Experimental and Density Functional Theory Corroborated

May 22, 2019 - Here, we describe a rational design of nonnoble metal-embedding ... We find that face-centered cubic structures are dominant in Ni-CNF,...
0 downloads 0 Views 2MB Size
Download PDF
Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3109−3114

pubs.acs.org/JPCL

Experimental and Density Functional Theory Corroborated Optimization of Durable Metal Embedded Carbon Nanofiber for Oxygen Electrocatalysis Yoonhoo Ha,†,# Sinwoo Kang,‡,# Kahyun Ham,‡ Jaeyoung Lee,*,‡,§ and Hyungjun Kim*,†

Downloaded via RUTGERS UNIV on August 8, 2019 at 23:30:48 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-Ro, Daejeon 34141, Republic of Korea ‡ School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-Ro, Gwangju 61005, Republic of Korea § Ertl Center for Electrochemistry and Catalysis, GIST, 123 Cheomdangwagi-Ro, Gwangju 61005, Republic of Korea S Supporting Information *

ABSTRACT: There is a growing need for widespread deployment of hydrogen and fuel cell technology for the realization of a sustainable energy landscape. However, due to the high price of platinum (Pt) catalysts, it is necessary to develop highly active and stable non-Pt oxygen reduction reaction (ORR) catalysts. Here, we describe a rational design of nonnoble metal-embedding and nitrogen-containing carbon nanofiber (M-CNF) catalysts. Using a combined experimental and computational approach, we establish an ORR activity volcano of M-CNF using the work function of the embedded metal as the descriptor. Near the top of the activity volcano, the embedded metal is further optimized by tuning the Fe1−xCox alloy composition to simultaneously achieve high catalytic activity and durability. This work identifies the mechanistic importance of controlling the charge transfer between the metal and carbon layers, providing guidance for the design of non-Pt ORR catalysts using stable carbon-layer-protected metals.

T

Recently, our group has developed metal-embedded and nitrogen-containing carbon nanofibers (M-CNF) as non-Pt alkaline oxygen electrocatalysts.26−34 CNFs achieve a stable accommodation of the metal nanoparticles and disperse them homogeneously with carbon layers surrounding the nanoparticles. This action contributes to stability by preventing direct contact with the electrolyte that induces chemical degradation.5,35 The metal particles that support the N-doped carbon layer enhance graphitization and conductivity.32 In this work, we investigate the ORR activity of various MCNF catalysts using a combination of experimental and computational studies. We find that the activity of oxygen reduction at the M-CNF catalysts shows a volcano-like relationship with the work function of the embedded metals as a relevant descriptor. On the basis of this mechanistic understanding of the role of metals in M-CNF catalysts and the volcano map of ORR activity, we tune the embedded metals to develop an optimal M-CNF catalyst with high activity and prolonged stability. To obtain a systematic understanding of ORR activity arising from the embedded metal core, we synthesized CNFs embedding various types of metals/alloys (M-CNFs, M = Ag, Fe, Co, Ni, AgFe alloy, and FeCo alloy) using electrospinning

o date, platinum (Pt) or platinum-based alloys have demonstrated the highest catalytic activity toward oxygen reduction reaction (ORR) in fuel cells.1−5 On the basis of moderate oxygen binding strength on their surfaces, Pt and Pt alloys satisfy the Sabatier principle to accelerate the sluggish kinetics of ORR on fuel cell cathodes.6−8 However, the use of Pt also creates several critical drawbacks that limit the high durability and cost-effectiveness of the fuel cell catalyst, leading to a significant obstacle for the facile deployment of fuel cell technologies. Among the various attempts to overcome these hurdles by developing non-Pt catalysts,9−16 transition metal−nitrogen− carbon (Me−N−C) catalysts are considered to be one of the most promising alternatives reaching high ORR activity.17−21 A number of research efforts have been pursued to identify the chemical details of the catalytic active site in order to rationalize generalized design principles. On the basis of various spectroscopic measurements such as X-ray absorption and Mössbauer spectroscopies, the presence of N-doped carbon sites (CNx) and N-coordinated Me centers (MeN4) has been suggested, both of which can catalyze ORR.22−25 However, the detailed roles of different catalytic active sites have been still not fully understood, and this problem is rendered even more challenging by the high chemical/ structural heterogeneity of the Me−N−C catalysts prepared by different experimental fabrication techniques. © 2019 American Chemical Society

