Highly Defective Fe-Based Oxyhydroxides from Electrochemical

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Highly Defective Fe-based Oxyhydroxides from Electrochemical Reconstruction for Efficient Oxygen Evolution Catalysis Qun He, Hui Xie, Zia ur Rehman, Changda Wang, Ping Wan, Hongliang Jiang, Wangsheng Chu, and Li Song ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 14, 2018

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ACS Energy Letters

Highly Defective Fe-based Oxyhydroxides from Electrochemical Reconstruction for Efficient Oxygen Evolution Catalysis Qun He,† Hui Xie,† Zia ur Rehman, Changda Wang, Ping Wan, Hongliang Jiang,* Wangsheng Chu, and Li Song* National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei, Anhui 230026 (P. R. China) AUTHOR INFORMATION Corresponding Author *[email protected] (H.J.); [email protected] (L.S.).

Abstract: Transition metal Fe-dominated materials generally show inefficient electrocatalytic oxygen evolution reaction (OER), while Fe-incorporated multi-metallic compounds can exhibit highly catalytic activity. In fact, there is always a controversy about the main active sites in Feinvolved OER elelctrocatalysts. Herein, we demonstrate a highly active mono-Fe based OER electrocatalyst with very low overpotential of 283 mV at a current density of 10 mA cm-2 and a Tafel slope of 41.4 mV dec-1. Systematic characterization reveals that the OER performance stems from the Na dissolution during electrocatalytic oxidation process, enabling the

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electrocatalyst reconstruction to form highly defective oxyhydroxides as highly active catalytic sites. Notably, the electrocatalytic activity can be significantly promoted through the introduction of hetero-transition metal. This work provides insights for active sites in OER catalysis, along with a facile route to tune the intrinsic activity.

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Electrochemical water splitting is generally considered as a vital section in the area of clean and renewable energy conversion and storage.1,2 The sluggish reaction kinetics for anodic oxygen evolution half-reaction owing to the complex multiple proton-electron transfer reaction pathways always hinder the efficiency of energy-related devices.3,4 RuO2 and IrO2 are well known for being efficient oxygen evolution reaction (OER) catalysts, but the natural scarcity has severely impeded their large-scale applications.5-7 Therefore, it is imperative to develop earth-abundant and efficient OER electrocatalysts with low overpotentials and long-term durability. In this regard, various transition metal (Ni, Co, etc) based materials, including their oxides, hydroxides, borides, selenides, phosphides, etc, have been explored and regarded as the promising alternatives for OER to date.8-13 However, as for single transition metal Fe-based


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materials, there is always inefficient activity.14-16 Recently, many reports declare that the Fe introduction could further enhance the OER activity of multi-metallic electrocatalysts, yet the Fe could only serve as OER promotor.17-21 Some latest theoretical and experimental studies also propose that Fe is likely to be active site in some Fe-based electrocatalysts.22-27 However, it always needs an experimental cognition of Fe sites towards OER. Recently, Chen et al. demonstrate that ultrathin pyrrhotite Fe7S8 nanosheets could realize efficient OER activity due to the existence of highly active Fe sites.28 Unfortunately, it is not enough to explore the structures of pristine catalysts only, because the electrocatalysts inevitably interact with surrounding species and undergo electrochemical reconstruction in OER process.29 Therefore, the exploration towards the Fe-based catalysts after undergoing OER process is highly desirable to understand the catalytic sites. Herein, taking the Na-containing Prussian blue analogues (PBAs) as precursors, we developed a systematic oxidation-phosphorization processing procedure to prepare transition metal phosphides nanoparticles-constructed cubes. As illustrated in Scheme 1, the Fe-based PBAs precursors were first prepared by mild coprecipitation method (recorded as PBA-Fe or PBAFeCo-x cubes, see more synthesis details in Supporting Information). Then the obtained PBAs were oxidized in air (recorded as Fe3O4 cubes, or Fe3O4/Co3O4-x cubes). Finally, FeP4 cubes or FeP4/CoP2-x cubes were obtained by high-temperature phosphorization treatment. The asprepared materials before and after OER catalysis were elaborately characterized. Superior OER performances were delivered.


