Cooperation between holey graphene and NiMo alloy for hydrogen

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Cooperation between holey graphene and NiMo alloy for hydrogen evolution in an acidic electrolyte Yoshikazu Ito, Tatsuhiko Ohto, Daisuke Hojo, Mitsuru Wakisaka, Yuki Nagata, Linghan Chen, Kailong Hu, Masahiko Izumi, Jun-ichi Fujita, and Tadafumi Adschiri ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04091 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 18, 2018

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Cooperation between holey graphene and NiMo alloy for hydrogen evolution in an acidic electrolyte Yoshikazu Ito†#*, Tatsuhiko Ohto‡*, Daisuke Hojo§, Mitsuru Wakisaka#∥, Yuki Nagata⊥, Linghan Chen∇, Kailong Hu†, Masahiko Izumi†, Jun-ichi Fujita†, Tadafumi Adschiri§∇¶ †

Institute of Applied Physics, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-

8573, Japan. #

PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan.

‡

Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka 560-8531, Japan.

§

New Industrial Creation Hatchery Center, Tohoku University, Sendai 980-8577, Japan.

∥

Graduate School of Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama 939-0398,

Japan. ⊥

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.

∇

WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.

¶

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan.

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ABSTRACT

The development of noble-metal-free hydrogen-evolution-reaction (HER) materials for electrochemical water splitting is the key to achieving low-cost and efficient electrocatalysis that drives electrochemical hydrogen evolution. However, the electrocatalytic activities of most nonnoble metals decrease in acidic electrolytes. Here, we fabricate non-noble-metal electrodes using a bicontinuous and open porous NiMo alloy covered by nitrogen-doped (N-doped) graphene with nanometer-sized holes. This noble-metal-free HER catalyst exhibits almost identical performance to that of a Pt/C electrode, while preserving its original catalytic activity even in acidic electrolytes. Density functional theory calculations indicate that the interfacial fringes between the nanoholes and NiMo surface induce charge transfer and promote hydrogen adsorption and desorption. The nanometer-sized holes simultaneously provide minimal surface area for chemical reactions, while delaying NiMo dissolution in excessive amounts of acidic electrolyte. Our method for the fabrication of the NiMo alloy provides a route to a promising class of electrochemical hydrogen-producing electrodes.

Keywords: NiMo alloy, non-noble metal, holey graphene, porous graphene, hydrogen evolution, acidic electrolyte, water splitting


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Main text Hydrogen, as a renewable energy source, is becoming increasingly important for realizing sustainable societies. Hydrogen gas, however, is currently produced from fossil fuels by energy reforming methods, such as steam reforming above 900 °C,1 with the emission of by-product gases. Clearly, hydrogen gas produced through this route is not environmentally friendly and is therefore far from being a “green” source of energy. In contrast, the electrolysis of water is more efficient; when driven by renewable energy it provides more environmentally friendly energy storage solutions.2,3 To realize energy-efficient and inexpensive electrodes, the overpotential associated with hydrogen evolution needs to be reduced. The performance of non-noble-metal (Ni, Fe, Co, Mo, etc.) electrodes in alkaline electrolytes is close that of Pt metal.4,5 Conversely, most electrodes that work in acidic electrolytes require the use of expensive noble-metal-based (Pt, Pd, Ru, etc.) materials, which increases the total cost of hydrogen production. Since the electrolysis of water in acidic electrolytes consumes less electricity than the analogous process in alkaline electrolytes, acidic electrolytes are more suitable for achieving high energy efficiency. The development of non-noble-metal-based electrodes that can be applied to the electrolysis of water in acidic electrolytes is therefore on the horizon.4,5 Non-noble metals are reported to have lower overpotentials than Pt metal for the electrochemical hydrogen evolution reaction (HER) in acidic electrolytes.4,5 Several methods have been proposed to ensure that these non-noble-metal electrodes are compatible with acidic electrolytes. Some of the successful methods use chemically stable metal carbides, nitrides, sulfides, or phosphides, such as Mo2C,6 NiMoN,7,8 MoS2,9 MoP,10 and Ni2P,11 while others use chemically stable sheets or tubes, such as graphene or carbon nanotubes, to enwrap the nonnoble metal/nanoparticles. In this encapsulation method, the carbon materials protect the non-


