Chemical Stability of P-substituted

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Operando Unraveling the Structural/Chemical Stability of P-substituted CoSe2 Electrocatalysts toward Hydrogen/ Oxygen Evolution Reactions in Alkaline Electrolyte Yanping Zhu, Hsiao-Chien Chen, Chia-Shuo Hsu, Ting-Sheng Lin, ChiaJui Chang, Sung-Chun Chang, Li-Duan Tsai, and Hao-Ming Chen ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00382 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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ACS Energy Letters

Operando Unraveling the Structural/Chemical Stability of Psubstituted CoSe2 Electrocatalysts toward Hydrogen/Oxygen Evolution Reactions in Alkaline Electrolyte Yanping Zhu,1† Hsiao-Chien Chen,1,2† Chia-Shuo Hsu,1 Ting-Sheng Lin,1 Chia-Jui Chang, 1 Sung-Chun Chang,3 Li-Duan Tsai,3 Hao Ming Chen1* 1Department 2Center

of Chemistry, National Taiwan University, Taipei 106, Taiwan

of Applied Nanomedicine, National Cheng Kung University, No. 1, University Rd., Tainan

70101, Taiwan. 3Materials

Chemical Research Laboratories, Industrial Technology Research Institute, Chutung 310,

Taiwan. †Contribution

equally

ABSTRACT A question whether the metal chalcogenides (phosphides) that have been acknowledged to be efficient materials as bifunctional electrocatalyst really perform the active species or just “pre-catalysts” has been debated. Herein, a series of operando measurements including in-situ X-ray absorption spectroscopy, liquid-phase transmission electron microscopy and in-situ Raman spectroscopy were conducted to real-time unravel the structural and chemical stability of P-substituted CoSe2 electrocatalysts under both hydrogen and oxygen evolution reactions in alkaline electrolyte. A conclusive suggestion can be revealed that, in alkaline electrolyte, Psubstituted CoSe2 electrocatalyst was acting as the “pre-catalyst” rather than the real reactive species. The introduction of phosphorus is speculated to generate more vacancy or defects around Co cations in the initial CoSe2 and considerably facilitate the structural transformation into the “real reactive species”, such as metallic cobalt (for HER) and cobalt oxyhydroxide (for OER).

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Hydrogen gas (H2) is extensively regarded as the most promising energy vector because of its high energy density and environmental benignity.1 Electrochemical water splitting has been proposed to be the cleanest technology for the hydrogen generation, in which the scalable water electrolysis requires some efficient electrocatalysts to boost its two core reactions: cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). The state-of-the-art platinum-based HER catalysts and iridium/ruthenium-based OER catalysts, however, are still suffering from their high cost and/or limited availability.2-4 It thus remains highly attractive to develop the alternative water-splitting electrocatalysts based on precious-metal-free and/or earth abundant elements. Accordingly, numerous efforts have been devoted to designing alternative catalysts based on earth-abundant and low-cost transition metals for HER (carbides, phosphides, chalcogenides and nitrides)5-8 and OER (oxides, hydroxides, chalcogenides and phosphates).9-12 Although various kinds of electrocatalysts with superior performance have been acknowledged, deep understanding of the catalytic process and precise identification of the active sites are still considerable dispute. In fact, it has been noticed that many catalysts may undergo a structural self-reconstruction or phase transformation during the oxidation or reduction processes.13-18 For instance, it has been found that both Co and P would dissolve in acidic electrolyte with remaining CoP2 (orthorhombic, JCPDS no. 032-0306) surface once CoP2 acting as HER electrocatalyst in acidic electrolytes.15 In alkaline condition, P was preferentially dissolved while the remaining Co on the surface would form cobalt hydroxide. 15 An imperative issue has been pointed out that unless fully characterization and complete understanding of these 2 ACS Paragon Plus Environment

