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Fully Oxidized Ni-Fe Layered Double Hydroxide with 100% Exposed Active Sites for Catalyzing Oxygen Evolution Reaction Chunguang Kuai, Yan Zhang, Deyao Wu, Dimosthenis Sokaras, Linqin Mu, Stephanie Spence, Dennis Nordlund, Feng Lin, and Xi-Wen Du ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01935 • Publication Date (Web): 03 Jun 2019 Downloaded from http://pubs.acs.org on June 3, 2019
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Fully Oxidized Ni-Fe Layered Double Hydroxide with 100% Exposed Active Sites for Catalyzing Oxygen Evolution Reaction Chunguang Kuai1, 2, ‡, Yan Zhang1, 3, ‡, Deyao Wu1, Dimosthenis Sokaras3,*, Linqin Mu2, Stephanie Spence2, Dennis Nordlund3, Feng Lin2,*, Xi-Wen Du1,* 1.
Institute of New-Energy Materials, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China.
Department of Chemistry, Virginia Tech, Blacksburg, VA 24061, USA.
Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors contributed equally to this work
Supporting Information Placeholder ABSTRACT: Ni-Fe layered double hydroxides (LDHs) are promising for catalyzing oxygen evolution reaction (OER) in alkaline media. However, the OER mechanism is highly debated, partially due to the lack of an ideal catalyst with 100% exposed active sites for unambiguous characterization. Herein, we develop an alcohol intercalation method to prepare ultrathin Ni-Fe LDH with a 1/3 unit-cell thickness and 100% exposed active sites. The ultrathin LDH catalyst exhibits an intrinsic activity similar to the bulk LDH and allows a direct and reliable characterization of the catalyst without any interference from “bulk” inactive species. Operando synchrotron X-ray analysis indicates that the metallic ions in ultrathin Ni-Fe LDH are fully oxidized into tetravalance states at low applied potentials, and OER occurs on the tetravalent Ni and Fe ions following a decoupled proton/electron mechanism. Our findings demonstrate that a full oxidization of metal ions is crucial for highly active NiFe LDHs, and this can be accomplished by engineering ultrathin nanostructures. KEYWORDS: oxygen evolution reaction; catalysts; operando analysis; synchrotron X-ray absorption spectroscopy; catalytic mechanism; layer double hydroxide; active site
Solar, wind and hydro are among the most promising sustainable energy resources for substituting traditional fossil fuel energy.1 To overcome their intermittency, energy storage in chemical molecules, such as hydrogen,2 hydrocarbons, alcohols,3-4 and ammonium,5-8 is imperative. Most of these chemical processes involve electrochemical reduction of small molecules such as H2O, CO2 and N2, with the water oxidation being a counter reaction through the oxygen evolution reaction (OER). Since OER requires a four-electron transfer, it is known to be very sluggish,9 and therefore it is meaningful to develop scalable active water oxidation catalysts with optimized properties. Ni-Fe oxides and hydroxides are among the most efficient and earth-abundant OER catalysts in alkaline solution, with
