Chemistry of Multitudinous Active Sites for Oxygen Reduction

Article pubs.acs.org/JPCC

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 Chemical & Biological Engineering Department, University of New Mexico, Albuquerque, New Mexico 87131, United States S Supporting Information *

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 polymer-based, 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 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 four-electron (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 metric of M−N−C electrocatalytic activity in oxygen reduction reaction occurring through the preferred 4e− reduction to H2O.

1. 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 PEMFCs depend on the utilization of platinum metal electrocatalysts (usually supported on a highly dispersed carbon material). Significant research effort has been directed toward replacement of the platinum by alternative platinum-free (which may contain Ru or Pd)1−5 or entirely non-platinum group metal (non-PGM) materials mainly consisting of metal−nitrogen−carbon (M−N−C) networks. Heteroatomic polymeric precursors,6−11 metal macrocyclic compounds,12−21 and the metal salt and nitrogen/carbon organic molecules22−25 have been reported as sources 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. 1.1. Reaction Pathways. Possible reaction pathways must be discussed as these have direct implications for designing the most active material. The most efficient oxygen reduction reaction (ORR) mechanism that will lead to the highest efficiency in power generation with reduction of O2 to H2O via a four-electron (4e−) single site mechanism. Alternatively, reduction of O2 may happen via two-step two-electron (2e−) reactions (2 × 2), in which oxygen is first reduced to H2O2, © 2015 American Chemical Society

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 O2 → H2O2, the first 2e− step reduction (S1) (2) site 2 for H2O2 → H2O, the second 2e− step reduction (S2) (3) site 1/2 for two steps of the first and second 2e− reductions (S*) (4) site 3 for a single step 4e− O2 → H2O reduction (S), and (5) be catalytically inactive and serve only as a conductive open framework 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. 1.2. Active Site Hypothesis. The relationship between activity and the amount and types of species (surface moieties) present in M−N−C materials is usually established through spectroscopic correlations with such analytical methods as Xray photoelectron spectroscopy (XPS), X-ray absorption nearedge spectroscopy (XANES), time-of-flight secondary ion mass spectrometry (TOF-SIMS), Mössbauer spectroscopy, and others.6,21,26−33 XPS is the workhorse of surface spectroscopic Received: August 6, 2015 Revised: October 28, 2015 Published: October 29, 2015 25917

DOI: 10.1021/acs.jpcc.5b07653 J. Phys. Chem. C 2015, 119, 25917−25928

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The Journal of Physical Chemistry C

Figure 2. Nitrogen and Fe−Nx functionalities existing in M−N−C materials.

bifunctional mechanism was elucidated in detail.19,27,38 In a recent work, Ozkan et al. have attempted to distinguish between metal-containing catalysts FeNC in which the 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 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 57Fe Mössbauer spectroscopy, a direct correlation between Fe−Nx centers 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 a dual-site 2 × 2 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 toward the oxygen reduction reaction.55 2. Pyridinic nitrogen (Npyridinic) also plays a more pivotal role than any other kind of nitrogen-containing species.25,56 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

Figure 1. Possible reaction pathways and active sites of ORR. (a) Dual site 2 × 2 e−, (b) single site 2 × 2 e−, and (c) single site direct 4e− mechanism.

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 nonPGM 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 of 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 nanoparticles, as 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. There are two main hypotheses: one claiming that nitrogen functionalities on/in carbon-based 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,46 but nitrogen atoms are not catalytically active toward a direct reduction of oxygen.47,48 Mechanistic studies of ORR on nitrogen-doped carbon nanotubes (CNTs) showed that only the first step of oxygen reduction reaction O2 → H2O2 is occurring without metal.49 The importance of metal for complete oxygen reduction by 25918


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variety of compositions that are tested electrochemically using the same fixed protocol and analyzed spectroscopically using the same acquisition and data processing approach. 1.4. 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 paper, 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

