Nitrogen-Coordinated IronâCarbon as Efficient Bifunctional

Article

Nitrogen-Coordinated Iron-Carbon (FeNC) as Efficient Bifunctional Electrocatalysts for Oxygen Reduction Reaction and Oxygen Evolution Reaction in Acidic Media Kuldeep Balram Mamtani, Deeksha Jain, Anne C. Co, and Umit S. Ozkan Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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Energy & Fuels

Nitrogen-Coordinated Iron-Carbon (FeNC) as Efficient Bifunctional Electrocatalysts for Oxygen Reduction Reaction and Oxygen Evolution Reaction in Acidic Media

Kuldeep Mamtani1, Deeksha Jain1, Anne C. Co2, Umit S. Ozkan1* 1

William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus OH 43210 2

Department of Chemistry and Biochemistry,

The Ohio State University, Columbus OH 43210

* Corresponding Author [email protected] 614-292-6623

ACS Paragon Plus Environment

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Abstract The ORR and OER performances of N-coordinated iron-carbon (FeNC), Ir/C and Pt/C catalysts were compared using half-cell measurements. The results suggest significant ORR and OER activity for FeNC materials unlike state-of-the-art Ir/C or Pt/C which exhibited high catalytic activity only for one of the two reactions. Bifunctionality analysis on these three catalyst materials reveals significantly lower combined overpotential for FeNC catalysts compared to that for commercial Ir/C and Pt/C samples.

Keywords: ORR, OER, bifunctional, FeNC, regenerative


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1. Introduction Increasing world energy needs and decreasing fossil fuel reserves require the development of sustainable and efficient technologies for power generation.1 Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are very important electrochemical reactions in this regard. Oxygen reduction reaction (ORR) occurs at the cathode of a hydrogen-based proton exchange membrane (PEM) fuel cell where the only by-product from the reaction is water. The low temperature of operation (< 80 °C) and slow kinetics of ORR necessitate Pt-based electrocatalysts.2,

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Similarly, the slow

kinetics of oxygen evolution reaction (OER) for water electrolysis necessitates the use of ruthenium or iridium-based catalysts

4, 5

. Both reactions use precious metal-based

electrocatalysts which are expensive and available only in limited reserves. Furthermore, Pt exhibits poor OER activity whereas Ru (or Ir) has low ORR activity. Thus, development of low-cost catalyst materials which exhibit high activity for ORR as well as OER is essential for the success of regenerative fuel cells which can operate as fuel cells as well as electrolyzers. Consequently, low-cost, bifunctional electrocatalyst development for ORR and OER has received considerable attention recently. Jaramillo and co-workers developed Mn2O3 and attributed its high ORR and OER activity to its nanostructured nature.6,

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On the

other hand, heteroatom-doped carbon materials with or without any metal have also been reported to have bifunctional activity for ORR and OER.8,

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Dai and co-workers8

have demonstrated bifunctionality of a high surface area, metal-free mesoporous carbon doped with nitrogen and phosphorus. They have also proposed the availability of graphite edges to be crucial in improving the catalytic activity.8 Similarly, Ajayan and co-


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used chemical vapor deposition of various C, N sources to synthesize N-

doped carbon nanotubes which exhibited high ORR and OER activity. They also studied the effect of tube diameter and the distribution of various nitrogen species in the synthesized catalysts 9. Similarly, Muhler and co-workers10,

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reported high ORR and

OER activity to MxOy (M: metal such as Co, Fe, Ni) particles embedded in porous and amorphous nitrogen rich carbon shell. The improved catalytic performance was believed to be a result of enhanced electronic conductivity and the synergistic effect between nitrogen-doped carbon sites and metal-based sites. Perovskites are another reported class of bifunctional catalyst materials for ORR and OER.12, 13 In spite of such recent research efforts in the field, bifunctional catalyst development for ORR and OER continues to remain a major challenge. The catalytic performance of the materials discussed above still needs significant improvement. In addition, all of these studies have been performed in alkaline medium. Thus, research efforts aimed at investigating ORR and OER electrocatalysts in a low pH (acidic) environment, which would pose additional challenges in terms of catalyst degradation, are essential. With this motivation, in this manuscript we report the ORR and OER performance of Ncoordinated iron-carbon (FeNC) materials in an acidic electrolyte. Though these materials have been previously studied for ORR14, 15, there is no study discussing their OER activity or bifunctional characteristics for ORR and OER. Systematic comparisons with the state-of-the-art catalyst materials, namely Pt/C and Ir/C are also presented using three bases, namely equal geometric area, equal catalyst weight and equal metal amounts. Electrochemical half-cell testing results demonstrate suitability of FeNC


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materials as efficient electrocatalysts, not only for ORR but also for OER, hence making them promising candidates for regenerative fuel cells. 2. Experimental 2.1.

