H9N3O2

Imidazolidinyl urea


11/8

C11H16N8O8

Piperazine


4/2

C4H10N2

Diketo pyrrolo-pyrrole (DPP)


6/2

C6H2N2O2

Aminotetrazole monohydrate (ATM)


1/6

CH3N5 H2O

4-aminoantipyrine (AAPyr)


11/3

C11H13N3O

Hydroxysuccinimide


4/1

C4H5NO3

FeTPP

macrocycle (porphyrin)

44/4

C44H28FeN4

Poly-ethylene imine(PEI)

Polymer

2/1

(C2H5N)n

Poly-ethylene imine with urea

Polymer

2/1-1/2

(C2H5N)n+CH4N2O

Poly(vinyl pyridine) (PVP)

Polymer

7/1

(C7H7N)n

Polyaniline (PANI)

Polymer

6/1

[C6H4NH]n

Polypyrrole (PP)

Polymer

4/1

(C4H2NH)n

2. Experimental Section 9 ACS Paragon Plus Environment


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Catalysts preparation. Modified sacrificial support method was used to prepare all catalysts.

5, 8, 24-25, 66-72

First, 10g of silica (Cab-O-Sil® L90, surface area 90 m2 g-1) was

dispersed in 100ml of water in an ultrasound bath. Then, suspension of 25g organic precursor (Table 1) in the appropriate solvent was added to the silica and ultrasonicated for 20 minutes. Finally, a solution of 2.5g iron nitrate (Fe(NO3)3*9H2O, Sigma-Aldrich) was added to SiO2precursor solution and ultrasonicated for 8 hours (the total metal loading on silica was calculated as ~15wt.%). After ultrasonication, viscous solution of silica, iron nitrate and the organic precursor was dried overnight at T=85 °C. The obtained solid was ground to a fine powder in an agate mortar, and then subjected to heat treatment (HT). The HT was undertaken under normal conditions at ultra-high pure (UHP) nitrogen atmosphere (flow rate 100 ccm), 20 deg min-1 temperature ramp rate, and 1.5 hour pyrolyzation time. The HT temperatures were 750°C, 800°C, 850°C and 900°C as listed in Tables S1 and S2 in Supporting Information. After heat treatment, silica was leached by 25 wt.% HF or 7M KOH overnight. Finally, electrocatalysts were washed with DI water until neutral pH and dried at T=85°C. Ring Disk Electrode. Electrochemical analysis for synthesized catalysts was performed using the Pine Instrument Company electrochemical analysis system. The rotational speed reported was 1200 RPM, with a scan rate of 5 mV sec-1. The electrolyte was 0.5 M H2SO4 saturated in O2 at room temperature. A platinum wire counter electrode and an Ag/AgCl reference electrode were used. Working electrodes were prepared by mixing 5 mg of the electrocatalyst with 850 µL of a water and isopropyl alcohol (4:1) mixture, and 150 µL of Nafion® (0.5% wt., DuPont). The mixture was sonicated before 30 µL was applied onto a glassy carbon disk with a sectional area of 0.2474 cm2. The loading of the catalyst on the electrode was 0.6 mg cm-2. For a subset of samples, RRDE experiments were performed to measure the hydrogen peroxide yield. Ring


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efficiency was 37%, and potential of 1.5V was used to detect H2O2 electroreduction. Hydrogen peroxide yield was calculated according to: %H2O2 = 100*(2*(IR/N)/(ID+ IR/N))

(1)

where IR, ID and N are the ring current, disk current and ring collection efficiency (0.37), respectively. Characterization. XPS spectra were acquired on a Kratos Axis DLD Ultra X-ray photoelectron spectrometer using an Al Kα source monochromatic operating at 150W with no charge compensation. The base pressure was about 2x10–10 torr, and operating pressure was around 2x10-9torr. Survey and high-resolution spectra were obtained at pass energies of 80 eV and 20 eV respectively. Acquisition time for survey spectra was 2 minutes, for C1s and O1s spectra - 5 minutes, for N 1s and Fe 2p –30 minutes. For non-conductive samples, charge neutralization was used. For these samples, Au powder was deposited on the sample, and Au 4f spectrum was acquired and used for spectral calibration. For conductive samples, no charge neutralization was used. Data analysis and quantification were performed using CasaXPS software. A linear background subtraction was used for quantification of C1s, O1s and N1s spectra while a Shirley background was applied to Fe 2p spectra. Sensitivity factors provided by the manufacturer were utilized to obtain atomic % of Fe, N, C and O present in samples. A 70% Gaussian/30% Lorentzian line shape was utilized in the curve-fit of spectra. Averages from 3 areas per sample are presented. 3. Results and Discussion 3.1. Microscopic analysis



