NiCoMo Hydroxide Nanosheet Arrays Synthesized via Chloride

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NiCoMo Hydroxide Nanosheet Arrays Synthesized via Chloride Corrosion for Overall Water Splitting Shaoyun Hao, Luchuan Chen, Chunlin Yu, Bin Yang, Zhongjian Li, Yang Hou, Lecheng Lei, and Xingwang Zhang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00333 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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NiCoMo Hydroxide Nanosheet Arrays Synthesized via Chloride Corrosion for Overall Water Splitting Shaoyun Hao+, Luchuan Chen+, Chunlin Yu, Bin Yang, Zhongjian Li, Yang Hou, Lecheng Lei, Xingwang Zhang* Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang Province 310027, China

*E-mail:[email protected]

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ABSTRACT: Designing the non-precious electrocatalysts with multiple active sites and prolonged durability in an integrated electrolyte towards water splitting is momentous for renewable energies being reserved in chemical fuels. Herein, we developed a method for synthesizing multimetallic hydroxide nanosheets by corroding the nickel foam with chloride ions, which enabled the screening and discovery of various multimetallic hydroxide electrocatalysts towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).

We

discovered

that

Ni5Co3Mo-OH

nanosheets

exhibited

electrocatalytic

performances towards OER (ƞ100 = 304  mV) and HER (ƞ10 = 52  mV). Moreover, Ni5Co3Mo-OH can be employed as acive bifunctional catalysts towards overall water splitting with a low cell voltage of 1.43 V at 10 mA·cm-2 (1.60 V at 100 mA·cm-2) and stable operation for 100 h (100 mA·cm-2). This work provides a method to develop the multimetallic hydroxides for electrocatalysis and energy conversion. TOC GRAPHICS

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Electrochemical

water

splitting

is

viewed

as

a

prospective

technology

for

environmental-friendly energy usage and storage.1-5 The noble catalysts including Pt/C, IrO2, and RuO2 have exhibited excellent performance towards water splitting until now.6,

7

However, the scarcity of the noble catalysts severely hampered their wide and practical application. Accordingly, the non-precious metal electrocatalysts, including transition metal phosphide,8-11 oxides,12-14 hydroxides,1,

15-18

alloy,19-21 and perovskite oxides22 that could

efficiently catalyze oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) have emerged as the prospective alternatives. Nevertheless, preparation of a two electrode system and application in an integrated electrolyser is still challenging due to the fact that most of electrocatalysts cannot stay most active and stable in the mismatching pH ranges.23, 24 Additionally, different electrocatalyts employed towards OER and HER need diverse preparation equipments and processes, raising the cost.23 Hence, rationally designing the bifunctional catalysts with exceptional activities towards OER and HER applied in an integrated alkaline solution is still in great demand.25 Among various electrocatalysts based on transition metals, the Ni-based hydroxides are the promising candidates towards water splitting due to their high active surface area and charge-transfer capability, endowing them high electrocatalytic performance.26 Recently, many reports revealed that a third metal (Cr, V, Ce, W) inserted into the Ni-based bimetallic (oxy)hydroxide forming the trimetallic materials can further boost their electrocatalytic performance,1, 15, 27-29 which should be attributed to the fact that the doped metal will incorporate into the lattices of the hybrid and interplay with other metal to modulate their local coordination environment and electronic structures. For 3