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Scheme 1. Schematic program of the consecutive two-step treatments to prepare transition metal phosphides-constructed cubes from PBAs (M = Fe or Co in this work). X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) and transmission electron microscope (TEM) images (Figure S1, S2) indicate the successful preparation of PBAFe cubes and PBA-FeCo-x cubes. After oxidation treatment in air, typical metal oxides with obvious cubic structures were obtained (Figure S3, S4). To study the Fe sites in OER process, the single FeP4 cubes were clearly analysed first. Figure 1a shows the XRD pattern of FeP4 cubes, which is in accordance with the standard data (JCPDS No. 34-0995). The TEM images of FeP4 cubes reveal its hollow and cubic structure constituted with nanoparticles (Figure 1b). The high-resolution TEM (HRTEM) image presents clear lattice fringe spacing values of 0.253 and 0.297 nm, assigned to the (-132) and (-112) crystal planes of the FeP4, respectively (Figure 1c). This result was further confirmed by the selected area electron diffraction (SAED) pattern (inset


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in Figure 1c). The elemental mapping of FeP4 cubes indicates homogeneous distribution of Fe, Na, and P elements in the entire sample. It is worth noting that the relatively weak O signal should stem from the partial surface oxidation in air. The detailed surface chemical states information of FeP4 cubes was revealed by X-ray photoelectron spectroscopy (XPS). The highresolution Fe 2p and P 2p XPS spectra reveal the existence of Fe-P, Fe-O, and P-O bonds, which are consistent with the O 1s XPS spectrum (Figure S5a-d). This result confirms the partial surface oxidation again. The detected XPS signal further indicates the existence of element Na (Figure S5e). The above analyses clearly demonstrate the structure and component information of the final FeP4 cubes. Brunauer-Emmett-Teller (BET) analysis shows the increase of specific surface area of FeP4 cubes relative to pristine PBA-Fe precursor (Figure S6).


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Figure 1. a) XRD pattern of FeP4 cubes. b) Typical TEM image of FeP4 cubes. c) HRTEM image of FeP4 cubes (inset: SAED pattern). d) Elemental mapping data of FeP4 cubes. The OER performances of our samples were evaluated in 1.0 M KOH electrolyte with a threeelectrode system (see more details in Supporting Information). The measured linear sweep voltammetry (LSV) curves (Figure 2a) suggest both PBA-Fe and Fe3O4 cubes exhibit low OER activities. However, high OER activity with very low overpotential was obtained for FeP4 cubes, even surpassing the commercial IrO2 catalyst and most of Fe-based catalysts reported to date (Figure 2a and Table S1). The activity difference can be further corroborated from the Tafel slopes. The lowest Tafel slope of 41.4 mV dec-1 is achieved for FeP4 cubes (Figure 2b). To further investigate the electrode reaction kinetics, the electrochemical impedance spectroscopy (EIS) measurements (Figure S7) were carried out. The obtained Nyquist plot of FeP4 cubes at open potential exhibits a much smaller charge transfer resistance (Rct, 132 Ω) than that of PBAFe cubes (330 Ω) and Fe3O4 cubes (320 Ω), revealing very favourable charge transfer kinetics.11 In order to present an explicit comparison among these electrocatalysts, the overpotentials, turnover frequencies (TOFs), and true values which exclude the effect of surface area, were evaluated.30 In Figure 2c, the FeP4 cubes possess an overpotential of 283 mV at a current density of 10 mA cm-2, obviously lower than PBA-Fe cubes (348 mV) and Fe3O4 cubes (336 mV), suggesting the high OER activity of FeP4 cubes. TOFs and true values were calculated from LSV curves, transition metal moles, electrochemical surface areas (ECSAs, Figure S8), and roughness factor (Rf) (see more details in Supporting Information). Assuming that all the Fe sites are catalytically active, the Fe contents in different samples were estimated by inductively coupled plasma-optical emission spectrometry (ICP-OES, Table S2). A high TOF value of 0.0386 s-1 at 300 mV is obtained for FeP4 cubes, which is much higher than that of PBA-Fe cubes (0.0023 s-1)