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Figure 3. TEM micrographs for a) Fe-AApyr, b) Fe-CBDZ, c) Fe-PP and d) Fe-PVP. Representative EDS spectra (e) and HREELS spectra (f). TEM shows open pore structure, EDS confirms the presence of Fe and N, HREELS shows high graphitic character. Through sacrificial support method, it is possible to obtain open-frame porous structures as shown in Figure 3a and b). For some polymer-based precursors, the open-frame structure is not obtained, but rather smaller porous structures (Figure 3c) or sheet-like morphologies (Figure 3d) are created. The presence of iron and nitrogen is confirmed by EDS (Figure 3e). The M-N-C network consists of the mixture of amorphous and graphitic carbon as shown on the inset and confirmed by HREELS (Figure 3f). From fitting HREELS spectra, the range of the graphitic character is between 60 and 80%. Most of the iron presents in clusters with size below the resolution of TEM images with only a few larger Fe particles present as shown on the inset image.

Obvious differences in morphology have a dramatic influence on the

performance directly through the effect on transport characteristics, accessibility, and


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conductivity. Morphology also influences the chemical structure whose effect on activity and mechanism of ORR in M-N-C electrocatalysts is the focal point of this study. 3.2. XPS analysis Initially, the reference spectra from standards such as polypyrrole and polypyridine have been acquired. High resolution N 1s spectra were fitted using single peak with FWHM of 1.1 eV at 398.8 eV for polypyridine and 400.7 eV for the polypyrrole. For consistency among all other spectra from electrocatalysts, the peaks during spectral curve fitting of N 1s spectra were constrained to have an FWHM between 1 and 1.2 eV. Figure 4a) shows high resolution N 1s spectrum for Fe aminoantipyrine (Fe-AApyr) curve fitted with six peaks. Figure 5 shows moieties and corresponding binding energies (BE’s) for N 1s spectra. The peak at the lowest BE of 398.0 eV comes from either imine or cyano groups. The peak at 398.8 eV is accepted to be pyridinic N contributing to the π system with one π-electron as confirmed from acquiring reference spectra from polypyridine. It is also understood that it is present at the perimeter of a vacancy defect in the graphene-like structure. Pyrrolic N with two π electrons is present at 400.7 eV as established by acquiring reference spectra from the polypyrrole. The binding energy of the peak at 401.7 eV corresponds to a mixture of graphitic and quaternary nitrogen, but the exact functionality is not clearly understood. It does not necessarily represent classical quaternary structure as shown in Figure 5 such as quaternary anion. It may be a protonated pyridinic N or any other nitrogen atom with a formal charge of +1 as shown in Figure 2. For a graphitic nitrogen single site defect, we have predicted a core level shift of 3.3 eV relative to pyridinic-N with an estimated binding energy of 402.1 e.V.73 So, peak at 401.7 eV will have contribution from multiple nitrogen moieties. To distinguish from the other peak discussed below at 402.7 eV, we will use quaternary as a definition for this peak observed at 401.7 eV as accepted widely in the literature 27, 30, 34, 38, 56.



a)

3100

3000

pyridinic pyrrolic

b)

Fe oxides

3000

Nx-Fe

Fe-Nx

2500

cps

cps

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quaternary

2900 2800

nitrile

2000

2700

Fe

satellites

graphitic 1500 404

402

400

398

2600 720

396

Binding energy, eV

715

710

705

Binding energy, eV

Figure 4. High resolution XPS spectra for Fe-AAPyr electrocatalyst a) N 1s and b) Fe 2p. The range of binding energies for each type of species is shown in Figrue 5. Nitrogen coordinated to Fe is highlighted in N 1s and Fe 2p spectra.

