原子配列が制御された多孔性電極による水素生成反応の効率化
Electrochemical hydrogen evolution reaction using porous nanostructured electrodewith controlled atomic surface structure研究代表者 国立研究開発法人産業技術総合研究所 研究グループ長 三重 安弘
Yasuhiro Mie, Group leader, National Institute of Advanced Industrial Science andTechnology (AIST)
1．Introduction
Due to global environmental issues, there has been a strong demand for thedevelopment of sustainable technology in many fields. Bearing this in mind,
electrocatalysis with an electrode is a promising approach in chemical production
because it can efficiently utilize electricity from renewable energy [1]. As the hydrogenmolecule is a viable energy source, efficient electrochemical hydrogen evolution at anelectrode catalyst is expected be a crucial factor in developing a hydrogen economy. In
electrochemical methods, an electrode works as a catalyst and an electronserver/receiver. Therefore, in addition to the nature of the electrode materials, its
surface structure is a key factor that determines the catalytic activity. Owing to thegrowth of nanotechnological methods, electrodes composed of nanomaterials and thenanostructurization of electrode surfaces have been developed [2]. Now, it is wellrecognized that the nanoarchitecture of electrodes significantly enhances theelectrocatalytic activity, and one of the reasons is the abundant low-coordinated surface
atoms at the steps/kinks [3]. Among many types of nanostructured electrodes,
nanoporous metals have been attracted in many fields owing to their large surface area[4]. Nanoporous gold (NPG) is promising in electrochemical applications of catalysis andsensing, owing to its advantages, such as high conductivity, catalytic activity, chemical
inertness, physical stability, reusability, and facile surface modification. Besides
nanostructure effect, the surface atomic structure (crystallographic orientation) of theelectrode catalyst plays a crucial role in determining the activity for electrochemicalreactions. Fairly recently, we have developed a facile method to prepare NPG and its
surface atomic structure can be controlled by changing the conditions in preparation
procedure [5].
Currently, it is recognized that Pt-based materials catalyze the HER most efficiently.
However, the scarcity of Pt limits its use and requires to develop another electrodecatalyst for HER. Although gold material is stable and can be useful electrode catalyst,
its HER activity is very poor (Fig. 1). In the present study, we prepared NPG electrode－ 261 －

with different atomic surface structure, and examined their HER activity, aiming theimprovement in efficiency of electrochemical HER.

2．Experimental
Electrochemical measurements were mostly performed with a normal three-electrodeconfiguration consisting of an Ag|AgCl|sat.KCl reference electrode, a Pt auxiliaryelectrode, and a gold electrode.
Anodization was used to prepare nanoporous structure on the gold electrode catalyst
according to our previous reports [5]. NPGs with different atomic surface structures
were obtained by changing the electrolyte solution in the anodization process. Thesurface morphology and crystallographic orientation were analyzed using scanningelectron microscopy (SEM) and cyclic voltammetry discussed below, respectively.
Gold nanoparticles (AuNPs) were also used to produce nano(porous) structured
electrode surface. Solution plasma method was conducted to prepare 10-20 nm sizes ofAuNPs [6].
3．Results and Discussions
3.1 HER at anodized NPG with different surface atomic structure
Anodization of commercially available gold electrode produced nanoporous structure
as shown in Fig. 2a,b, composed of ligaments and pores. The morphology of NPG
obtained under different electrolyte condition (35 and 500 mM chloride ion) was similar,
and that of gold electrode surface before anodization process exhibited the planarsurface (Fig. 2c).

－ 262 －

While having the similar morphologies, the voltammetric responses in the presenceof Pb2+ (Pb underpotential deposition (UPD) and stripping properties) differedsignificantly between the aforementioned NPGs, as shown in Fig. 2d,e. Pb-UPD andstripping measurements are generally used to evaluate the crystallographicorientations (atomic structure) of gold electrode surfaces. NPG prepared from thesolution containing 35 mM Cl– (NPG-35) exhibited main anodic peaks at 0.444 and at
approximately 0.600 V (Fig. 2d). The former was sharp and close to the potentials of thestripping of Pb on Au(111). On the other hand, NPG prepared from the solution
containing 500 mM Cl– (NPG-500)