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noble metal/nanoparticles. This strategy has been employed in energy harvesting devices such as fuel cells, Li ion batteries, and water electrolysis.12-16 Their excellent performance arises from the large surface areas of the graphene or carbon nanotubes, high mechanical flexibilities, excellent electrical conductivities, and tunable catalytic activities.17,18 However, the graphene layers that cover the non-noble-metal surfaces limit the original catalytic activities of the non-noble metal. Here, a challenge is the further improvement of the catalytic properties of graphene- or carbonnanotube-protected non-noble-metal electrodes in acidic electrolytes. In this study, we systematically investigated the hydrogen-evolution performance of a porous NiMo cathode with its surface covered by holey porous graphene layers, in an acidic electrolyte, and compared it to that of a porous NiMo cathode fully covered with graphene layers. We generated multiple holes in the graphene layers during their growth by tuning the number of catalytically inert SiO2 nanoparticles on the NiMo surface (Figure 1). We found that NiMo covered by graphene with nanometer-sized holes exhibits significantly improved HER performance and lifetime in an acidic electrolyte, compared to the porous NiMo cathode fully covered with graphene layers. Density functional theory (DFT) calculations suggest that the fringes (hole region) between the graphene nanoholes and the NiMo surface enhances hydrogen adsorption and desorption, while the non-holey graphene layer (non-hole region) prevents excess impregnation and corrosion of the non-noble metal by the acidic electrolyte (see Figure 1). Our results reveal that the combination of a non-noble metal with holey graphene is a promising design route for noble-metal-free catalysts that are chemically stable in acidic electrolytes.

Results and discussion


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NiMo composites covered with holey graphene were synthesized by reducing NiMoO4 nanofibers19 following chemical vapor deposition (CVD); the NiMoO4 nanofibers were also used as CVD templates (Figure 1). In comparison with solid-template-based (dealloyed nanoporous Ni) CVD approaches,20-23 our approach provides homogeneously mixed precursors of 300-nmdiameter NiMoO4 nanofibers decorated with 18–27 nm SiO2 nanoparticles (Figure 1). Firstly, NiMoO4 nanofibers were prepared by a hydrothermal synthesis.24,25 The NiMoO4 nanofibers (Figure S1) were then mixed with SiO2 nanoparticles and drop-cast onto a Cu sheet (substrate) after which they were annealed at 950 °C for 20 min under a mixed atmosphere of argon and hydrogen. During this process, the following sequential transformations occurred: (1) the reduction of NiMoO4, (2) the reconstruction of the reduced NiMoO4 ligaments into porous NiMo structures, and (3) the segregation of SiO2 nanoparticles on the porous NiMo structures, most likely driven by a mismatch between NiMo and SiO2 in the alloy. The resulting threedimensional (3D) porous NiMo structure, with a bicontinuous and open porosity, and segregated SiO2 nanoparticles on its surface (Figure 2a) were continuously employed as CVD templates for graphene growth. These templates were annealed at 750 °C to ensure the uniform growth of Ndoped graphene on the surface of the NiMo ligaments under a mixed atmosphere of hydrogen and argon, while using pyridine as the source of both carbon and nitrogen. The holes expected in the N-doped graphene on the NiMo porous structure are illustrated in Figure 1. Controlling the amount of catalytically inert SiO2 nanoparticles on the surface enables the hole sizes to be tuned. The holey graphene on the NiMo surface are expected to provide two functions: to minimize the surface area for desirable chemical reaction in the holes, and to delay NiMo dissolution in the acidic electrolyte by avoiding corrosion with graphene covering (Figure 1).