overpotentials of only 250-300 mV at a 10 mA/cm2 current density.10-14 Despite their high performance, the catalytic mechanism of Ni-Fe based catalysts remains highly controversial due to the lack of an ideal catalyst with 100% exposed active sites for unambiguous investigation. Operando synchrotron X-ray absorption spectroscopy (XAS) has been widely used to study the local chemical environment, oxidation states, and catalytic behavior of OER catalysts.15-22 Although these techniques are element specific, their probing depth provides a cumulative information from the entire studied material; namely, the detected signals may correspond to both the active ions at the surface and the non-active ions at the catalyst sub-surface and bulk. This may lead to an ambiguous determination of the metal ions oxidation state in the OER active range, and thus of the catalytic mechanism. One possible solution is to engineer ultrathin catalysts, e. g. NiFe LDHs with single or double layer, which can expose all of metallic ions on the surface for operando XAS analysis. Previous works demonstrated that the thickness of NiFe LDH could be reduced via anion-exchange by weakening the interlayer van der Waals force.23, 24 However, such a method, besides being time consuming, also reduces the number of active sites due to the adsorption of residual exchanged anions on the catalyst surface.23 Therefore, it remains a challenge to synthesize few-layer NiFe LDHs for mechanistic study. Herein, we develop an alcohol intercalation method to prepare ultrathin Ni-Fe LDH with a 1/3 unit-cell thickness (Scheme S1). The resulting catalyst achieves overpotentials of 210 mV and 320 mV at 10 mA/cm2 and 500 mA/cm2, respectively, outperforming all NiFe LDHs reported in literature.10, 23, 25, 26 The turnover frequency (TOF) of ultrathin Ni-Fe LDH is similar with that of the bulk counterpart, indicating the same OER mechanism for both cases. The ultrathin nature of the catalyst (1/3 unit-cell) ensures that all its metal sites are at the surface and hence it has 100% exposed active sites. Therefore, this structure enables
Figure 1. Transmission electron microscope images of (a) ultrathin and (b) bulk LDHs. Atomic force microscope images and thickness profiles of (c) ultrathin and (d) bulk LDHs, (e) XRD patterns for ultrathin and bulk LDHs. The standard α-Ni(OH)2 was extracted from JCPDS #38-0715. (f) Raman spectra of ultrathin and bulk LDHs.
unambiguous XAS studies for the catalytic mechanism of mixed NiFe catalysts without the “bulk” interference. Our findings here show that ultrathin LDH can be fully oxidized under low OER potentials, and fully oxidized metal ions can boost OER which proceeds on NiFe LDH in a decoupled proton/electron way. The as-prepared ultrathin NiFe LDH exhibits a hexagonal morphology (Fig 1a) and a thickness of ~1.2 nm (Fig 1c). Meanwhile, a bulk LDH sample, synthesized by a hydrothermal method (see details in Experimental Methods), also presents a hexagon-like morphology (Fig 1b) but a larger thickness of about 9.5 nm (Fig 1d). By comparing the thickness of ultrathin LDH with the (001) interplanar spacing of α-Ni(OH)2 (Scheme S2), we find that the total thickness of ultrathin LDH accords with a height of 1/3 unit-cell of α-Ni(OH)2, implying that the ultrathin LDH consists of 2 unit layers, and hence has 100% of its metal sites exposed to surface. In the X-ray diffraction (XRD) pattern of ultrathin LDH (Fig 1e), only two peaks are observed at ~11.5 and ~23, corresponding to the (003) and (006) planes of α-Ni(OH)2. Hence, the ultrathin LDH exposes its (001) basal surface. In contrast, bulk LDH shows weak peaks at higher diffraction angles, corresponding to the (101), (012), (015), (018), (110), and (113) planes. The Raman spectra of both samples (Fig 1f) include three peaks at 295, 430, and 530 cm-1 which can be assigned as E-type, M-O(H), and M-O vibrations, respectively.27-36 The I530/I430 ratio of