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 Kurungot gave evidence that this type of moiety is essential for oxygen adsorption and reduction to water via a 4e− mechanism.62 4. In other reports, graphitic31 and quaternary nitrogen species63,64 were positively correlated with ORR activity. Ruoff et al. studied the mechanism on metal-free nitrogen-doped graphene-based catalysts and came to the conclusions that pyridinic nitrogen improves offset potential and converts mechanism from 2e− to 4e− while activity depends on the amount of graphitic nitrogen.43 1.3. Limitations of Reported Findings. In all of the 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 as current at a fixed voltage, e.g., I at 0.8 V, and concentration of certain species as determined by a spectroscopic method such as XPS. In these structure-toperformance studies, the type of precursor and synthetic approach is fixed while parameters such as temperature and duration of pyrolysis, ratio of precursors, and loadings of precursors are varied. The cross-comparison between different reports discussed above (refs 14, 17, 19, 27, 29, 31, 38, 43, 48, 49, and 56−64) 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 different acquisition and data processing 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 sites 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 vary 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 bionanocomposites. The only difference between bionanocomposites 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 the 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 a cross-laboratory study published in 2009,28 there is no systematic study comparing the performance and structure of electrocatalysts obtained from different precursors at a

Table 1. Precursor Types Used as Nitrogen and Carbon Sources name carbendazim (CBDZ) imidazolidinyl urea piperazine diketo pyrrolo-pyrrole (DPP) aminotetrazole monohydrate (ATM) 4-aminoantipyrine (AAPyr) hydroxysuccinimide FeTPP poly(ethylene imine) (PEI) poly(ethylene imine) with urea poly(vinylpyridine) (PVP) polyaniline (PANI) polypyrrole (PP)

class

C/N ratio

formula

9/3

C9H9N3O2

11/8

C11H16N8O8

4/2

C4H10N2

6/2

C6H2N2O2

1/6

CH3N5·H2O

11/3

C11H13N3O

4/1

C4H5NO3

44/4

C44H28FeN4

2/1

(C2H5N)n

polymer

2/1−1/2

polymer

7/1

(C2H5N)n + CH4N2O (C7H7N)n

polymer polymer

6/1 4/1

[C6H4NH]n (C4H2NH)n

small organic molecule small organic molecule small organic molecule small organic molecule small organic molecule small organic molecule small organic molecule macrocycle (porphyrin) polymer

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, at different ratios between the precursor and iron salt, and with two ways of leaching the support, KOH or HF. Each of the electrocatalysts was electrochemically tested using exactly the same rotating disk electrode (RDE) protocol and analyzed structurally by XPS for deriving structure-to-property relationships for the identification of active sites.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A modified sacrificial support method was used to prepare all catalysts.5,8,24,25,66−72 First, 10 g of silica (Cab-O-Sil L90, surface area 90 m2 g−1) was dispersed in 100 mL of water in an ultrasonic bath. Then, a suspension of 25 g of organic precursor (Table 1) in the appropriate solvent was added to the silica and ultrasonicated for 20 min. Finally, a solution of 2.5 g of iron nitrate (Fe(NO3)3·9H2O, SigmaAldrich) was added to SiO2−precursor solution and ultrasonicated for 8 h (the total metal loading on silica was calculated as ∼15 wt %). After ultrasonication, the 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 25919


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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.

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 C 1s, O 1s, and N 1s spectra while a Shirley background was applied to Fe 2p spectra. Sensitivity factors provided by the manufacturer were utilized to obtain atomic percentages of Fe, N, C, and O present in samples. A 70% Gaussian/30% Lorentzian line shape was utilized in the curve fitting of spectra. Averages from three areas per sample are presented.

treatment (HT). The HT was undertaken under normal conditions in an ultrahigh pure (UHP) nitrogen atmosphere (flow rate 100 ccm), with 20 deg min−1 temperature ramp rate and 1.5 h pyrolyzation time. The HT temperatures were 750, 800, 850. and 900 °C as listed in Tables S1 and S2 in the Supporting Information. After heat treatment, silica was leached by 25 wt % HF or 7 M KOH overnight. Finally, electrocatalysts were washed with deionized (DI) water until neutral pH and dried at T = 85 °C. 2.2. Ring Disk Electrode. Electrochemical analysis for synthesized catalysts was performed using the Pine Instrument Co. electrochemical analysis system. The rotational speed reported was 1200 rpm, with a scan rate of 5 mV s−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, rotating ring disk electrode (RRDE) experiments were performed to measure the hydrogen peroxide yield. Ring efficiency was 37%, and a potential of 1.5 V was used to detect H2O2 electroreduction. Hydrogen peroxide yield was calculated according to %H 2O2 = 100(2(IR /N )/(ID + IR /N ))