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The porous NiMo alloy and the NiMo composites covered with N-doped graphene and SiO2 nanoparticles show bicontinuous and open porous morphologies, with ligament radii of 250–500 nm and nearly identical pore sizes, by scanning electron microscopy (SEM) (Figure 2a and S2). We confirmed that the SiO2 nanoparticles do not influence the morphology and are successfully segregated on the porous NiMo surface in the cross-sectional SEM image (Figure 2 and

S3). The average specific surface areas were determined by the nitrogen-

adsorption/desorption method and Brunauer–Emmett–Teller (BET) theory;26 a BET surface area of 2.81 m2/g was determined for the pristine porous NiMo, while values of 2.43 and 2.36 m2/g were obtained for the NiMo composite covered with N-doped graphene and low levels (10−5 wt%) of SiO2 nanoparticles, and for the NiMo composite covered with N-doped graphene with high levels (10−4 wt%) of SiO2 nanoparticles, respectively (Figure S4). Scanning transmission electron microscopy (STEM) confirmed the presence of single SiO2 nanoparticles on the porous NiMo composite covered by graphene layers (10−4 wt% SiO2 nanoparticles). We observed that these segregated single SiO2 nanoparticles in the 6–15 graphene layers on the NiMo surface successfully blocked further graphene growth at the interface (Figure 2b). Moreover, no interspace between the graphene layers and the NiMo surface was observed, which protects against chemical etching in acidic electrolytes. The SiO2-nanoparticle level (from 10−5 to 10−1 wt%) was also adjusted in order to control hole size (Figures 2 and S5) during hole-size optimizations, and 6–16 nm SiO2 nanoparticles were employed for this purpose; some graphene layers were filled with the smaller SiO2 nanoparticles (Figure S6). The N-doped holey porous graphenes were examined by STEM following etching of the porous NiMo. The low-magnification dark-field (DF) STEM image of the holey porous graphene created using high levels of SiO2 nanoparticles (10−4 wt%) shows the presence of holes with micrometer diameters on the graphene surface


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(Figure 2c and Figure S7). The inset in Figure 2c reveals sharp diffraction spots in the corresponding selected-area electron diffraction pattern of the multi-layer graphene, verifying its high crystallinity. The high-magnification DF-STEM image of the holey porous graphene created by low levels of SiO2 nanoparticles (10−5 wt%) shows 10–15 nm holes on the graphene surface (Figure 2d). Moreover, STEM-EDS elemental analyses reveal that the Ni and Mo are homogenously distributed in the porous NiMo composites bearing the SiO2 nanoparticles (Figure S8), which benefit catalytic reactions. As such, the amount of SiO2 nanoparticles segregated on the homogenously distributed NiMo alloy surface plays an important role in determining hole size and number during the creation of catalytic reaction fields. The structures of the NiMo composites covered by holey porous N-doped graphene were characterized by X-ray diffraction (XRD) and Raman spectroscopy. The XRD spectrum of the pristine porous NiMo displays peaks that mainly correspond to NiMo (JCPDS 48-1745); it also shows the presence low amounts of Ni4Mo (JCPDS 65-5480) and pure Mo (JCPDS 04-0809) (Figure 3a). The XRD spectra of the NiMo composites covered by holey porous N-doped graphene exhibit peaks that correspond mainly to NiMo, while some peaks are related to Mo2C (JCPDS 65-8766) and Ni4Mo;27-31 small amount of NiO (JCPDS 71-1179), MoO2 (JCPDS 724534), and MoO3 (JCPDS 74-7911) were detected in both samples (Figure S9). The Raman spectra reveal that the intensity ratio (ID/IG) of the D and G bands of the holey N-doped porous graphene was 1.22 after the NiMo had been dissolved; this intensity ratio is larger than that of the fully covered N-doped graphene (ID/IG = 0.50) (Figure 3b and Table S1), suggesting that the SiO2 nanoparticles induce structural defects, such as fringes and edges. These defects are created during the formation of the holes on the graphene (Figure S10). Indeed, the NiMo composite created using the high level of SiO2 nanoparticles shows the largest ID/IG intensity ratio (1.39),