Received: April 1, 2019 Accepted: May 20, 2019 Published: May 22, 2019 3109

DOI: 10.1021/acs.jpclett.9b00927 J. Phys. Chem. Lett. 2019, 10, 3109−3114

Letter

The Journal of Physical Chemistry Letters

the embedded metal nanoparticle is primarily determined by the relative difference of their Fermi levels. As schematically shown in the level alignment plot of Figure 2a, when the work

and ball-milling processes. First, from the transmission electron microscope (TEM) images shown in the insets of Figures 1

Figure 1. ORR polarization curve for different embedded metals in CNF in O2-saturated 0.1 M KOH with the rotating speed of 1600 rpm. (Inset) TEM image of encapsulated Co metal in several graphite layers on the surface of Co-CNF.

and S1, we confirm that the core metal particles with sizes in the 12−20 nm range are well-dispersed in M-CNF due to homogeneous mixing during the fabrication process and are also fully encapsulated by the graphitized carbon layers around the rim of the CNF. This clearly demonstrates that the metal nanoparticles are entirely embedded in the CNF, making an indirect participation of metal nanoparticles in ORR likely, in agreement with a previous study.35 Second, we investigate the crystallinity of the M-CNFs by means of the X-ray diffraction (XRD) patterns (Figure S2). We find that face-centered cubic structures are dominant in NiCNF, Co-CNF, and Ag-CNF (04-1027, 89-4307, and 04-0783 JCPDS patterns, respectively) while the Fe-CNF consists of mainly Fe3C (JCPDS: 77-0255). Considering the catalytic graphitization in heat treatment, we assume that the three metals are dissolved in the amorphous carbon phase and then separated from carbon during a cooling step. However, iron combined with carbon and transformed to carbide form.36 In the case of AgFe and FeCo alloys, we find that there is a shift of the main peak located at 43° compared to the Ag (111) peak and Co (111) peak, respectively. However, peaks from the metal redox peak are not observed in cyclic voltamoggrams (CVs) except for Co-CNF (Figure S3). Most importantly, we observe a distinct dependence of the ORR activity on the embedded metal in CNF. Figure 1 shows the ORR polarization curves for the prepared M-CNFs that were obtained using linear sweep voltammetry (LSV) conducted in O2-saturated 0.1 M KOH with a rotating speed of 1600 rpm. The potential at a given current density of 0.3 mA/cm2 shows that the ORR activity follows the trend NiCNF < Ag-CNF < Ag0.5Fe0.5-CNF < Co-CNF < Fe0.3Co0.7CNF < Fe0.5Co0.5-CNF < Fe0.7Co0.3-CNF < Fe-CNF. We also find that this trend is consistent at different rotating speeds, 2500, 1600, 900, and 400 rpm (Figure S4). It was previously suggested that the metal nanoparticles influence the surrounding carbon layers by electron transfer (ET).37,38 The amount of ET between the graphitic layers and

Figure 2. (a) Schematic band alignments between the metal and nitrogen-doped graphene (N-Gr) of the M-CNF catalyst for different work functions. Direction and the amount of charge transfer can be determined in terms of the relative Fermi level between the metal surface and the N-Gr. (b) Volcano plot of the potential at a current density of 0.3 mA/cm2 as a function of the calculated work function (WDFT) using the density functional theory. WDFT was obtained from the optimized close-packed surface models of the embedding metals, except for the M = Fe case. For Fe-CNF, we calculated the WDFT of Fe3C on the basis of the XRD result (Figures S2 and S8).