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catalysts can be provided to establish conclusively that the catalytic active species on the surface are still the original compounds, we should not refer to those unstable materials as OER or HER catalysts.16,18 Generally, the structural or compositional changes of catalysts were revealed by various ex-situ characterization methods which were unable to actually capture the dynamic structures or intermediate information during the catalytic process. In contrast, in-situ or operando manners are gaining an increasing interest as judicious tools to track self-reconstruction and real-time clarify the reactive species.19-21 Cobalt selenide (CoSe2) has been intensively studied as a typical earth-abundant catalyst for both HER and OER because of its outstanding catalytic performance.22,23 For example, thin CoSe2 sheets with an orthorhombic phase were synthesized by exfoliating a lamellar CoSe2-DETA hybrid, while the density functional theory (DFT) calculations and the temperature-dependent resistivities suggested its metallic behavior, demonstrating the remarkable OER activity.24 Furthermore, a phosphorus substitutioninduced phase transition from cubic CoSe2 (c-CoSe2) to orthorhombic CoSe2 (o-CoSe2) was also demonstrated to be an efficiently bifunctional electrocatalysts.25 It is worth saying that, however, both the calculation models and relative analyses based on the initial species are becoming questionable if the actual reactive species under reaction condition cannot be still the original ones.18,26,27 In present work, through a phosphorization-induced cubic-orthorhombic phase transformation, we demonstrate a P-substituted CoSe2 nanostructure with a dramatically improved electrocatalytic performance. More significantly, a series of operando measurements including in-situ X-ray absorption spectroscopy (XAS), liquid-phase transmission electron microscopy (LP-TEM), and in-situ Raman spectroscopy were carried out to capture the dynamic structural evolution under electrochemical conditions, from which we can draw a clear conclusion that substantial changes take place during both the HER and OER processes. Based on these in-situ investigations, some suggestions regarding the resulting phenomenon are further proposed to illustrate the correlation between the geometrical structure and the electrochemical behavior of catalysts. The P-substituted CoSe2 nanostructures were synthesized through a combined selenization and phosphorization on the precursor of cobalt hydroxide carbonate hydrate (CHCH, Co(OH)(CO3)0.5·xH2O) (Figure S1). The phase composition of the as3 ACS Paragon Plus Environment


prepared samples were examined by X-ray diffraction (XRD) as revealed in Figure 1a. A broad peak at the two theta value of ~26o can be assigned to the substrate of carbon cloth (CC).28 Without phosphating treatment, the as-prepared sample can be clearly indexed to a cubic phase of CoSe2 (c-CoSe2, JCPDS no. 09-0234). After substituting with phosphorus (the sample of CoSe2.01P0.49), a mixed phase is confirmed by presenting a new pattern belonging to an orthorhombic phase of CoSe2 (o-CoSe2, JCPDS no. 53-0449), suggesting the existence of a phase transition. It can be found a complete phase transition from c-CoSe2 into o-CoSe2 with increasing the introduction of phosphorus substitution (CoSe1.64P0.54, CoSe1.26P1.42 and CoSe0.45P1.18). A typical SEM image of c-CoSe2 as shown in Figure 1b indicates a morphology that there are highly dense and vertically aligned nanowires uniformly covering the entire surface of CC. These vertically aligned nanowires can be ascribed to the CHCH precursor, since the CHCH precursor is clarified to possess a similar morphology as revealed in SEM images of Figure S2b and S2c while a SEM image of the bare carbon cloth (Figure S2a) shows a smooth surface with an average dimeter of ~10 um. The corresponding HRTEM image (Figure 1c) gives a lattice spacing of 0.585 nm that corresponds to the (100) plane of the cubic CoSe2 structure (JCPDS no. 09-0234). Phosphating treatment caused a morphology change as observed in Figure 1d and Figure S2d-f, where the Psubstituted CoSe2 nanowires are characteristic of the rough surface with presence of numerous nanocrystals. The representative HRTEM image of CoSe1.26P1.42 (Figure 1e) confirms a clear lattice spacing of 0.355 nm which is consistent with the value of (110) plane for the orthorhombic CoSe2 (JCPDS no. 53-0449), and the inserted selected area electron diffraction (SAED) pattern with bright spots clarifies its polycrystalline nature. The elemental compositions of the as-prepared samples were conducted by an inductively coupled plasma mass spectrometry (ICP-MS) measurement and were normalized by the atomic ratio of cobalt (Table S1). It is worth noting that the resultant c-CoSe2 and o-Co(Se,P)2 have relatively high Se/Co or Se+P/Co atomic ratios. The excess Se or Se+P in the both c-CoSe2 and o-CoSe2 may be derived from the adsorbed species on its surface, since the coordination number of Co-Se in c-CoSe2 (~ 4.2) is similar to that of Co-(Se+P) in o-Co(Se,P)2, indicating no Se or P exist in the 4 ACS Paragon Plus Environment