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example, Wang et al. designed the trimetallic Ni3Fe0.5V0.5 (oxy)hydroxide, which significantly enhanced the activity towards OER due to vanadium doping into NiFe-based hydroxide as well as the synergistic effects among Ni, V, and Fe cations.26 Despite much progress has been achieved, discovery of novel bifunctional metal hydroxides towards overall water splitting with stable operation and low overpotentials in the alkaline media are still requisite. On the other hand, the common approaches to synthesizing metal hydroxide nanosheets are co-precipitation,30 hydrothermal synthesis,26 ion exchange method,31 reconstruction method,32 and corrosion by Fe3+ ions.33 However, most of the approaches to synthesizing metal hydroxide nanosheets are still limited to finite types of multimetallic hydroxides34due to the fact that those methods are greatly affected by temperature, anion ions (SO42-, CO32-, NO32-), and pH (8-10).35 Particularly, the emerging layered double hydroxides (LDH) must contain trivalent and bivalent metal cations with strict atomic ratio,36 which seriously restricted regulation of composition of multimetallic hydroxides. Nevertheless, increasing the chemical complexity of hydroxides is one of the most effective approaches to exploring the novel electrocatalysts, since the multimetallic hydroxides endow the promise of the electrocatalyts with tunable properties and features that could exceed monometallic hydroxides.37 Up to now, there remains lack of a general and facile approach to preparing multimetallic hydroxides with flexible compositions. Consequently, developing a novel method for preparation of multimetallic hydroxides to obtain efficient electrocatalysts is highly desired.

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It is well-known that metal corrosion is an electrochemical process, which is the total reaction that occurs on the surface of metal with generating an electrolyte layer.38 Chloride ion is one of the most important inducers of local corrosion for metal because chloride ions can be effortlessly adsorbed on the surface of metal and damage the passivation film.39 Being hygroscopic, chloride species could accelerate the electrochemical reaction and promote formation of electrolytes.40 Therefore, the metal substrate will be corroded under moisture film with reduction of O2 from water or air in the cathodic site. Meanwhile, the formed hydroxide ions (OH-) shifted to anodic areas generating metal hydroxide.41 As far as we know, despite decades of efforts and research, most of the researchers have been committing themselves to protect metal against chloride corrosion, the research that converts chloride ions corrosion to a favorable side for fabrication of novel nanomaterials has rarely been reported. Herein, we take inspiration from chloride corrosion of metal to demonstrate a general and facile route for development and discovery of the novel multimetallic hydroxides for OER and HER catalysis. As a proof concept, a novel bifunctional electrocatalyst Ni5Co3Mo-OH discovered via this method exhibited excellent performances towards water splitting: an overpotential of 304  mV (100 mA·cm-2) for OER,  52  mV (10 mA·cm-2) for HER, and a cell voltage of 1.43 V (10 mA·cm-2) towards overall water splitting. Synthesizing multimetallic hydroxide nanosheet arrays supported on NF by fully utilizing high concentration of chloride ions (Cl-) to corrode NF in a one-step route was illustrated in Figure 1. It is well known that the Cl- ions can be effortlessly adsorbed to metal surfaces due 5

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to the strong penetrating ability and depassivation performance.42 The conductivity of the aqueous solutions turned stronger with increasing the concentration of Cl-. Moreover, the higher concentration of Cl- (e.g. 0.5 M) would be in favor of the Cl- ions caught on the surface of metal, and expedites the local corroding process. The function of the Cl- ions and oxygen were investigated in detail (as illustrated in SI, Figure S1-S3), suggesting the metal hydroxides could not be formed without them. Thus, the mechanism for fabricating multimetallic hydroxide nanosheets supported on NF can be proposed as follows:43, 44 Firstly, the NF was corroded to Ni2+ ions in a high concentration of Cl- solution occurring in the anode region. Then, the chloride ions continuously migrated and enriched to the anode region. Simultaneously, the oxygen from the air or water reacted with water to form hydroxide (OH-) on the cathode. Finally, the Mx+ cations and the diffusion of Ni2+ synchronously reacted with OH- to precipitate and form multimetallic hydroxide nanosheet arrays supporting on NF (Figure 1).

