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Cite This: ACS Appl. Energy Mater. 2019, 2, 3999−4007
Polarized Electronic Configuration in Transition Metal−Fluoride Oxide Hollow Nanoprism for Highly Efficient and Robust Water Splitting HyukSu Han,*,†,⊥ Jungwook Woo,‡,⊥ Yu-Rim Hong,§ Yong-Chae Chung,*,‡ and Sungwook Mhin*,∥ †
Department of Materials Science and Engineering, Hongik University, 2639 Sejong-ro, Sejong, 339-701, Republic of Korea Division of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea § Korea Institute of Industrial Technology, 137-41 Gwahakdanji-ro, Gangneung-si, Gangwon 25440, Republic of Korea ∥ Korea Institute of Industrial Technology, 156 Gaetbeol-ro, Yeonsu-gu, Incheon 406-840, Republic of Korea
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‡
S Supporting Information *
ABSTRACT: Metal−fluoride possesses a high potential as new high-performance water oxidation catalysts due to a highly polarized electronic configuration. However, low conductivity, related to high iconicity in metal−fluorine bonds, and instability of metal−fluoride in alkaline solution act as major roadblocks for using metal−fluoride as a highly efficient electrocatalyst. Here, we first disclose a novel strategy to design the electrochemically active and stable metal− fluoride electrocatalysts, nickel−cobalt fluoride oxide (NCFO), for water oxidation. The incorporation of F leads to substantial increase of the number of surface active sites with unsaturated electronic structure, which is beneficial for boosting electrocatalytic activity.
KEYWORDS: water splitting, oxygen evolution reaction, electrocatalyst, hydrogen production, metal fluoride table. Thus, fluorine can effectively abstract electrons from neighboring metals, resulting in abundant CUSs. However, the electrical conductivity of metal−fluorides is poor due to the high iconicity of metal−fluorine bonds,9,13 which can hamper catalytic activity. Importantly, the intrinsically low conductivity of metal fluorides can be enhanced by introducing metal− oxygen bonds. Metal−fluoride oxides possess both high electrical conductivity and electrochemical robustness due to the coexistence of metal−O and metal−F bonds.14,15 In addition, the hollow nanostructure of these oxides can further improve the catalytic activity of metal−fluorides by decreasing the ionic diffusion length and increasing the contact area with an electrolyte, thus facilitating electron/mass transfer.16 We first disclose nickel−cobalt fluoride oxide (NCFO), whose nanostructure intriguingly consists of a hollow nanoprism, as an excellent OER electrocatalyst in alkaline media. Hollow nickel−cobalt−boride was first prepared via a selective ion exchange strategy using a nickel−cobalt−hydroxide (NCO) nanoprism as a template and sodium borohydride (NaBH4) as an ion exchanger (Figure 1a,b). The resulting nickel−cobalt−boride with a partially oxidized phase (NCBO)
E
lectrochemical water splitting is the most promising green approach for producing renewable energy,1−8 and it includes the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Among these two reactions, the OER is considered the bottleneck in the water splitting process due to the sluggish electron transfer among the four protons and the strong oxygen−oxygen bonding.9 Novel metal-based catalysts such as RuO2 and IrO2 are often employed to expedite the OER. However, the scarcity and high cost of these catalysts considerably hinder practical applications. Hence, developing an earth-abundant catalyst that accelerates water oxidation becomes imperative, though challenging. Water oxidation on a catalyst takes place via two major processes: dissociation of water and chemisorption of intermediates. The dissociation of water molecules is strongly related to the coordinatively unsaturated sites (CUSs) on the surface of a catalyst.10 In general, a large number of CUSs on the catalyst with an electron-deficient configuration would boost water oxidation. Accordingly, extensive research has pursued design of transition metal-based catalysts such as oxides, phosphides, and sulfides with electron-deficient structures.9−12 Metal−fluorides are promising candidates for high-performance OER catalysts, given that fluorine has the highest electronegativity among all of the atoms in the periodic © 2019 American Chemical Society
Received: March 1, 2019 Accepted: May 21, 2019 Published: May 22, 2019 3999
DOI: 10.1021/acsaem.9b00449 ACS Appl. Energy Mater. 2019, 2, 3999−4007
Letter
ACS Applied Energy Materials
Figure 1. Schematic of the formation of hollow NCFO: (a) NCO nanoprism, (b) hollow NCBO after ion exchange, and (c) hollow NCFO after fluorination.
