Intensified Field-Effect of Hydrogen Evolution Reaction

Yin Xie; Liyang Zhang; Peijin Ying; Jiacheng Wang; Kuan Sun; Meng Li

doi:10.7536/PC200812

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (9) : 1571-1585 DOI: 10.7536/PC200812

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Intensified Field-Effect of Hydrogen Evolution Reaction

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Abstract

There are many factors affecting energy consumption and hydrogen evolution efficiency in the process of hydrogen evolution by water electrolysis, among which interface resistance and bubble coverage are two of the most significant factors. It's found that the interface resistance and bubble coverage of the catalyst can be effectively reduced by strengthening external field-effect in the process of water electrolysis. For instance, thermal field can introduce energy into the charge transfer during the reaction process, thereby reducing the charge transfer resistance and overpotential. Besides, it is able to directly adjust the electronic structure of the catalyst or induce the redistribution of electrolyte ions by using the electric field, which promotes the interfacial charge transfer. What's more, polarization of water molecules can be induced in a optical field. Therefore, the introduction of thermal field, magnetic field, ultrasonic field, electric field, high-gravity field and optical field into the electrolytic cell are effective strategies to reduce energy consumption and hence improve the efficiency of hydrogen evolution. In recent years, although some works focusing on the field-effect of water electrolysis are reported, there are few reviews to systematically introduce the field-effect in water electrolysis. In this paper, the research progress of field-effect in hydrogen evolution reaction in recent years is reviewed. Moreover, this paper introduces the basic principles of various field-effect of water electrolysis, analyzes the effects of various fields through experimental cases, and summarizes the challenges as well as prospect of field enhanced water electrolysis hydrogen evolution.

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Contents

1 Introduction

2 Hydrogen evolution reaction

3 Intensified field-effect of HER

3.1 Thermal field

3.2 Magnetic field

3.3 Ultrasonic field

3.4 Electric field

3.5 Gravitational field

3.6 Optical field

3.7 Multiple field

4 Conclusion and outlook

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field-effect / water electrolysis / hydrogen evolution reaction / hydrogen energy

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Yin Xie, Liyang Zhang, Peijin Ying, Jiacheng Wang, Kuan Sun, Meng Li. Intensified Field-Effect of Hydrogen Evolution Reaction. Progress in Chemistry, 2021, 33(9): 1571-1585 DOI:10.7536/PC200812

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1 Introduction

With the increasing demand for energy and the increasingly serious environmental pollution, it has become a trend of energy utilization in recent years to develop clean and efficient renewable energy utilization technology and to meet the power management measures of "peak shaving and valley filling" as far as possible. Hydrogen, as an ideal clean fuel with very high mass energy density (142 MJ/kg), is considered to be one of the most promising energy carriers in the future^{[1⇓⇓⇓~5]}. hydrogen evolution reaction (hydrogen evolution reaction, HER) by electrolysis of water is one of the simplest, most efficient and cleanest ways to produce high-purity hydrogen energy. However, at present, only about 5% of industrial hydrogen is obtained through electrolysis of water, and the other 95% mainly comes from fossil fuels^[6⇓~8]. In the long run, hydrogen production from non-renewable traditional fossil fuels is not a sustainable way. In the future, hydrogen energy must be converted from renewable energy, which can not only solve the problem of uneven spatial and temporal distribution of renewable energy, but also avoid the use of fossil fuels and their carbon dioxide emissions.

