Highly Efficient Electrochemical Hydrogen Evolution Reaction at Insulating Boron Nitride Nanosheet on Inert Gold Substrate

It is demonstrated that electrochemical hydrogen evolution reaction (HER) proceeds very efficiently at Au electrode, an inert substrate for HER, modified with BNNS, an insulator. This combination has been reported to be an efficient electrocatalyst for oxygen reduction reaction. Higher efficiency is achieved by using the size controlled BNNS (<1 μm) for the modification and the highest efficiency is achieved at Au electrode modified with the smallest BNNS (0.1–0.22 μm) used in this study where overpotentials are only 30 mV and 40 mV larger than those at Pt electrode, which is known to be the best electrode for HER, at 5 mAcm−2 and at 15 mAcm−2, respectively. Theoretical evaluation suggests that some of edge atoms provide energetically favored sites for adsorbed hydrogen, i.e., the intermediate state of HER. This study opens a new route to develop HER electrocatalysts.

1 Center for Green Research on Energy and Environmental Materials and Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan. 2 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan. 3 Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan. 4 Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan. Correspondence and requests for materials should be addressed to K.U. (email: uosaki.kohei@ nims.go.jp) showing that BNNS indeed acts as an electrocatalyst for HER at Au electrode as does for ORR, although BNNS shows negative and no effect at Pt (iii, iv) and GC (v, vi) electrodes, respectively, as observed for ORR 15,16 . The overpotential for HER at BNNS/Au is larger than those at Pt electrode by ca. 120 mV and 200 mV at 5 mA/cm 2 and 15 mA/cm 2 , respectively. It is interesting to compare these values with those at transition metal dichalcogenides, which recently attract much attention as HER electrocatalyst on various substrates as mentioned above. Overpotential for HER at 15 mA/cm 2 at WS 2 nanosheets on GC electrodes is larger than that at Pt electrode by 200 mV 11 , which is the same as that at the BNNS/Au. MoS 2 showed lower overpotential for HER than WS 2 and the BNNS/Au. It is about 150 mV at amorphous MoS 2 on GC 12 , and nanocrystalline MoS 2 both on gold 13 and reduced graphene oxide 14 substrates.

Results and Discussion
To understand why BNNS modification provides Au with improved electrocatalytic activity and to find ways to further reduce the overpotential for HER, HER mechanism is considered theoretically. HER is a 2-electron process with at least two elemental steps. In acidic solution the first step is a discharge of proton as: where H(a) represents hydrogen atom adsobed on an electrode surface. This process is followed by either to form molecular hydrogen [4][5][6][7][8][9] . Since H(a) is the intermediate state, energetics of this state should play cruicial role in determining the HER rate. The importance of free eneegy/heat of adsorption of hydrogen atom on the elctrode surface was pointed out long time ago 3-5 and so-called "volcano" relations between rate, or exchange current density, of HER and various forms of interaction between electrode and adsorbed hydrogen were demonstarted almost 60 years ago for the first time 4,5 . Recently DFT calculation was applied to obtain hydrogen chemisorption energies and volcano curve was obtained between the calculated hydrogen chemisorption energies and measured exchange current densities of HER 9 . The best electrode for HER, which situates at the top of the volcano relation, should have free energy of intermediate state, i.e., adsorbed hydrogen, close to 0 with respect to the intial state, H + + e − , and the final state, H 2 , at equilibrium potential. Here DFT calculations are performed to determine the free energy of adsorbed hydrogen at various substrate, Δ G H(a) . Δ G H(a) at atoms at the terrace of free h-BN is calculated to be + 2.25 eV, which is much larger than that at Au(111), which is + 0.2 eV, and binding of H to the atoms at the edges of the island is very strong, i.e., too negative Δ G H(a) : − 2.1 eV for boron atom at the edge and − 2.8 eV for nitrogen atom at the edge. Thus, HER at free h-BN is not possible as H binds hardly on the terrace and too strongly at the edges. It must be noted that Nørskov et al. reported + 0.4 eV for Δ G H(a) at Au(111) 9 . This discrepancy arises from the difference in DFT functionals. While they used RPBE functional, we used WC functional because RPBE cannot reproduce BN-metal interaction.  Based on this theoretical consideration, further improvement of electrocatalytic activity for HER can be expected by increasing the fraction of atoms at the edges of BNNS on Au substrate. Fraction of edge atoms should be increased by decreasing the size of each BNNS at a given amount of BNNS on the surface. Figure 3 (i) and (ii) shows LSVs of bare and BNNS modified Au (BNNN/Au) electrode, respectively, as already presented in Fig. 1. In this case the size of BNNS was not controlled and is distributed from less than 0.01 μ m to more than 10 μ m as shown in the inset (ii).  Table 1. The overpotential at BNNS(0.1-0.22 μ m)/Au electrode is only 30 and 40 mV larger than that at Pt electrode at 5 mAcm −2 and 15 mAcm −2 . These values are better than those at WS2 11 and MoS 2 modified electrodes 12-14 as mentioned above and that at Ni 2 P on Ti substrate 24 .
Not only exchange current densities but also Tafel slopes are affected by BNNS modification. While that at bare Au electrode is 70 mV/decade, it decreases to 40 mV/decade at BNNS(unfiltered)/Au and those at BNNS  In Summary, we have demonstrated that HER proceeds very efficiently at Au electrode, which is an inert substrate for HER, modified with BNNS, which is an insulator. Higher efficiency is achieved by using the size controlled BNNS (< 1 μ m) for the modification and the highest efficiency was achieved at Au electrode modified with the smallest BNNS (0.1-0.22 μ m) where overpotentials were only 30 mV and 40 mV larger than those at Pt electrode at 5 mAcm −2 and at 15 mAcm −2 , respectively. The Tafel slopes at Au electrode modified with size controlled BNNS were around 30 mV/decade, suggesting HER proceeds via Volmer-Tafel mechanism. DFT calculation suggests that the origin of small overpotential and Volmer-Tafel mechanism is the existence of energetically favored sites for adsorbed hydrogen, i.e., the intermediate state of HER. This work opens a new route to develop HER electrocatalysts and the development of more efficient electrocatalysts for HER is under way.