function (W) of the embedded metal nanoparticle decreases (i.e., when the Fermi level of the metal increases), more electrons are likely to be transferred to the surrounding graphitic layers. Notably, we find that the potential at 0.3 mA/cm2 and the current density measured at 0.85 VRHE follow a volcano-curve behavior with respect to WDFT, which is the W calculated from density functional theory (DFT) as shown in Figures 2b and S5, respectively. This suggests that the work function of the embedded metal can serve as a good ORR activity descriptor of M-CNF catalysts. Using DFT calculations, we further elucidate a connection between the metal work function and the amount of ET with the oxygen binding energy (ΔEO) that is the most widely accepted ORR activity descriptor. From DFT calculations on the interfacial systems shown in Figure S6, we find that the magnitude of the surface area normalized ET from metal to Ndoped graphene (ΔQET) is strongly correlated with the change in WDFT (Figure 3a). Using Bader charge analysis,39 we calculated the charge of the N-doped graphene layer, which is normalized by the metal−graphene interfacial area, yielding 3110

DOI: 10.1021/acs.jpclett.9b00927 J. Phys. Chem. Lett. 2019, 10, 3109−3114

Letter

The Journal of Physical Chemistry Letters

monotonically with increasing Fe content (dark bars in Figure 4a and Figure S7). This finding implies that Fe-CNF is located

Figure 3. (a) Surface area normalized electron transfer amount (ΔQET) as a function of the DFT-calculated work function (WDFT) of metal surfaces. ΔQET is quantified using Bader charge analysis. DFToptimized distances between the metal surface and the graphitic layer are shown in parentheses. (b) Calculated oxygen binding energies (ΔEO) at the N-doped carbon site of the graphene layer (see Figure S6 and Table S3) as a function of ΔQET. ΔEO is defined by the reaction energy of 0.5O2 + * → *O.

Figure 4. (a) Trends of the current density of Fe1−xCox-CNF @ 0.85 V with a number of potential cycles. (b) Change in the normalized ORR current density of Fe1−xCox-CNF with increasing Co content of x from 0 to 1. Comparison of the current densities before (●) and after 20, 000 potential cycles in 0.6−1.0 V (▽). The values before the current density line are the initial current densities of Fe1−xCox-CNF.

ΔQET. ΔQET values for Ni, Co, Fe0.3Co0.7, Fe0.5Co0.5, Fe0.7Co0.3, Fe, and Ag0.5Fe0.5 show a linear correlation with WDFT. For Ag, which has relatively small WDFT values, DFT calculations show virtually no ET from the metal to the carbon layer (Table S2). This is attributed to the different nature of the chemical interaction between the surface and N-doped graphene. Less chemically active surfaces containing Ag yield larger separation distances of around 2.7 Å, and significantly suppress the ET due to the minimized orbital overlaps, while in the other cases the separation distances are around 2 Å (Figure S6). Figure 3b also shows that the ΔQET is correlated with the ΔEO of the graphitic layers. Although the correlation is slightly rendered, which is presumably due to the strain imposed during the DFT simulation cell construction of interfacial systems, we find a trend of increasing ΔEO with increasing ΔQET. Therefore, we conclude that the embedded metal modulates (1) the oxophilicity and thereby (2) the ORR activity of the carbon layers by transferring electrons. This explains why the W of the embedded metal is a suitable ORR activity descriptor for the M-CNF catalysts as well. With the activity volcano in hand, we now optimize the catalytic performance of the M-CNF catalysts. Since the top of the volcano is located at W ≈ 4.6 eV, we investigate the Fe1−xCox-CNF catalysts with x ranging from 0 to 1 in detail. Assuming for simplicity that W of the FeCo alloy follows Vegard’s law, we find that the higher Fe content results in a better ORR activity since Fe−Co alloys locate at the right leg of the volcano. Indeed, we find ORR activity to increase nearly