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intercrystalline spaces (vide infra). The electrocatalytic performance of the as-prepared samples toward both HER and OER in alkaline electrolyte (1M KOH) as illustrated in Figure 2, where bare CC and the benchmark Pt/C as well as RuO2 were also conducted to serve as the reference catalysts. The blank CC shows a negligible activity whereas the Pt/C catalyst demonstrates a highly active HER performance, as indicated by the lowest initial overpotential (Uonset) of 0 V and an overpotential of 43 mV to reach a representative current density of 10 mA cm-2 (η10).21 As compared to the c-CoSe2 sample with a poor catalytic activity (Uonset = 155 mV, η10 = 204 mV), the various P-substituted CoSe2 samples exhibit the significant enhancements in their HER activities. Among all samples, the CoSe1.26P1.42 sample performs the best HER activity with requiring 92 mV only to achieve the current density of 10 mA cm-2. Further increasing the amount of P cannot give rise to a HER activity even better and thereby leading to a volcano-shaped dependence of P amount (Figure S3). Notably, the current of the CoSe1.26P1.42 sample has outperformed the commercial Pt/C at a higher cathodic voltage (above 230 mV), meaning this electrocatalyst possesses an even better nature especially for practical applications (normally require 500 mA cm-2 at least).29 Furthermore, kinetic insights into the HER process can be gained through the Tafel plots constructed from steady polarization curves (Figure 2b). The CoSe1.26P1.42 catalyst achieves the lowest value of Tafel slope (90 mV dec-1) as compared with those of c-CoSe2 (196 mV dec-1), CoSe2.01P0.49 (118 mV dec-1), CoSe1.64P0.54 (109 mV dec-1) and CoSe0.45P1.18 (93 mV dec-1) samples. This observation can evidently explicate that the H2 production on the CoSe1.26P1.42 electrode follows the Heyrovsky-Volmer mechanism with a rate-determining step of the electrochemical desorption.30,31 Furthermore, a prolonged test of the CoSe1.26P1.42 electrode was performed at a constant cathodic potential of 92 mV, a current density of ~10 mA cm-2 without noticeable decay could be obtained for more than 15 h (Figure S4). In terms of OER activity to realize a bifunctional electrocatalyst, the P-CoSe2 catalysts were evaluated as illustrated in Figure 2c where the bare CC as well as the commercial RuO2 were also performed as the reference. Among all catalysts, the CoSe1.26P1.42 sample that requires an 5 ACS Paragon Plus Environment


overpotential of 255 mV to reach the current density of 10 mA cm-2 (Figure S3) still exhibits the best OER activity. Such performance is comparable to many leading earthabundant HER and OER catalysts in alkaline electrolyte reported recently (Table S2 and S3). Meanwhile, a Tafel slope of approximately 87 mV dec-1 was found for CoSe1.26P1.42 sample, which was slightly lower than those of the rest P-substituted CoSe2 electrocatalysts (Figure 2d). The chronoamperometric examination presents that the OER current density for the CoSe1.26P1.42 sample almost remained unchanged for 15 hours at least (Figure S5) and further indicates its robust stability. For a bifunctional electrolyzer, a two-electrode system was constructed and showed an remarkably bifunctional performance (Figure S6). Notably, we have to point out a fact that numerous reports commonly explicated the enhanced activities as a consequence of increase in reactive-site population that can be ascribed to a larger value of the electrochemical surface area (ECSA) estimated from the corresponding electrochemical double-layer capacitances (Cdl).5,9,31 The higher Cdl values of the CoSe1.26P1.42 sample for both OER (118.39 mF cm-2) and HER (1.16 F cm-2) can be referred to the largest electrochemically active surface area as well as the reactive sites (Figure S7 and S8). It can be found that both CoSe2.01P0.49 and CoSe1.64P0.54 samples possess a similar ECSA value, but the CoSe1.64P0.54 sample performs rather better OER activity. This suggests that the increase of reactive-site population is not the only factor to result in the OER enhancement among oxidized chalcogenide samples (i.e., pristine CoSe2 and P-substituted CoSe2). Furthermore, to consider the polymorphic CoSe2, the orthodromic CoSe2 (o-CoSe2) was also prepared to serve as control sample. Its electrocatalytic performance toward both HER and OER was greatly inferior to that of c-CoSe2 (Figure S9 and S10). Note that even if the polarization curves were normalized by electrochemical active surface area (Figure S11), the CoSe1.26P1.42 sample can show the best OER/HER activity among all catalysts. It can be concluded that the exceptional HER/OER performance of CoSe1.26P1.42 sample may arise to the effect of phosphorus substitution. As mentioned above, this study aims for revealing what roles the initial electrocatalysts are acting (i.e., active species or pre-catalysts). First, the situation that 6 ACS Paragon Plus Environment