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Figure 1. (a) The schematic diagram for formation of multimetallic hydroxide nanosheets. (b) The reaction mechanism for preparation of Ni5Co3Mo-OH and its application towards OER and HER. Interestingly, various multimetallic hydroxide nanosheets can be easily obtained via the Clcorrosion reaction (Figure 2 and Figure S4), indicating the Cl- corrosion is a general and facile method. The crystalline nature of hydroxide nanosheets was analyzed via X-ray diffraction (XRD) (Figure S5). The synthesized monometallic (Ni(OH)2) and bimetallic (e.g. NiCo-OH) hydroxides exhibited no characteristic peak, indicating their amorphous nature. Additionally, the multimetallic (e.g. NiCoMo-OH) hydroxides exhibited a widely weak peak around 25°, revealing the low crystallinity of these multimetallic hydroxide nanosheets. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was respectively applied to observe the morphology, distribution of each element, and composition of Ni-M-OH nanosheet arrays growing on NF, respectively (Figure 2 and Figure S4-S9). It was seen that each element in the monometallic and multimetallic hydroxide nanosheets was homogeneously distributed, revealling that the formation of multimetallic hydroxides.

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Figure 2. The SEM, HAADF images, and TEM elemental maps of monometallic and multimetallic hydroxide nanosheets. To reveal the chemical valence and bonding states in these metal hydroxides, the typical Ni-M-OH hydroxides (NiCo-OH, NiMo-OH and Ni5Co3Mo-OH) were characterized with X-ray photoelectron spectroscopy (XPS) (Figure 3 and Figure S10a). For Ni 2p spectrum of NiCo-OH, NiMo-OH, and Ni5Co3Mo-OH, 2p3/2 and 2p1/2 peaks situate at 855.6 and 873.2 eV (Figure 3a). The other two characteristic peaks of 861.6 and 879.6 eV should be ascribed to Ni-O.45 Therefore, the intense satellite peaks demonstrated that most of Ni elements in the obtained nanosheets lay Ni2+ oxidation state. With respect to the Co 2p (Figure 3b), it illustrated two different doublets situated at Co 2p3/2 (a lower energy band, 781.1 eV) and Co 2p1/2 (a higher energy band, 796.8 eV).46 Meanwhile, the gap between Co 2p3/2 and Co 2p1/2 was more than 15 eV, implying that the oxidation state of Co2+ and Co3+.47 The peak 8

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appeared at 775 eV suggests that the Co species in Ni5Co3Mo-OH existed more than coordination mode, which revealed Co cations possessed the higher affinity in an octahedral structure featuring with inverse spinel character, indicating that Mo cations might substitute for partial Co cations in trimetallic Ni5Co3Mo-OH. Besides that, three satellite peaks were also observed (786.1 eV, 791.2 eV, and 803.3 eV) (Figure 3b). On the other hand, the spectra of Mo 3d include two energy bands locating at 231.7 and 234.9 eV, revealing the coexisting Mo4+ 3d5/2 and Mo6+ 3d3/2, respectively (Figure 3c).48 For O 1s spectra of NiCo-OH, NiMo-OH, and Ni5Co3Mo-OH, the peaks at 531 eV are the typical metal-oxygen bonds (Figure 3d).49 It is also worthy note that the binding energies of Mo 3d and O 1s for Ni5Co3Mo-OH moved to lower binding energies, which are in comparison with the bimetallic NiMo-OH hydroxide. This observation implies that the partial electron transfers from the Ni to Co or Mo in Ni5Co3Mo-OH via the oxygen bridges among the metal ions.26

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Figure 3. The XPS spectra for (a) Ni 2p. (b) Co 2p. (c) Mo 3d. (d) O 1s of NiCo-OH, NiMo-OH, and Ni5Co3Mo-OH. In order to identify the structural benefits of the trimetallic NiCoMo-OH nanosheets, the electrochemical performance of Ni(OH)2, NiCo-OH (typically Ni5Co3-OH), NiMo-OH (typically Ni5Mo-OH), and NiCoMo-OH towards OER was characterized by the linear scan voltammetry (LSV) in a solution of KOH (1 M) employing a three-electrode system with the scan rate of 3 mV·s-1. All the obtained potentials were directly calibrated into reversible hydrogen electrode (RHE) and performed with iR correction. Firstly, we investigated the performances of NiCoMo-OH samples with different Ni:Co:Mo ratios supported on NF towards OER and HER. The performance of Ni5Co3Mo-OH for OER was higher than that of other NiCoMo-based hydroxides (Figure S11-S12). Thus, Ni5Co3Mo-OH was chosen as the 10