ultrathin LDH is higher than that of the bulk one, indicating the number of M-O motifs is higher than that of Ni-OH motifs. X-ray photoelectron spectroscopy (XPS) results illuminate that Ni/Fe ratios in both samples are almost the same (Figs. S1a and S1b), and iron valance state in ultrathin LDH is higher than that of the bulk counterpart (Figs. S1c and S1d). To understand the electronic structure of the ultrathin LDH, we first investigate the starting materials by ex-situ XAS. For the Ni cations, no obvious difference is observed between ultrathin and bulk LDHs in both K-edge and L-edge
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Figure 2. XAS results for the as-prepared samples. (a) main edge of Ni K-edge, (b) zoomed in pre-edge region of Ni K-edge, (c) soft XAS of Ni L-edge in total electron yield(TEY) mode, (d) main edge of Fe K-edge, (e) zoomed in pre-edge region of Fe K-edge, (f) soft XAS of Fe L-edge TEY mode. Ni(OH)2, FeCl2 and Fe2O3 were used as the reference samples for Ni2+, Fe2+ and Fe3+ ions respectively. spectra (Fig. 2a, b, c). In contrast, the Fe K-edge position of ultrathin LDH is 1.38 eV higher than that of bulk LDH (Fig. 2d), which may arise from different Fe oxidation state or Fe-O covalency37. The Fe oxidization state could be figured out by analyzing the pre-edge region of K-edge (Fig. 2e) and L-edge (Fig. 2f) spectra. As shown in Fig. 2e, two well-separated peaks are observed for ultrathin LDH, which is typical for Fe3+ in the octahedral coordination (Oh)38. Comparatively, bulk LDH displays one wide peak which may originate from the superposition of both Fe3+ and Fe2+ peaks. This hypothesis is verified by L-edge spectra (Figure 2f). According to the FeCl2 reference, Fe2+ ions are featured by two shoulders on L2 and L3 edges (indicated by the arrows in Fig. 2f) which are absent in the spectrum of ultrathin LDH but present in that of bulk LDH, implying almost pure Fe3+ ions in ultra-thin LDH but mixed Fe2+/ Fe3+ ions in bulk LDH. Quantitative analysis shows that ratios of Fe3+:Fe2+ in ultrathin and bulk LDHs are 99.8:0.2 and 85.7:14.3, respectively. The difference is not pronounced enough to explain the relatively large change in Fe K-edge positions (1.38 eV) in Fig. 2d, hence, the reason is more likely to be the higher Fe-O covalency in ultrathin LDH37. This assumption is confirmed by the charge-density wave calculation. As shown in Fig. S2, the electrons in ultrathin LDH are more delocalized than those in bulk LDH, indicating the higher Fe-O covalency in ultrathin LDH. It has been reported that the electron delocalization can promote the electron transfer and lower oxidization barrier39-40, thus facilitate the formation of Fe3+ ions in ultrathin LDH. We then compare the electrochemical water oxidation properties of the two samples, with commercial RuO2 as a reference. Ultrathin LDH presents much higher performance than bulk LDH and RuO2 in 1 M KOH (Figure 3a). Specifically, the overpotentials of ultrathin LDH are 210 mV @ 10 mA/cm2 and 320 mV @ 500 mA/cm2, much lower than
Figure 3. Electrochemical properties of ultrathin and bulk LDHs. (a) Cyclic voltammetry curves at 5mV/s with 85%iR correction. R represents the resistance of the electrolyte, determined by EIS test. (b) Tafel plots. (c) EIS profiles. (d) Amount of active Ni sites determined by the Ni reduction peak. (e) TOF plot. (f) Specific activity of bulk and ultrathin LDH with 5% iR correction. R represents the resistance of the electrolyte, determined by EIS test.