Materials

N-coordinated iron-carbon (FeNC) catalysts were synthesized according to the procedure described in literature

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. Briefly, 500 mg of phenanthroline dissolved in a 1:2

ratio by volume of ethanol: water was mixed with 31.2 mg of iron (II) acetate. When a deep red color, indicative of the formation of [Fe(phen)3]2+ complex was observed, 500 mg of Black Pearls 2000 was added to obtain a nominal Fe loading of 1%. This mixture was stirred in a water bath at 70 °C until the solution volume was reduced to one third of the original volume and then kept overnight in the oven. The resulting material was then ball-milled at 200 rpm for 3h using a rotary ball-mill. The ball-milled precursor underwent two pyrolysis steps, first in argon at 1050 °C for 1h and then in ammonia at 950 °C for 20 minutes. FeNC catalysts were also synthesized with a nominal metal loading of 5%. 10% Pt/C (ETEK) or 5% Pt/C (Fuel Cell Store) and 20% Ir/C (Premetek Co.) or 5% Ir/C (Fuel Cell Store) were used as received as the state-of-the-art catalyst materials for ORR and OER respectively. The nomenclature employed here indicates the nominal metal loading and the total catalyst amount (including the mass of carbon) as the first and the second term in the parenthesis, respectively, following the sample name. For example, Pt/C (X%, Y µg/cm2) refers to Pt/C sample with X as the nominal Pt loading and Y as the weight/cm2 of catalyst (Pt + C).


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2.2.

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Electrochemical Testing

All electrochemical measurements were performed at room temperature in a standard three-electrode system comprising of a working electrode (5.61 mm glassy carbon disk), a hydrogen reference electrode (ET070 Hydroflex) and a counter electrode (Pt coil) in a 0.5 M H2SO4 electrolyte. Electrochemical measurements were collected using a BioLogic potentiostat. To prepare the catalyst ink for FeNC, 95 µL of 5 wt% Nafion® solution and 350 µL ethanol (200 proof) were added to 10 mg of catalyst weighed in a 2 mL vial. The vial was then kept for ultra-sonication in an ice bath until the catalyst was well-dispersed. 9 µL of the ink was then pipetted onto the glassy carbon disk which corresponded to a catalyst loading of approximately 800 µg/cm2geometric. The catalyst ink preparation procedure was similar for Pt/C and Ir/C and the catalyst loading (including the mass of carbon) was either 200 µg/cm2geometric or 800 µg/cm2geometric. A lower catalyst loading for the precious metal-based catalysts is typically used in the literature.17-19 To measure ORR activity, cyclic voltammograms (CVs) were first collected at 50 mV/s from 1.2 V to 0.0 V to 1.2 V with the working electrode rotating at 1000 rpm until reproducible CVs were obtained in the oxygen saturated electrolyte. CVs at 10 mV/s were then collected at 400, 800, 1000, 1200 and 1600 rpm on the disk again from 1.2 V to 0 V to 1.2 V. CVs were also collected in an argon-saturated electrolyte between the same potential region at 10 mV/s, serving as a blank. The current from the blank was subtracted from the oxygen saturated current to obtain the oxygen reduction reaction (ORR) data. ORR performance of various samples in this study was evaluated by comparing (i) onset potential defined as the potential at a background-subtracted