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and Fe3O4 cubes (0.0018 s-1) (Figure 2c, Table S2). Similar result for true values were also obtained (Figure 2c, Figure S8). Later, the rotating ring-disk electrode (RRDE) measurements were employed to further understand the reaction mechanism. As shown in Figure 2d, a very low ring current (Ir, red line) is obtained, which is far lower than that of the disk current (Id, blue line), indicating negligible formation of coproduct (peroxide intermediates) at the surface of catalyst during OER process. Additionally, the Ir gradually decreases when the potential increases to the high values, indicating the generation of less coproducts at the high potential range. This result verifies that the fast enhancement of current is predominantly attributed to a satisfactory 4-electron transfer pathway for generating oxygen molecules for the OER process.11 Furthermore, the Faradaic efficiency (FE) evaluation was carried out to verify that rapid increase of current was caused by water oxidation rather than additional reactions. First, an RRDE with a Pt ring electrode was employed to achieve a successive OER (disk electrode) – ORR (ring electrode) process. The Id values were fixed at potentials ranging from 100 µA to 340 µA to produce oxygen molecules from the catalyst, then the generated oxygen molecules were further reduced by a Pt ring electrode with an ORR potential of -0.78 V. As exhibited in Figure 2e, the collected Ir is approximately 33.0 µA when the chosen Id was fixed at 100 µA, corresponding to a high FE value of 89%. These results prove that our catalyst can provide the 4-electron reaction process to generate oxygen at relatively low potential. Moreover, a rapid FE increase was achieved with the increase of disk electrode potential, and nearby 100% FE can be obtained under high Id (340 µA), indicating that the measured oxidation current catalysed by FeP4 cubes catalyst can be ascribed to OER process (Figure 2f).11 Furthermore, electrochemical stability, one of very important criterions in judging OER performance, is necessary for any electrode materials. To probe the durability of FeP4 cubes catalyst, the long-term stability was assessed


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over 100 hours under a constant potential (0.30 V). It is noteworthy that the current density only displays a negligible change for FeP4 cubes catalyst, which is more stable than IrO2 (Figure 2g).

Figure 2. a) Polarization curves of FeP4 cubes, together with PBA-Fe cubes, Fe3O4 cubes, IrO2 and b) corresponding Tafel slopes of these samples. c) The comparison of overpotentials at 10 mA cm-2, TOFs at 300 mV, and true values at 300 mV among PBA-Fe cubes, Fe3O4 cubes, and FeP4 cubes. d) Disk and ring current of PBA-Fe cubes catalyst on an RRDE (1600 rpm) with a ring potential of 0.27 V in 1.0 M KOH electrolyte. e) Ring current of FeP4 cubes catalyst on an RRDE (1600 rpm) with a ring potential -0.78 V in 1.0 M KOH electrolyte at different disk current values. f) The calculated Faradaic efficiency of FeP4 cubes catalyst at different disk


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potentials in 1.0 M KOH electrolyte. g) Long-term current density-time measurements of FeP4 cubes and IrO2 in 1.0 M KOH electrolyte with the same set potential of 0.30 V.

To get a deep insight into the root for the enhanced electrochemical activity of FeP4 cubes, the structures, components, and chemical states analyses were carried out for galvanostatic activation (GA)-treated FeP4 cubes with the aging in 1.0 M KOH for 20 h at a current density of 20 mA cm-2 (see more details in Supporting Information). XRD measurement in Figure 3a shows that the GA-treated FeP4 cubes catalyst contains obvious diffraction peaks assigned to the FeOOH (JCPDS No. 22-0353). This result demonstrates the occurrence of distinct reconstruction. Further HRTEM and SAED observations also demonstrate the existence of FeOOH (Figure 3b-c). In the elemental mapping, the oxygen signal is obviously enhanced (Figure 3d), suggesting the formation of excessive oxygen-containing species originating from electrochemical reconstruction and oxidation.15,17,29 Notably, the Na signal is obviously weak (Figure 3d and Table S3). As reported, soluble metal dissolution during electrochemical process would create nanoporous catalysts.31 To confirm the role of Na, the ECSAs of pristine and GAtreated FeP4 cubes catalysts were evaluated in detail. Significant increase of surface area for GAtreated FeP4 cubes relative to pristine FeP4 cubes can be seen (Figure 3e, Figure S9), which could be attributed to the dissolution-derived nanoporous structure (Figure S10). This result can be further proven from a relatively small ECSA of alkali metal-free catalyst and its comparatively low OER activity with large overpotential and low TOF (Figure S11, S12, Table S2).32 Chemical states analyses in pristine and GA-treated FeP4 cubes were systematically performed. XPS survey spectra show the obvious decrease of P and Na on the surface of GA-treated FeP4 cubes, further validating the existed electrochemical reconstruction probably benefited from the