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which indicates that more fringes and edges were created with increasing levels of SiO2 nanoparticles. Although the holey N-doped graphene contains defects that are evident from the intense Raman D band, it retains the high crystallinity of the graphene structure (I2D/IG = 0.98– 1.2), consistent with the observed electron diffraction patterns. As such, the levels of SiO2 nanoparticles used during fabrication could be used to tailor the graphene hole size without affecting the NiMo structure, providing a route for exploring the relationship between the effect of the nanoholes and the catalytic performance of the non-noble-metal electrodes. The binding states and quantitative chemical compositions of the NiMo composites were investigated by X-ray photoelectron spectroscopy (XPS) (Figure 3c–f). For the holey graphenecovered NiMo composite bearing the nanoholes, the Mo 3d XPS spectrum reveals Mo(0) (227.6 eV, 230.8 eV) with small amounts of the Mo oxide (233.6 eV, 235.5 eV) and Mo2C byproduct (228.5 eV), similar to that reported for Mo2C systems.32,33 The Mo(0) peak position was shifted to lower values than that of Mo metal (277.8–228.0 eV).34-36 The Ni 2p XPS spectrum exhibits Ni(0) (852.9 eV, 870.1 eV) and Ni(II) peaks (856.1 eV, 875.7 eV, Ni(OH)2 by moisture) without any obvious Ni oxides. The position of the Ni(0) (852.9 eV) peak was shifted to a higher value that those of Ni metal peaks (852.4–852.7 eV),37-39 which is due to charge transfer and interactions between the Ni and Mo on the graphene layers. Considering the XRD and XPS results, we conclude that small amounts of MoO2, MoO3, and NiO exist inside the NiMo ligaments, rather than on their surfaces. The C 1s XPS spectrum displays high-quality graphene peaks and a small peak corresponding to Mo2C (283.6 eV).32 These XPS results are in good agreement with the XRD data, revealing that Mo, Ni, and Mo2C co-exist at very low concentrations on the surface. Moreover, the N 1s XPS spectrum reveals different N-doping configurations of graphitic, pyridinic, and oxide structures,40,41 without the formation of any


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nitrides. Since the short CVD time does not significantly affect carbide and nitride formation, which can contribute to HER activity, the NiMo character is expected to be well preserved in the non-graphene growth regions where the SiO2 nanoparticles are intact. The HER performance of the holey porous N-doped graphene-covered NiMo composites were tested and compared with a fully graphene-covered NiMo composite (referred to as “NiMoFG”) and a commercial Pt/C catalyst. The iR drops for all samples were corrected by the ohmic resistance measured at an electrode potential of +200 mV (vs. RHE). Figure 4a displays hydrodynamic voltammograms of the electrochemical HER at NiMo cathodes tested in an acidic 0.5 M H2SO4 electrolyte. The porous NiMo electrode, and both holey graphene-covered-NiMo electrodes bearing nanoholes (abbreviated to “NiMo-NHG”) and micrometer-scale holes (abbreviated to “NiMo-MHG”), each loaded at 5 mg, exhibit high HER activities compared to that of NiMo-FG. Overpotentials, η, at a current density of 10 mA/cm2, were determined to be 22 mV for porous NiMo, 30 mV for NiMo-NHG, 40 mV for NiMo-MHG, and 114 mV for NiMoFG. Moreover, the approximate turnover frequencies (TOFs)42,43 at electrode potentials of −100 mV (vs. RHE) were determined. NiMo-NHG (1.1 H2/s at 69 mA/cm2) exhibited a higher TOF than NiMo-MHG (0.8 H2/s at 53 mA/cm2), and an almost identical TOF to that of the porous NiMo (1.3 H2/s at 85 mA/cm2). These values are higher than those reported for Ni- and Mobased materials in 0.5 M H2SO4 solution, as summarized in Table S2–S3. To exclude the influence of the Mo2C, the Mo2C-rich NiMo-FG (in which over 99% of the Mo had been converted to Mo2C near the surface, as shown in Figure S11–S12) and the Mo2C-rich porous graphene following dissolution of the NiMo from the Mo2C-rich NiMo-FG (Figure S13) were similarly tested (Figure S14), see further discussion in the Supporting Information. Whereas the Mo2C-rich NiMo-FG exhibited HER performance between that of NiMo-FG and NiMo-MHG,