Figure 2. (a) SEM (inset) and TEM images of NCBO. (b) HR-TEM image of NCBO with corrsponding SAED patter (upper inset). The lower inset shows the lattice fringe of the metal boride phase. (c, d) TEM images of NCFO with the corresponding SAED pattern (inset). (e, f) HR-TEM images of NCFO (marked region as white rectangle in Figure 1d), which clearly shows highly crystalline structure with lattice fringe of metal− fluoride. (g) EDX images for Ni, Co, F, and O in NCFO . (h) XRD patterns of NCBO (black line) and NCFO (red line) with standard pattern of (Co, Ni)F2 (blue line).
was converted to NCFO through reaction with fluorine vapor, where ammonium fluoride (NH4F) was used as the fluorine source (Figure 1c). In the proposed material design strategy, NCO was chosen as the initial template in order to induce a partial oxidation phase after the ionic exchange. In addition, NCBO was used as the precursor for the formation of NCFO because boron has an electronegativity and ionic radius similar to those of fluorine, and therefore, the partial substitution of boron by fluorine may take place easily. In addition, boron ions have a much smaller ionic radius than nickel and cobalt ions, which is beneficial for the formation of the hollow structure at
the surface of NCO during the selective ion exchange by Kirkendall effect. To the best of our knowledge, NCFO has never been studied as a water oxidation catalyst despite its high potential. The catalyst exhibited a very low overpotential of 250 mV, affording a current density of 70 mA cm−2 when it was loaded onto nickel foam. In addition, the exceptional durability of the catalyst in an alkaline medium was demonstrated. Density functional theory (DFT) calculations revealed that the Co and Ni atoms are active catalytic sites, while the F atoms tune the surface electrical structure, and Co facilitates charge transfer, significantly lowering the energy barriers for water oxidation. 4000
DOI: 10.1021/acsaem.9b00449 ACS Appl. Energy Mater. 2019, 2, 3999−4007
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Figure 3. High-resolution XPS of electron orbitals of Co 2p and Ni 2p for (a) NCO, (b) NCBO, and (c) NCFO, respectively. The two core-level signals of Co 2p and Ni 2p are attributed to 2p3/2 and 2p1/2.
shows the selective area electron diffraction (SAED) pattern of NCBO, revealing the low crystalline nature of the as-prepared hollow nanoprism. Energy dispersive X-ray (EDX) mapping was performed for the NCBO nanoprism. Ni, Co, B, and O were homogeneously distributed in the NCBO nanoprism (Figure S4). Substantial morphological change was observed in the NCBO after fluorination (NCFO). The hierarchical nanosheet surface morphology was changed to an intriguing pinecone-like surface structure after fluorination (Figure 2c). The SAED pattern indicates that randomly oriented small crystallites are present in NCFO. Ring patterns corresponding to (011) and (110) planes of the CoxNi1−xF2 phase were distinctly observed (upper inset of Figure 1c). Figure 2d more clearly shows the pinecone-like surface morphology of NCFO. The HR-TEM image was taken on a single subunit with a size of ∼10 nm (marked as a white rectangle), revealing the highly crystalline nature of NCFO (Figure 2e). A lattice spacing of 3.3 Å with highly ordered atomic arrangement was observed for NCFO, corresponding to the (110) interplanar spacing of CoxNi1−xF2 phase. Two different planes of (011) and (110) were clearly observed in the HR-TEM image and the interplanar angle of
The morphologies of NCO nanoprisms with Ni:Co molar ratios of 2:1 are presented in Supporting Information Figure S1, which shows a homogeneous tetragonal nanoprism structure with an average particle size of approximately 300 nm. In addition, the particle size was not significantly changed after boronizaiton and fluorization as revealed in the lowmagnification SEM images (Figure S1). Figure S2 shows that the X-ray diffraction (XRD) pattern of the NCO closely matches that of a tetragonal Ni−Co acetate hydroxide phase, (Ni,Co)5(OH)2−(CH3COO)8·2H2O.17,18 The NCO precursors were then hydrothermally reacted with NaBH4 at 120 °C for 1 h to convert them into metal borides. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images clearly reveal the unique, hollow nanoprism morphology and crystalline structure of NCBO (Figure 2a). The evolution of a hierarchical nanosheet subunit-decorated nanostructure after boronization is clear in both scanning electron microscopy (SEM) and TEM images in Figure S3 and Figure 2b, respectively. The outer shell of NCBO consists of thin metal−boride layers with a lattice spacing of 2.11 Å (lower inset in Figure 2b), corresponding to the (002) interplanar distance of the Co2B phase. The upper inset in Figure 2b 4001
