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Dual Tuning of Ni-Co-A (A = P, Se, O) Nanosheets by Anion Substitution and Holey Engineering for Efficient Hydrogen Evolution Zhiwei Fang, Lele Peng, Yumin Qian, Xiao Zhang, Yujun Xie, Judy J. Cha, and Guihua Yu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01548 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018
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Dual Tuning of Ni-Co-A (A = P, Se, O) Nanosheets by Anion Substitution and Holey Engineering for Efficient Hydrogen Evolution Zhiwei Fang,†§ Lele Peng,†§ Yumin Qian,† Xiao Zhang,† Yujun Xie,‡ Judy J. Cha‡ and Guihua Yu†* †
Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States
‡
Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06520, United States Supporting
Information
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KEYWORDS: mixed-transition-metal phosphides, two-dimensional holey nanosheets, metallic, electrocatalyst, hydrogen evolution. ABSTRACT: Seeking earth-abundant electrocatalysts with high efficiency and durability has become the frontier of energy conversion research. Mixed-transition-metal (MTM)-based electrocatalysts, owing to the desirable electrical conductivity, synergistic effect of bimetal atoms and structural stability, have recently emerged as new-generation HER electrocatalysts. However, the correlation between anion species and their intrinsic electrocatalytic properties in MTM-based electrocatalysts is still not well understood. Here we present a novel approach to tuning the anion-dependent electrocatalytic characteristics in MTM-based catalyst for HER, using holey Ni/Co-based phosphides/selenides/oxides (Ni-Co-A, A=P, Se, O), as the model materials. The electrochemical results, combined with the electrical conductivity measurement and DFT calculation reveal that P substitution could modulate the electron configuration, lower the hydrogen adsorption energy and facilitate the desorption of hydrogen on the active sites in Ni-CoA holey nanostructures, resulting in superior HER catalytic activity. Accordingly we fabricate the NCP holey nanosheet electrocatalyst for HER with an ultralow onset overpotential of nearly zero, an overpotential of 58 mV and long-term durability, along with an applied potential of 1.56 V to boost overall water-splitting at 10 mA cm─2, serving as one of the best electrocatalysts among all the reported non-noble-metal catalysts to date. This work not only presents a deeper understanding of the intrinsic HER electrocatalytic properties for MTM-based electrocatalyst with various anion species, but also offers new insights to better design highly efficient and durable water splitting electrocatalysts.
INTRODUCTION Water electrolysis, as an important approach to convert electricity into clean energy stored by molecular hydrogen from unlimited water supply, plays a pivotal role in the development of alternative, carbon-neutral energy technologies.1-3 Even though noble metals (e.g., Pt, Pd) are adopted as the most active electrocatalysts to facilitate hydrogen generation, their prohibitive costs and scarcity greatly restrict the global scalability of electrolysis technology.4, 5 Thus, designing highefficiency and robust electrocatalyst for HER and overall water splitting based on earth-abundant elements has become the frontier of catalysis research. A variety of non-noble metal catalyst systems, such as oxides, sulfides, carbides, and phosphides, have recently emerged as promising materials for newgeneration electrocatalysts, due to their unique d electron configurations, high corrosion resistance, and earth-abundant nature.6-11 Among the precious-metal-free electrocatalysts, dichalcogenides and phosphides have sparked intensive research interests due to their attractive electrocatalytic properties. For instance, the metallic 1T-MoS2 nanosheets exhibited excellent stability in water and satisfactory HER activity, due to the desirable electronic conductivity and excellent hydrophilic
property.7 Similarly, nanostructured Ni2P, with a large accessible surface area and a high density of exposed (001) facets, exhibits excellent activity for catalyzing the HER.12 It is generally believed that anion in dichalcogenides and phosphides act as the base to trap proton during the hydrogen evolution process and modify the electronic structure of electrocatalyst,11, 12 but the active sites for hydrogen adsorption/desorption is still not well understood. Despite the fact that substantial efforts have been devoted to rational design of advanced HER electrocatalysts, revealing the correlation between the anion species in the electrocatalysts and catalytic properties remains a great challenge, and in-depth research into the electronic properties/structures of MTM-based electrocatalysts is still lacking. To fully realize the potential of the transition-metal-based electrocatalysts, strategies such as hetero-metal doping, nanostructure engineering, and surface modification have been proposed.13-15 MTM-based electrocatalysts, thanks to the desirable electrical conductivity, synergistic effect of bimetal atoms and structural stability, have emerged as promising earth-abundant electrocatalysts. For instance, ternary CoMoSx electrocatalyst was reported as a low-cost alternative to noble metal catalysts for efficient hydrogen evolution in both alkaline and acidic media, due to the synergistic effect of
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Figure 1. Scheme of the synthesis of NCO, NCS and NCP holey nanosheets and corresponding STEM images (scale bar: 200 nm, green, blue and purple balls in NCO, NCP and NCS, respectively, represent NCA nanoparticles).