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the Mo2C porous graphene exhibited much poorer performance than the Mo2C-rich NiMo-FG. Hence, the contribution of Mo2C to the HER is not very high when compared with the contribution of NiMo. The nitrogen-doping effects were also examined by comparing the Ndoped NiMo-NHG, holey N-doped porous graphene devoid of NiMo, and NiMo-NHG devoid of N-doping (Figure S14). Whereas the holey N-doped porous graphene devoid of NiMo did not exhibit any HER activity, the NiMo-NHG devoid of N-doping produced a 36% lower current density than the N-doped NiMo-NHG. Therefore, the combination of N-doped nanoholes and the NiMo surface contributed to the observed enhancement in HER performance. Furthermore, electrochemical impedance spectroscopy was employed to investigate the transport performance of the NiMo samples for the HER (Figure S15). The half circles in the Nyquist plots of these samples were fitted to evaluate their charge-transfer resistances (Rct) at an electrode potential of −200 mV (vs. RHE), assuming an equivalent circuit consisting of a parallel combination of an Rct and an electrochemical double-layer capacitance (Cdl). Rct values of 3.8, 5.1, 5.4, and 219 Ω were determined for the porous NiMo, NiMo-NHG, NiMo-MHG, and NiMo-FG, respectively. Although the graphene covering clearly increases the Rct, both holey graphene samples exhibit lower charge-transfer resistances that significantly benefit their HER reaction kinetics and catalytic activities, which are close to those of porous NiMo. To understand the reaction mechanism, the Tafel plots, exchange current densities, and double layer capacitances (Cdl) of the NiMo samples were investigated. The Tafel plots exhibit slopes of 37–49 mV/dec (Figure 4b), which suggest that the HERs occur via mixed Volmer and Heyrovsky mechanisms; i.e., both H3O+ discharging and the electrochemical desorption of Hads to form molecular hydrogen determine the HER rate.12,44 Moreover, the exchange current densities obtained from the Tafel equation are 1.0–2.8 mA/cm2. A higher exchange current


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density was observed for NiMo-NHG compared to NiMo-FG, which suggests that charge transfer is promoted on the NiMo surface near the activated fringes of the nanoholes. The electrochemical surface area (ECSA), which is almost proportional to Cdl, can be qualitatively compared by evaluating the Cdl values of the NiMo samples on the basis of cyclic voltammetry (CV) at various sweep rates (Figure 4c and S16). The measured Cdl values are 22.0 mF/cm2 for the porous NiMo, 15.4 mF/cm2 for NiMo-NHG, 12.8 mF/cm2 for NiMo-MHG, and 5.5 mF/cm2 for the NiMo-FG. The graphene covering clearly decreases the ECSA in spite of the similar BET surface areas; however the ECSA of NiMo-NHG is significantly greater than those of NiMoMHG and NiMo-FG. Considering the differences in the wettabilities of NiMo and graphene, the ECSA of NiMo-NHG should be lower than that of NiMo-MHG. The observed discrepancy arises from: (1) the total size of the hole area (large NiMo surface area) and/or (2) the hydrophilic character of fringes that contain many defective structures. As the amount of SiO2 nanoparticles on the NiMo-MHG sample depicted in Figure S5 (loading amount: 10-4 wt%, larger hole area) is higher than that of the NiMo-NHG shown in Figure 2 (loading amount: 10-5 wt%, smaller hole area), we consider that the origin of the ECSA enhancement is mainly due to the hydrophilic character of the fringes, which also contribute to improved HER performance. The cycling stabilities of the NiMo samples were examined in 0.5 M aqueous H2SO4 (Figure 4d). The HER activity of the porous NiMo was significantly lower after durability testing, and became even lower than that of NiMo-FG. This remarkable loss of HER activity is explained by the dissolution of NiMo during the cycles of testing in the acidic electrolyte. In contrast, NiMo-FG exhibited very stable performance over 2000 cycles, even though its intrinsic HER activity was the lowest among the samples, indicating that the reaction kinetics and catalytic ability are dominated by the graphene layer. When the effects of the nanoholes and the