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the sample contacts with the electrolyte has to be considered. Accordingly, a liquidphase transmission electron microscopy (LP-TEM) was carried out, as shown in Figure 3a, the sequential in-situ STEM images of the CoSe1.26P1.42 catalyst were recorded after immersing in the alkaline electrolyte. Initially, the irregular particles possess a smooth surface and the representative HRTEM image (Figure 3b) reveals a clear lattice spacing of 0.36 nm consistent with the value of (011) plane of o-CoSe2. After contacting with the alkaline electrolyte (approximately 60 s), the initial particles remarkably changed with shrinking in size and forming a lot of tiny particles (below 10 nm) around the surface. After contacting with electrolyte for 150 s, the catalyst has transformed into numerous small nanocrystals, but partial catalysts still remain. These small nanocrystals can be confirmed to be the formation of Co(OH)2 around the surface as clarified by corresponding HRTEM image that gives a lattice spacing of 0.28 nm which corresponds to the (010) plane of Co(OH)2 (Figure 3c). A captured video as presented in Movie S1 has recorded the entire transformation for this duration. This phenomenon confirms that the as-prepared CoSe1.26P1.42 catalyst is unstable in the alkaline electrolyte and thereby resulting in a structural transformation into the Co(OH)2 phase on the surface. In contrast, as revealed in Figure S12, the morphology of the c-CoSe2 remains as initial condition even after contacting with the alkaline electrolyte for the same duration (i.e., 150 s). This suggests that the c-CoSe2 is more stable than the CoSe1.26P1.42 sample in alkaline solution while the corresponding video is displayed in Movie S2. Besides, a control experiment without introducing the alkaline electrolyte was also performed on CoSe1.26P1.42 catalyst to exclude the effect caused by electron beam irradiation (Movie S3). These findings have elucidated a paramount fact that P-substituted cobalt chalcogenide is undergoing a remarkably structural transformation within an even shorter duration (< 60 sec) once contacting with alkaline electrolyte, which may be as a consequence of a preferential dissolution of P on CoP2 phase in alkaline electrolyte.15 Guided by all information above, we can conclude that P-substituted CoSe2 catalyst may be acting as the “pre-catalyst” rather than the real reactive species. With the aim of revealing the transformation of these “pre-catalyst” during the bifunctional HER and OER, in-situ X-ray absorption near edge structure (XANES) 7 ACS Paragon Plus Environment


spectroscopy for the as-prepared c-CoSe2 and o-CoSe1.26P1.42 samples were conducted to clarify the role of the presence of phosphorus. In the case of HER, Figure 4a and 4b display the in-situ Co K-edge XANES spectra of the c-CoSe2 and CoSe1.26P1.42 electrodes at the conditions of as-prepared, immersion into the KOH electrolyte (denoted as “KOH”) as well as cathodically increasing potentials. It is found that the white lines for both two catalysts intensify and shift to a higher energy after contacting with alkaline electrolyte, which in turn can be correlated with a slight oxidation of cobalt and be in accordance with the observation from LP-TEM. As compared with the case of the c-CoSe2 catalyst, after progressively increasing the cathodic potential to 0.36 V (vs RHE), the intensity of white lines for the CoSe1.26P1.42 sample have a more pronounced decline and the oxidation states are closer to that of Co foil, indicating that the enhanced HER performance of the CoSe1.26P1.42 catalyst may be ascribed to the near-metallic state of cobalt.21,32 A similar observation has been demonstrated in a reported NiS2 electrocatalyst, the metallic state of nickel was revealed to act as real reactive species.32 In the case of OER, a similar phenomenon can be confirmed that both c-CoSe2 and o-CoSe1.26P1.42 samples, with increasing the anodically applied potential, possess the higher oxidation states evidenced by the increased intensity of white lines and the shift toward higher energy (Figure 4c and 4d). The oxidation states of cobalt cations for the c-CoSe2 and CoSe1.26P1.42 samples at various potentials during the OER process are plotted in Figure S13. It’s worthy to say that the chemical state of cobalt cations in CoSe1.26P1.42 sample is characteristic of an even higher valence than that of Co(3+) at the applied potential of 1.44 V (vs RHE), whereas the c-CoSe2 sample reached a similar chemical state at an even higher potential of approximately 1.54 V. This feature evidently explicates that the Co cations in CoSe1.26P1.42 sample is mostly oxidized to Co(3+) under a rather low potential of 1.44 V as compared to the case of cCoSe2, which can be referred to the onset potential of polarization curves for OER in Figure 2. Regarding the elemental distribution of the pre- and post-catalysts for CoSe1.26P1.42 sample, the corresponding Co 2P XPS spectra, SEM images as well as EDX mappings of the catalysts indicated that P and Se signals remained in the postcatalyst after HER, whereas P and Se dissolved and mostly disappeared in the OER 8 ACS Paragon Plus Environment