the highly active CoSx units and highly stable MoSx units.13 Density functional theory (DFT) calculation showed that the Co-doped nickel phosphides, compared with pure Ni2P, offered the moderate trapping of atomic hydrogen and facile desorption of the generated H2 due to the H-poisoned active sites.11, 16 On the other hand, two dimensional (2D) nanosheets have shown promise for enhancing the catalytic activities due to the increased surface area for adsorption/desorption process, but the irreversible restacking of nanosheets during the electrode fabrication lead to the decreased electrochemical active area and lengthened electrolyte diffusion pathway.14 To make the best of layered nanostructures in electrocatalysis, porosity engineering serves as an effective strategy to introduce more active sites and continuous mass/charge transport pathway with interconnected open structures and structural stability.17-19 Here we present novel insight into tuning the HER electrocatalytic properties in MTM-based HER electrocatalysts, using Ni/Co-based phosphides/selenides/oxides, Ni-Co-A holey nanosheet system (NCA, A = P, Se, O anion atoms), as the model materials. Extensive electrochemical measurements combined with modeling calculation, suggest that P substitution could modulate the electron configuration, lower the hydrogen adsorption energy and promote the facile desorption of hydrogen on active sites. By means of P substitution and holey engineering, we fabricate the NCP holey nanosheet catalyst with a low overpotential of 58 mV to motivate the hydrogen evolution at 10 mA cm─2 and a low Tafel slope of 57 mV/dec. In addition, NCP holey nanosheets exhibit an ultralow applied potential of 1.56 V at 10 mA cm─2 and long-term durability, serving as one of the best water-splitting electrocatalysts to date. This work not only presents a deeper understanding of the correlation between anion species and intrinsic HER catalytic properties for mixed-metal-based electrocatalyst, but also offers new insights to better design earth-abundant water splitting electrocatalysts with high efficiency and durability.
RESULTS AND DISCUSSIONS The general synthesis of MTM-based phosphides/selenides 2D holey nanosheets, through a one-step phase transformation process from the holey oxide precursor, is schematically shown in Figure 1. NCO (NiCo2O4) holey nanosheets are first prepared via a template-directed approach based on graphene oxide (GO) nanosheets followed by a controlled calcination, according to the previously reported method.15, 17, 20 GO sheets act as flexible 2D substrates for uniform anchoring of metal ions (pink and green spheres represent Co2+ and Ni2+). After thermal treatment in air, 2D holey oxides composed of the interconnected nanoparticles (8-12 nm) were synthesized and meanwhile decomposition of rGO was confirmed by scanning transmission electron microscopy (STEM, inset i of Figure 1) and Raman spectroscopy (Figure S1). The selenization process can be accomplished through the reaction with Na2SeO3 in the reductive environment (See Experimental Section, Supporting Information, SI). Unlike layered dichalcogenides, the preparation of metal phosphide nanosheets, due to the relatively strong bonds between the layers, is still a great challenge.21 Here for the first time, we report the NiCoP nanosheets with interconnected holey structure converted from holey Ni/Comixed oxide precursor via the PH3-plasma chemical vapor deposition. According to the microscopic images, holey nanostructure in both NCS (NiCo2Se4) and NCP (inset ⅱ, ⅱ of Figure 1, Figure S2) can be maintained after phase transformation. The lateral size of nanosheets is between 2-3 µm and the thickness is around 20 nm, made up of interconnected nanoparticles (about 15 nm) with no obvious aggregation, and the average diameter of holes is about 15 nm (Figure S2, S3). Notably, this route can be generally applied to other metal selenides (such as Co3Se4, NiSe, etc.), and phosphide (including CoP, Ni2P), and the holey nanoarchitecture and interconnected nanoparticles could be retained from holey oxide precursor (Figure S4, S5, S6).