Figure 5. Nitrogen moieties nomenclature. A detailed discussion of nitrogen species is in the text. The identification of component at a rather high BE (402.7 eV) is also contentious. This value is quite close to BE expected for N-O functional groups. Interpretation of the high-BE component of the N 1s peak in terms of “graphitic” N was supported by accurate ab initio model calculations of the core level ionization potentials and the core level shifts.

63

It has

been shown that these rather high N 1s binding energies in N-containing compounds may be observed without N-O functional groups in the sample. This peak is, therefore, identified as graphitic N. We should keep in mind, however that graphitic N may have a significant spread


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in binding energies positions, i.e. between 401.5-403.0 eV depending on the chemical type of other nitrogens in direct proximity. When fixed width of ~1.1 eV is used during peak decomposition, another peak, shifted up by ~1 eV from the BE of pyridinic N, has to be included in the fit. This corresponds to the binding energy of 399.8 eV. This is the energy where reference materials such as iron bipyridine have peak due to Fe---N bond.74 Iron bound to pyrrolic N such as in porphyrins appears at lower BE of 398.5 eV. Ab initio studies of energetics and geometries of metal nitrogen sites in CNTs show that binding of pyridinic N to Fe is more energetically favorable that of pyrrolic to Fe.

75

This peak is at the energy at which amines would also appear. In

metal-free electrocatalysts, indeed there is a peak at this BE due to amines present. For confirming the presence of Fe---N types of bond in N 1s spectra, we have done the analysis of Fe 2p spectra for the same sample (Figure 4b). In addition to a small peak due to metallic Fe (707.0 eV) and peaks due to various forms of iron oxides (710-713 eV), there is a distinct peak due to Fe-Nx bond at 708.4 eV.22 We have also confirmed the presence of Fe-Nx species in the range of 399.8-400 eV by DFT calculations of binding energy shifts for M-N-C graphene-based materials.52 As was recently suggested by Bogdanoff et.al. 53, some nitrogen atoms are bonded to the metal centers with a higher probability that the others. Mossbauer spectroscopy confirmed the existence of two types of Fe-Nx centers, i.e. one being a mesomeric Fe-N4 center and the other disordered Fe-N4 center. Through correlation of XPS and Mossbauer spectra, the peak due to the mesomeric Fe-N4 center where all nitrogen atoms are equally bonded to the metal center was assigned to M-N peak in N 1s XPS spectrum. Other types of Fe-Nx, centers, such as Fe-N2 and Fe-N3, can also be present and contribute to the same peak. According to DFT calculations, the binding energy of these Fe-Nx moieties lies in the range of 0.9-1.5 eV higher than the position of the pyridinic peak.


76

To conclude,


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even though, some amount of amines may contribute to the same BE of 399.8 eV, the presence of Fe-Nx types of species is confirmed by N 1s and Fe 2p spectra. Tables S1 and S2 in Supporting information show all XPS data for all samples included in this study. Very different amounts of nitrogen is detected – from 1.2 to 14.5 atomic % with an average around 4-5% (Figure 6). The concentration of iron was found to be in the range of 0.1-1.4% with an average around 0.2-0.4%. For some samples, the amount of iron was lower than the detection of limit, which is ~0.02% for Fe 2p. It is important to remember that attenuation length of Fe 2p and N 1s photoelectrons are remarkably different. The atomic percent of nitrogen detected comes from 10-15 nm of the top surface of the sample while the iron is only detected from top 2-4 nm of the surface. For samples for which we report atomic percent of Fe, the level of S/N in the Fe 2p spectra was high enough for curve-fitting the spectra as shown in Figure 4b.