at the top of the activity volcano. We further investigate the durability of Fe1−xCox-CNF by comparing the change in the ORR activity after 20 000 potential cycles in the 0.6−1.0 VRHE range (open bars in Figure 4a). Fe-CNF showed the largest activity loss after a full cycling test despite its highest initial activity. Furthermore, we observed that the activity loss decreases in the presence of Co. Considering that carbon oxidation of Me−N−C catalysts occurs in the high potential range over 0.9 VRHE,40 demetalation is considered to be more relevant to the observed activity loss. As such, the formation of carbide species, which will facilitate the metal diffusion into the carbon layers and obstruct the full chemical protection of the embedded metal, can be intimately related to the stability degradation of the MCNF catalysts. However, when Co is added to the Fe lattice, the carbide formation tendency is noticeably suppressed (Figure S2b), resulting in enhanced stability. Considering the trade-off relation between the initial activity and the stability (Figure 4b), we thus conclude that the Co content of 10% is optimal for dramatically reducing the activity loss after 20 000 cycles from 63.4% (x = 0) to only 32.4% (x = 0.1), while showing a high initial activity of 1.3 mA/cm2 that is still 90% of the value at the top of the activity volcano. In summary, we elucidated the role of the embedded metal of M-CNF catalysts using a combined experimental and computational approach. We established a volcano-like correlation of ORR activity and the work function of the 3111

DOI: 10.1021/acs.jpclett.9b00927 J. Phys. Chem. Lett. 2019, 10, 3109−3114

Letter

The Journal of Physical Chemistry Letters embedded metal. We find that the amount of the electron transfer between the metal and carbon layer is described by the metal work function of the metal that modulates the oxygen binding energy and ORR activity of the carbon layers. Using the activity volcano, we optimized the chemical composition of the M-CNF catalyst and found Fe0.9Co0.1-CNF to be a suitable stable and active ORR catalyst. We expect that the current study will provide guidance for a rational design of nonnoble metal-based electrocatalysts to approach the high catalytic activity and product selectivity required for the commercialization of fuel-cell technology.41

originated from carbon and metal redox reaction. To evaluate the durability of M-CNFs, we repeated potential cycling ranging from 0.6 to 1.0 V (vs RHE) with the 50 mV/s in N2saturated 0.1 M KOH. LSV for ORR polarization curve was measured after every 1, 100, 1000, 3000, 5000, 10 000, 15 000, and 20 000 potential cycling. Computational Details. We performed spin-polarized density functional theory (DFT) calculations using a revised Perdew− Burke−Ernzerhof (RPBE)42 functional with D3 dispersion corrections43 as implemented in the Vienna ab initio software package (VASP).44 We used projector-augmented-wave (PAW)45 method with a plane-wave cutoff energy of 450 eV. We prepared bulk unit cells of various metals (Ag, Co, and Ni), iron carbide (Fe3C), body-centered-cubic based iron−cobalt alloys46 (Fe11Co5, FeCo, Fe5Co11), and Ag−Fe alloy with R-3 M symmetry (Figure S8). For the optimizations of the bulk unit cells, the reciprocal spaces were sampled using the Monkhorst−Pack scheme by employing the k-mesh sizes summarized in Table S1. To verify the structure of the bulk unit cells, we carried out a powder XRD simulation of the optimized bulk unit cells. In the case of the bulk unit cell of Ag−Fe alloy, which has not been reported, we chose AgFe with R3̅M symmetry model as a Ag−Fe bulk unit cell after examining DFT energetics of the R3̅M model and three Ag16Fe16 random alloy models consisting of 32 atoms in the face-centered cubic unit cell (Figure S9). On the basis of the optimized bulk structures, we prepared surface slab models consisting of three or four layers of close-packed surfaces with an additional vacuum layer of 25 Å along the c-axis. Then the slab models were optimized by applying dipole correction along the c-axis and fixing atoms at their bulk structure positions except for the topmost two layers. In the case of the Fe3C(001) surface, we allowed the top three Fe layer and three C layers to be relaxed.47,48 Details about the cell dimensions and Γ-centered k-meshes employed in DFT calculations are listed in Table S2. We further investigated the interaction between a metal slab and nitrogen-doped graphene (N-Gr) monolayer. To quantify an amount of charge transfer between the metal surface and the N-Gr, we performed Bader charge analysis.39 Detailed DFT results are fully tabulated in Tables S1−S5.