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current density of -0.1 mA/cm2geometric or -0.125 mA/mgcatalyst or -2.5 mA/mgmetal and (ii) half-wave potential (E1/2). The calculation of selectivity towards water formation was made using the KoutechyLevich equation (equation I). 1 1 1 = + () where i is the measured current density, iK is the kinetic current density and ilim is the limiting current density defined by = 0.62/ / Here, n is the number of electrons transferred per molecule of oxygen, F is the Faraday’s constant (96485 C/mole of electrons), is the bulk concentration of oxygen (1.13 x 10-6 mol/cm3) 20, 21 , is the diffusion coefficient of oxygen (1.8 x 10-5 cm2/s), is the kinematic viscosity of the electrolyte (1.009 x 10-2 cm2/s).22 To evaluate OER activity, linear sweep voltammograms (LSVs) were collected from 0.0 to 2.0 V in an argon-saturated electrolyte with the catalyst-coated electrode rotating at 1600 rpm. The scan rate used was 10 mV/s. The current-time decay plots at 1.1, 1.2 and 1.3 V show current decaying to negligible values within a few seconds and therefore the current measured between 1.1 to 1.3 V at 10 mV/s was assumed to be due to the capacitive current and subtracted from the OER current. All OER current densities reported herein are already corrected for the corresponding capacitive current.


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For comparing OER activity, the potential at an OER current density of 10 mA/cm2geometric or 12.5 mA/mgcatalyst or 250 mA/mgmetal was considered. The OER stability was evaluated by cycling the catalyst-coated electrode from 1.0 to 1.6 V

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. Argon was continuously bubbled into the electrolyte and the electrode was rotated

at 1600 rpm while cycling was performed at 100 mV/s. OER activity was measured again after 25, 50, 75, 100, 200 and 500 cycles. All potentials referred to in this work are referenced with respect to a reversible hydrogen electrode (RHE). 3. Results and Discussion 3.1.

ORR and OER Activity Measurements

ORR and OER activity measurements were made to allow comparisons of catalysts activity based on equal geometric surface area, equal catalyst weight, and equal weight of metal. 3.1.1. Comparisons based on equal geometric surface area The ORR activity for FeNC catalyst as well as that for commercial Ir/C and Pt/C samples was measured. In these experiments, higher metal loadings and lower catalyst amounts (active metal + the mass of carbon) are used for the precious metal-based catalysts relative to FeNC. The loading and amount used are typical of those ORR activity measurements on precious metals and non-precious metal catalysts in the literature

17-19

. Thus, the samples chosen for comparison here are Pt/C (10%, 200

µg/cm2), Ir/C (20%, 200 µg/cm2) and FeNC (1%, 800 µg/cm2). As seen in Figure 1(a), Ir/C exhibited the lowest ORR activity whereas Pt/C showed the highest ORR activity. The ORR activity for FeNC catalyst was also found to be approaching that of Pt/C sample. The onset potential and half-wave potential (E1/2) values for the former were


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lower than the latter sample by only 50 mV and 40 mV, respectively (Table 1). On the other hand, Ir/C sample showed significantly lower onset potential and E1/2 compared to Pt/C and FeNC catalysts. We also measured the selectivity of these catalysts towards water formation. Figure 1(b)

presents the corresponding Koutechy-Levich plots at several potentials for Ir/C,

Pt/C and FeNC samples. The plots were found to be linear and parallel at all potentials ranging from 0.1 to 0.6 V for all three samples. The slope of these plots were used to obtain selectivity (n) which was found to be close to 4 for all catalysts tested, suggesting that oxygen is primarily reduced via a four-electron pathway (either direct four-electron reduction to H2O or a two-electron reduction to H2O2 as the intermediate and another two-electron reduction to water). The selectivity values for all these samples were found to be weakly dependent on the potential as represented in the inset of Figure 1(b). To investigate if this apparent selectivity of 4 for FeNC indeed corresponds to a direct 4 e- pathway or a 2+2 e- pathway, we performed additional measurements. KoutechyLevich analysis when a lower catalyst loading (200 µg/cm2) was used suggests a selectivity of only 3.4 (Table S1 in the supplementary information). Thus, the results here suggest a strong dependence of n on the catalyst loading used and that ORR occurs most likely via a 2+2 e- pathway on FeNC materials. For the OER activity measurements, anodic linear sweep voltammograms (LSVs) were collected in argon saturated 0.5 M H2SO4. The methodology used for subtracting the capacitive contribution from the overall measured current is illustrated in Figure S1. The inset in Figure 3 represents the current-time plots at 1.1 V, 1.2 V and 1.3 V. The current