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dissolution of Na (Figure S13a). The high-resolution XPS spectrum of Fe shows that the signal from Fe-P almost disappeared (Figure 3f). Meanwhile, positive shifts of Fe 2p peaks indicated the oxidation of Fe to a higher valence state.17 The consistent shifts were also observed for O 1s and P 2p XPS spectra (Figure S13b, Figure S13c). The existence of P-O signal and the absence of Fe-P signal imply the complete reconstruction of FeP4 phase. The P-O bonds are probably assigned to the adsorbed P-related species.15 In General, the synthesized FeOOH shows low OER activity (Figure S14, Figure S15).14-16 Therefore, the detailed atomic structures of the GAtreated FeP4 cubes should be explored. The X-ray absorption structure (XAS) spectroscopy, one of the most advanced spectroscopic techniques, was carried out to detect the structures of a series of the obtained samples. The X-ray absorption near-edge structure (XANES) spectra show the absorption edge of GA-treated (20 h) sample is very close to that of the FeOOH, suggesting a high average Fe oxidation state in GA-treated (20 h) sample (Figure 3g). This result further confirms the occurrence of high-degree reconstruction. Notably, the corresponding k-space oscillations show the obvious difference between GA-treated (20 h) sample and FeOOH, indicating the structural differences in the coordination environment around the Fe atoms (Figure S16). This difference can be clearly observed from the Fourier transformation (FT) analysis. There is an obvious decrease of coordination number of the GA-treated (20 h) sample relative to that of as-prepared FeOOH, suggesting the highly defective structure of post electrocatalyst (Figure 3h, Figure S17). The obtained defective structure with low coordination environment would improve the electronic conductivity and facilitate the adsorption process of H2O onto metal cations, thereby increasing the intrinsic catalytic activity of FeOOH.3,33-35 The produced phosphates in electrochemical reconstruction would also benefit the OER process.15 The preceding detailed analyses strongly demonstrated that the high OER activity of PBA-Fe cubes-


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derived FeP4 cubes catalyst should be attributed to the high-efficiency electrochemical reconstruction to generate considerable defective active sites on nanoporous FeOOH.

Figure 3. a) XRD pattern of GA-treated FeP4 cubes. b) HRTEM and c) SAED images of GAtreated FeP4 cubes. d) Elemental mapping data of GA-treated FeP4 cubes. e) The calculated Cdl values of pristine and GA-treated FeP4 cubes. f) High-resolution XPS spectra of Fe 2p. g) Fe kedge XANES of pristine, GA-treated samples together with Fe foil, FeO, and FeOOH. h) Fe kedge FT analysis of pristine and GA-treated samples. Recently, some theoretical and experimental researches have revealed that dual-transitionmetal based OER electrocatalysts would exhibit more favourable reaction kinetics in comparison with their counterparts due to their optimized electronic structures.17-21 Here, FeCo dual metal


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phosphides with adjustable Fe/Co ratios were also prepared by a similar method (see more synthesis details in Supporting Information). The characterization towards the dual metal phosphides indicates the formation of the FeP4/CoP2-x cubes (Figure S18 - Figure S20). The OER performance of dual-transition metal PBAs derived phosphides were also measured. Significantly, the obtained FeP4/CoP2-x cubes catalysts display enhanced OER performance compared with FeP4 cubes. Among them, the optimized FeP4/CoP2-2 cubes catalyst achieves the highest OER activity with an overpotential value of 256 mV at a current density of 10 mA cm-2 (Figure 4a). This high activity is further authenticated by the low Tafel slope (34.7 mV dec-1), small EIS value (Figure 4b, Figure S21), making it as one of the most efficient OER catalysts reported to date (Table S4).