3. RESULTS AND DISCUSSION 3.1. Microscopic Analysis. Through a sacrificial support method, it is possible to obtain open-frame porous structures as shown in Figure 3a,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 energy-dispersive X-ray spectroscopy (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 highresolution electron energy loss spectroscopy (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 transmission electron microscopy (TEM) images with only a few larger Fe particles present as shown on the inset image (Figure 3a). Obvious differences in morphology have a dramatic influence on the performance directly through the effect on transport characteristics, accessibility, and conductivity. Morphology also influences the chemical structure whose effect on the 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 a single peak with a full width at half-maximum (fwhm) of 1.1 at 398.8 eV for polypyridine and 400.7 eV for the polypyrrole. For

(1)

where IR, ID, and N are the ring current, disk current, and ring collection efficiency (0.37), respectively. 2.3. Characterization. XPS spectra were acquired on a Kratos Axis DLD Ultra X-ray photoelectron spectrometer using an Al Kα source monochromatic operating at 150 W with no charge compensation. The base pressure was about 2 × 10−10 Torr, and operating pressure was around 2 × 10−9 Torr. Survey and high-resolution spectra were obtained at pass energies of 80 and 20 eV, respectively. Acquisition times were 2 min for survey spectra, 5 min for C 1s and O 1s spectra, and 30 min for N 1s and Fe 2p spectra. For nonconductive samples, charge 25920


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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 Figure 5. Nitrogen coordinated to Fe is highlighted in N 1s and Fe 2p spectra.

identified as graphitic N. We should keep in mind, however, that graphitic N may have a significant spread in binding energy positions, i.e., between 401.5 and 403.0 eV, depending on the chemical type of other nitrogens in direct proximity. When a 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 a peak due to the Fe−N bond.74 Iron bound to pyrrolic N such as in porphyrins appears at a 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 the Fe−Nx bond at 708.4 eV.22 We have also confirmed the presence of Fe−Nx species in the range 399.8−400 eV by density functional theory (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 than others. Mössbauer spectroscopy confirmed the existence of two types of Fe−Nx centers, i.e., one being a mesomeric Fe−N4 center and the other being a disordered Fe−N4 center. Through correlation of XPS and Mössbauer spectra, the peak due to the mesomeric Fe−N4 center where all nitrogen atoms are equally bonded to the metal center was assigned to the M−N peak in the 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, 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 the Supporting Information show all XPS data for all samples included in this study. Very different amounts of nitrogen are detected: from 1.2 to 14.5 atom % with an average around 4−5% (Figure 6). The concentration of iron was found to be in the range 0.1−1.4% with an average around 0.2−0.4%. For some samples, the amount of iron was lower than the detection limit, which is ∼0.02% for Fe 2p. It is

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 the high resolution N 1s spectrum for Fe aminoantipyrine (Fe-AApyr) curve fitted with six peaks. Figure 5

Figure 5. Nitrogen moiety nomenclature. A detailed discussion of nitrogen species is in the text.

shows moieties and corresponding binding energies (BEs) 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 a classical quaternary structure as shown in Figure 5 such as a 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 eV.73 Therefore, the peak at 401.7 eV will have contributions 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 The identification of component at a rather high BE (402.7 eV) is also contentious. This value is quite close to the 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 Ncontaining compounds may be observed without N−O functional groups in the sample. This peak is, therefore, 25921


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the same loadings on the electrode is representative of the mechanism of ORR. Metal-free electrocatalyst based on carbendazim has an 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%.25 Imidazolidinyl urea based electrocatalysts have the best performance. As Tables S1 and S2 in the Supporting Information show, the 43 metal-containing samples studied have a range of E1/2 values between 0.35 and 0.79 V and a range of H2O2 yields 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. 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-wave potential 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 Figures 6 and 8−11 is an average of three areas analyzed by XPS. It is important to acknowledge general trends in chemistry-performance plots pointing to 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 M−N−C-based electrocatalysts has always been acknowledged,11,33,41,43,49,53,77−79 and this observation of higher activity with higher overall amount of nitrogen is important confirmation. The role of individual types of nitrogen in the mechanism of oxygen should be evaluated by looking at the 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 signals from different sampling depths. In particular, N 1s comes from ∼11 nm while Fe 2p provides composition from only 1.5−2 nm of the material. Atomic percentages 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 amounts 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 to overall activity. 3.4.1. Metal-Free Electrocatalysts. Figure 8 plots the 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 values reported between 0.35 and 0.60 V corresponds to the potential of reduction of oxygen to hydrogen peroxide via a 2e− reaction. 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 water and/or no or small amount species that reduce hydrogen peroxide generated by the first step further into oxygen. This observation is consistent with mechanistic studies of ORR on N-doped CNTs without