exhibited anodic currents at 0.492 and around0.660 V with a shoulder peak at 0.622 V. The former peak was close to that representing
Pb stripping on the Au(100) surface. The voltammogram of the normal planar (beforeanodization) gold electrode showed two broad stripping peaks around 0.444 and 0.610
V shown in Fig. 2f. These differences in peak positions for each surface indicate that theNPGs and normal gold surfaces have different facet contributions. We also confirmed a
considerable amount of Au(111) and Au(100) facets in NPG-35 and NPG-500,
respectively, by electrochemical reductive desorption peaks of self-assembled monolayer
film on these gold surfaces. All results indicate that the normal, NPG-35, and NPG-500－ 263 －have Au(110)-, Au(111)-, and Au(100)-rich surfaces, respectively.
Since the NPG electrodes having different facet contributions were confirmed, thenwe examined the electrochemical HER with these NPG electrodes to investigate theeffect of the surface atomic structure of nanostructured catalyst on the reaction. Fig. 3A
represents the obtained voltammograms at NPGs in a 0.5 M H2SO4 solution with iR
compensation in comparison with that of normal gold electrode, where the current
density, j, in mA cm–2, was calculated from the actual surface area estimatedelectrochemically from the reductive charge of AuO. As shown in Fig. 3A, NPG-35
exhibited a larger catalytic current than NPG-500 and normal gold. The current density
in the voltammogram obtained for NPG-35 was more than 2 and 15 times larger thanthose for NPG-500 and normal gold, respectively, at –0.1 V. The potentials providing thecurrent density of 3 mA cm–2 were –0.048 V for NPG-35, whereas those of NPG-500 andnormal gold were –0.073 and –0.149 V, respectively. The overpotential value for theNPG-35 electrode was considerably lower compared with those of other gold-based
catalysts [7], indicating the effective enhancement in the hydrogen production by thepresent nanostructurization. Because NPG-35 and NPG-500 have similar morphologies
when they have similar electrochemical surface areas, as shown in Fig. 2, the differencein current density for the HER may be ascribed to the difference in the facet
compositions of each surface. Previous report, which used single-crystal gold electrode,showed that Au(111) exhibited about 2.3 times larger catalytic current than Au(100) [8].
Further, gold nanospikes electrode having Au(111) dominated surface is also reported
to show significant increase in the current density for HER [9]. Our results areconsistent with these phenomena.

－ 264 －Fig. 3B represents the Tafel plot for further understanding of the reaction [10]. The
slopes of both NPGs and normal gold varied between lower and higher overpotential
regions, similar to those in previous reports for gold-based catalysts [10,11]. The slopes
for normal gold were 69 and 150 mV dec–1 in the lower and higher overpotential regions,respectively. These values are within the reported region for gold-based catalysts,
indicating that the formation of a surface-adsorbed hydrogen atom (the proton
discharge reaction known as the Volmer reaction) is the rate-determining step. The
NPG-35 and NPG-500 exhibited the slopes of 42 and 57 mV dec–1 in the lower region,and 118 and 170 mV dec–1 in the higher region, respectively. The decrease of the slope
in the lower region indicates the enhanced H adsorption process on the NPG surface
owing to the nanostructured effect. The slope variation for NPG-35 is close to those of
the simulated curve (40 and 120 mV dec–1 in the lower and higher overpotential regions,respectively); here, an electrochemical desorption reaction (Heyrovsky reaction) is therate-determining step, indicating the HER occurs through Volmer-Heyrovsky
mechanism at NPG-35. These observation reveals the importance of the control of thecrystallographic orientation in nanostructured catalysts in electrocatalytic reactions.
Thus, the obtained outcomes clearly demonstrate that the preparation of NPG utilizing
the present method is remarkably effective for the development of efficient catalysts.
3.2 HER at AuNP deposited nano(porous)structured electrode
The HER was also investigated using AuNPs deposited thin film electrode with acertain atomic surface structure. AuNPs used in this study mostly had diameters
between 10 and 20 nm, as shown in the Transmission Electron Microscopy (TEM) image(Fig. 4a). The concentrated aqueous solution with around 0.2 wt% dispersed AuNPs wasdropped on the GC electrode surface and left under air to evaporate the remaining
aqueous solution. The drying process finished within 48 h. A AuNP thin film exhibitinga golden metallic color was obtained, as shown in Fig. 4b, with a diameter of 6.0 mm.
Fig. 4c and 4d show SEM images of the commercial planar and the AuNP thin film goldsurfaces. The AuNP thin film was formed by drying the concentrated aqueous
dispersion of AuNPs on the GC surface, so the AuNPs were piled on top of each other.
Consequently, the top view of the SEM image shows the true appearance. It has beensuggested that there should be nanostructures based on AuNPs inside the AuNP thinfilm, compared with the commercial planar gold electrode (see Fig. 4c and d). The XRD
pattern of a AuNP thin film on a glass substrate prepared using the same method as
the present study exhibited four main diffraction peaks corresponding to the (111), (200),(220), and (311) planes of gold metal with 2θ between 30 and 80 degrees (JCPDS, No.