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post-catalyst (Figure S14-S16). Note that after OER process, the O signal significantly presented while the O signal slightly increased in the case of HER. This suggests the transformation into cobalt oxyhydroxide (CoOOH) which is acknowledged to be the real active site for OER.33-35 This finding is further confirmed by the ICP-MS and XPS analyses which indicate that P and Se have vanished after OER but remain partially with an amorphous nature in the HER post-catalyst (Figure S17, S18, Table S1 and S4). Consequently, the in-situ XANES can unambiguously conclude that the presence of phosphorus is able to significantly boost the structural transformation of the precatalysts into the real reactive species in both HER (metallic state) and OER (oxidized state) in alkaline electrolyte. To further verify the surface states of the c-CoSe2 and o-CoSe1.26P1.42 electrodes, insitu Raman was employed to clarify the real-time evolution using a special designed setup shown in Figure S19. The spectrum for the as-prepared c-CoSe2 shows a sharp peak at 192 cm-1 (Figure S20a) which is a consequence of the Se-Se stretching mode of cubic CoSe2 phase.36 For the CoSe1.26P1.42 sample, the shift of Se-Se stretching mode (176 cm-1) indicates the cubic-orthorhombic phase transition (Figure 5a),36 while the structural component retains the initial condition in c-CoSe2 electrode during the HER process. The CoSe1.26P1.42 electrode transformed into Co(OH)2 once contacting with alkaline electrolyte according to the additional Raman bands located at 250 and 503 cm-1.37 The declined intensity of the peak at 250 cm-1 means the reduction of Co cations to the metallic state that has no characteristic Raman peak, which is in accordance with the above in-situ XANES result. Note that the mostly transformation into CoOOH (498 and 600 cm-1) was realized at 1.54 V for the c-CoSe2 catalyst in the OER case (Figure S20b), but only a smaller potential of 1.44 V was required to complete the phase transition for the CoSe1.26P1.42 catalyst (Figure 5b), verifying again that the cobalt cation in CoSe1.26P1.42 was more easily oxidized into CoOOH which acted as the active species for most cobalt-based OER catalysts.34,38 The findings of in-situ Raman also concluded that the original Co environment of CoSe1.26P1.42 was easily altered because of the lattice distortion caused by phosphorus substitution. We suggest this phenomenon can be attributed to a disparity in coordination environment of cobalt that 9 ACS Paragon Plus Environment


may arise to the phosphorus substitution. EXAFS curve fitting is employed to extract the coordination environment of absorbing Co as shown in Table 1 and Figure S21. In the as-prepared samples, we can obtain a coordination number (CN) of ~1.5 for Co-P path in the CoSe1.26P1.42 catalyst, while there is no detectable scattering contribution from P in the c-CoSe2. Once contacting with alkaline electrolyte and applying the anodically potential to drive the OER, the CN values for Co-O path, with accompanying the increase of Co-Co(CoOOH) contribution, gradually raise in both cases. Nonetheless, the CN value of Co-Co (CoOOH) path for CoSe1.26P1.42 catalyst (i.e., 4.6) is larger than that of c-Co-Se2 (i.e., 2.5). This difference is related to a larger amount of CoOOH phase in the P-substituted CoSe2 during the OER, which in turn can be correlated with the higher stability of c-CoSe2 in comparison with the P-substituted case. This suggestion is also validated by the CN values of Co-Se path, in which the CN value of Co-Se path is revealed to be ~2.2 for c-CoSe2 while that of P-substituted CoSe2 case is about 0.6 only during OER. It is also worth mentioning that, to interpret the HER situation, the metallic Co (i.e., real reactive species for HER) seems to result from the P induced defects since the CN value of Co-Se path (~3.4) remains unchanged during HER as compared to that of as-prepared condition (~3.7). This result unveils that the stable cubic structure and the presence of selenium may plague its transformation into the active CoOOH for OER or metallic Co for HER. Contrarily, the presence of P around Co cation is suggested to offer more vacancy/defect that is introduced by substituted phosphorus, which allows the Co cations to be easily oxidized or reduced and thereby leading to a facile transformation to CoOOH or metallic Co, respectively (as schematically shown in Figure 6). The c-CoSe2, o-CoSe2, and CoP2 crystal structures theoretically have the same Co-coordination numbers of 6 for Co-Se and Co-P paths. Nevertheless, the asprepared c-CoSe2 and CoSe1.26P1.42 have lower Co-coordination numbers of ~5.2 for Co-Se(P) path than the theoretical value, this lower CN indicates the presence of anionic vacancies that can further induce the cationic defects (Co species with low oxidation states) in the resultant crystal structures. According to all observation above, the electrocatalyst can be described as a fully oxidized structure, and the ECSA normalized activity of all samples and reference Co(OH)2 exhibits a similar intrinsic 10 ACS Paragon Plus Environment