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 the Supporting Information. Higher activity is observed for electrocatalysts with higher overall amounts of nitrogen.

important to remember that attenuation lengths of Fe 2p and N 1s photoelectrons are remarkably different. The atomic percent of nitrogen detected comes from 10 to 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 signal-to-noise in the Fe 2p spectra was high enough for curve-fitting the spectra as shown in Figure 4b. 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

Figure 7. RRDE results for carbendazim (CBDZ) derived catalysts and imidazolidinyl urea (IMZU) material. Conditions: 0.5 M H2SO4 saturated with O2, 1200 rpm, 5 mV s−1, and catalyst loading 0.6 mg cm−2. Metal-free catalyst has much lower E1/2 (∼0.4 V) and higher ring current due to hydrogen peroxide production than metalcontaining catalyst.

three samples while Tables S1 and S2 in the Supporting Information have half-wave potentials and hydrogen peroxide yields. Due to the hydrophobic nature of these materials, loading of 0.6 mg/cm2 was necessary to obtain full coverage of catalyst on the 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 with the electrochemical reduction of H2O2. However, the relative comparison between H2O2% values detected for various types of materials tested at exactly 25922


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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 toward oxygen reduction to hydrogen peroxide.

Figure 9. E1/2 (●) and %H2O2 (○) vs pyridinic and pyrrolic N for Fe-containing electrocatalysts. Larger amounts of pyridinic nitrogen and lower amounts of pyrrolic nitrogen result in better ORR. Pyrrolic nitrogen contributes to higher hydrogen peroxide production.

metal showing that only the first step of the oxygen reduction reaction O2 → H2O2 is occurring.49 We have direct evidence that pyrrolic N serves as an active site for the first step of the oxygen reduction reaction. 3.4.2. 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 values and a high peroxide yield, but in addition, there is a group of electrocatalysts with higher E1/2 between 0.7 and 0.8 V 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 toward another possible explanation for 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 contribute. We will address this hypothesis later. The amount of pyrrolic N does not improve the activity of metal-containing electrocatalysts (Figure 9c); quite the opposite occurspyrrolic N causes partial reduction of oxygen to hydrogen peroxide (Figure 9d) thereby reducing the overall activity as expressed by a 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 metal-containing electrocatalysts and is opposite from the report in which pyrrolic N was shown to be an efficient active site for complete 4e− reduction of oxygen.62 The conclusions in this report, however, were based on spectra from only four samples fitted inconsistently with peaks of varied width. Moreover, careful examination of XPS results in 25923


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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 N is a good metric 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.

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.

decreases the yield of hydrogen peroxide. This conclusion is supported by a previous report that pyridinic nitrogen is the one that is needed for converting the mechanism from 2e− to 4e−.53 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 amounts of these species as determined from N 1s spectra, percent of Nx−Fe, and as determined from Fe 2p spectra, percent of Fe−Nx, both half-wave potential and hydrogen peroxide yield. There is an obvious trend indicating that there is an 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 to hydrogen peroxide production, therefore suggesting that Fe−Nx moieties either reduce oxygen to water directly via 4e− reaction or reduce hydrogen peroxide to water. As we have discussed above, through correlation between Mössbauer 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

ref 62 report shows that one out of four samples does not follow the trend in increasing activity with the amount of pyrrolic nitrogen, making the conclusion derived unjustified (Figure 4 in ref 62). From our observations in Figures 8 and 9, it is clear that the ratio of pyridinic to pyrrolic type of N is a parameter that is an indicative of the electrocatalytic activity of the M−N−C type of catalyst. Figure 10 plots this ratio versus the 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, halfwave 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 larger pyridinic/pyrrolic ratio for both metal-containing and metal-free samples, indicating that pyridinic N may actually be the site that reduces hydrogen peroxide to water; therefore, more significant amounts of pyridinic nitrogen with respect to pyrrolic nitrogen 25924