those of bulk LDH (251 mV @ 10 mA/cm2 and 460 mV @ 500 mA/cm2) (Figs. 3a and S3). The lower overpotential of the ultrathin LDH could be ascribed to its higher electrochemically active surface area (ECSA) (Fig. S4 and Table S2), which increases the contact area between the catalysts and the electrolyte, and facilitates the mass transfer. In addition, the Tafel slope of ultrathin LDH is only ~30 mV/Dec, lower than the corresponding slopes of bulk LDH (46 mV/Dec) and RuO2 (106 mV/Dec) (Fig. 3b). In EIS spectra, ultrathin LDH exhibits the lowest semicircle, indicating a fast charge transfer between the electrode and electrolyte (Fig. 3c). When compared with latest published data, ultrathin LDH achieves a top performance among the state-of-the-art NiFe LDH materials (Table S1). Next, we calculate TOF to determine the intrinsic OER activity (Figs. 3d, 3e, S5 and S6). Two methods are employed to determine the number of active sites. The first one takes all the metal ions as catalytically active sites (TOFTM). In this case, TOF is usually underestimated because only part of metal ions can serve as active sites.43 The second method assumes the charged nickel ions as the active sites (TOFint)41. The number of active sites is determined by integrating the reduction peak of Ni2+/Ni3+/4+(Fig. 3d), so as to avoid the influence of the OER current on the anodic peak during the positive scan. The TOFTM of the ultrathin LDH is higher than that of the bulk LDH, and both LDHs are superior to the commercial RuO2(Fig. S6). In contrast, TOFint values of ultrathin and bulk LDHs are similar (~0.6 s-1), which implies similar OER mechanisms and active sites for the ultrathin and bulk LDH samples. We also calculate the specific activity by normalizing total current density with ECSA (Fig.S4 and Table S2). As shown in Fig. 3f, the specific current density of ultrathin LDH is still higher than that of bulk LDH. Moreover, the long-term durability was investigated by performing chronopotentiometric test at 10 mA/cm2. The results show that the ultrathin LDH can endure the
Figure 4. Operando XAS results of Ni and Fe element in ultrathin LDHs. (a) Ni K-edge, (b) Fe K-edge, (c) M-O bond length and (d) Coordination number evolution with increasing applied potential during the OER process. The error bars were generated based on the EXAFS fitting errors. long-term testing without obvious degradation of the performance (Fig. S7). Soft XAS analysis shows that the Ni L-edge and Fe L-edge of ultrathin LDH keep unchanged after 80 CV cycles (Fig. S8), indicating a high reversibility upon the oxidization and reduction process. To understand the chemical origin of the performance variation between ultrathin LDH and bulk counterpart, we conducted operando hard XAS to evaluate the changes of oxidation states for metal cations at different working potentials. For Ni ions, operando XAS illustrates that the Ni K-edge of ultrathin LDH shows an obvious positive shift as the applied potential increases, while that of bulk LDH changes slightly (Figs. 4a and S9). Since the K-edge position represents the oxidation state,37, 42 we conclude that Ni ions in ultrathin LDH are oxidized more easily than those in bulk LDH. According to the Ni K-edge data reported in the literature43 and delithiated LiNiO2 data (Fig. S10), the Ni2+ cations in ultrathin LDH could be fully oxidized to Ni4+ at the applied potential of 1.4 V vs reversible hydrogen electrode (RHE) (see Figs. 4a, S9 and S10), while for bulk LDH, the average oxidation state is less than 3+ at the same potential (Figs. 4a, S9 and S10). Operando EXAFS reveals that the Ni-O bonds in ultrathin LDH shrink from 2.06 to 1.89 Å upon the applied potential (Fig. 4c, S11 and Table S3, S4), confirming the conversion of Ni2+ into Ni4+.43-48 As for Fe ions, the operando Fe K-edge spectra of ultrathin and bulk LDHs shift gradually to positive direction as the applied potential increases (Figs. 4b and S12). For ultrathin LDH, the doublet peak in the pre-edge region becomes a singlet peak as the applied potential is above 1.4 V vs RHE (Fig. S12a); meanwhile, the intensity of the white line gradually decreases and an obvious shoulder emerges at 7150 eV (Fig. S12b), indicating a dramatic increase in Fe oxidation state. In addition, operando EXAFS spectrum of ultrathin LDH indicates that Fe-O bonds contract from 1.99 to 1.88 Å (Fig. 4c, Fig. S13 and Tables S3, S5), suggesting an iron