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activity among all samples as illustrated in Figure S22. Accordingly, the superior OER activity in oxidized chalcogenides (i.e., pristine CoSe2 and P-substituted CoSe2) dominantly comes from the large number of active sites per unit area of the electrode surfaces. A similar finding was also observed in a study of fully oxidized Co3C (amorphous CoOx) toward OER.39 In summary, based on all findings above, a fact can be concluded that the stable cubic structure and the presence of selenium may plague its transformation into the reactive species. The introduction of phosphorus can generate more vacancy or defect in the initial CoSe2 and considerably facilitate the structural transformation into the “real reactive species”, such as metallic cobalt (for HER) and cobalt oxyhydroxide (for OER). Most importantly, the reactive species that are really in charge of the target reactions (i.e., HER and OER) are evidently not the initial phases in alkaline electrolyte. Psubstituted CoSe2 electrocatalysts were revealed to be acting the “pre-catalysts” rather than the real reactive species, this may be the paramount concept for further designing the exceptional electrocatalysts. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Experimental Details, Figure S1-S22 and Table S1-S4 are included in the text. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author contributions Y.P.Z. and H.C.C. contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We acknowledge support from the Ministry of Science and Technology, Taiwan (Contracts No. MOST 107-2628-M-002- 015-RSP and MOST 107-2113-M-003-007-), National Taiwan University (NTU-108L880113), and the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. REFERENCES 11 ACS Paragon Plus Environment


(1) Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K.; Uchimura, M.; Paulikas, A.; Stamenkovic, V.; Markovic, N. Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces. Science 2011, 334, 12561260. (2) Suntivich, J.; May, K.; Gasteiger, H.; Goodenough, J.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383-1385. (3) Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nat. Nanotechnol. 2015, 10, 444-452. (4) Hunter, B.; Gray, H.; Müller, A. Earth-Abundant Heterogeneous Water Oxidation Catalysts. Chem. Rev. 2016, 116, 1420-14136. (5) Zhu, Y.; Chen, G.; Xu, X.; Yang, G.; Liu, M.; Shao, Z. Enhancing Electrocatalytic Activity for Hydrogen Evolution by Strongly Coupled Molybdenum Nitride@Nitrogen-Doped Carbon Porous Nano-Octahedrons. ACS Catal. 2017, 7, 3540-3547. (6) Zhu, Y.; Chen, G.; Zhong, Y.; Zhou, W.; Liu, M.; Shao, Z. An Extremely Active and Durable Mo2C/Graphene-Like Carbon Based Electrocatalyst for Hydrogen Evolution Reaction. Mater. Today Energy 2017, 6, 230-237. (7) Zhu, X.; Liu, M.; Liu, Y.; Chen, R.; Nie, Z.; Li, J.; Yao, S. Carbon-Coated Hollow Mesoporous FeP Microcubes: An Efficient and Stable Electrocatalyst or Hydrogen Evolution. J. Mater. Chem. A 2016, 4, 8974-8977. (8) Wu, H.; Lu, X.; Zheng, G.; Ho, G. Topotactic Engineering of Ultrathin 2D Nonlayered Nickel Selenides for Full Water Electrolysis. Adv. Energy Mater. 2018, 8, 1702704. (9) Suen, N.; Hung, S.; Quan, Q.; Zhang, N.; Xu, Y.; Chen, H. Electrocatalysis for The Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337-365. (10)Zhu, Y.; Chen, G.; Zhong, Y.; Chen, Y.; Ma, N.; Zhou, W.; Shao, Z. A SurfaceModified Antiperovskite as An Electrocatalyst for Water Oxidation. Nat. Commun. 2018, 9, 2326. (11)Xu, X.; Chen, Y.; Zhou, W.; Zhu, Z.; Su, C.; Liu, M.; Shao, Z. A Perovskite Electrocatalyst for Efficient Hydrogen Evolution Reaction. Adv. Mater. 2016, 28, 6442-6448. (12)Wang, J.; Tan, C.; Zhu, T.; Ho, G. Topotactic Consolidation of Monocrystalline CoZn Hydroxides for Advanced Oxygen Evolution Electrodes. Angew. Chem. Int. Ed. 2016, 55, 10326-10330. (13)Mabayoje, O.; Shoola, A.; Wygant, B.; Mullins, C. The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as “Pre-catalysts”. ACS Energy Lett. 2016, 1, 195-201. (14)Chen, G.; Hu, Z.; Zhu, Y.; Gu, B.; Zhong, Y.; Lin, H.; Chen, C.; Zhou, W.; Shao, Z. A Universal Strategy to Design Superior Water-Splitting Electrocatalysts Based on Fast In Situ Reconstruction of Amorphous Nanofilm Precursors. Adv. Mater. 2018, 30, 1804333. 12 ACS Paragon Plus Environment