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micrometer-scale holes are compared, NiMo-NHG maintained 68% of its current density after 1000 cycles, whereas porous NiMo and NiMo-MHG retained only 2.5% and 39%, respectively (Figure S17). The chronoamperometric durability of NiMo-NHG is evidenced by retention of the −20 mA/cm2 current density for ~130 hours, without any decrease in HER activity (Figure S18). Hence, the holey graphene covering successfully improves the performance of the nonnoble metal in the acidic electrolyte. The relationship between the HER activity and structural changes were investigated by SEM and TEM (Figure S19 and S20) in order to shed light on the degradation mechanism. The SEM images provide evidence that NiMo-FG retained its original structure after cycling, while NiMo-NHG and NiMo-MHG exhibited partial NiMo dissolution. It is noteworthy that NiMo dissolution in NiMo-NHG was significantly suppressed compared to that of NiMo-MHG. The TEM images reveal the NiMo below the graphene layers gradually dissolved over cycles 1–500, and the NiMo surface finally lost contact with the graphene layer and the fringes near the holes. Moreover, XRD, leaching tests, Raman spectroscopy, and XPS provided further confirmation after 1000 cycles (Figure 3b, S21, and S22, and Table S1). XRD and XPS following HER testing reveal the existence of NiMo and Ni4Mo structures with generated Ni(OH)2 and oxidized Mo species on their surfaces. We consider that the main structures were well preserved and that other regions near the holes were involved in surface redox process during HER testing, resulting in the leaching of NiMo (5–10 at% weight loss). On the other hand, NiMo-NHG exhibited the same graphene characteristics as as-prepared graphene. Considering the similar D- and G-band intensity ratios before and after HER testing (ID/IG: 1.22 to 1.26) and the small concentrations of oxidized carbon and nitrogen species, the original character of the N-doped graphene layer is well preserved. This means that the graphene layers act as supporters and, more importantly, that additional holes were not created and the graphene


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sheets did not break; consequently, we conclude that the degradation mechanism involves the decrease in contact area between NiMo and the graphene fringes, rather than structural changes in the graphene layer; consequently the degree of dissolution is controlled only by the pristine holes. Gibbs free energy profiles were calculated by DFT in order to understand the underlying mechanism involving the graphene covering on these non-noble metal electrodes. A structural model of NiMo(100) and graphene, with and without a ~1 nm hole on the NiMo(100) (Figure S23), was simply constructed using a 1.8 × 1.9 nm surface unit cell. The overall HER mechanism in an acidic electrolyte has been reported to involve three stages: (1) an initial stage involving H+ + e-; (2) an intermediate stage involving H* adsorption; and (3) a final stage that generates 1/2H2.45-47 It is well-known that highly efficient catalysts have Gibbs free energies for H* adsorption, |∆GH*|, close to zero; ∆GH* for Pt (∆GH*Pt) is approximately −0.08 eV. Figure 4e reveals that the ∆GH* values of a pure graphene layer (+0.80 eV) and NiMo(100) fully covered with graphene (+0.78 eV) are too positive for effective H* adsorption, while and ∆GH* value of Ni(111) (−0.55 eV) is too negative; consequently, excellent HER performance cannot be expected from these catalysts. On the other hand, the ∆GH* of NiMo(100) (∆GH*NiMo = −0.11 eV) is close to that of the Pt catalyst, consistent with reported experimental data.4,5 Moreover, a model of a graphene layer with ~1 nm holes distributed over the entire surface of NiMo(100) exhibits a ∆GH* of −0.19 eV calculated in the nanohole-region of the NiMo(100); ∆GH*NiMo-hole is improved compared with the ∆GH* value of NiMo(100) fully covered with graphene, MoS2 (−0.28~+0.13 eV),48−49 and Mo2C (−0.82 eV)50. The higher value of |∆GH*NiMo-hole| compared to |∆GH*NiMo| is attributed to the influence of the graphene covering, and indicates that the catalytically active nanoholes are important for enhancing HER. The ∆GH* of each catalytically