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at potentials below 1.3 V decays rapidly to zero within a few seconds confirming that no OER current is observed in the potential range between 1.1 and 1.2 V. Thus, the constant current measured between 1.1 and 1.2 V was considered to be contribution from the capacitive nature of the electrode and this capacitive current contribution was consequently subtracted from the OER current and therefore the OER activity reported herein are all corrected for the capacitive current. The capacitive current subtracted OER voltammograms for Pt/C (10%, 200 µg/cm2), Ir/C (20%, 200 µg/cm2) and FeNC (1%, 800 µg/cm2) samples are shown in Figure 2. Among the three samples investigated here, Pt/C exhibited the lowest OER activity as indicated by its significantly higher overpotential requirements. On the other hand, the potential at an OER current density of 10 mA/cm2, which is considered here as a measure of OER activity, was found to be comparable for FeNC and Ir/C (1.55 V vs. 1.54 V respectively). This is an important observation and demonstrates the promise of FeNC materials for practical applications related to OER 24. The inset in Figure 2 shows the OER current densities at 1.63 V. 3.1.1.1.

Stability of FeNC under OER conditions

One of the questions that come to mind when dealing with carbon-containing catalysts is the carbon corrosion. To address the question of catalyst degradation, the two most active OER catalysts namely Ir/C and FeNC were evaluated for their stability. Figure 3 compares the % retention of normalized OER currents for the two samples at 1.45 V, 1.50 V and 1.55 V after several cycles. At all potentials, FeNC exhibited significantly higher retention of OER current and consequently higher stability relative to Ir/C. It is


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interesting to note that both samples showed higher retention in the current at higher potentials.

3.1.2 Comparisons based on equal catalyst weight We also compared the ORR and OER performance of Pt/C, Ir/C and FeNC when equal weights of catalysts were used. As pointed out before, the term “catalyst” also includes the mass of the carbon support even for Pt/C and Ir/C. Though this basis using equal catalyst weights is not commonly used in the literature, such a comparison is still useful. The samples compared are Pt/C (5%, 800 µg/cm2), Ir/C (5%, 800 µg/cm2) and FeNC (1%, 800 µg/cm2). Thus, the catalyst loading was 800 µg/cm2 in all cases. The nominal metal loadings for Pt/C and Ir/C were purposefully chosen such that they are similar to that for FeNC. It should be noted that the geometric area of the disk electrode in addition to the catalyst amounts is also the same for all samples here. A comparison for the ORR activity for these three samples is presented in Figure 4. As a first observation, the general trends observed here are the same as before. The onset potential for Ir/C, FeNC and Pt/C were 0.72 V, 0.85 V and 0.97 V. Similarly, the E1/2 values for the three samples respectively were 0.47 V, 0.73 V and 0.83 V (Table 1). It is interesting to note that the ORR activity of both Ir/C and Pt/C increased relative to the case when lower catalyst amounts were used. For example, the onset potential for Pt/C increased from 0.89 to 0.97 V when catalyst loading increased from 200 to 800 µg/cm2. This is quite expected considering the concomitant increase in the amount of metal (Pt or Ir) on the electrode when higher catalyst amount is used.


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The OER voltammograms (after subtracting the capacitive current) for Pt/C (5%, 800 µg/cm2), Ir/C (5%, 800 µg/cm2) and FeNC (1%, 800 µg/cm2) are shown in Figure 5. The trends for OER activity between the three catalysts were again similar to those shown in Figures 1 and 3. The potential values for Ir/C, Pt/C and FeNC at 12.5 mA/mgcatalyst were found to be 1.59 V, 1.72 V and 1.52 V, respectively. Furthermore, the OER current normalized to the total catalyst amount (including the weight of C for all samples) at 1.63 V is presented in the inset of Figure 5. As evident, the OER current was observed to be similar for FeNC and Ir/C. On the other hand, Pt/C exhibited significantly lower OER current. 3.1.3 Comparisons based on equal metal loadings and equal catalyst amounts A third basis used for comparison of the ORR and OER performance was equal nominal metal loading and equal catalyst amount. Though such a basis is also not commonly used in literature, this comparison is quite informative and insightful. Furthermore, it is advantageous because it allows for direct comparison between samples. It should again be noted that geometric surface area of the electrode is also constant here for all samples. Figure 6 presents the comparison of ORR activity for Ir/C, Pt/C and FeNC. The nominal metal loading and catalyst loading respectively were 5% and 800 µg/cm2 for all samples. Thus, 40 µg of the metal was used on the disk electrode for all three samples. The ORR activity trends were again the same as before with Ir/C as the least active and Pt/C as the most active. The ORR activity for FeNC was found to be higher than Ir/C but lower than that for Pt/C as evident by comparison of onset and half-wave potentials for various samples (Table 1). It is interesting to note that there is no significant difference in the ORR activity between FeNC samples synthesized using two