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(15)Zhang, Y.; Gao, L.; Hensen, E.; Hofmann, J. Evaluating the Stability of Co2P Electrocatalysts in the Hydrogen Evolution Reaction for Both Acidic and Alkaline Electrolytes. ACS Energy Lett. 2018, 3, 1360-1365. (16)Wygant, B.; Kawashima, K.; Mullins, C. Catalyst or Precatalyst? The Effect of Oxidation on Transition Metal Carbide, Pnictide, and Chalcogenide Oxygen Evolution Catalysts. ACS Energy Lett. 2018, 3, 2956-2966. (17)Chen, W.; Liu, Y.; Li, Y.; Sun, J.; Qiu, Y.; Liu, C.; Zhou, G.; Cui, Y. In Situ Electrochemically Derived Nanoporous Oxides from Transition Metal Dichalcogenides for Active Oxygen Evolution Catalysts. Nano Lett. 2016, 16, 7588-7596. (18)Jin, S. Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts? ACS Energy Lett. 2017, 2, 1937-1938. (19)Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J.; Gul, S.; Webb, S.; Yachandra, V.; Yano, J.; Jaramillo, T. In Situ X-ray Absorption Spectroscopy Investigation of A Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction. J. Am. Chem. Soc. 2013, 135, 8525-8534. (20)Zheng, X.; Zhang, B.; Luna, P.; Liang, Y.; Comin, R.; Voznyy, O.; Han, L.; Arquer, F.; Liu, M.; Dinh, C.; et al. Theory-Driven Design of High-Valence Metal Sites for Water Oxidation Confirmed Using In Situ Soft X-Ray Absorption. Nat. Chem. 2018, 10, 149-154. (21)Hu, C.; Ma, Q.; Hung, S.; Chen, Z.; Ou, D.; Ren, B.; Chen, H.; Fu, G.; Zheng, N. In Situ Electrochemical Production of Ultrathin Nickel Nanosheets for Hydrogen Evolution Electrocatalysis. Chem 2017, 3, 122-133. (22)Chen, P.; Xu, K.; Tao, S.; Zhou, T.; Tong, Y.; Ding, H.; Zhang, L.; Chu, W.; Wu, C.; Xie, Y. Phase-Transformation Engineering in Cobalt Diselenide Realizing Enhanced Catalytic Activity for Hydrogen Evolution in an Alkaline Medium. Adv. Mater. 2016, 28, 7527-7532. (23)Zhang, H.; Yang, B.; Wu, X.; Li, Z.; Lei, L.; Zhang, X. Polymorphic CoSe2 with Mixed Orthorhombic and Cubic Phases for Highly Efficient Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2015, 7, 1772-1779. (24)Liang, L.; Cheng, H.; Lei, F.; Han, J.; Gao, S.; Wang, C.; Sun, Y.; Qamar, S.; Wei, S.; Xie, Y. Metallic Single-Unit-Cell Orthorhombic Cobalt Diselenide Atomic Layers: Robust Water-Electrolysis Catalysts. Angew. Chem. Int. Ed. 2015, 54, 12004-12008. (25)Zheng, Y.; Wu, P.; Gao, M.; Zhang, X.; Gao, F.; Ju, H.; Wu, R.; Gao, Q.; You, R.; Huang, W. Doping-Induced Structural Phase Transition in Cobalt Diselenide Enables Enhanced Hydrogen Evolution Catalysis. Nat. Commun. 2018, 9, 2533. (26)Seo, B.; Sa, Y.; Woo, J.; Kwon, K.; Park, J.; Shin, T.; Jeong, H.; Joo, S. SizeDependent Activity Trends Combined with in Situ X‑ray Absorption Spectroscopy Reveal Insights into Cobalt Oxide/Carbon Nanotube-Catalyzed Bifunctional Oxygen Electrocatalysis. ACS Catal. 2016, 6, 4347-4355. (27)Fabbri, E.; Nachtegaal, M.; Binninger, T.; Cheng, X.; Kim, B.; Durst, J.; Bozza, F.; Graule, T.; Schaublin, R.; Wiles, L. Dynamic Surface Self-Reconstruction Is The Key of Highly Active Perovskite Nano-Electrocatalysts for Water Splitting. 13 ACS Paragon Plus Environment