Figure 6. E1/2 vs total atomic percentage of nitrogen detected by XPS. Each circle is an individual catalyst as shown in Tables S1 and S2 in Supporting Information. Higher activity is observed for electrocatalysts with higher overall amount of nitrogen. 16 ACS Paragon Plus Environment

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3.3. RRDE testing. All of the electrocatalysts have been tested using the same protocol as described in the Experimental section. Figure 7 shows the representative RRDE results for three samples while Tables S1 and S2 in Supporting information has half-wave potential and hydrogen peroxide yield. Due to hydrophobic nature of these materials, loading of 0.6 mg/cm2 was necessary to obtain full coverage of catalyst on working electrode. For consistency, this loading, which is widely utilized,

7, 10, 45

was used for all materials. The hydrogen peroxide

yield that is obtained at this higher than optimal loading may be underestimated due to disproportionation of hydrogen peroxide within the catalyst which is a competing mechanism to the electrochemical reduction of H2O2. However, the relative comparison between H2O2% detected for various types of materials tested at exactly the same loadings on the electrode is representative of the mechanism of ORR. Metal-free electrocatalysts based on carbendazim has extremely low half-wave potential and a high peroxide yield of 30%. A material made from the same precursor with the addition of metal shows remarkably better performance with a much lower peroxide yield of 10%.

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Imidazolidinyl urea based electrocatalysts have the best performance. As Tables S1 and S2 in Supporting information show, the 43 metal-containing samples studied have a range of E1/2 between 0.35 and 0.79 V value of and range of H2O2 yield between 7 and 37%. Next we will study the relationship between activity as determined by RRDE and structure as determined by XPS for metal-free and metal-containing electrocatalysts.



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Figure 7. RRDE results for carbendazim (CBDZ) derived catalysts and Imidazolidinyl urea (IMZU) material. Conditions: 0.5M H2SO4 saturated with O2, 1200RPM, 5 mV s-1, catalyst loading 0.6mg cm-2. Metal-free catalyst has much lower E1/2 (~0.4 V) and higher ring current due to hydrogen peroxide production than metal-containing. 3.4. Structure-to-performance correlations. In this section, we study possible correlations between various types of nitrogen moieties detected and electrocatalytic activity metrics such as half-way potentials and hydrogen peroxide yield. When doing so, it is important to remember, that the materials under study are very heterogeneous in nature, and each point in the Figures 6, 8-11 is an average of three areas analyzed by XPS. It is important to acknowledge general trends in chemistryperformance plots pointing on the possible relationship between structure and performance. There is an overall increase in activity with the amount of atomic N detected as shown in Figure 6. The role of nitrogen in ORR of MNC-based electrocatalysts has always been acknowledged11, 33, 41, 43, 49, 53, 77-79 and this observation of higher activity with higher overall amount of nitrogen is important confirmation. The role of individual types of nitrogen into 18 ACS Paragon Plus Environment

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the mechanism of oxygen should be evaluated by looking at relative distribution of nitrogen moieties within total nitrogen present. Absolute amounts of nitrogen as provided by XPS depend on the amount of oxygen and carbon present as well, as the total composition is reported as totaled to 100%. Moreover, carbon, oxygen, nitrogen and iron represent signal from different sampling depths. Particularly, N 1s comes from ~11 nm while Fe 2p provides composition from only 1.5-2 nm of the material. Atomic % reported by XPS, therefore, have to be treated with great care. To eliminate errors caused by the difference in sampling depths and influence of the different absolute amount of carbon and oxygen that can be present in the sample, the relative distribution of nitrogen within total nitrogen detected should be utilized. This allows understanding how the relative distribution of individual types of nitrogen contribute into overall activity. 3.4.1. Metal-free electrocatalysts. Figure 8 plots relationship between half-wave potential (Figure 8a) and peroxide yield (Figure 8b) for metal-free electrocatalysts versus the amount of pyrrolic nitrogen detected. Peroxide yield determination was performed for a subset of metal-free and metal-containing electrocatalysts as described in the Experimental Section. The range of E1/2 reported between 0.35-0.60 V corresponds to the potential of reduction of oxygen to hydrogen peroxide via 2ereaction. The half-wave potential is higher for samples with the more considerable amount of pyrrolic N. Moreover, there is also a weak correlation between the hydrogen peroxide yield and activity confirming that in metal-free electrocatalysts, there are no species that convert oxygen directly to the water and/or no or small amounts species that reduce hydrogen peroxide generated by the 1st step further into oxygen. This observation is consistent with mechanistic studies of ORR on N-doped CNTs without metal showing that only the 1st step of



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oxygen reduction reaction O2H2O2 is occurring.49 We have a direct evidence that pyrrolic N serves as an active site for the first step of the oxygen reduction reaction.