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different metal loadings. This observation can be attributed to a site saturation effect where it is demonstrated that ORR activity increases with metal loading upto a particular value beyond which it levels-off or decreases with further increase in the metal content 25

.

The capacitive-current-subtracted OER data are presented in Figure 7 (a). The OER activity was again found to be lowest for Pt/C. On the other hand, Ir/C and FeNC samples exhibited similar OER activities. The specific OER current at 1.63 V for the latter was about 90% of that of Ir/C as evident from the inset of Figure 7 (a). This is a very significant result from a practical viewpoint if we consider the enormous difference in the cost of Ir vs. that of Fe. Potential vs. log (i), or Tafel plots of the OER process on Pt/C, Ir/C and FeNC were also plotted. The Tafel slopes were found to be 330, 194 and 305 mV/dec, respectively (Figure 7 (b)). These values are comparable to Tafel slopes reported for several other OER catalysts in the literature 9, 26-28. 3.2 Bifunctionality Analysis The bifunctional electrocatalytic activity to catalyze both the ORR and OER on the three catalysts namely Ir/C, Pt/C and FeNC was evaluated. The literature often measures the bifunctionality of a catalyst by the total overpotential requirement for ORR and OER. For this, the current density typically chosen are -3 mA/cm2geometric for ORR and 10 mA/cm2 geometric

for OER 29. These values correspond to -3.75 mA/mgcatalyst and 12.5 mA/mgcatalyst

respectively for a catalyst loading of 800 µg/cm2. Similarly, on equal metal weight basis, these values are -75 mA/mgmetal and 250 mA/mgmetal for a nominal metal loading of 5% and catalyst loading of 800 µg/cm2. Table 2 presents the results from this analysis. The


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combined ORR and OER overpotential for Ir/C (20%, 200 µg/cm2) and Pt/C (10%, 200 µg/cm2) was found to be 1.16 V and 1.14 V respectively. On the other hand, FeNC (1%, 800 µg/cm2) sample required a combined overpotential of only 0.83 V. These results are expected considering that Ir/C showed poor ORR activity but good OER activity whereas Pt/C exhibited excellent ORR activity, but very low OER activity. Thus, each of these precious metal-based catalysts performed well only for one of the two reactions. On the other hand, FeNC catalyst materials exhibited high activity for both reactions demonstrating their promise as efficient bifunctional electrocatalysts for both ORR and OER. This observation is also valid in general when other bases of comparisons are used. For example, when higher catalyst loadings namely 800 µg/cm2 were employed for Ir/C and Pt/C, the total overpotential requirement was found to be 1.32 and 0.93 V respectively. These values are again significantly higher than that exhibited by FeNC (1%, 800 µg/cm2), 0.82 V. Comparison of Ir/C, Pt/C and FeNC on equal nominal metal (5%) and equal catalyst loading (800 µg/cm2) basis also reveals significantly higher overpotential for Ir/C and Pt/C relative to that for FeNC. Sulfuric acid electrolyte used in the present study may be believed to hinder the catalytic performance of Pt/C sample. Therefore, we measured the ORR and OER activity for Pt/C in 0.1 M HClO4 which is typically considered as a non-adsorbing electrolyte. Though the ORR activity for Pt/C was found to be relatively higher in HClO4 than that in H2SO4 electrolyte, its combined overpotential for ORR and OER was found to be similar in both electrolytes. Thus, the conclusions reported here remain unaffected by the choice of the electrolyte used for measurements. 4. Conclusions