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Figure 4. a) Polarization curves of CoP2/FeP4-x cubes, together with FeP4 cubes and b) corresponding Tafel slopes of these samples in 1.0 M KOH solution. c) Electrochemical durability measurements of CoP2/FeP4-2 cubes at applied potentials of 0.26 V, 0.27 V in 1.0 M KOH solution. d) Schematic illustration of the obtained electrocatalysts for OER process.

The activity enhancement observed in the FeP4/CoP2-x cubes catalysts can be understood by the further structure and chemical states analyses. XRD pattern of GA-treated FeP4/CoP2-2 cubes catalyst also shows the same diffraction peaks of FeOOH, similar with that of GA-treated FeP4


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cubes (Figure S22, Figure 3a). This result further proves the consistent electrochemical reconstruction procedure occurred for these two samples. This can be further confirmed by the detailed XPS analysis, in which that the according shifts and disappearance of Fe 2p, O 1s, Co 2p, P 2p spectra were observed (Figure S23). Therefore, the highly enhanced OER activity of FeP4/CoP2-x cubes catalysts originate from the Co-doped FeOOH. A deep activity evaluation was performed by the comparison of overpotentials, TOFs, and true values (Figure S24, Table S5). Our statistic results depict that significantly enhanced OER activity with high TOFs (the highest to be 0.167 s-1) and true values (the highest to be 0.854) were achieved for FeP4/CoP2-x catalysts (Figure S25). The electrochemical stability was also measured, proving the high stability of the dual metal catalyst (Figure 4c). Based on the above analysis, the synergy effects in OER process towards FeP4/CoP2-x catalysts can be illustrated in Figure 4d. The Na dissolution-derived nanoporous structures would provide large electrochemical active surface area. The electrochemical reconstruction-derived defective oxyhydroxides serve as highly active OER sites. In conclusion, we have successfully developed one class of high-efficiency Fe-based OER catalysts with low overpotentials, small Tafel slopes, and long-term durability using alkali metalcontaining PBAs as precursors. It represents one of the best OER catalysts reported to date. Our detailed characterization demonstrated that the highly defective oxyhydroxides derived from electrochemical reconstruction can serve as highly active catalytic sites to achieve favourable catalytic kinetics. These results should expand our understanding towards OER catalysis. It is also believed that this work would open up new pathways to produce earth-abundant and highly active catalysts for future energy applications.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental Section, Figure S1-S4, S10, S11, S14, S18, S19, S22 showing the XRD, SEM, TEM, and mapping results of different materials, including precursors, final products, and material after test. Figure S5, S13, S20, S23 showing XPS results of these samples. Figure S6 showing the BET result of samples. Figure S7-S9, S12, S14, S15, S21, S24, S25 showing electrochemical results of these samples. Figure S16 and Figure S17 showing the XAFS data for different materials. Table S1-S5 exhibiting the activity comparison and ICP-OES results of different samples. (PDF) AUTHOR INFORMATION Author Contributions †

Q.H. and H.X. contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is financially supported by MOST (2017YFA0303500, 2014CB848900), NSFC (U1532112, 11574280, 11605201, 21706248), CAS Key Research Program of Frontier Sciences (QYZDB-SSW-SLH018), China Postdoctoral Science Foundation (BH2310000033), CAS Iterdisciplinary Innovation Team, Innovative Program of Development Foundation of Hefei Center for Physcial Science (T6FXCX003). L. S. acknowledges the recruitment program of global experts, the CAS Hundred Talent Program, Key Laboratory of Advanced Energy


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Materials Chemistry (Ministry of Education) Nankai University (111 project，B12015), Key Laboratory of the Ministry of Education for Advanced Catalysis Materials and Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces (Zhejiang Normal University). We thank the Shanghai synchrotron Radiation Facility (14W1, SSRF), the Beijing Synchrotron Rad·iation Facility (1W1B and soft-X-ray endstation, BSRF), the Hefei Synchrotron Radiation Facility (Photoemission, MCD and Catalysis/Surface Science Endstations, NSRL), the USTC Center for Micro and Nanoscale Research and Fabrication for helps in characterizations, and Dr. Shuangming Chen for helps on XAFS tests.

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