Methods
BN powder was sonicated in IPA with 3 mg/ml as initial concentration in an ultrasonic bath for 96 h. The dispersions were centrifuged at 3000 rpm for 30 min after sonication and the 1/2 of supernatant was collected and the collected dispersion was diluted by IPA by 3 times further to be used to prepare BNNS(unfiltered)/Au electrode (1 cm x 1 cm). Size distribution of BNNS in IPA solution was determined by dynamic light scattering (DLS) method using laser scattering particle size distribution analyzer (HORIBA-LA-950V2).
Size controlled h-BNNS was obtained by filtration using MF-Millipore filter (Merck Millipore, VSWP type) of various pore size. The diluted BNNS dispersion mentioned above was filtered by a filter of 1 μ m pore size filter followed by the filtration using a filter of 0. Surface modification by h-BNNS was carried out by self-evaporation of IPA from a h-BNNS dispersion on substrates as follows. 4 to 5 gold substrates (1 cm × 1 cm) were placed perpendicularly in a 10 ml glass beaker, in which 5 ml of BNNS dispersed isopropyl alcohol (IPA) was filled. The beaker was covered by aluminum foil with small holes on the top surface and it was left at room temperature until IPA was fully evaporated (ca. 24 h) and the gold  Table 1. Summary of electrocatalytic activity of various electrodes for HER.
surface was covered with BNNS. The gold substrates were then heated at 120 °C in a vacuum chamber (10 −6 Pa) for about 2 h. The gold electrode was characterized by SEM, Raman and electrochemical techniques. Raman measurements suggest majority of BNNS on Au are of monolayer as previously reported 15 . All electrodes were pre-treated by cycling the potential between − 0.1 and + 1.5 V in Ar saturated 0.5 M H 2 SO 4 electrolyte solution at a sweep rate of 100 mV s −1 for 100 cycles to remove any surface contaminants before the HER activity. Geometric surface area (0.5 cm 2 ) was used to calculate the current density.
LSVs were recorded by varying the potential from 0.2 to − 0.9 V with a scan rate of 1 mV s −1 . All the electrochemical measurements were carried out in a 0.5 M H 2 SO 4 an aqueous solution at room temperature. The electrolyte solution was deaerated by passing ultrapure Ar gas for at least for 1 h.
The calculations are performed using DFT with the gradient-corrected exchange-correlation functional of Wu and Cohen as implemented in the SIESTA code 15 . Computational details are given in the Supporting Information.