■

METHODS Preparation of M-CNFs, Electrospinning, and Ball-Milling Process. Four grams of polyacrylonitrile (PAN, Sigma-Aldrich) and 1.2 g of metal precursor were mixed in N,N-dimethylformamide (DMF, Junsei) for 8 h until it formed the homogeneous polymer solution. Silver(I) nitrate (AgNO3, Sigma-Aldrich), Ni(II) acetylacetonate (Ni(Acac)2, Sigma-Aldrich), Co(III) acetylacetonate (Co(Acac)3, Sigma-Aldrich), and Fe(III) acetylacetonate (Fe(Acac)3, Sigma-Aldrich) were used as metal precursors. In the case of alloy M-CNFs, we controlled the metal ratio in M-CNF by the weight ratio of the metal precursors. Then, high electric voltage (24 kV) was applied between a drumlike collector and a syringe that contained wellmixed polymer solution with metal precursor. The collector was rotated with 500 rpm of rotating speed to collect the electrospun nanofibers. All these steps were carried out at room temperature and relative humidity under 30%. The ascollected electrospun nanofiber was pyrolyzed under 280 °C in air with 1 °C/min of ramping rate and kept for 1 h for stabilization. Continually, pyrolyzed electrospun nanofiber was carbonized in N2 atmosphere at 1000 °C with 5 °C/min of ramping rate and kept for 1 h. To uniformize the length of MCNF, the as-prepared M-CNF was pulverized using a ball-mill with 5 mm diameter zirconia beads in a zirconia container. The ball-milling step was repeated 10 times with a set of 20 min for milling and 10 min for idle. Characterization of M-CNFs. The morphologies of M-CNF were characterized by a transmission electron microscope (TEM; JEOL JEM-2100F). The crystal structures on the MCNF were investigated with X-ray diffraction (XRD; Rigaku Miniflex II). Electrochemical measurements were evaluated using a three-electrode system connected to a potentiostat/ galvanostat (Biologic, VSP). Pt wire was used as the counter electrode and Hg/HgO was used as reference electrode dipped into 1 M KOH solution. The working electrode was prepared by the drop-coating method with catalyst ink. The ink was fabricated by dispersing 10 mg of M-CNF powder and 10 μL of 10 wt % Nafion dispersion solution (Sigma-Aldrich) in 1.2 mL of deionized water and 0.8 mL of isopropyl alcohol (Junsei) in the sonication bath. After 30 min, 9.9 μL of catalyst ink was loaded on glassy carbon in a rotating disk electrode (loading amount = 0.2 mg/cm2, area = 0.2475 cm2). Cyclic voltammograms (CVs) were obtained at a scan rate of 20 mV/ s from 1.2 to 0.2 V versus reversible hydrogen electrode (RHE) in 0.1 M KOH electrolyte deaerated with nitrogen gas for 30 min at 25 °C. To evaluate ORR activity of M-CNF, a linear sweep voltammetric (LSV) measurement was conducted in O2-saturated 0.1 M KOH with rotating speeds of 2500, 1600, 900, and 400 rpm. Then, we gained an ORR polarization curve by subtracting from LSV performed in O2 to N2, eliminating the peaks of other reactions such as capacitance

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00927. TEM images, XRD images, CVs, ORR polarization curves, volcano plots as a function of DFT-calculated work function, DFT-optimized structures of bulk and interfacial models, trends of the catalytic activity as a function of composition ration, cell parameters, properties of optimized metal surfaces, atomic binding energies and bond distances, and optimized xyz coordinates (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*[email protected] (J. Lee). *[email protected] (H. Kim). ORCID