DOI: 10.1021/acsaem.9b00449 ACS Appl. Energy Mater. 2019, 2, 3999−4007
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ACS Applied Energy Materials
Figure 4. Electrocatalytic OER properties of NCFO with different catalysts (NCBO, NF-NCFO, and RuO2) in 1 M KOH. (a) LSV curves with a scan rate of 5 mV s−1 and (b) corresponding Tafel plots. (c) TOFs at an overpotential of 400 mV. (d) RRDE data for NCFO, with the inset showing the schematic setting of the RRDE measurements. (e) Chronoamperometry curve of NCFO under an applied voltage of 1.6 V. (f) OER polarization curves of NCFO measured initially and after 1000 CV cycles in the potential range from 1.23 to 1.6 V.
The electron structure evolution in the samples was studied by systemically analyzing XPS data of NCO, NCBO, and NCFO. As can be seen in Figure 3a, the Co 2p3/2 spectrum of NCO can be resolved fairly well with two spin−orbit doublets from two different Co entries of Co−O (781.1 eV) and Co(OH)2 (783.9 eV) with the corresponding satellite. Similarly, for the Ni 2p3/2 spectrum of NCO, two spin−orbit doublets from Ni−O (855.6 eV) and Ni(OH)2 (857.9 eV) were observed with the corresponding satellites. In addition, negative peak shifts were clearly detected in major Co 2p3/2 (780.7 eV) and Ni 2p3/2 (855.2 eV) spectra after boronization (Figure 3b), indicating the electron transfer from O to Co or Ni may take place during the formation of Co−Ni−B compounds. In sharp contrast, significant positive peak shifts, compared to NCBO and even NCO, were found for the Co 2p3/2 (781.8 eV) and Ni 2p3/2 (855.8 eV) spectra of NCFO, which demonstrates that substantial charge transfer occurred from Co or Ni to F atoms after fluorination. The formation of cobalt−nickel−fluoride in NCFO can be clearly seen from the Co 2p3/2 (781.8 eV) XPS spectra, corresponding to the Co−F bond, in Figure 3c.19 Similarly, the coexistence of Ni−F and Ni−O bonds is detected in Ni 2p3/2 spectra (Figure 3c). In F 1s spectra, a distinct peak is observed at 684.5 eV, corresponding to metal−fluorine bond (Figure S8a).20 This further indicates that the F atoms, which possess the highest electronegativity among all elements, pull the valence electron density from the metal atoms (Co or Ni), resulting in electron delocalization in the metal sublattice of NCFO. The broad fitted peak at 530.9 eV is associated with Co−OH or Ni−OH bonds that frequently are observed for cobalt and nickel oxides after fluorination (Figure S8b).20 The deconvoluted peak at
66.3° was calculated, which is very close to the theoretical value of 67° for the CoxNi1−xF2 phase (Figure 2f). EDX spectroscopy analysis of NCFO revealed the existence of Ni, Co, F, and O (Figure 2g). The Ni:Co atomic ratio in NCFO was determined to be 2.38 (Figure S5), which is in good agreement with the experimental stoichiometry. The content of fluorine was 35.07 at. %, indicating a significant amount of fluoride formed. EDX mapping detected 10 at. % oxygen, indicating that partially oxidized (oxy)hydroxide phases may exist. Noted that no distinct peak for B element was detected in the EDX spectra (Figure S5), revealing most of the boron is substituted by fluorine during the fluorination process. In addition, the nitrogen adsorption−desorption isotherm was measured on NCFO resulting in Brunauer−Emmett−Teller (BET) surface area of 124.6 m2g−1 (Figure S6). XRD (Figure 2h) and X-ray photoelectron spectroscopy (XPS, Figure 3) were employed to identify the chemical conversions from transition metal−boride to fluoride complexes. XRD analysis showed that