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example towards OER. As shown in Figure 4a, the electron-transfer process around 1.35 V should be the redox reaction by Ni(II)/Ni(III or IV). Notably, the Ni5Co3Mo-OH exhibited the overpotentials about 304 mV (280 mV) at 100 mA·cm-2 (50 mA·cm-2), which compared with NiMo-OH (329, 312 mV), NiCo-OH (376, 343 mV), Ni(OH)2 (487, 443 mV), confirming the remarkable activity of Ni5Co3Mo-OH for OER. Moreover, the performance of Ni5Co3Mo-OH was also compared with the hydroxide/oxide OER catalysts (Table S1). The Ni5Co3Mo-OH exhibited the lowest Tafel slope of 56 mV·dec-1, compared with NiMo-OH (64 mV·dec-1), NiCo-OH (113 mV·dec-1), Ni(OH)2 (140 mV·dec-1), suggesting the doping Mo significantly boosted the kinetic of NiCo-OH for OER (Figure 4b). Meanwhile, the high electrocatalytic performance of Ni5Co3Mo-OH for OER was also confirmed by its smallest Rct (Figure 4c). Additionally, Ni5Co3Mo-OH also revealed the highest electrochemical active surface area (ECSA), which can be estimated by double layered capacitances (Cdl), proving that the fabricated Ni5Co3Mo-OH possessed the highest active surface area (Figure 4d, Figure S14). More significantly, Ni5Co3Mo-OH remained stable at 100 mA·cm-1 for 100 h, proving its high stability (Figure S13a).

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Figure 4. The electrochemical activities of Ni(OH)2, NiCo-OH, NiMo-OH, and Ni5Co3Mo-OH towards OER. (a) LSV (3 mV·s-1) of these multimetallic hydroxides towards OER. (b) Tafel plots inferred from (a). (c) Nyquist plots towards OER. (d) Cdl measurements in OER for these electrodes. Similarly, to better prove their utility and understand the influence of chemical compositions of Ni-M-OH, the fabricated Ni(OH)2, NiCo-OH, and Ni5Co3Mo-OH electrodes were also applied in HER (Figure 5). Notably, the Ni5Co3Mo-OH nanosheets revealed the highest electrochemical activity towards HER with an overpotential (ηHER) about 52 mV to attain 10 mA·cm-2 (η10 for Ni(OH)2, NiCo-OH, NiMo-OH, and Pt/C were 150 mV, 86 mV, 72 mV, and 44 mV, respectively), as displayed in Figure 5a. The electrochemical activity of Ni5Co3Mo-OH for HER was adjacent with Pt/C. Simultaneously, the gap of η10 between 12

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Ni5Co3Mo-OH and Pt/C was only 8 mV. Additionally, Ni5Co3Mo-OH was more active than the hydroxide-based electrocatalysts reported to data (Table S2). The value of Tafel slope for Ni5Co3Mo-OH was 59 mV·dec-1 (Figure 5b). Compared with NiMo-OH (74 mV·dec-1), NiCo-OH (78 mV·dec-1), and Ni(OH)2 (119 mV·dec-1), it is implied the electrochemical activity towards HER occurred on the electrode by the Volmer−Heyrovsky mechanism with a faster kinetic rate50. Nyquist plots were used to confirm Rct (charge transfer resistance) of these multimetallic hydroxide nanosheets (Figure 5c). As displayed in Figure 5c, Ni5Co3Mo-OH exhibited the smallest Rct among these nanosheet arrays due to the Mo doping in NiCo-OH, facilitating the charge transfer and enhancing more active sites. Besides that, the high activity of Ni5Co3Mo-OH for HER was also confirmed by ECSA (Figure S14), which was assessed by Cdl. The Cdl of Ni5Co3Mo-OH (27.7 mF·cm-2, 692.5 cm2) was higher than that of NiCo-OH (23.5 mF·cm-2, 587.5 cm2), NiMo-OH (24.3 mF·cm-2, 607.5 cm2), and Ni(OH)2 (17.7 mF·cm-2, 442.5 cm2), suggesting that Ni5Co3Mo-OH featured with high exposure of active sites for HER (Figure 5d). As shown in Figure S13b, the working electrode of Ni5Co3Mo-OH was operated by chronoamperometric test about 100 h at 100 mA·cm-2 without obvious degradation. To identify the structural or compositional changes, Ni5Co3Mo-OH was characterized with SEM, XRD, and TEM after OER and HER tests (Figure S15-S17), confirming the stability of the multimetallic hydroxides.