Figure 8. E1/2 (●) and %H2O2(◌) vs. pyrrolic N for metal-free materials. In metal-free electrocatalysts, high amounts of pyrrolic nitrogen result in better activity towards oxygen reduction to hydrogen peroxide 3.4.1. Metal-containing electrocatalysts. Figure 9 plots the same electrochemical testing metrics for the same types of species but in metal-containing electrocatalysts. Among the metal-containing samples, there are some that have quite low E1/2 and a high peroxide yield, but in addition, there is a group of electrocatalysts with higher E1/2 between 0.7 and 0.8 corresponding to the potential of the full reduction of oxygen to water. There is an obvious trend between the amount of pyridinic nitrogen and activity (Figure 9a). The amount of pyridinic N does not show a correlation with hydrogen peroxide yield (Figure 9b). This observation supports previously published indications that pyridinic N is either a possible active site or a marker for the edge plane.23, 43, 48, 57-58, 60

It is important to note that no such correlation is found for pyridinic N in samples

without metal. Edge planes exist in metal-free materials as well, but no correlation between pyridinic nitrogen and activity is found. That points towards another possible explanation for 20 ACS Paragon Plus Environment

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a positive correlation between the amount of the pyridinic nitrogen and ORR activity suggested by Bogdanoff et.al.53 It is important to stress than the binding energy of pyridinic nitrogen is also where disordered Nx-Fe centers such as Fe-N, Fe-N2, and Fe-N3 contributes. We will address this hypothesis later.

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Figure 9. E1/2 (●) and %H2O2 (◌) vs. pyridinic and pyrrolic N for Fe containing electrocatalysts. Larger amounts of pyridinic and lower amounts of pyrrolic nitrogens result in better ORR. Pyrrolic nitrogen contributes to higher hydrogen peroxide production. Amount of pyrrolic N does not improve the activity of metal-containing electrocatalysts (Figure 9c); quite the opposite – pyrrolic N causes partial reduction of oxygen to hydrogen peroxide (Figure 9d) thereby reducing an overall activity as expressed by lower half-wave potential The observation that pyrrolic N is an active site for the first step 2e- reduction reaction of oxygen to hydrogen peroxide is confirmed by both metal-free and metalcontaining electrocatalysts and is opposite from the report in which pyrrolic was shown as an 21 ACS Paragon Plus Environment


efficient active site for complete 4e- reduction of oxygen.62 The conclusions in this report, however, were based on spectra from only 4 samples fitted inconsistently with peaks of varied width. Moreover, careful examination of XPS results in Ref.62 report shows that 1 out of 4 samples does not follow the trend in increasing activity with the amount of pyrrolic nitrogen making the conclusion derived unjustified. (Figure 4 in 62).

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0.60 0.50

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Figure 10. E1/2 (●) and %H2O2(◌) vs ratio of pyridinic to pyrrolic N for all electrocatalysts. The ratio of pyridinic to pyrrolic is a good metrics of electrocatalytic activity. The decrease in the ratio for metal-free electrocatalysts indicates that pyridinic nitrogen reduces hydrogen peroxide to water which is also confirmed by lower %H2O2 for samples that have a higher ratio. From our observations in Figures 8 and 9, it is clear that the ratio of pyridinic to the pyrrolic type of N is a parameter that is an indicative of electrocatalytic activity of M-N-C type of catalyst. Figure 10 plots this ratio versus half-wave potential (Figure 10a) and hydrogen peroxide yield (Figure 10b) for metal-free and metal-containing electrocatalysts. Clearly, the larger the ratio, the higher the half-wave potential, and the lower the hydrogen peroxide yield. For metal-free samples, half-wave potential decreases with larger pyridinic/pyrrolic ratio, indicating that pyridinic N is not an active site for partial oxygen reduction to hydrogen peroxide. At the same time, hydrogen peroxide yield decreases with 22 ACS Paragon Plus Environment

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larger pyridinic/pyrrolic ratio for both metal-containing and metal-free samples, indicating that pyridinic may actually be the site that reduces hydrogen peroxide to water, therefore, more significant amounts of pyridinic nitrogen with respect to pyrrolic decreases yield of hydrogen peroxide. This conclusion is supported by previous report that the pyridinic nitrogen is the one that is needed for converting the mechanism from 2e- to 4e-.53 a)