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Figure 2. a) XRD of a series of Ni/Co-based holey nanosheet samples. b) HRTEM and SA-ED (inset) of NCP holey nanosheets. c) Elemental mapping of NCP holey nanosheets. d) Ni 2p3/2 spectrum, e) Co 2p3/2 spectrum, and f) P 2p spectrum of NCP holey nanosheets. Scale bar: 10 nm in b, 10 nm−1 in the inset of b, 200 nm in c.
The phase purity of the series of NCA is first characterized by the X-ray diffraction (XRD, Figure 2a). The peak positions of the XRD patterns of the as-prepared NCA holey nanosheets match well with those of standard PDF card of NiCoP, NiCo2Se4 and NiCo2O4. High-resolution transmission electron microscopy (HR-TEM) image of NCP holey nanosheets, provided in Figure 2b, shows the clear lattice fringes of 0.22 nm, 0.19 nm and 0.17 nm correspond well to the (111), (210) and (300) facets of the hexagonal NCP, respectively, which is consistent with the XRD results. While the selected area electron diffraction (SAED) pattern (Figure 2b inset) shows that a diffuse set of concentric rings indicate the formation of hexagonal NCP with the polycrystalline structure. The interparticle connection between the adjacent NCP nanoparticles can also be clearly observed in Figure S8. These chemically interconnected structures between the adjacent nanoparticles can boost the electron transfer through the entire nanosheet structure and promote the mechanical stability during the electrocatalysis. Energy-dispersive X-ray spectroscopy (EDX) analysis (Figure 2c) of NCP holey nanosheets confirms the uniform distribution of Ni, Co, P in NCP, and the complete formation of bimetallic phosphide compounds. X-ray photoelectron spectroscopy (XPS) tests (Figure 2d, e, f, Figure S10) are further conducted to confirm the composition of NCA holey nanosheets. In holey NCP, the binding energy at 853.2 eV is assigned to Ni-P bonding,22 which is close to that of metallic Ni (852.6 eV), suggesting the presence of partially charged Ni species (Niδ+, δ is close to 0). The peak at 857.0 eV can be attributed to Ni-POx with its shake-up satellite peak at 861.3 eV. Similarly, the binding energy at 778.6 eV in Co 2p3/2 spectra is related to Co-P bonding, which is also close to that of metallic Co (778.2 eV). For P 2p region, the peak at 129.1eV corresponds to the formation of metal phosphides, slightly lower than that of elemental P (130.0 eV), which suggests the P atom is partially negatively charged (Pδ-). In this case, the metal cation can act as a water(hydroxide)acceptor center, and the P atoms act as the proton-acceptor center during the HER process in alkaline media. Moreover,
peak at 132.9 eV is assigned to the phosphate species (PO43-, etc.), possibly due to the partial oxidation after air exposure according to previous studies.22 Similarly, Ni-Se and Co-Se bonding can be confirmed in NCS, but the intensity is relatively lower than the one in NCP (Figure S10). The HER performance of a series of NCA holey nanosheets as electrocatalysts is first measured on a three-electrode configuration in 1.0 M KOH electrolyte, as described in the Experimental Method (SI). The 95% IR-corrected polarization curves (See Figure S11 for more details), measured on the Ni foam (Figure 3a) and Glassy carbon electrode (GCE, Figure S12) as the electrodes respectively, demonstrate the normalized current density versus voltage (j versus V) for holey NCP, NCS and NCO nanosheets along with the reference, bare electrode and commercial 20% Pt/C, for comparison. Phosphide sample shows significantly enhanced HER properties with a low onset overpotential (nearly zero at 1 mA cm─2), and 58 mV to achieve 10 mA cm─2 (Figure 3a). In comparison, selenide exhibits inferior electrocatalytic activity with an overpotential of 150 mV, and oxide shows poor HER performance with an overpotential over 260 mV. As a reference, hydrogen evolution using Ni foam requires an overpotential of over 330 mV to achieve 10 mA cm─2. To better understand the electrocatalytic activity of the NCA holey nanosheets, Tafel slopes for all catalysts are calculated in Figure S13. The Tafel slope of NCP holey nanosheets is 57 mV/dec, markedly smaller than that of NCS (122 mV/dec) and NCO holey nanosheets (127 mV/dec). The intrinsic activity of the NCA holey nanosheets is further studied by the normalized HER properties and turnover frequency (TOF). It is believed that the current density normalized by the electrochemical active surface area (ECSA, Figure S14) reflect the intrinsic catalytic property of the catalyst, unlike the one normalized by the geometric surface area.23 Among this series of Ni/Co-based electrocatalysts, phosphide samples exhibit the highest normalized catalytic property (Figure S15) and the largest value of TOFs of 0.732 s−1 (at an overpotential of 200 mV, Table S1), which is much larger than that of corresponding selenide and oxide sample. The
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Figure 3. a) HER performance of a series of NCA holey nanosheets and bare Ni foam in 1.0 M KOH aqueous solution. b) Temperaturedependent electrical resistance of NCP, NCS, NCO holey nanosheets. c) Atomic structure of the hydrogen-covered surfaces and d) the corresponding energy diagram for hydrogen adsorption on the crystal face of NCO (111), NCS (010) and NCP (001). (Pink and green spheres represent Co2+ and Ni2+, brown, red and blue sphere in represent O, Se and P).
results clearly reveal that the kinetics of hydrogen evolution on Ni/Co-based electrocatalysts is strongly dependent on the anionic species in the NCA. To understand the HER activity trend for this series of NCA holey nanosheets, electrical properties and hydrogen adsorption/desorption modeling are systematically studied. Based on the temperature-dependent electrical resistance curves from 150K to 300 K, the electrical resistance of NCP and NCS keep increasing as temperature increases, displaying the metalliclike behavior and enhanced electron delocalization (Figure 3b), which is consistent with the density of states for NCP and NCS in recent papers.11, 15 Conversely, the electrical resistance of NCO decreases as the temperature increases with about six orders of magnitude higher than that of corresponding phosphide and selenide at room temperature. Generally, moderate electron delocalization is expected to be beneficial for the HER process. Electron delocalization can lower the charge transfer resistance between catalysts and current collector.15 On the other hand, too strong electron delocalization is likely to decrease the electron density on anion species (the smaller value of δ in Xδ−), leading to the less affinity for proton.24, 25 The electrical results indicate that phosphorus/selenium substitution of oxygen can lead to the enhanced electron delocalization, which can also be confirmed by the peaks of Niδ+, Coδ+ and Xδ− (Pδ−, Seδ−) in XPS spectrum (Figure S10). Moreover, the electrical conductivity is relatively higher for selenide, which indicates the greater electron delocalization, slowing the kinetics of proton trapping. The result is consistent with the relatively lower intensity of peaks of Niδ+ and Coδ+ in XPS spectra. Therefore, moderate electron delocalization in NCP could facilitate the charge transfer and also reduce the barrier of proton binding on Pδ−.