－ 265 －04-0784), revealing the excellent crystallinity of AuNP on the substrate. A similar result
was expected for the present AuNP thin film electrode on the GC substrate.

The AuNP thin film produced the typical cyclic voltammograms of a gold electrode in
H2SO4 solution, as shown in Fig. 4e. The peak currents are clearly recognizable,originating from surface oxide formation and their reduction. The surface area of thefilm electrode prepared from the 2 mg/mL AuNPs solution was estimated from thereductive charge of the surface oxide film in the voltammogram to be 2.4 cm2, giving asurface roughness of 8.6 using the geometric area of the electrode with a diameter of 6mm. Importantly, the voltammograms in a 0.5 M H2SO4 solution were almost
unchanged after 200 cycles (measurements), although no binder was used for theAuNP/GC disk. This showed that the AuNP thin film electrode exhibited a stableelectrochemical response (dotted and solid lines in Fig. 4e). We also confirmed that theAuNP thin film electrode prepared on GC is stable when stored at room temperature－ 266 －for more than 1 year [12].
Fig. 5a shows the linear sweep voltammograms recorded in a 0.5 M H2SO4 solution
at AuNP thin film and planar gold electrodes. The currents are represented by thecurrent density using the actual/electrochemical electrode surface area estimatedelectrochemically from the reductive charge of the surface AuO of these gold electrodes.
It can be clearly seen that the AuNP thin film electrode exhibited a much higher
catalytic current than the planar gold electrode. The current density of 27 mA cm–2 inthe voltammogram obtained at the AuNP thin film electrode was more than 10 times
larger than that at the planar gold electrode at −0.13 V vs RHE, and was close to that
(30 mA cm–2) reported for nanoporous gold electrodes prepared by an anodization
technique [5]. The potential providing the current density of 3 mA cm–2 was −0.094 and−0.194 V for the AuNP thin film and commercial gold electrodes, respectively. The
overpotential value obtained at the AuNP thin film electrode was considerably lower
than those of planar and other gold-based catalysts similar to the previous case. Theseresults indicate that electrode surface nanostructuring by reagent-free AuNPs
effectively enhanced the electrochemical activity towards HER. Perez et al. studied
HER with gold single-crystal electrodes. They suggested that the activity for HER
increases on Au(111) > Au(100) > Au(110) planes [8], highlighting the significance of thesurface atomic structure for this reaction. We compared the voltammetric features ofplanar and AuNP thin film gold electrodes to obtain information on the crystal planes.
Compared with the planar gold electrode, the AuNP thin film exhibits increasedoxidation peaks at 1.1 and 1.3 V, which correspond to Au(100) and Au(111) planes,respectively. Therefore, the enhanced (111) and the surface of the present AuNP film－ 267 －gold electrode may improve HER performance.
A Tafel analysis was conducted to further understand the HER reaction at the AuNP
thin film electrode. The Tafel plot is shown in Fig. 5b, together with the results for theplanar gold electrode. The slope obtained for planar gold in the higher overpotential
region was 134 mV dec–1. This value is within the reported range measured at goldbased electrodes. This indicates that the proton discharge reaction (the formation of asurface-adsorbed hydrogen atom), known as the Volmer reaction, determined the rate.
By contrast, the AuNP thin film electrode exhibited a slope of 39 mV dec–1. The lower
slope indicates a faster H adsorption process on the AuNP thin film electrode surface.
The value of the slope suggests that the electrochemical desorption reaction (known asthe Heyrovsky reaction) was the rate determining step. Hence, HER is expected to takeplace through the Volmer–Heyrovsky mechanism at the AuNP thin film electrode. Thisphenomenon is similar to the aforementioned anodized NPG case. Due to the lower freeenergy for hydrogen adsorption, the abundant low-coordinated surface atoms at thesteps/kinks in the nanostructured surface would cause enhanced H adsorption at theAuNP thin film surface [13]. Thus, the nanostructured and facet effects would enhance
HER activity at the present AuNP thin film electrode.
3.3 -Keto acid reduction at anodized NPG with controlled atomic surface structure
Concomitantly, we also found that the anodized NPG-500 shows improved catalytic
activity for -keto acid reduction to the corresponding amino acid [14]. A reductive
voltammetric signal at the NPG electrode appeared at a more positive potential by 0.18–
0.79 V, compared with those at the planar-gold electrode without anodization and otherpreviously reported electrode systems (as shown in Fig. 6), indicating the high activityof the prepared nanostructure for the electrochemical reaction. Maximum Faradaic
efficiencies (FEs) of 74–93% in the reductive molecular conversion to amino acids of Ala,Asp, Glu, Gly, and Leu were obtained under the optimized conditions. The FE values
were strongly dependent on the applied potential in the electrolysis, suggesting that thehydrogen evolution reaction at the electrode surface was more significant as the applied
potential became more negative. The effect of potential at the NPG was lower than thatat the planar-gold electrode. These results indicate that nanostructurization decreases
the overpotential for the electrochemical reductive amination, resulting in high FE.