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Figure 5. The electrochemical activities of Ni(OH)2, NiCo-OH, NiMo-OH, and Ni5Co3Mo-OH towards HER. (a) LSV (3 mV·s-1) of these multimetallic hydroxides towards HER. (b) Tafel plots inferred from (a). (c) Nyquist plots towards HER. (d) Cdl measurements in HER for these electrodes. Since the high activities of Ni5Co3Mo-OH towards OER and HER, the prepared Ni5Co3Mo-OH electrodes were respectively applied as the anode and cathode towards water splitting in KOH (1 M) using a two-electrode system. The polarization curve of Ni5Co3Mo-OH reveals that it can reach 1.43 V at 10 mA·cm-2, while the fabricated NiMo-OH and NiCo-OH electrodes need to increase to 1.50 V and 1.60 V (Figure 6a). Impressively, the performance of Ni5Co3Mo-OH towards overall water splitting was much higher than Pt/C and IrO2/C (1 mg∙cm-2) supported on NF (Figure 6a). The performance (e.g. 1.60 V at 100 14

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mA·cm-2) of Ni5Co3Mo-OH towards overall water splitting was superior to the other hydroxide-based electrocatalysts reported presently (Table S3). As is known to all, stability for catalysts is a crucial parameter to evaluate catalysts towards water splitting in an integrated electrolyser. As revealed in Figure 6b, the Ni5Co3Mo-OH was stable more than 100 h at 10 and 100 mA·cm-2 towards overall water splitting, respectively. The mass-loading of Ni5Co3Mo-OH determined by weighing the powder stripped with ultrasonication from NF was 0.2 mg·cm-2.

Figure 6. (a) Polarization curves of NiCo-OH, NiMo-OH, Ni5Co3Mo-OH, and Pt/C||IrO2/C towards overall water splitting. (b) Chronopotentiometry of Ni5Co3Mo-OH at constant 10 and 100 mA·cm-2. As previous reported, the NiCo-based (oxy)hydroxide and MoOx have been viewed as active sites towards water splitting.48, 51-53 Thus, the high performance of Ni5Co3Mo-OH for water splitting might be due to the fact that the trimetallic Ni5Co3Mo-OH was synthesized in a one-step route with the Mo element effectively doping into metal hydroxide. Consequently, the coordination environment and electronic interaction among Ni, Co, and Mo may interplay with other more strongly. Although the synergistic effect among them could not be directly 15