Jaeyoung Lee: 0000-0002-9392-5576 3112

DOI: 10.1021/acs.jpclett.9b00927 J. Phys. Chem. Lett. 2019, 10, 3109−3114

Letter

The Journal of Physical Chemistry Letters

reaction (ORR): a density functional theory (DFT) study. J. Power Sources 2015, 287, 431−438. (16) Ahmad, E.; Liborio, L.; Kramer, D.; Mallia, G.; Kucernak, A.; Harrison, N. Thermodynamic stability of LaMnO 3 and its competing oxides: A hybrid density functional study of an alkaline fuel cell catalyst. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, No. 085137. (17) Jiang, W.-J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L.-J.; Wang, J.-Q.; Hu, J.-S.; Wei, Z.; Wan, L.-J. Understanding the high activity of Fe−N−C electrocatalysts in oxygen reduction: Fe/Fe3C nanoparticles boost the activity of Fe−N x. J. Am. Chem. Soc. 2016, 138, 3570−3578. (18) Chen, J.; Li, W.; Xu, B. Nitrogen-rich Fe-NC materials derived from polyacrylonitrile as highly active and durable catalysts for the oxygen reduction reaction in both acidic and alkaline electrolytes. J. Colloid Interface Sci. 2017, 502, 44−51. (19) Sa, Y. J.; Park, C.; Jeong, H. Y.; Park, S. H.; Lee, Z.; Kim, K. T.; Park, G. G.; Joo, S. H. Carbon Nanotubes/Heteroatom-Doped Carbon Core−Sheath Nanostructures as Highly Active, Metal-Free Oxygen Reduction Electrocatalysts for Alkaline Fuel Cells. Angew. Chem., Int. Ed. 2014, 53, 4102−4106. (20) Sa, Y. J.; Seo, D.-J.; Woo, J.; Lim, J. T.; Cheon, J. Y.; Yang, S. Y.; Lee, J. M.; Kang, D.; Shin, T. J.; Shin, H. S. A general approach to preferential formation of active Fe−N x sites in Fe−N/C electrocatalysts for efficient oxygen reduction reaction. J. Am. Chem. Soc. 2016, 138, 15046−15056. (21) Lu, Z.; Xu, G.; He, C.; Wang, T.; Yang, L.; Yang, Z.; Ma, D. Novel catalytic activity for oxygen reduction reaction on MnN4 embedded graphene: a dispersion-corrected density functional theory study. Carbon 2015, 84, 500−508. (22) Strickland, K.; Miner, E.; Jia, Q.; Tylus, U.; Ramaswamy, N.; Liang, W.; Sougrati, M.-T.; Jaouen, F.; Mukerjee, S. Highly active oxygen reduction non-platinum group metal electrocatalyst without direct metal−nitrogen coordination. Nat. Commun. 2015, 6, 7343. (23) Varnell, J. A.; Edmund, C.; Schulz, C. E.; Fister, T. T.; Haasch, R. T.; Timoshenko, J.; Frenkel, A. I.; Gewirth, A. A. Identification of carbon-encapsulated iron nanoparticles as active species in nonprecious metal oxygen reduction catalysts. Nat. Commun. 2016, 7, 12582. (24) Singh, D.; Tian, J.; Mamtani, K.; King, J.; Miller, J. T.; Ozkan, U. S. A comparison of N-containing carbon nanostructures (CNx) and N-coordinated iron−carbon catalysts (FeNC) for the oxygen reduction reaction in acidic media. J. Catal. 2014, 317, 30−43. (25) Singh, D.; Mamtani, K.; Bruening, C. R.; Miller, J. T.; Ozkan, U. S. Use of H2S to probe the active sites in FeNC catalysts for the oxygen reduction reaction (ORR) in acidic media. ACS Catal. 2014, 4, 3454−3462. (26) Shin, D.; Choun, M.; Ham, H. C.; Lee, J. K.; Lee, J. A graphitic edge plane rich meso-porous carbon anode for alkaline water electrolysis. Phys. Chem. Chem. Phys. 2017, 19, 21987−21995. (27) Vu, T. H.; Shin, D.; Jeong, J.; Lee, J. Porous and Conductive Fibrous Carbon for Enhanced Electrocatalytic Oxygen Reduction Reaction in Alkaline Media. J. Phys. Chem. C 2016, 120, 22342− 22348. (28) An, X.; Shin, D.; Jeong, J.; Lee, J. Metal-Derived Mesoporous Structure of a Carbon Nanofiber Electrocatalyst for Improved Oxygen Evolution Reaction in Alkaline Water Electrolysis. ChemElectroChem 2016, 3, 1720−1724. (29) Jeong, B.; Shin, D.; Choun, M.; Maurya, S.; Baik, J.; Mun, B. S.; Moon, S.-H.; Su, D.; Lee, J. Nitrogen-deficient ORR active sites formation by iron-assisted water vapor activation of electrospun carbon nanofibers. J. Phys. Chem. C 2016, 120, 7705−7714. (30) Shin, D.; An, X.; Choun, M.; Lee, J. Effect of transition metal induced pore structure on oxygen reduction reaction of electrospun fibrous carbon. Catal. Today 2016, 260, 82−88. (31) An, X.; Shin, D.; Ocon, J. D.; Lee, J. K.; Son, Y.-i.; Lee, J. Electrocatalytic oxygen evolution reaction at a FeNi composite on a carbon nanofiber matrix in alkaline media. Chin. J. Catal. 2014, 35, 891−895.