synthesized NCBO has a mixture phase of Ni−Co acetate hydroxide and metallic boride (Figure 2h). In contrast, NCFO is crystallized in (Ni1−xCox)F2 phase (JCPDS No. 81-2270) with residual Ni−Co hydroxide that may be present on the surface. An increased peak intensity in the XRD pattern of NCFO also demonstrates the enhanced crystallinity after fluoridation, which is consistent with the TEM analysis. XPS was performed to elucidate the chemical states of the NCFO. The wide XPS survey spectrum of NCFO is shown in Figure S7a, confirming the presence of F, Ni, Co, and O elements. Distinct XPS spectra for boron was not detected (Figure S7b), revealing that most of the metal−boride was transformed to metal−fluoride. 4002
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dec−1), NF-NCFO (56 mV dec−1), and RuO2 (54 mV dec−1) imply that the reaction kinetics are much faster in NCFO, which has a rate-limiting step associated with coupled chemical charge transfer.28 The water oxidation properties of NCFO were also compared with those of some recently reported state-of-theart, nonprecious metal OER catalysts, including NF-supported catalyst (Table S1). For example, NixB and NiCoP catalysts supported by NF delivered current densities of only 20 and 10 mA cm−2, respectively, at an overpotential of 280 mV in 1.0 M KOH, while NF-NCFO presented about 120 mA cm−2 at the same overpotential.29,30 Hence, the water oxidation performance of NCFO is promising and outstanding among the recently reported, transition metal-based catalysts. Quantitative evaluation for the water oxidation activity of NCFO was performed by calculating the turnover frequency (TOF). TOF of the catalyst can be obtained using E eq 1:
532.9 eV can be assigned to the CoO groups or surfacedominated oxygen species.21 These results reflect the successful formation of cobalt−nickel−fluoride complexes. Figure 4a shows that NCFO exhibited remarkable catalytic activity for water oxidation. The overpotential (η) required to deliver a current density of 10 mA cm−2 (η10) is generally used as a reference to evaluate the electrocatalytic OER performance in terms of solar-to-fuel conversion.22 NCFO required an η10 of 350 mV, while the benchmark RuO2 electrocatalyst required 350 mV, indicating that the former had superior water oxidation activity. Also, NCFO exhibited a smaller η10 than NCBO (η10 = 390 mV) and NCO (η10 = 380 mV, Figure S9a). In addition, as a control experiment, nickel−cobalt−fluoride (NCF) with bulk morphology (Figure S10a) was prepared and OER activity was measured to study the influence of morphology on the catalytic performance of NCFO. The measured LSV curve revealed that η10 of (370 mV), which is higher than NCFO but lower than NCO and NCBO, was observed for NCF (Figure S10b). Hence, the enhanced OER activity of NCFO indicates that physicochemical interactions between metals and fluorine, resulting in delocalization of electron density in the metal sublattice, in assistance with intriguing nanostrucutre may play a crucial role in improving the water oxidation activity. To find the optimal fluorination conditions, we performed control experiments by varying the experimental conditions for fluorination. First, different temperature conditions (300, 400, and 500 °C) were tested for fluorination, and the corresponding LSV curves for the OER were measured (Figure S11a). The highest water oxidation activity was obtained when fluorination was performed at 400 °C. Phase transformation from boride to fluoride may not be completed at temperatures lower than 400 °C, while particle aggregation may occur at temperatures higher than 400 °C. Time for fluorination was also tested, and Figure S11b indicates that fluorination for longer than 45 min is desired to improve catalytic activity. Lastly, the amount of fluorine source (NH4F) was optimized, and the results revealed that 0.4 g is the optimal amount for fluorination (Figure S11c) in terms of OER activity. Nickel foam (NF) is an excellent three-dimensional support for catalysts thanks to its large surface area, robustness, and synergetic catalytic effect.23−27 To further enhance the OER activity, we loaded 0.28 