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Figure 12. 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 oxide 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 activity. Defect formation in the carbon graphitic network that is accompanying by oxidation of carbon also creates possible sites for the formation of moieties that are ORR active. On the other hand, the amounts of pyrrolic and graphitic nitrogens show an inverse relationship with the number of defect sites as manifested by the amount of carbon−oxygen functionalization. This may indicate that these are plain defects that do not change an overall sp2 character in the 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 a possible reaction pathway of ORR: Based on E1/2 of reaction and correlation between the ratio of pyridinic to pyrrolic nitrogen, we can suggest that, in metalfree materials, there are no species that convert oxygen directly into water or a small amount of species that convert hydrogen peroxide generated by the first 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 first step of O2 reduction to hydrogen peroxide: active site 1. The ratio of pyridinic to pyrrolic N 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 the following occurs: Pyrrolic nitrogen catalyzes the first 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− direct reduction of oxygen to water (active site S) or also catalyzes the second step of H2O2 reduction to H2O (active site S2). There is a correlation between the peak due to pyridinic N and activity. This may be due to (a) pyridinic nitrogen being the active site for H2O2 reduction to H2O as confirmed by the ratio of pyridinic to pyrrolic nitrogen (active site S2) and/or (b) the peak assigned as pyridinic having a contribution from disordered Fe−Nx centers (such as Fe−N2, Fe−N3) which serve as active sites for either direct 4e− ORR (active site S) or second step 2e− reaction (active site S2).

obtained by summing areas of three peakspyridinic, pyrrolic, and Me−N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 the 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 the pyrrolic peak shows no correlation with activity, so if there is any contribution of disordered metal−nitrogen moieties to 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 Fe−N3 coordinations may, therefore, contribute to the lower BE, where pyridinic N is detected, due to a smaller shift of electron density from nitrogen to iron in disordered coordination versus the mesomeric Fe−N4 environment, in which larger reduction of electron density on the nitrogen atom results in a larger binding energy. Hence, the correlation observed between the peak 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. An even deeper understanding of 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. A carbon network that has a significant amount of graphitic carbon as confirmed by 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 the C−C graphitic network. 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 the total amount of these functional groups, labeled as CxOy, as a function of the total amount of pyridinic and Fe−Nx groups 25925


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The Journal of Physical Chemistry C Notes

Figure 13 shows the suggested pathways and possible active sites.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the DOE-EERE Fuel Cell Technology Program (subcontract to Northeastern University, with PI Sanjeev Mukerjee).

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(1) Antolini, E. Palladium in Fuel Cell Catalysis. Energy 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 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. J. 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 Air-Breathing 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 Polyaniline-Iron-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 Oxygen-Reduction Reaction in Polymer Electrolyte Fuel Cells. Energy 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. Appl. Catal., B 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. Synthesis-Structure-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. HighPerformance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 2011, 332, 443−447. (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. (14) Cheng, N. C.; Kemna, C.; Goubert-Renaudin, S.; Wieckowski, A. Reduction Reaction by Porphyrin-Based Catalysts for Fuel Cells. Electrocatalysis 2012, 3, 238−251. (15) Cheon, J. Y.; Kim, T.; Choi, Y.; Jeong, H. Y.; Kim, M. G.; Sa, Y. J.; Kim, J.; Lee, Z.; Yang, T. H.; Kwon, K.; et al. Ordered Mesoporous Porphyrinic Carbons with Very High Electrocatalytic Activity for the Oxygen Reduction Reaction. Sci. Rep. 2013, 3, 2715. (16) Kasahara, K.; Tsuboi, H.; Koyama, M.; Endou, A.; Kubo, M.; Del Carpio, C. A.; Miyamoto, A. Tight-Binding Quantum Chemical Molecular Dynamics Study on First Proton Transfer Process of Orr

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 2e− reaction. Fe−Nx centers can serve as active sites that catalyze either direct 4e− ORR (S), 2 × 2 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 the sacrificial support method from different classes of nitrogen precursors, the role of different types of nitrogen in ORR has been established. Pyrrolic nitrogen catalyzes the first step of oxygen reduction to hydrogen peroxide. Pyridinic nitrogen serves as a second 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. Identifying synthetic routes of designing materials with the smallest amount of pyrrolic nitrogen and the largest amounts of metal−nitrogen centers in the pyridinic environment will result in 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 nitrogen-containing charge-transfer salt precursors.55

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07653. Tables S1 and S2 showing relative amounts of nitrogen and iron species derived from high resolution N 1s and Fe 2p XPS spectra along with E1/2 values and H2O2 yields for metal-free and metal-containing samples (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [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. 25926


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Chemistry of Multitudinous Active Sites for Oxygen Reduction

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