Up to now, noble metal Pt-based catalysts have the highest catalytic efficiency for HER, but the high cost and scarcity of Pt limit its large-scale practical application^[9⇓~11]. Therefore, we need to find alternative catalysts based on abundant and economical elements^[7]. So far, a variety of economical and abundant transition metal compounds, such as metal sulfides, selenides, phosphides, carbides and nitrides, have been proved to be promising substitutes for precious metal Pt-based catalysts^[12⇓~14]^{[15⇓⇓⇓~19]}^{[20⇓⇓~23]}^[24⇓~26]^{[27⇓⇓~30]}. However, most of the current non-noble metal-based catalysts are significantly less efficient or less stable than Pt-based catalysts, and most of the catalysts involve complex preparation methods, making the cost enormous. Therefore, in addition to finding more efficient, reliable and economical alternatives to Pt-based catalysts, the use of external fields to enhance low-cost catalysts for hydrogen evolution from water electrolysis has become a hot research topic in recent years. It is found that the external field can effectively reduce the charge transfer resistance and the bubble coverage on the surface of the catalyst in the process of hydrogen evolution from water electrolysis. The temperature field can inject energy into the charge transfer in the reaction process, thus reducing the charge transfer resistance and overpotential^[31,32]. The magnetic field can promote the circulation flow of the electrolyte, thereby strengthening the mass transfer process in the electrolyte and reducing the bubble coverage on the surface of the electrode^[33⇓~35]; Ultrasonic field can promote the rapid detachment of hydrogen bubbles on the surface of the electrode and enhance the forced convection of the electrolyte^[36,37]. The electric field can directly adjust the electronic structure of the catalyst or induce the redistribution of electrolyte ions, thus promoting the interfacial charge transfer^[38,39]. The supergravity field promotes the release of hydrogen bubbles by providing an external force along the direction of bubble buoyancy^[40⇓~42]; The optical field can induce the polarization of water molecules, increase the residual electric field, and stretch the hydrogen bonds in water molecules, which is conducive to the decomposition of water to produce hydrogen and oxygen^[43]. Therefore, the application of external fields, including temperature field, magnetic field, ultrasonic field, electric field, high gravity field and light field in the process of hydrogen evolution by electrolysis of water can significantly promote the mass transfer process in the electrolyte, reduce the total impedance of the system, and produce much better performance than the traditional hydrogen evolution by electrolysis of water. In the process of hydrogen evolution by electrolysis of water, the reasonable use of external field for strengthening will produce great benefits.

The purpose of this paper is to make a comprehensive review of the recent progress in the enhancement of hydrogen evolution from water electrolysis by external fields, and to introduce the basic principles of various kinds of enhancement of hydrogen evolution from water electrolysis by external fields. At the same time, the performance of each external field means is analyzed through experimental cases, and the challenges and development direction of external field enhancement of hydrogen evolution from water electrolysis are summarized.

2 Brief Introduction of Hydrogen Evolution by Electrolysis of Water

The process of hydrogen evolution by electrolysis of water has been deeply analyzed by a large number of studies. By summarizing the experimental data and analysis results of hydrogen evolution by electrolysis of water with different catalysts, the researchers put forward the corresponding reaction mechanism, which is now generally recognized as the process of hydrogen evolution in electrolyte:^[44⇓~46]

1。 Electrochemical reaction (Volmer reaction):

H₃O⁺+e^-+M→M—H+H₂O (acid)

H₂O+e^-→M—H+OH^- (medium/alkaline)

2。 Complex desorption (Tafel reaction):

M—H+M—H→2M+H₂

3。 Electrochemical desorption (Erovsky reaction):

M—H+H₃O⁺+e^-→M+H₂+H₂O (acid)

H₂O+e^-+M—H→H₂+H₂O (medium/alkaline)

Where M is the active free surface site of the catalyst and M — H is the hydrogen atom adsorbed on the catalyst site. It can be seen that HER begins with the electrochemical adsorption of a proton (Volmer reaction), followed by the combination of two adsorbed hydrogen atoms (Tafel reaction), or the combination of an adsorbed hydrogen atom and a dissolved proton (Heyrovsky reaction) to produce a hydrogen molecule (Fig. 1A)^[47]. The reaction mechanism depends on the reaction kinetics of the selected catalyst. Under alkaline conditions, the OH^- and M — H adsorbed on the surface of the catalyst are produced by the electrochemical reduction of the H₂O (Volmer reaction). Before the formation of M — H, the catalyst needs to destroy the bond of H — OH; Under acidic conditions, M — H adsorbed on the surface of the catalyst is reduced by the H₃O⁺ of hydronium ions^[48,49].