mg cm−2 of NCFO onto NF (NFNCFO). The LSV polarization curve of NF-NCFO was measured at a scan rate of 0.5 mV s−1. Surprisingly, NF-NCFO showed a remarkably high water oxidation current with very low overpotential (Figure 4a). η10 is not valid for NF-NCFO because a current density of 10 mA cm−2 overlaps with the huge oxidation wave of Co2+ /Ni2+ → Co3+ /Ni3+ + e− originating from the NF support. Notably, a current density of 70 mA cm−2 was attained only at an overpotential of 280 mV. The LSV curve of bare NF exhibited negligible water oxidation activity (Figure S12), indicating that the high OER activity of the NF-NCFO resulted from the loaded NCFO catalyst. The Tafel slope is a representative indicator for reaction kinetics; a smaller Tafel slope corresponds to faster kinetics for electrochemical reactions involving a charge transfer. The Tafel plots of the catalysts, shown in Figure 4b, were derived from the measured LSV curves via the Tafel equation (η = b log j + a), where η is the overpotential, j is the current density, and b is the Tafel slope. NCFO showed a smaller Tafel slope (23 mV dec−1) than NCO (62 mV dec−1, Figure S9b). NCBO (52 mV
TOF = jA /(4FNs)
(1)
where j is the current density (A cm−2) at the given overpotential, A is the area of the electrode (cm2), F is the Faraday constant (96,485 C mol −1 ), and N s is the concentration of active sites in the catalyst (mol cm−2).31,32 Ns is estimated from the linear relationship between the peak current of the Co2+,Ni2+/Co3+,Ni3+ oxidation wave and scan rate (see the Supporting Information for experimental details).32 Ns for NCFO and NCBO were calculated to be 1.69 × 10−7 and 2.13 × 10−7 mol cm−2 (Figure S13), which is an upper limit for the number of active sites because heterogeneous catalysis occurs only on the surface while the bulk material remains electroactive. The TOF of NCFO is 0.035 s−1 at an overpotential of 400 mV, which is about four times higher than that of NCBO (0.009 s−1) (Figure 4c). Furthermore, a rotating ring disk electrode (RRDE) technique was applied to determine the Faradaic efficiency (FE) of NCFO for the OER (see the Supporting Information for experimental details), and results are shown in Figure 4d. For the RRDE measurements, a ring potential of 0.4 V was applied to detect O2 molecules generated by the oxygen reduction reaction (ORR). A ring current of 0.12 mA was detected when a constant current of 0.60 mA was applied to the disk electrode. The FE can be calculated using eq 2: FE = iring /(idiskN )
(2)
where idisk, iring, and N are, respectively, the disk current, ring current, and current collection efficiency, which is normally about 0.2.17 The FE of NCFO for water oxidation was calculated as 99.9%. Thus, the observed water oxidation current for NCFO mostly originates from the oxygen evolution with a high FE. In addition, the double-layer capacitance (Cdl) was calculated using cyclic voltammetry (CV) in the non-Faradaic region to estimate the electrochemical surface area (ECSA). The current in the non-Faradaic region is produced from the charging of the electrical double layer, which has a linear relationship with the active surface area (Figure S14).22,33 For NCFO, Cdl was calculated as 0.76 mF, which is comparable to the counterpart catalyst RuO2 powder (0.87 mF). Electrochemical impedance spectroscopy (EIS) is used to investigate the electrochemical behaviors of interfaces between electrocatalysts and electrolytes. In EIS, a relevant equivalent circuit can reveal meaningful electrochemical information regarding 4003
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Figure 5. (a) Calculated OER free energy landscape on Co0.292Ni0.708F2 at pH 0. Inset: Top view of most stable adsorbed structure of OH, O, and OOH on Co0.292Ni0.708F2. (b) Charge difference maps for OH, O, and OOH on Co0.292Ni0.708F2 (isovalue = 0.02 e/Å3). Isosurfaces in yellow indicate increases in charge density, while those in blue indicate decreases. Deep blue, silver, light blue, red, and white atoms are cobalt, nickel, fluorine, oxygen, and hydrogen, respectively. (c) Partial density of states of NiF2 and (d) co-doped NiF2 (Ni0.708 Co0.292F2).