－ 268 －4．Conclusion
In developing a hydrogen society, efficient electrochemical hydrogen evolution
reaction (HER) at an electrode catalyst can be a crucial strategy. Although gold is stable
and can be useful electrode catalyst, its HER activity is very poor. In this study, porous
nanostructured gold surfaces with controlled atomic surface structures were preparedand their HER activity was investigated. We found that nanostructured gold surface
with Au(111)-rich surface showed considerably larger (more than 15 times and 2 times)
activity in current density for HER compared with those at polycrystalline planar gold
and nanostructured gold with Au(100)-rich surfaces, respectively. On the other hand,electrochemical reductive reaction of -keto acid to the corresponding amino acid wassignificantly improved at the nanostructured gold electrode catalyst with Au(100)surface. Results suggest that the present method to control the atomic surface structureof electrode surface is promising to develop efficient electrode catalyst forelectrochemical reactions.

－ 269 －

Acknowledgement
The author acknowledges the financial supports from JFE 21st Century Foundation.
謝辞
本研究は、公益社団法人 JFE21 世紀財団より技術研究助成の支援を受けて行いました。
ここに謝意を表します。
References
1)

Wang, F. M.; Li, W. J.; Wang, R.; Guo, T. Q.; Sheng, H. Y.; Fu, H. C.; Stahl,S. S.; Jin, S., Joule 2021, 5 , 149-165.

2)

Liang, J.; Liu, Q.; Li, T. S.; Luo, Y. L.; Lu, S. Y.; Shi, X. F.; Zhang, F.; Asiri,A. M.; Sun, X. P., Green Chem., 2021, 23, 2834-2867.

3)

Biener, J.; Biener, M. M.; Madix, R. J.; Friend, C. M., Acs Catal., 2015, 5 , 62636270.

4)

Bhattarai, J. K.; Neupane, D.; Nepal, B.; Mikhaylov, V.; Demchenko, A. V.;
Stine, K. J., Nanomaterials 2018, 8 , 171.

5)

Mie, Y.; Takayama, H.; Hirano, Y., J. Catal. 2020, 389, 476-482.

6)

Matsuda, N.;

Nakashima, T.;

Okabe, H.;

Yamada, H.;

Shiroishi, H.;

Nagamura, T., Mol. Crys. Liq. Crys., 2017, 653, 137-143.
7)

Renjith, A.; Roy, A.; Lakshminarayanan, V., J. Coll. Int. Sci., 2014, 426, 270-279;
Siddhardha, R. S. S.;

Lakshminarayanan, V.; Ramamurthy, S. S., J. Power

Sources 2015, 288, 441-450.
8)Perez, J.; Gonzalez, E. R.; Villullas, H. M., J. Phys. Chem. B 1998, 102, 1093110935.

9)

Plowman, B.;

Ippolito, S. J.;

Bansal, V.;

Sabri, Y. M.;

O'Mullane, A. P.;

Bhargava, S. K., Chem. Commun., 2009, 5039-5041.
10) Zou, X. X.; Zhang, Y., Chem. Soc. Rev., 2015, 44, 5148-5180.
11) Khanova, L. A.; Krishtalik, L. I., J. Electroanal. Chem., 2011, 660, 224-229.
12) Mie, Y.; Okabe, H.; Mikammi, C.; Motomura, T.; Matsuda, N. Electrochem.,

Commun., 2022, 146, 107415.
13) Lu, J.; Zhou, W. J.; Wang, L. K.; Jia, J.; Ke, Y. T.; Yang, L. J.; Zhou, K.;
Liu, X. J.; Tang, Z. H.; Li, L. G.; Chen, S. W., Acs Catal., 2016, 6, 1045-1053.
14) Mie, Y.; Katagai, S.; Mikammi, C. Int. J. Mol. Scie., 2021, 22, 9442.

－ 270 －