However, whether the reaction proceeds via the Volmer-Tafel or Volmer-Heyrovsky mechanism, the hydrogen atoms adsorbed on the catalyst sites, i.e., M — H, are all reaction intermediates. It can be seen that the electrochemical performance is closely related to the M-H bonding in the process of water electrolysis. As shown in Figure 1b, the researchers concluded that the rate of hydrogen evolution reaction initially increases with the increase of M — H bonding, but if the adsorption is too strong, the adsorbed hydrogen is difficult to desorb from the electrode, resulting in a decrease in the relative rate of M — H bond breakage to form H₂ and a decrease in the overall reaction rate. Therefore, only when the M-H bonding is moderate, the rate of H₂ precipitation reaction can reach the maximum, which is called "volcanic effect"^[51]. It is also evident from fig. 1b that precious metals are mostly located near the top of the volcanic relationship graph, and the precious metal-based catalysts have superior catalytic activity compared to other non-precious metal-based catalysts^[50]. Therefore, in recent years, the search for low-cost and high-performance catalysts for water electrolysis has become a research hotspot, and a variety of economical and abundant transition metal compounds have been proved to be promising substitutes for noble metal-based catalysts. However, at present, the hydrogen evolution efficiency or stability of many non-noble metal-based catalysts is not as good as that of noble metal-based catalysts, and most of them involve complex and harsh preparation processes, and the preparation cost is also very high.

In addition to the screening of HER catalysts, it can also be found that hydrogen evolution by electrolysis of water is a gas evolution reaction occurring at the gas-liquid-solid three-phase interface, so the enhancement of mass transfer in the electrolyte is also an important strategy to improve the HER rate. During the HER process, hydrogen bubbles are formed on the surface of the cathode, which can be separated from the electrode surface only when they reach a critical size. With the increase of hydrogen release, the bubbles formed will cover the electrode surface, reduce the active reaction area of the electrode, and poison the electrode, that is, the increase of bubble coverage on the electrode surface will increase the total impedance of the system and reduce the hydrogen evolution activity of the catalyst^[52]. In addition, the bubble layer formed on the surface of the electrode also greatly slows down the release rate of hydrogen, and the accumulation of hydrogen bubbles increases the overpotential required for hydrogen evolution from water electrolysis and the ohmic voltage drop of the system. In order to enhance the process of hydrogen evolution by electrolysis of water, the released gas should be removed from the active sites in time to increase the active reaction area available for hydrogen bubble generation. Therefore, in addition to finding an economical and effective catalyst, it is also an effective strategy to improve the performance of the catalyst by enhancing the water electrolysis process through the external field to achieve the rapid removal of hydrogen bubbles on the electrode surface. In addition to promoting the rapid removal of bubbles and directly accelerating the gas/liquid mass transfer process, some external fields can also directly act on the catalyst or reactant molecules (ions) themselves to reduce the energy barrier of the reaction and promote the hydrogen evolution process from the aspects of chemical reaction kinetics or thermodynamics.

3 External field enhanced electrolysis of water for hydrogen evolution

In the process of hydrogen evolution from water electrolysis, the external field can accelerate the generation and release of hydrogen bubbles on the electrode surface and reduce the total impedance of the system, thus reducing the energy consumption of the system and improving the efficiency of hydrogen evolution from water electrolysis^[53]. As shown in Table 1, the mechanisms of different external fields to enhance the process of hydrogen evolution from electrolytic water are different. For example, the temperature field can inject energy into the charge transfer in the reaction process, thereby reducing the charge transfer resistance and reducing the overpotential of hydrogen evolution from electrolytic water. The ultrasonic field mainly accelerates the separation of hydrogen bubbles from the electrode surface, strengthens the mass transfer in the electrolyte solution, and reduces the bubble coverage on the electrode surface, thereby reducing the energy consumption of hydrogen evolution from electrolytic water and improving the efficiency of hydrogen evolution from electrolytic water. However, no matter what kind of mechanism, it can effectively enhance the process of hydrogen evolution from electrolytic water. Therefore, the application of external fields, including temperature field, magnetic field and electric field, has become a common strategy to enhance hydrogen evolution in water electrolysis in recent years.