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active site, and hole-size and hydrogen-adsorption-site dependence of ∆GH*, were examined; the H* adsorption sites were found to be the NiMo surface and the graphene-hole fringes, rather than the graphene itself (Figure S23–S32). The charge distribution of the model graphene layer with a ~1 nm hole positioned on the NiMo(100) surface was investigated in order to understand the effect of the hole (Figure 4f). The interface is negatively charged (−0.2 to −0.4 e) at the graphene fringe and positively charged (+0.2 to +0.6 e) at the NiMo surface adjacent to the fringe. We propose that the positively and negatively charged fringe regions induced by the defective structures create additional ECSA-containing catalytic sites (Figure 4) on the NiMoNHG that tune the electronic densities of state (Figure S28) in order to enhance electrical conductivity and charge transfer, which is consistent with the high exchange current density (Table S2) exhibited by the nanohole sample, resulting in the formation of additional active sites for H* adsorption and desorption during electrochemical hydrogen production. Therefore, the interplay of the holey graphene fringe and the NiMo surface leads to high HER activity, and is associated with the electronic structures induced at the interface of the graphene hole. The high HER performance and enhanced chemical stability in the acidic electrolyte apparently arises from the holey graphene covering of the non-noble-metal surface. The DFTcalculated Gibbs free energy and charge population suggest that the graphene-hole fringe in NiMo-NHG improves charge transfer compared to porous NiMo and NiMo-FG, thereby benefitting H* adsorption and desorption and promoting the HER processes. Moreover, the nanosized fringe-rich holey sample has an enhanced ECSA. Indeed, the Cdl value for NiMoNHG is enhanced compared to that for NiMo-MHG or NiMo-FG; hence the TOF of the NiMoNHG catalyst (1.1 H2/s) is very close to that of the porous NiMo (non-graphene) (1.3 H2/s). Moreover, the combination of the nanosized holes and NiMo surface plays an important role in


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HER activity, and is experimentally supported by the lower performance of NiMo-FG devoid of holes and holey N-doped graphene devoid of a NiMo surface. Furthermore, the conductive and bicontinuous porous networks possess many open pore channels that facilitate mass and electron transport, resulting in low Tafel slopes. Consequently, the holey N-doped graphene on the porous NiMo composite creates additional electrochemically active sites that are induced by the fringes adjacent to the nanoholes, while maintaining high HER performance and enhancing chemical stability against acidic electrolytes. Conclusion We successfully synthesized a NiMo composite with its surface covered by nanohole-sized holey N-doped graphene and systematically investigated the interplay between the non-noble metal and the graphene covering in an acidic electrolyte. The nanohole fringes of the holey graphene enhance electrochemical hydrogen evolution as well as chemical stability compared with NiMo devoid of the graphene covering. The interplay between the holey graphene fringe at a nanohole and the NiMo surface adjacent to the fringe can be used to effectively tune local electronic structures leading to Gibbs free energies for H* adsorption that are close to zero. Such holey graphene coverings result in the bifunctional creation of electrochemically active sites and a delay in the dissolution of the non-noble metal in the acidic electrolyte, thereby realizing high catalytic performance. Although the non-noble metal electrode still suffers from dissolution in the acidic electrolyte, the holey graphene covering technique represents a new design direction that exploits the intrinsic catalytic performance of non-noble metals even in acidic electrolytes by tuning the hole sizes and the numbers of nanoholes on graphene/non-noble-metal electrodes.