The HER process in an alkaline media, at the atomic level, can be demonstrated from the initial catalyst-water (hydroxyl ion) state, to the catalyst-H intermediate state, and the final catalyst-H2 state.13 To further gain mechanistic aspects into the HER on the surface of the bimetallic compound, the catalytic pathways on NCA catalysts for HER are further investigated by using DFT method to calculate the hydrogen adsorption enthalpy of catalysts. To study the effect of the anion on H adsorption energy in bimetallic compounds, (001) facet of NCP, (010) facet of NCS and (111) facet of NCO is chosen as the surface model, due to their high catalytic properties based on the previous papers (Figure 3c, S16).11, 15, 16, 26 The first principles calculations based on the DFT method are performed within spin-polarized generalized gradient approximation. As shown in Figure 3d, bimetallic phosphide holey nanosheets possess the smallest hydrogen adsorption enthalpy (absolute value, Table S2) than that of NCS and NCO at almost all sites. For NCO, hydrogen would be first adsorbed on oxygen site (O-Hads), due to the largest adsorption. However, the bond strength between O and H is too high, indicating the energy of O-Hads intermedia is too low, leading to the increased reluctance shown toward desorption of Hads from the oxygen site. This result is consistent with the large O-H bonding energy (463 kJ/mol, compared with P-H, 322 kJ/mol and Se−H, 276 kJ/mol) and the largest electronegativity of oxygen (3.5), compared with Se (2.6) and P (2.2). Similarly, due to the relatively large electronegativity of selenium, the adsorption of hydrogen is stronger than that of phosphide. Based on the calculation result of hydrogen adsorption in NCP, hydrogen atom tends to be first adsorbed on Co three-fold hollow site with phosphorus atom beneath (site 3, P-hollow site), due to its largest adsorption energy among all possible sites. After all Ph sites covered with hydrogen, the rest of sites (site 1, 2, and 4 stands for P, Co, NH, respectively) would start to adsorb pro-
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Figure 4. a) Polarization curves and b) Tafel plots of NCP holey nanosheets, c-NCP (control nanosheets without holes) and bare Ni foam (NF) in 1.0 M KOH aqueous solution. c) Polarization curves of a series of phosphide holey nanosheets in 1.0 M KOH solution. d) HER performance of NCP holey nanosheets in acidic (0.5 M H2SO4), neutral (1.0 M PBS, pH = 7.4) and alkaline (1.0 M KOH) ─ media. e) Chronopotentiometric measurement of NCP holey nanosheets and NCP nanosheets at current density of 10 mA cm 2 in 1.0 M KOH solution. f) Cycling stability of NCP holey nanosheets in 1.0 M KOH solution.
ton. Considering the moderate absorption energy of atomic hydrogen, desorption of on Co, NH, P sites is significantly easier than the active sites in comparative selenides and oxides. The above calculated results indicate NCP is more favorable for facile desorbing hydrogen and thus facilitating the HER kinetics, which is in good agreement with the experimental results. To sum up, our studies suggest the general design protocol for HER electrocatalysis: moderate electron delocalization, abundant active sites (both water-acceptor and proton-acceptor center), and lowest H adsorption energy (for facile hydrogen absorption/desorption). The anion substitution in the holey NCA nanosheet electrocatalysts can achieve the above goals for enhanced HER electrocatalysis. Based on the above understanding, the HER performances of NCP holey nanosheets and control samples (c-NCP, nanosheets without holes) are investigated to study the underlying mechanisms of NCP electrocatalyst. NCP control sample requires an additional overpotential of 130 mV to reach a same current density of holey NCP, shown in Figure 4a. The difference can be ascribed to the introduction of holey nanostructure into the nanosheets. The unique holey structure, due to the higher surface area and open structure, has a higher wettability than control sample, which could enhance the electrolyte penetration and augment the contact degree between reactants and active sites, and thus facilitate the HER kinetics. Tafel slopes for all catalysts are also investigated, as shown in Figure 4b. The Tafel slope of NCP holey nanosheets is 57 mV/dec, much smaller than that of NCP control sample (109 mV/dec), which indicates the enhanced kinetics in holey structures. To study the synergistic effect of bimetal atoms, a series of phosphide holey nanosheets, including CoP, Ni2P and NiCoP, are also tested for HER in 1.0 KOH solution (Figure 4c). Compared with binary Ni2P, NiCoP holey nanosheets reveals significantly increased HER performance after the introduction of heteroatom Co. Binary CoP and Ni2P exhibit inferior electrocatalytic activity with the overpotential of 82 mV and 99 mV, respectively (Figure S17). The results can be