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Figure 11. E1/2 (●) and %H2O2(◌) vs N coordinated to Fe determined from N spectra (a and b) and from Fe spectra (c and d). The relative amount of nitrogen coordinated with Fe as observed by both N 1s and Fe 2p spectra is correlated with higher E1/2 and smaller H2O2 yield. Lastly, it is necessary to evaluate the hypothesis that Fe in N environment is an active site for oxygen reduction to water. Figure 11 plots amount of these species as determined from N 1s spectra, % of Nx-Fe, and as determined from Fe 2p spectra, % of Fe-Nx, both half-wave potential and hydrogen peroxide yield. There is an obvious trend indicating that there is an 23 ACS Paragon Plus Environment


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increase in activity with an increase of the amount of iron coordinated with nitrogen. In addition, it is clear that these species do not contribute into hydrogen peroxide production, therefore, suggesting that Fe-Nx moieties either reduce oxygen to water directly via 4ereaction and/or reduce hydrogen peroxide to water. As we have discussed above, through correlation between Mossbauer and XPS data, the existence of mesomeric and disordered metal-nitrogen centers was recently reported.53 In this report, the relative concentration of Fe-N4 centers was obtained by summing areas of three peaks - pyridinic, pyrrolic and Me-N peaks assuming they can be assigned to distorted and mesomeric Fe-N4 centers. The peak due to Fe-N4 in N 1s XPS spectra was assigned to the center in which nitrogen atoms are mesomerically bonded to iron. This is a peak due to Fe-Nx center that shows the correlation with activity in Figure 11. Introduction of disorder in the mesomeric metal-nitrogen coordination results in a shift of electron density from iron to nitrogen which is reflected in the shift in the binding energy to the positions where both pyrrolic and pyridinic nitrogen are observed. We have shown that pyrrolic peak shows no correlation with activity, so if there is any contribution of disordered metal-nitrogen moieties into this binding energy region, it is suppressed by the pyrrolic nitrogen itself. On the other side, the peak due to the pyridinic nitrogen may indeed have a contribution of disordered metal-nitrogen centers as there is a strong positive relationship between the amount of this peak and ORR activity (Figure 9a) and the correlation is only observed for metal-containing samples. DFT calculations of binding energy shifts in N 1s spectra of Fe-Nx moieties indicated that the shift due to Fe-N2 and Fe-N3 is less than that for Fe-N4.52, 76 Fe-N2 and FeN3 coordinations may, therefore, contribute into the lower BE, where pyridinic N is detected, due to smaller shift of electron density from nitrogen to iron in disordered coordination versus mesomeric Fe-N4 environment, in which larger reduction of electron density on the nitrogen atom is resulting in a larger binding energy. Hence, the correlation observed between peak 24 ACS Paragon Plus Environment

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identified as pyridinic nitrogen may be not only due to pyridinic nitrogen itself but also due to metal coordinated to nitrogen. This is only true for metal-containing samples. For metal-free samples, pyridinic nitrogen is purely pyridinic.

Figure 12. The amount of CxOy% as a function of (a) N pyridinic + Nx-Fe and b) N pyrrolic + N graphitic. Nitrogen defects in a graphene-like network that result in better activity (pyridinic and Nx-Fe) are linked to larger amount of surface oxides groups. Even deeper understanding into why certain types of nitrogen functionalities show a positive correlation with activity comes from looking into types of carbon functionalities that exist in these materials. Carbon network that has a significant amount of graphitic carbon as confirmed by both EELS, HRTEM, and XPS, also has many carbon-oxygen functionalities. These include C-OH, carbonyls, epoxy and carboxyl functional groups, and they tend to form whenever there is a defect in C-C graphitic network occurs. The amounts of carbon-oxygen functionalities can be easily obtained from C 1s high-resolution spectra, and they can be used as a measure of defects or edge sites in the graphene-like network. Figure 12 plots total amount of these functional groups, labeled as CxOy, as a function of the total amount of pyridinic and Fe-Nx groups (Figure 12a) and as a function of the total amount of pyrrolic and graphitic N (Figure 12b). A positive correlation exists between the degree of carbon functionalization and the total amount of species that show a positive contribution to ORR 25 ACS Paragon Plus Environment