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In summary, the ORR and OER performance of FeNC catalysts was compared with state-of-the-art catalysts for these reactions, namely Pt/C and Ir/C, respectively. Pt/C showed excellent ORR activity, but poor OER activity whereas Ir/C had good OER activity, but very poor ORR activity. FeNC materials, on the other hand, displayed significant catalytic activity not only for ORR, but also for OER. Evaluation of bifunctional characteristics suggests significantly lower combined ORR and OER overpotential for FeNC catalysts compared to the state-of-the-art material for each of the reactions discussed. These trends did not change whether the basis for comparison was equal geometric surface area, equal catalyst weight or equal metal amount and equal catalyst weight. Thus, the results from this study demonstrate FeNC materials as potential oxygen electrodes for regenerative fuel cell systems. Acknowledgements This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-07ER15896. We would also like to thank Ohio Coal Research Consortium for their financial support under Subcontract No. OCRC-C-04.

References 1. Chu, S.; Majumdar, A., Opportunities and challenges for a sustainable energy future. Nature 2012, 488, 294-303. 2. Mukerjee, S.; Srinivasan, S.; Soriaga, M. P., Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction. Journal of Electrochemical Society 1995, 142, 1409-1422.


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3. Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T., Review - Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl. Catal. B-Environ. 2005, 56, 9-35. 4. Zhao, Y.; Hernandez-Pagan, E. A.; Vargas-Barbosa, N. M.; Dysart, J. L.; Mallouk, T. E., A High Yield Synthesis of Ligand-Free Iridium Oxide Nanoparticles with High Electrocatalytic Activity. The Journal of Physical Chemistry Letters 2011, 2, 402406. 5. Nakagawa, T.; Beasley, C. A.; Murray, R. W., Efficient Electro-Oxidation of Water near Its Reversible Potential by a Mesoporous IrOx Nanoparticle Film. J. Phys. Chem. C 2009, 113, 12958-12961. 6. Gorlin, Y.; Jaramillo, T. F., A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. Journal of American Chemical Society 2010, 132, 13612-13614. 7. Pickrahn, K. L.; Park, S. W.; Gorlin, Y.; Lee, H.-B.-R.; Jaramillo, T. F.; Bent, S. F., Active MnOxElectrocatalysts Prepared by Atomic Layer Deposition for Oxygen Evolution and Oxygen Reduction Reactions. Advanced Energy Materials 2012, 2, 1269-1277. 8. Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L., A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat. Nanotechnol. 2015, 10, 444452. 9. Yadav, R. M.; Wu, J.; Kochandra, R.; Ma, L.; Tiwary, C. S.; Ge, L.; Ye, G.; Vajtai, R.; Lou, J.; Ajayan, P. M., Carbon Nitrogen Nanotubes as Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions. ACS Applied Materials & Interfaces 2015, 7, 11991-12000. 10. Aijaz, A.; Masa, J.; Rosler, C.; Xia, W.; Weide, P.; Botz, A. J.; Fischer, R. A.; Schuhmann, W.; Muhler, M., Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted Nitrogen-Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode. Angew Chem Int Ed Engl 2016, 55, 4087-4091. 11. Masa, J.; Xia, W.; Sinev, I.; Zhao, A.; Sun, Z.; Grutzke, S.; Weide, P.; Muhler, M.; Schuhmann, W., Mn(x)O(y)/NC and Co(x)O(y)/NC nanoparticles embedded in a nitrogen-doped carbon matrix for high-performance bifunctional oxygen electrodes. Angew Chem Int Ed Engl 2014, 53, 8508-8512. 12. Hardin, W. G.; Slanac, D. A.; Wang, X.; Dai, S.; Johnston, K. P.; Stevenson, K. J., Highly Active, Nonprecious Metal Perovskite Electrocatalysts for Bifunctional Metal– Air Battery Electrodes. The Journal of Physical Chemistry Letters 2013, 4, 1254-1259. 13. Chen, Z.; Yu, A. P.; Higgins, D.; Li, H.; Wang, H. J.; Chen, Z. W., Highly Active and Durable Core-Corona Structured Bifunctional Catalyst for Rechargeable Metal-Air Battery Application. Nano Lett. 2012, 12, 1946-1952. 14. Kramm, U. I.; Herranz, J.; Larouche, N.; Arruda, T. M.; Lefevre, M.; Jaouen, F.; Bogdanoff, P.; Fiechter, S.; Abs-Wurmbach, I.; Mukerjee, S.; Dodelet, J. P., Structure of the catalytic sites in Fe/N/C-catalysts for O-2-reduction in PEM fuel cells. Phys. Chem. Chem. Phys. 2012, 14, 11673-11688. 15. Kramm, U. I.; Lefevre, M.; Larouche, N.; Schmeisser, D.; Dodelet, J. P., Correlations between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for the ORR in PEM Fuel Cell via Fe-57 Mossbauer Spectroscopy and Other Techniques. J. Am. Chem. Soc. 2014, 136, 978-985.