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Figure 1. (a) XRD patterns of the as-prepared P-substituted CoSe2 with various amount of phosphorous. (b) SEM and (c) HRTEM images of c-CoSe2. The inset in panel c shows the corresponding SAED pattern. (d) SEM and (e) HRTEM images of CoSe1.26P1.42, and the inset in panel e shows the corresponding SAED pattern.

Figure 2. (a) Electrochemical water-splitting activities of the CoSe1.26P1.42, CoSe0.45P1.18, CoSe1.64P0.54, CoSe2.01P0.49, c-CoSe2, 20% Pt/C, RuO2 and blank CC electrodes in 1M KOH electrolyte: (a) LSV polarization curves for the HER; (b) corresponding Tafel plots for the HER; (c) LSV polarization curves for the OER; (d) corresponding Tafel plots for the OER. 15 ACS Paragon Plus Environment


Figure 3. (a) In-situ STEM images of the CoSe1.26P1.42 catalyst taken at different times after immersing into the KOH solution. HRTEM images of the selected region of (b) initial and (c) final state of the CoSe1.26P1.42.

Figure 4. In-situ Co K-edge XANE spectra of (a) c-CoSe2 and (b) CoSe1.26P1.42 at various potentials for the HER process, (c) c-CoSe2 and (d) CoSe1.26P1.42 at various potentials for the OER process. 16 ACS Paragon Plus Environment

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ACS Energy Letters

Figure 5. In-situ Raman spectra of the CoSe1.26P1.42 catalyst at various potentials for (a) HER and (b) OER process.

Figure 6. Schematic models of P-induced structural transformation in CoSe2 catalyst towards HER/OER in alkaline electrolyte.



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Table 1. Fitting parameters of in-situ Co K-edge EXAFS spectra for c-CoSe2 and CoSe1.26P1.42 catalysts. c-CoSe2 catalyst As-prepared KOH

OER @ 1.44 V HER @ -0.36 V

Path

CN

R

𝚫E

DW

Co-Se

5.2 (2)

2.40 (2)

-2.3 (5)

0.0076 (1)

Co-Se

4.1 (1)

2.42 (2)

-0.1 (5)

0.0051 (1)

Co-O

0.9 (1)

1.80 (3)

-7.5(5)

0.0015 (9)

Co-Se

2.2 (5)

2.44 (1)

0.4 (7)

0.0098 (9)

Co-O

2.4 (4)

1.89 (2)

4.1 (2)

0.0036 (3)

Co-Co(CoOOH)

2.5 (4)

2.81 (1)

2.9 (2)

0.0047 (5)

Co-Se

4.8 (1)

2.41 (2)

-2.6 (5)

0.0067 (1)

CoSe1.26P1.42 catalyst

As-prepared

KOH

OER @ 1.44 V

HER @ -0.36 V

Path

CN

R

𝚫E

DW

Co-Se

3.7 (2)

2.37 (2)

-1.3 (2)

0.0076 (1)

Co-P

1.5 (2)

1.91 (1)

10.1 (2)

0.0096 (2)

Co-Se

3.0 (1)

2.35 (3)

-6.3 (2)

0.0058 (3)

Co-P

0.4 (4)

1.93 (2)

10.8 (5)

0.0059 (9)

Co-O

2.3 (2)

1.80 (1)

-6.1 (5)

0.0085 (7)

Co-Se

0.6 (5)

2.43 (2)

-9.3 (9)

0.0099 (7)

Co-P

0.1 (5)

1.92 (3)

5.0 (5)

0.0095 (9)

Co-O

3.1 (2)

1.88 (2)

2.5 (2)

0.0043 (2)

Co-Co(CoOOH)

4.6 (1)

2.80 (1)

-0.6 (1)

0.0077 (1)

Co-Se

3.4 (4)

2.32 (3)

-3.2 (5)

0.0082 (5)

Co-P

0.1 (6)

1.91 (4)

0.3 (9)

0.0075 (9)

Co-Co(foil)

1.8 (5)

2.42 (3)

-6.5 (4)

0.0097 (7)


Chemical Stability of P-substituted

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