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activity. Defect formation in the carbon graphitic network that is accompanying with oxidation of carbon also creates possible sites for the formation of moieties that are ORR active. On the other hand, the amount of pyrrolic and graphitic nitrogens show an inverse relationship with the number of defect sites as manifested by the amount of carbon-oxygen functionalization. It may indicate that these are plain defects that do not change an overall sp2 character in graphite structure. 3.5. Summary In summary, through direct, comprehensive structure-to-performance correlations for a large number of metal-free and metal-containing NC electrocatalysts, we have derived potential active sites participating in possible reaction pathway of ORR: Based on E1/2 of reaction and correlation between ratio of pyridinic to pyrrolic nitrogen, we can suggest that in metal-free materials, there are no species that convert oxygen directly into water or a small amount of species that convert hydrogen peroxide generated by the 1st step further into water. Some hydrogen disproportionation can also happen in the catalyst layer causing lower hydrogen peroxide detected than is expected from materials that only reduce water to hydrogen peroxide. Pyrrolic nitrogen catalyzes the 1st step of O2 reduction to hydrogen peroxide – active site 1. The ratio of pyridinic to pyrrolic suggests that the pyridinic nitrogen does convert hydrogen peroxide further to water (active site 2), but its amount is small in comparison to the amount of pyrrolic nitrogen that produces H2O2. In metal-containing electrocatalysts: Pyrrolic nitrogen catalyzes the 1st step of O2 reduction to hydrogen peroxide – active site S1. There is a correlation between Fe-Nx centers and activity. It is not conclusive whether this moiety only catalyzes the 4e- a direct reduction of oxygen to water (active site S) or also catalyzes the 2nd step of H2O2 reduction to H2O (active site S2).


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There is a correlation between peak due to pyridinic N and activity. This may be due to: (a) pyridinic nitrogen being active site for H2O2 reduction to H2O as confirmed by the ratio of pyridinic to pyrrolic nitrogen (active site S2) and/or (b) peak assigned as pyridinic having a contribution from a disordered Fe-Nx centers (such as Fe-N2, Fe-N3) which serve as either active sites for direct 4e- ORR (active site S) or 2nd step 2e- reaction site (active site S2). Figure 13 shows the suggested pathways and possible active sites.

Figure 13. Active sites and reaction pathways identified. Pyrrolic (s1) reduces oxygen to hydrogen peroxide in 2e- reaction. Pyridinic (s2) reduces hydrogen peroxide to water in 2ereaction. Fe-Nx centers can serve as active sites that catalyze either direct 4 e- ORR (s), 2x2 single site ORR(s*) and/or 2e- reduction of H2O2 to water (s2). 4. Conclusion Through systematic spectroscopic and electrochemical characterization of more than 40 different Fe-based nitrogen-carbon composites synthesized by sacrificial support method from different classes of nitrogen precursors, the role of different types of nitrogen in ORR has 27 ACS Paragon Plus Environment


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been established. Pyrrolic nitrogen catalyzes the first step of oxygen reduction to hydrogen peroxide. Pyridinic nitrogen serves as a 2nd step of hydrogen peroxide reduction to water. Metal coordinated to nitrogen in mesomeric and disordered form catalyzes 4e- a direct reduction of oxygen to water and/or the second step of hydrogen peroxide reduction similarly to the pyridinic nitrogen. Through identifying synthetic routes of designing materials with the smallest amount of pyrrolic nitrogen and largest amounts of metal-nitrogen centers in the pyridinic environment, will result in the ORR non-PGM electrocatalysts with the highest activity. As a result of this understanding, thorough optimization resulted in unprecedented active and durable catalysts derived from a nitrogen containing charge-transfer salt precursors. 55 Corresponding Author *[email protected], Tel: 505-277-2304 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported in part by the DOE-EERE Fuel Cell Technology Program (subcontract to Northeastern University, with PI Sanjeev Mukerjee). Supporting Information. Tables S1 and S2 shows relative amount of nitrogen and iron species derived from high resolution N 1s and Fe 2p XPS spectra along with E1/2 and H2O2 yield for metal-free and metal-containing samples. 28 ACS Paragon Plus Environment