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16. Lefevre, M.; Proietti, E.; Jaouen, F.; Dodelet, J. P., Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells. Science 2009, 324, 71-74. 17. Meng, H.; Jaouen, F.; Proietti, E.; Lefevre, M.; Dodelet, J.-P., pH-effect on oxygen reduction activity of Fe-based electrocatalysts. Electrochem. Commun. 2009, 11, 1986-1989. 18. Li, Q.; Wu, G.; Cullen, D. A.; More, K. L.; Mack, N. H.; Chung, H. T.; Zelenay, P., Phosphate-Tolerant Oxygen Reduction Catalysts. ACS Catalysis 2014, 4, 3193-3200. 19. 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, 443-447. 20. Rahman, M. R.; Awad, M. I.; Kitamura, F.; Okajima, T.; Ohsaka, T., A comparative study of ORR at the Pt electrode in ammonium ion-contaminated H2SO4 and HClO4 solutions. J. Power Sources 2012, 220, 65-73. 21. Wei, Y.-C.; Liu, C.-W.; Wang, K.-W., Improvement of oxygen reduction reaction and methanol tolerance characteristics for PdCo electrocatalysts by Au alloying and CO treatment. The Royal Society of Chemisty: Chemical Communications 2011, 47, 11927-11929. 22. Gottesfeld, S.; Raistrick, I. D.; Srinivasan, S., Oxygen reduction kinetics on a platinum RDE coated with a recast Nafion film. J. Electrochem. Soc. 1987, June, 14651462. 23. Riese, A.; Banham, D.; Ye, S.; Sun, X., Accelerated Stress Testing by Rotating Disk Electrode for Carbon Corrosion in Fuel Cell Catalyst Supports. Journal of the Electochemical Society 2015, 162, F783-F788. 24. Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z., Heteroatom-Doped GrapheneBased Materials for Energy-Relevant Electrocatalytic Processes. ACS Catalysis 2015, 5, 5207-5234. 25. Jaouen, F.; Dodelet, J.-P., Average turn-over frequency of O2 electro-reduction for Fe/N/C and Co/N/C catalysts in PEFCs. Electrochim. Acta 2007, 52, 5975-5984. 26. Mao, S.; Wen, Z.; Huang, T.; Houa, Y.; Chen, J., High-performance bi-functional electrocatalysts of 3D crumpled graphene–cobalt oxide nanohybrids for oxygen reduction and evolution reactions. Energy and Environmental Science 2014, 7, 609616. 27. Chen, S.; Duan, J.; Ran, J.; Jaroniec, M.; Qiao, S. Z., N-doped graphene filmconfined nickel nanoparticles as a highly efficient three-dimensional oxygen evolution electrocatalyst. Energy Environ. Sci. 2013, 6, 3693. 28. Chen, S.; Duan, J.; Jaroniec, M.; Qiao, S. Z., Three-dimensional N-doped graphene hydrogel/NiCo double hydroxide electrocatalysts for highly efficient oxygen evolution. Angew Chem Int Ed Engl 2013, 52, 13567-13570. 29. McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F., Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977-16987.