Hyungjun Kim: 0000-0001-8261-9381 Author Contributions #

Y.H. and S.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by Technology Development Program to Solve Climate Changes through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (2017M1A2A2092564, 2018M1A2A2063861). We also acknowledge Dr. Stefan Ringe (KAIST) and Mr. Sujik Hong (GIST) for their fruitful discussions and also for helping us to proofread the manuscript.

■

REFERENCES

(1) Huang, J.; Sen, R.; Yeager, E. Oxygen reduction on platinum in 85% orthophosphoric acid. J. Electrochem. Soc. 1979, 126, 786−792. (2) Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. Enhancement of the electroreduction of oxygen on Pt alloys with Fe, Ni, and Co. J. Electrochem. Soc. 1999, 146, 3750−3756. (3) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Marković, N. M. Improved oxygen reduction activity on Pt3Ni (111) via increased surface site availability. Science 2007, 315, 493−497. (4) Greeley, J.; Stephens, I.; Bondarenko, A.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of platinum and early transition metals as oxygen reduction electrocatalysts. Nat. Chem. 2009, 1, 552. (5) Lee, J.; Jeong, B.; Ocon, J. D. Oxygen electrocatalysis in chemical energy conversion and storage technologies. Curr. Appl. Phys. 2013, 13, 309−321. (6) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 2004, 108, 17886−17892. (7) Xu, Y.; Ruban, A. V.; Mavrikakis, M. Adsorption and dissociation of O2 on Pt− Co and Pt− Fe alloys. J. Am. Chem. Soc. 2004, 126, 4717−4725. (8) Stephens, I. E.; Bondarenko, A. S.; Grønbjerg, U.; Rossmeisl, J.; Chorkendorff, I. Understanding the electrocatalysis of oxygen reduction on platinum and its alloys. Energy Environ. Sci. 2012, 5, 6744−6762. (9) 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. (10) Jaouen, F.; Proietti, E.; Lefèvre, 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. (11) Dai, L.; Xue, Y.; Qu, L.; Choi, H.-J.; Baek, J.-B. Metal-free catalysts for oxygen reduction reaction. Chem. Rev. 2015, 115, 4823− 4892. (12) Dombrovskis, J.; Palmqvist, A. Recent Progress in Synthesis, Characterization and Evaluation of Non-Precious Metal Catalysts for the Oxygen Reduction Reaction. Fuel Cells 2016, 16, 4−22. (13) Cheng, F.; Zhang, T.; Zhang, Y.; Du, J.; Han, X.; Chen, J. Enhancing electrocatalytic oxygen reduction on MnO2 with vacancies. Angew. Chem., Int. Ed. 2013, 52, 2474−2477. (14) Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y. Toward the rational design of nonprecious transition metal oxides for oxygen electrocatalysis. Energy Environ. Sci. 2015, 8, 1404−1427. (15) Feng, L.-y.; Liu, Y.-j.; Zhao, J.-x. Iron-embedded boron nitride nanosheet as a promising electrocatalyst for the oxygen reduction 3113