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FIGURES

Figure 1. Schematic illustration of the fabrication of a porous NiMo composite covered with holey graphene layers for hydrogen evolution in acidic electrolytes.


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Figure 2. Morphologies of porous NiMo composites covered by holey graphene. (a) SEM image of the pristine porous NiMo decorated with SiO2 nanoparticles. (b) BF-STEM image of a single SiO2 nanoparticle attached to the graphene-layer-covered NiMo surface. The SiO2 nanoparticle blocks further graphene growth. (c) DF-STEM image of porous graphene with large micrometer -diameter holes created by high levels of SiO2 nanoparticles after NiMo dissolution. The inset shows selected-area diffraction spots observed in TEM mode. (d) DF-STEM image of porous graphene with nanoholes created by low levels of SiO2 nanoparticles after NiMo dissolution.


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Figure 3. Structural characterization of NiMo composites covered by holey graphene. (a) XRD spectra of the porous NiMo annealed at 950 °C for 20 min, with and without a covering of graphene. The bottom spectrum displays NiMo, Ni4Mo, and Mo2C reference peaks. (b) Raman spectra of the NiMo fully covered by graphene, the holey graphene-covered NiMo composite with low and high levels of SiO2 nanoparticles, and the holey graphene-covered NiMo composite with low levels of SiO2 nanoparticles after HER testing. (c) Mo 3d, (d) Ni 2p, (e) C 1s XPS spectra, and (f) N 1s and Mo 3p spectra of the NiMo composite covered by holey porous graphene and low levels of SiO2 nanoparticles.


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Figure 4. HER activities of NiMo composites covered by holey graphene. (a) Hydrodynamic voltammograms of samples with and without graphene holes in 0.5 M aqueous H2SO4. (b) Tafel plots of the various samples. (c) Double layer current densities as functions of scan rate. (d) Hydrodynamic voltammograms before and after durability testing. (e) Gibbs free energy profiles calculated by DFT for NiMo composites, with and without graphene covering. The calculated free energy diagram of HER at the equilibrium potential for a Pt catalyst; Ni(111); NiMo(100) with and without graphene covering, or graphene with a ~1 nm nanohole; and a graphene sheet. (f) Charge population analysis of NiMo(100) covered by graphene with a ~1 nm nanohole. The legend displays the various color codes.


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ASSOCIATED CONTENT Supporting Information. Figures S1-S32, Table S1-S3 and detailed experimental methods and discussion. AUTHOR INFORMATION Corresponding Author Yoshikazu Ito Email: [email protected] Tatsuhiko Ohto Email: [email protected] Author Contributions Y.I. conceived the experiments. Y.I., D.H., T.A. fabricated the NiMoO4. Y.I., C.L., H.K., M.I., J.F. characterized the NiMo electrode. Y.I. performed HER experiments. T.O., Y.N. performed DFT calculations. M.W. provided valuable suggestions and discussions. The manuscript was written by Y.I., T.O., Y.N., and discussed, edited and approved by all authors. ACKNOWLEDGMENT We thank Ms. Kazuyo Omura at the Institute for Material Research in Tohoku University for XPS measurements. This work was sponsored by JST-PRESTO “Creation of Innovative Core Technology for Manufacture and Use of Energy Carriers from Renewable Energy” (JPMJPR1541, JPMJPR1444); JSPS KAKENHI Grant Number JP15H05473, JP16H06367, JP23246063, JP15H02195, JP16K17855); SIP (Cross-Ministerial Strategic

Innovation

Promotion Program) conducted by CSTI (Council for Science, Technology and Innovation),


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Cabinet Office, the Government of Japan.; World Premier International Research Center Initiative (WPI), MEXT, Japan; NIMS microstructural characterization platform as a program of "Nanotechnology Platform" of the MEXT, Japan.; University of Tsukuba Basic Research Support Program Type S; Kumagai Foundation for Science and Technology.


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TOC


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Cooperation between holey graphene and NiMo alloy for hydrogen

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