ascribed to the enhanced hydrogen adsorption energy of bimetallic compound and greater electron delocalization.11, 15 Remarkably, these 2D NCP holey nanosheets could serve as the HER electrocatalyst over a wide range of pH 0−14. The catalytic activity towards HER is investigated in acid, neutral, alkaline media, as shown in Figure 4d. Elemental Ni, Co and their corresponding oxides are known as a chemically unstable metal in acidic media. After P substitution, NiCoP only needs an overpotential of 80 mV in 0.5 M H2SO4 solution, thanks to the enhanced chemical stability of phosphides in acid solution. Compared with commonly adopted acidic and alkaline solutions for water splitting, the neutral solution serves as a more environmentally benign media with practical perspectives, but the applications in neutral media still suffer from the low current density and slow kinetics. The mixed metal phosphide holey nanosheet electrocatalyst can obtain the current density of 10 mA cm−2 at a cell voltage as low as 170 mV, with a low onset overpotential of 50mV, in 1.0 M phosphate buffered saline (PBS). Moreover, stability is another essential indicator to evaluate the performance of electrocatalysts. Chronopotentiometric measurement and long-term cycling test in 1.0 M KOH are conducted to investigate the stability property of NCP holey nanosheet electrocatalyst. Figure 4e demonstrate that NCP holey nanosheets could retain < 90 mV over 24 h, which is as stable as the NCP nanosheets without hole structures. Additionally, after the long-term cycling test of over 2000 cycling, no obvious degradation is observed for NCP holey nanosheets (Figure 4f). After the assessment of their exceptional HER performance, we further evaluated the OER performance of NCP holey nanosheets in 1.0 M KOH solution. Recently, Ni/Co-based oxygen-evolving catalysts have shown desirable electrocatalytic activity for alkaline OER after the electrochemically anodic condition (AC) treatment, during which involves the formation of amorphous oxide layer on the surface of electrocatalyst.27-29 After 20 cycles of activation between 0-0.8 V (vs
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electron configuration and two-dimensional holey nanoarchitecture. First, compared with selenide and oxide, phosphide displays the lowest hydrogen adsorption energy, and facile desorption of hydrogen on the abundant active sites. Second, the metallic nature of phosphide and the synergistic effect of Ni/Co atoms favor the electron transfer kinetics between the catalyst and current collector, and the moderate electron delocalization reduces the barrier of proton binding, thus favoring the HER kinetics. Third, holey nano-architecture could enhance the electrolyte diffusion, and augment the contact degree between reactants and active sites. This unique structure enables facilitated electrolyte penetration and facile release of evolved H2 and O2 bubbles. Within the 2D holey nanosheets, chemically interconnected nanoparticles could not only facilitate electron transfer, but also enhance the structural stability of electrocatalyst. Thanks to these advantageous features, NCP holey nanosheets demonstrate to be one of the best electrocatalysts with exceptional electrocatalytic performance for hydrogen generation and overall water splitting among the reported Ni/Co-based dichalcogenides (Table S3, S4, S5).
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Figure 5. a) OER performance of NCP and NCO holey nanosheets on Ni foam in 1.0 M KOH aqueous solution. b) LSV curve of NCP holey nanosheets for overall water splitting in a two-electrode configuration. c) Stability test of water splitting in 1.0M KOH for NCP holey nanosheets.