DOI: 10.1021/acs.jpclett.9b00927 J. Phys. Chem. Lett. 2019, 10, 3109−3114

Letter

The Journal of Physical Chemistry Letters (32) Jeong, B.; Shin, D.; Lee, J. K.; Kim, D. H.; Kim, Y. D.; Lee, J. The influence of a fibrous carbon envelope on the formation of CoFe nanoparticles for durable electrocatalytic oxygen evolution. Phys. Chem. Chem. Phys. 2014, 16, 13807−13813. (33) Shin, D.; Jeong, B.; Mun, B. S.; Jeon, H.; Shin, H.-J.; Baik, J.; Lee, J. On the origin of electrocatalytic oxygen reduction reaction on electrospun nitrogen−carbon species. J. Phys. Chem. C 2013, 117, 11619−11624. (34) Uhm, S.; Jeong, B.; Lee, J. A facile route for preparation of nonnoble CNF cathode catalysts in alkaline ethanol fuel cells. Electrochim. Acta 2011, 56, 9186−9190. (35) Hu, Y.; Jensen, J. O.; Zhang, W.; Cleemann, L. N.; Xing, W.; Bjerrum, N. J.; Li, Q. Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts. Angew. Chem., Int. Ed. 2014, 53, 3675−3679. (36) Ross, P. N.; Sattler, M. The corrosion of carbon black anodes in alkaline electrolyte III. The effect of graphitization on the corrosion resistance of furnace blacks. J. Electrochem. Soc. 1988, 135, 1464− 1470. (37) Deng, D.; Yu, L.; Chen, X.; Wang, G.; Jin, L.; Pan, X.; Deng, J.; Sun, G.; Bao, X. Iron encapsulated within pod-like carbon nanotubes for oxygen reduction reaction. Angew. Chem. 2013, 125, 389−393. (38) Choi, C. H.; Lim, H.-K.; Chung, M. W.; Park, J. C.; Shin, H.; Kim, H.; Woo, S. I. Long-range electron transfer over graphene-based catalyst for high-performing oxygen reduction reactions: importance of size, N-doping, and metallic impurities. J. Am. Chem. Soc. 2014, 136, 9070−9077. (39) Tang, W.; Sanville, E.; Henkelman, G. A grid-based Bader analysis algorithm without lattice bias. J. Phys.: Condens. Matter 2009, 21, No. 084204. (40) Choi, C. H.; Baldizzone, C.; Grote, J. P.; Schuppert, A. K.; Jaouen, F.; Mayrhofer, K. J. Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy. Angew. Chem., Int. Ed. 2015, 54, 12753−12757. (41) Jeong, B.; Ocon, J. D.; Lee, J. Electrode architecture in galvanic and electrolytic energy cells. Angew. Chem., Int. Ed. 2016, 55, 4870− 4880. (42) Hammer, B.; Hansen, L. B.; Nørskov, J. K. Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 7413. (43) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (44) Kresse, G.; Furthmüller, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169. (45) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953. (46) Díaz-Ortiz, A.; Drautz, R.; Fähnle, M.; Dosch, H.; Sanchez, J. Structure and magnetism in bcc-based iron-cobalt alloys. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 224208. (47) Chiou, W. C., Jr; Carter, E. A. Structure and stability of Fe3Ccementite surfaces from first principles. Surf. Sci. 2003, 530, 88−100. (48) Tian, X.; Wang, T.; Yang, Y.; Li, Y.-W.; Wang, J.; Jiao, H. Adsorption Structures and Energies of Cu n Clusters on the Fe (110) and Fe3C (001) Surfaces. J. Phys. Chem. C 2014, 118, 21963−21974.

3114

DOI: 10.1021/acs.jpclett.9b00927 J. Phys. Chem. Lett. 2019, 10, 3109−3114