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Table 1. Comparison of ORR kinetic parameters for various samples Nominal Basis

Sample

metal content (wt%)

Catalyst loading

Measures of ORR activity (V vs. RHE)

2

(µg/cm )

E @ -0.1 2

mA/cm

Equal geometric area

Equal catalyst amount

Equal metal amount

geometric

*

E1/2**

Ir/C

20

200

0.68

0.44

Pt/C

10

200

0.89

0.78

FeNC

1

800

0.84

0.74

E @ -0.125 mA/mgcatalyst*

E1/2**

Ir/C

5

800

0.72

0.47

Pt/C

5

800

0.97

0.83

FeNC

1

800

0.85

0.73

E @ -2.5 mA/mgmetal*

E1/2**

Ir/C

5

800

0.72

0.47

Pt/C

5

800

0.97

0.83

FeNC

5

800

0.85

0.75

*

standard deviation less than 0.01 V between three independent measurements

**



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Table 2. Bifunctional catalytic activity for various samples

Nominal Basis

Sample

metal content

Catalyst loading

Overpotential for ORR*

Overpotential for OER*

(µg/cm2)

(V)

(V)

(wt%)

Combined overpotential for ORR and OER as a measure of bifunctional catalytic activity (V)

Equal geometric area

Equal catalyst amount

Equal metal amount *

ηORR @ -3 mA/cm2geometric

ηOER @ 10 mA/cm2geometric

Ir/C

20

200

0.85

0.31

1.16

Pt/C

10

200

0.50

0.64

1.14

FeNC

1

800

0.51

0.32

0.83

ηORR @ -3.75 mA/mgcatalyst

ηOER @ 12.5 mA/mgcatalyst

Ir/C

5

800

0.96

0.36

1.32

Pt/C

5

800

0.43

0.50

0.93

FeNC

1

800

0.53

0.29

0.82

ηORR @ -75 mA/mgmetal

ηOER @ 250 mA/mgmetal

Ir/C

5

800

0.96

0.36

1.32

Pt/C

5

800

0.43

0.50

0.93

FeNC

5

800

0.54

0.31

0.85



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(a)

(b)

Figure 1. (a) ORR polarization curves for Ir/C, Pt/C and FeNC. The comparisons are based on equal geometric area (RDE, O2 saturated, 0.5 M H2SO4, 1600 rpm and 10 mV/s). (b) Koutechy-Levich plots at various potentials for Ir/C (purple), Pt/C (red) and FeNC (green). Also, included are the best fit lines (dashed) Selectivity as a function of potential for these samples is shown as an inset.


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Figure 2. OER voltammograms for Ir/C, Pt/C and FeNC corrected for capacitive current. The comparisons are based on equal geometric area. The dotted line shows the potential needed for a current density of 10 mA/cm2. Inset presents OER current at 1.63 V vs. RHE for each sample (Ar saturated, 0.5 M H2SO4, 1600 rpm and 10 mV/s).


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Figure 3. Comparisons of normalized OER currents before and after several cycles for Ir/C (solid) and FeNC (hollow) at (a) 1.45 V, (b) 1.50 V and (c) 1.55 V.


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Figure 4. ORR polarization curves for Ir/C, Pt/C and FeNC. The comparisons are based on equal catalyst amounts (RDE, O2 saturated, 0.5 M H2SO4, 1600 rpm and 10 mV/s).


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Figure 5. OER voltammograms for Ir/C, Pt/C and FeNC corrected for capacitive current. The comparisons are based on equal catalyst amounts. The dotted line shows the potential needed for a current of 12.5 mA/mgcatalyst. Inset presents OER current at 1.63 V vs. RHE for each sample (Ar saturated, 0.5 M H2SO4, 1600 rpm and 10 mV/s).


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Figure 6. ORR polarization curves for Ir/C, Pt/C and FeNC. The comparisons are based on equal metal loadings and equal catalyst amounts (RDE, O2 saturated, 0.5 M H2SO4, 1600 rpm and 10 mV/s).


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Figure 7. (a) OER voltammograms for Ir/C, Pt/C and FeNC corrected for capacitive current. The comparisons are based on equal metal loadings and equal catalyst amounts. The dotted line shows the potential needed for a current of 250 mA/mgmetal. Inset in (a) represents OER current at 1.63 V vs. RHE for each sample. (b) Tafel plots for Ir/C, Pt/C and FeNC. (Ar saturated, 0.5 M H2SO4, 1600 rpm and 10 mV/s.


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Graphical Abstract


27

Nitrogen-Coordinated IronâCarbon as Efficient Bifunctional

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Nitrogen-Coordinated IronâCarbon as Efficient Bifunctional