Ag/AgCl), the electrochemically oxidized-NCP holey nanosheets require an overpotential of only 280 mV for OER (Figure 5a, S18), which is 80 mV lower than that of NCO precursor, and even better than traditional RuO2 electrocatalyst (310 mV). To investigate the origin of excellent performance for holey NCP nanosheets, ex-situ XPS is carried out to analyze the surface chemical composition and electronic states, as shown in Figure S19. After the OER cycling, the P 2p peaks disappear, which agrees well with previous reports.29, 30 This phenomenon might be due to the electrochemical oxidation of P into phosphates during the OER and their further dissolution in the electrolyte. Similarly, the main peaks in Co 2p3/2 and Ni 2p3/2 can be attributed to CoOOH and NiOOH species formed after the OER cycling.31 This amorphous oxide layer formed after AC treatment not only protects the NCP phase from further oxidation (Figure S20) but offers abundant active sites for OER process.15, 31 The enhanced OER performance suggests that NCP holey nanosheets can serve as a potential bifunctional catalyst for overall water splitting. To further explore its suitability in the overall water splitting, an alkaline electrolyzer based on the NiCoP holy nanosheets as both anode and cathode catalyst is assembled for both HER and OER applied on Ni foam substrates. Figure 5b shows the 95% polarization curve of this electrolyzer in a two-electrode system. A current density of 10 mA cm−2 is obtained at around 1.56 V, which represents a combined overpotential of only 340 mV for OER and HER, while using Ni foam required a combined overpotential of 640 mV, for comparison. The potential is stable at 1.56 V without obvious degradation during a 16 h galvanostatic electrolysis when employing the electrolyzer of NCP/NCP holey nanosheets couple (Figure S21). Remarkably, as demonstrated in Figure 5c, this electrolyzer maintains the current density of 20 mA cm−2 at a cell voltage as low as 1.63 V, and 50 mA cm−2 at 1.78 V over 6 hours, respectively. Thus, NCP holey nanosheets enabled a high-performing overall water-splitting with excellent bifunctional electrocatalytic activity and stability. The superior electrocatalytic performance of MTM phosphide holey nanosheets can be attributed to the synergistic effects of the lowest hydrogen adsorption energy, desirable
CONCLUSION To conclude, we present a novel approach to tuning the anion-dependent electrocatalytic characteristics in the MTMbased HER electrocatalyst, using holey NCA holey nanosheet as the model materials. Extensive electrochemical characterization, combined with electrical test and DFT calculation, suggest that P substitution can modulate the electronic configuration and lower the hydrogen adsorption energy on active sites. Based on the above understanding, we fabricate the NCP holey nanosheet catalyst with a low overpotential of 58 mV at 10 mA cm─2 and a low Tafel slope of 57 mV/dec to catalyze HER. In addition, NCP holey nanosheets exhibit an ultralow applied potential of 1.56 V at 10 mA cm─2 and long-term durability, serving as one of the best water-splitting electrocatalysts to date. This work not only presents a deeper understanding of the HER electrocatalytic properties for MTM-based electrocatalysts with various anion species, but also offers new insights to better design highly efficient, durable and earthabundant water splitting electrocatalysts.
ASSOCIATED CONTENT Supporting Information. Experimental and computational details, additional XRD patterns, STEM images, EDX spectrum, XPS spectrum, electrical transport test, electrochemical characterizations and calculation results. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *
[email protected] (G.Y.)
Author Contributions §
Z.F. and L.P. contributed equally to this work.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT We thank Prof. J.B. Goodenough and Prof. J. Zhou at the University of Texas at Austin for valuable discussions and some instru-
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Journal of the American Chemical Society mental support. G.Y. acknowledges the funding support from the Welch Foundation Award F-1861, Sloan Research Fellowship, and Camille Dreyfus Teacher-Scholar Award.
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