solutions.34−36 Thus, we have performed detailed spectroscopic and microscopic studies of NCFO after long-term OER measurements. Figure S16 presents SEM images of NCFO after the long-term OER test. The NCFO particles retained the pristine nanoprism structure with the pinecone-like surface morphology. TEM analysis of post-OER NCFO indicated that amorphization of the materials did not occur, whereas the pristine cobalt−nickel−fluoride phase with the initial morphology was retained after long-term water oxidation (Figure S17). In the crystalline area, a lattice fringe of 3.3 Å, which is associated with the (110) crystallographic plane of (NixCo1−x)F2 phase, was clearly observed (the inset of Figure S17b). Energy dispersive spectroscopy (EDS) images (Figure S18) reveal the presence of Co, Ni, F, B, and O in NCFO after the long-term OER test, demonstrating the excellent robustness of NCFO in alkaline water oxidation. We also carried out XPS analysis on a post-OER NCFO sample. After catalysis, The binding energies for major peaks of Co 2p3/2 (781.3 eV) and Ni 2p3/2 (856.1 eV) were comparable with those of the pristine NCFO sample, demonstrating the presence of metal−F bonds after OER, which is also consistent with TEM analysis on the post-OER sample (Figure S17). For the O 1s spectra, a peak corresponding to a surface hydroxide group (∼531. eV) was still observed and C−O bonding (∼535.0 eV) becomes more distinct. Notably, a third peak appeared at around 530.0 eV due to the formation of metal− oxygen bonding. In addition, a significant change in the F 1s spectrum was detected after catalysis. The peak intensity for a metal−F bond (∼684.0 eV) was reduced. A new peak corresponding to a C−F bond (∼688.0 eV) was observed, which may be present due to an organic binder (i.e., Nafion) used for the preparation of the catalyst ink. However, the clear
the respective components. Generally, the charge transfer resistance (Rct) can be determined from the semicircle diameter in the high-frequency region of a Nyquist plot (Z′ vs −Z″). The experimental data were fitted using a Randles equivalent circuit model to calculate Rct for the samples. Figure S15 presents the EIS data of the samples measured at an open circuit voltage. NCFO shows Rct of approximately 350 Ω, which is similar to that of NCBO. Two different semicircles were observed for NCBO, which may originate from the electrochemical responses of metal−boride and surface hydroxide phases, respectively. Besides good activity, the stability of the catalyst is another important factor for the practical application of electrochemical water splitting. The electrocatalytic durability of NCFO for water oxidation was evaluated by chronoamperometric (CA) and continuous CV measurements. Notably, NCFO displayed 100% of the current density output without any degradation during 10 h of water oxidation at 1.58 V in 1.0 M KOH, verifying its excellent stability in an alkaline solution (Figure 4e). The increase of current density of NCFO during chronoamperometry test may be due to the formation of Co, Ni (oxy)hydroxide phases on the surface, which has high catalytic activity for OER. Co, Ni (oxy)hydroxide phases are readily formed during OER under alkaline conditions, where adsorption/desorption of intermediates such as OH, O, and OOH are favored. Moreover, Figure 4f presents the polarization curve of NCFO after 1,000 CV cycles in the OER potential window (1.23−1.60 V) at a scan rate of 50 mV s−1. NCFO exhibited a negligible increase in overpotential (only 0.3%) for affording 10 mA cm−2. Most first-row transition metal-based catalysts transform into oxy/hydroxides during water oxidation in alkaline 4004
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the dissociative adsorption mode for water molecules is thermodynamically more preferred than the molecular adsorption mode on the surface, boosting the Volmer reaction in NCFO. Density of states (DOS) analysis was performed to investigate the effects of Co doping in NCFO. The partial DOSs of NiF2 and Co-doped NiF2 (Ni0.708 Co0.292F2) are presented in Figure 5c,d. The total DOS near the Fermi level (EF) substantially increased after Co replacement at the Ni site, presented as a distinct downshifting of DOS from the conduction band toward the valence band. As can be seen in the partial DOS plots in Figure 5d, Co mainly contributes to the enhancement of DOS near EF. Electrons near the EF are readily involved in metallic conduction and, thus, facilitate charge transfers between the catalyst’s surface and the adsorbed reactants, resulting in high electrocatalytic activity. Hence, Co doping improves metallic or conductive properties of NCFO, directly leading to enhanced OER activity. Consequently, the individual roles of the elements in NCFO can be defined: Co and Ni are active adsorption and catalytic sites for the reactants, Co is the charge carrier supplier enhancing charge transfer, and F is the surface electronic configuration modifier that subtracts electrons from active sites. The combination of these contributions from each of the elements within the intriguing nanostructure can render NCFO a highly efficient, robust electrocatalyst for water oxidation.