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verified, according to the analysis of XPS (Figure 3), the peaks of Mo and O also shifted and their densities were lowered significantly due to strong electronic structure among the Ni, Co, and Mo. The electronic interaction was benefit to the activity of electrocatalyst towards overall water splitting.19 Additionally, the higher active surface area of multimetallic hydroxide nanosheet arrays also contributed to the superior electrochemical performance towards water splitting (Figure 4d and Figure 5d). In summary, we demonstrated a novel one-step method for preparation of multimetallic hydroxide nanosheets via making full use of chloride ions to corrode the nickel foam, the facility of which enabled the screening and fabrication of multimetallic hydroxides for OER and HER catalysis. We discovered that trimetallic Ni5Co3Mo-OH nanosheets showed high electrocatalytic performances towards OER (ƞ100 = 304  mV) and HER (ƞ10 = 52  mV). More significantly, the trimetallic Ni5Co3Mo-OH electrodes were applied as a cathode and an anode, achieving 1.43 V (10 mA·cm-2) with a sustainable stability more than 100 h (100 mA·cm-2), ranking the best as compared to other bifunctional catalysts. Simultaneously, the high performance of Ni5Co3Mo-OH grew on NF towards water splitting was related with the interacted influence among the elements (Ni, Mo, and Co) and large active surface area. Multimetallic hydroxide electrocatalysts fabricated by this facile method possess the promise in the practical application for energy conversion. ASSOCIATED CONTENT Supporting Information. The detailed experimental methods were illustrated in SI. SEM images of NiCo-OH fabricated under different conditions. XRD patterns of the multimetallic 16

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hydroxide nanosheets grew on NF. Additionally, the XPS, TEM, and electrochemical data of some multimetallic hydroxide nanosheets were presented in SI. Meanwhile, the information in SI is also available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Xingwang Zhang *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by Zhejiang Provincial Natural Science Foundation of China under Grant (No. LR17B060003). The work was also financially supported by the Natural Science Foundation of China (Project No. 21776248). We are also very grateful to Mrs. Na Zheng from the State Key Laboratory of Chemical Engineering in Zhejiang University for giving the convenience of taking SEM. REFERENCES (1) Zhang B.; Zheng X.; Voznyy O.; Comin R.; Bajdich M.; García-Melchor M.; Han L.; Xu J.; Liu M.; Zheng L.; García de Arquer F. P.; Dinh C. T.; Fan F.; Yuan M.; Yassitepe E.; Chen N.; Regier T.; Liu P.; Li Y.; De Luna P.; Janmohamed A.; Xin H. L.; Yang H.; Vojvodic A., Sargent E. H., Homogeneously Dispersed Multimetal Oxygen-Evolving Catalysts. Science 2016, 352 (6283), 333-337. 17

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(2) Seh Z. W.; Kibsgaard J.; Dickens C. F.; Chorkendorff I.; Nørskov J. K., Jaramillo T. F., Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355 (6321), eaad4998. (3) Duan H.; Li D.; Tang Y.; He Y.; Ji S.; Wang R.; Lv H.; Lopes P. P.; Paulikas A. P.; Li H.; Mao S. X.; Wang C.; Markovic N. M.; Li J.; Stamenkovic V. R., Li Y., High-Performance Rh2P Electrocatalyst for Efficient Water Splitting. J. Am. Chem. Soc. 2017, 139 (15), 5494-5502. (4) Cabán-Acevedo M.; Stone M. L.; Schmidt J. R.; Thomas J. G.; Ding Q.; Chang H. C.; Tsai M. L.; He J. H., Jin S., Efficient Hydrogen Evolution Catalysis Using Ternary Pyrite-Type Cobalt Phosphosulphide. Nat. Mater. 2015, 14, 1245. (5) Zheng Y. R.; Wu P.; Gao M. R.; Zhang X. L.; Gao F. Y.; Ju H. X.; Wu R.; Gao Q.; You R.; Huang W. X.; Liu S. J.; Hu S. W.; Zhu J.; Li Z., Yu S. H., Doping-Induced Structural Phase Transition in Cobalt Diselenide Enables Enhanced Hydrogen Evolution Catalysis. Nat. Commun. 2018, 9 (1), 2533. (6) Seitz, L. C.; Dickens, C. F.; Nishio, K.; Hikita, Y.; Montoya, J.; Doyle, A.; Kirk, C.; Vojvodic, A.; Hwang, H. Y.; Norskov, J. K.; Jaramillo, T. F., A Highly Active and Stable IrOx/SrIrO3 Catalyst for the Oxygen Evolution Reaction. Science 2016, 353 (6303), 1011-1014. (7) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y., Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3 (3), 399-404. 18

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