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ACKNOWLEDGMENT This work was supported in part by the DOE-EERE Fuel Cell Technology Program (subcontract to Northeastern University, with PI Sanjeev Mukerjee). REFERENCES 1. Antolini, E., Palladium in Fuel Cell Catalysis. Energ Environ Sci 2009, 2, 915-931. 2. Serov, A.; Kwak, C., Review of Non-Platinum Anode Catalysts for Dmfc and Pemfc Application. Appl Catal B-Environ 2009, 90, 313-320. 3. Serov, A.; Nedoseykina, T.; Shvachko, O.; Kwak, C., Effect of Precursor Nature on the Performance of Palladium-Cobalt Electrocatalysts for Direct Methanol Fuel Cells. Journal of Power Sources 2010, 195, 175-180. 4. Whipple, D. T.; Jayashree, R. S.; Egas, D.; Alonso-Vante, N.; Kenis, P. J. A., Ruthenium Cluster-Like Chalcogenide as a Methanol Tolerant Cathode Catalyst in AirBreathing Laminar Flow Fuel Cells. Electrochim Acta 2009, 54, 4384-4388. 5. Zalineeva, A.; Serov, A.; Padilla, M.; Martinez, U.; Artyushkova, K.; Baranton, S.; Coutanceau, C.; Atanassov, P. B., Self-Supported Pdxbi Catalysts for the Electrooxidation of Glycerol in Alkaline Media. J Am Chem Soc 2014, 136, 3937-3945. 6. Ferrandon, M.; Kropf, A. J.; Myers, D. J.; Artyushkova, K.; Kramm, U.; Bogdanoff, P.; Wu, G.; Johnston, C. M.; Zelenay, P., Multitechnique Characterization of a PolyanilineIron-Carbon Oxygen Reduction Catalyst. J Phys Chem C 2012, 116, 16001-16013. 7. Jaouen, F.; Proietti, E.; Lefevre, M.; Chenitz, R.; Dodelet, J. P.; Wu, G.; Chung, H. T.; Johnston, C. M.; Zelenay, P., Recent Advances in Non-Precious Metal Catalysis for OxygenReduction Reaction in Polymer Electrolyte Fuel Cells. Energ Environ Sci 2011, 4, 114-130. 8. Serov, A.; Robson, M. H.; Artyushkova, K.; Atanassov, P., Templated Non-Pgm Cathode Catalysts Derived from Iron and Poly(Ethyleneimine) Precursors. Applied Catalysis B: Environmental 2012, 127, 300-306. 9. Wu, G., Johnston, C. M., Mack, N. H., Artyushkova, K., Ferrandon, M., Nelson, M., Lezama-Pacheco, J. S., Conradson, S. D., More, K. L., Myers, D. J, et al., SynthesisStructure-Performance Correlation for Polyaniline-Me-C Non-Precious Metal Cathode Catalysts for Oxygen Reduction in Fuel Cells. J Mater Chem 2011, 21, 11392-11405. 10. Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 2011, 332, 443447. 11. Zhong, M.; Kim, E. K.; McGann, J. P.; Chun, S. E.; Whitacre, J. F.; Jaroniec, M.; Matyjaszewski, K.; Kowalewski, T., Electrochemically Active Nitrogen-Enriched Nanocarbons with Well-Defined Morphology Synthesized by Pyrolysis of Self-Assembled Block Copolymer. J Am Chem Soc 2012, 134, 14846-14857. 12. Buchner, F.; Flechtner, K.; Bai, Y.; Zillner, E.; Kellner, I.; Steinruck, H. P.; Marbach, H.; Gottfried, J. M., Coordination of Iron Atoms by Tetraphenylporphyrin Monolayers and Multilayers on Ag(111) and Formation of Iron-Tetraphenylporphyrin. J Phys Chem C 2008, 112, 15458-15465. 13. Charreteur, F.; Jaouen, F.; Dodelet, J. P., Iron Porphyrin-Based Cathode Catalysts for Pem Fuel Cells: Influence of Pyrolysis Gas on Activity and Stability. Electrochim Acta 2009, 54, 6622-6630. 29 ACS Paragon Plus Environment


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Chemistry of Multitudinous Active Sites for Oxygen Reduction

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