existence of a metal−F bond was observed after long-term catalysis. Overall, these results demonstrate that NCFO is an excellent catalyst for water oxidation, showing high activity and durability in alkaline media and performing better than or as well as recently developed, state-of-the-art OER catalysts, including the benchmark novel electrocatalyst, RuO2. To acquire a fundamental understanding of the individual roles of the metals and possible water oxidation reaction pathways, we performed systematic DFT calculations. First, we considered all possible adsorption sites of OH, O, and OOH species on nickel−cobalt−fluoride (Ni0.708 Co0.292F2), consistent with experimental stoichiometry, to calculate the reaction energy landscapes (Figure S20). The Gibbs free energies of the reactions (ΔG) were obtained using eq 3: ΔG = ΔE + ΔZPE − T ΔS + ΔG U
(3)
where ΔE is the calculated total energy difference, ΔZPE is the zero-point energy correction, TΔS is entropy, and ΔGU is free energy change by cell bias. The reaction steps (I−IV), respectively corresponding to OH, O, OOH, and OO adsorptions on the substrates, are described in eqs 4−7): step I: H 2O(l) → OH(ads) + H+ + e−
(4)
step II: OH(ads) → O(ads) + H+ + e−
■
(5)
step III: +
O + H 2O(l) → HOO(ads) + H + e
−
S Supporting Information *
(6)
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00449.
step IV: HOO → O2 (g) + H+ + e−
ASSOCIATED CONTENT
Experimental details such as material synthesis and characterization, DFT calculations, and electrochemical measurements (PDF)
(7)
The preferred adsorption sites for all reaction steps were determined to be Ni, Co, or F atom sites consisting of a nickel−cobalt−fluoride surface (Table S2). However, adsorption onto F sites was not favored for any reaction steps, and the final adsorptions occurred at the neighboring metal sites (Figure S20, bottom images). The calculated free energy diagrams for OH−, O−, OOH− adsorptions are presented in Figure 5a. The minimum voltage to render the reaction spontaneous was calculated to be 1.91 V. The step III reaction (OOH formation) was found to be the reaction ratedetermining step (RDS) for OER. Furthermore, the spatial distribution of an extra electron in each reaction step reveals that a strong charge redistribution occurred at the interface between the active Co atoms and adsorbates (Figure 5b). More specifically, the large electron transfer from Co atoms to adsorbates takes place leading to an accumulation of electron density near the adsorbates and simultaneously depletion of electrons at the interfacial Co atoms. Importantly, electrons from the Co atom also transferred to the bottom F atoms (noted as F3 and F4 in Figure 5b) owing to the high electronegativity of the F atoms. The calculated Bader charges at the individual reaction steps are summarized in Table S2, demonstrating the electron-deficient nature of the Co active site and, in contrast, the electron-sufficient nature of the F atoms located just below the Co atom. The hydrated alkalimetal cations can be stabilized easily at the electron accumulation region by noncovalent interactions in alkaline solution, which further promotes the interaction between the surface of the catalyst and the water molecules.37 In this case,
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AUTHOR INFORMATION
Corresponding Authors
*(H.H.) E-mail:
[email protected]. *(Y.-C.C.) E-mail:
[email protected]. *(S.M.) E-mail:
[email protected]. ORCID
HyukSu Han: 0000-0001-7230-612X Author Contributions ⊥
H.H. and J.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grants 2016R1A2B4010674 and 2018R1D1A1A02085938).
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REFERENCES
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DOI: 10.1021/acsaem.9b00449 ACS Appl. Energy Mater. 2019, 2, 3999−4007
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DOI: 10.1021/acsaem.9b00449 ACS Appl. Energy Mater. 2019, 2, 3999−4007
Letter
ACS Applied Energy Materials (37) Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K. C.; Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M. Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces. Science 2011, 334, 1256−1260.
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DOI: 10.1021/acsaem.9b00449 ACS Appl. Energy Mater. 2019, 2, 3999−4007