valance state transformation from Fe3+ to Fe4+.43-45, 49-51 In comparison, bulk LDH does not show any obvious change even the applied potential is up to 1.5 V vs RHE (Figs. S12c and S12d), implying that the Fe cations in bulk LDH are harder to oxidized. The electrochemical water oxidation reaction on the ultrathin sample starts at ~1.42V vs RHE, corresponding to the formation of tetravalent Fe and Ni (Fig.4 a, b, c). These results indicate that tetravalent Ni and Fe are prerequisites for OER. On the other hand, the coordination numbers (CNs) for Ni-O and Fe-O bonds in the ultra-thin sample were the same with those in the bulk material ( 6) prior to the OER test, implying that Ni and Fe ions are in the octahedral coordination. After the catalyst was OER activated, the CNs for Ni and Fe decreased to 5 (Fig. 4d), indicating that both Ni and Fe centered octahedra lose one of the bonded oxygen. The lattice-oxygen-involved OER process is further supported by the pH-dependent OER behavior of the ultrathin LDH (Fig. S14), which is known as the characteristic of the decoupled proton/electron transfer and the loss of lattice oxygen. 44, 52-54 Therefore, we conclude that OER occurs on the tetravalent Ni and Fe ions following a decoupled proton/electron mechanism. In summary, ultrathin Ni-Fe LDH with 100% exposed metal ions was successfully produced via an alcohol intercalation process and used as an ideal platform for operando synchrotron X-ray analysis to probe the catalytic mechanism without any “bulk” or sub-surface interferences. Under low OER potentials, all of metal ions in ultrathin LDH can be fully oxidized into tetravalency state, which facilitates the lattice-oxygen-involved OER in a decoupled proton/electron way. Moreover, since the high surface area improves the mass transportation during OER, eventually, the ultrathin LDH shows high activity towards water oxidation, with overpotentials of 210 mV @10 mA/cm2. Our findings indicate that the complete oxidization of metal ions is a prerequisite for highly active NiFe LDHs, which can be accomplished by ultrathin (single or double unit layer) structures.
ASSOCIATED CONTENT Supporting Information Synthesis of ultrathin Ni-Fe LDH, synthesis of bulk Ni-Fe LDH, characterization, electrochemical test, XAS test, DFT calculation and additional experimental results(PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interests.
ACKNOWLEDGMENT This work at Tianjin university was supported by the Natural Science Foundation of China (Nos. 51871160, 51671141, and 51471115). The work at Virginia Tech was supported by Department of Chemistry startup funds and the Institute for Critical Technology and Applied Science. The Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility is operated for the US Department of Energy Office of Science by Stanford University. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No.DE-AC02-76SF00515.
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Figure 1. Transmission electron microscope images of (a) ultrathin and (b) bulk LDHs. Atomic force microscope images and thickness profiles of (c) ultrathin and (d) bulk LDHs, (e) XRD patterns for ultrathin and bulk LDHs. The standard α-Ni(OH)2 was extracted from JCPDS #38-0715. (f) Raman spectra of ultrathin and bulk LDHs. 80x52mm (300 x 300 DPI)
Figure 2. XAS results for the as-prepared samples. (a) main edge of Ni K-edge, (b) zoomed in pre-edge region of Ni K-edge, (c) soft XAS of Ni L-edge in total electron yield(TEY) mode, (d) main edge of Fe K-edge, (e) zoomed in pre-edge region of Fe K-edge, (f) soft XAS of Fe L-edge TEY mode. Ni(OH)2, FeCl2 and Fe2O3 were used as the reference samples for Ni2+, Fe2+ and Fe3+ ions respectively. 80x66mm (300 x 300 DPI)
Figure 3. Electrochemical properties of ultrathin and bulk LDHs. (a) Cyclic voltammetry curves at 5mV/s with 85%iR correction. R represents the resistance of the electrolyte, determined by EIS test. (b) Tafel plots. (c) EIS profiles. (d) Amount of active Ni sites determined by the Ni reduction peak. (e) TOF plot. (f) Specific activity of bulk and ultrathin LDH with 5% iR correction. R represents the resistance of the electrolyte, determined by EIS test. 80x54mm (300 x 300 DPI)
Figure 4. Operando XAS results of Ni and Fe element in ultrathin LDHs. (a) Ni K-edge, (b) Fe K-edge, (c) MO bond length and (d) Coordination number evolution with increasing applied potential during the OER process. The error bars were generated based on the EXAFS fitting errors. 80x69mm (300 x 300 DPI)