3.1 Temperature field strengthening

The charge transfer resistance (charge transfer resistance,R_ct) is one of the important indexes for determining the rate of hydrogen evolution in water electrolysis. The smaller the charge transfer resistance is, the higher the rate of hydrogen evolution is. A temperature field is added in the hydrogen evolution process of the electrolytic water, and high temperature injects energy for charge transfer, so that the reduction of charge transfer impedance can be promoted, the removal of hydrogen bubbles on the surface of the electrode is facilitated, the bubble coverage rate on the surface of the electrode is reduced, and the mass transfer in the electrolyte is promoted, so that the overpotential is reduced, and the hydrogen evolution efficiency of the electrolytic water is improved^[54⇓~56]. At the same time, the temperature field increases the apparent activation energy to a certain extent, which makes the reaction easier and promotes the production of hydrogen^[57]. Many experimental results have confirmed that increasing the catalytic temperature will have a positive impact on the process of hydrogen evolution from water electrolysis^{[54⇓⇓⇓~58]}.

Piontek et al. studied the performance of hydrogen evolution by electrolysis of water in a 0.5 M H₂SO₄ at different temperatures (Fe_xNi_1-_x)₉S₈(x=0~1) (Fig. 2A)^[32]. They focused on the study of Fe_4.5Ni_4.5S₈ with periodic composition and active centers. It was found that under the same experimental conditions, the HER overpotential at -10 mA·cm^-2 gradually decreased from 377 mV to 138 mV as the temperature increased from 0 ℃ to 90℃,Fe_4.5Ni_4.5S₈ (Fig. 2b). When the temperature increased and decreased to the initial value, the catalytic activity did not change, indicating that the change in catalytic activity was only caused by the temperature and not by the activity of the catalyst during the electrochemical test. They also tested galvanostatic Electrochemical Impedance Spectroscopy (EIS) at a range of temperatures of -0.2 mA, as shown in Figure 2C. At this current, HER just happened, and the low-noise spectra of the generated Electrochemical Impedance Spectroscopy bubbles could be detected. However, it can be seen from the low-frequency Bode plot (inset of Fig. 2C) that the low-noise spectrum of the H₂ bubbles gradually disappears with the increase of temperature, that is, high temperature promotes the rapid release of hydrogen bubbles on the electrode surface, making the H₂ bubbles on the electrode surface more difficult to be monitored. It can also be seen from the Nyquist plot in Figure 2C that in the high frequency region, the plot has a small shift to the negative direction as the temperature increases, which is related to the slow decrease in the resistance of the solution. At lower temperatures, the whole semicircle tends to close, while at higher temperatures, a linear trend can be observed in the low-frequency range, indicating a diffusion-controlled process. In addition, from the Tafel slope analysis shown in fig. 2D, it can be seen that the hydrogen evolution mechanism at different temperatures remains unchanged, indicating that during the entire reaction process,The improvement of hydrogen evolution performance in water electrolysis is mainly due to the fact that the temperature field promotes the release of hydrogen bubbles on the electrode surface, reduces the bubble coverage on the electrode surface, and enhances the mass transfer in the electrolyte, thus reducing the overpotential of hydrogen evolution.

In another report, Sharma et al. studied the effect of temperature on hydrogen evolution from electrolysis of water on Ni-MoO₂/RGO catalysts, and found that the polarization curves of Ni-MoO₂/RGO at different temperatures were significantly different (Fig. 2e), that is, the overpotential decreased gradually with the increase of temperature^[31]. It can be seen from the Nyquist diagram in Figure 2F that the semicircle of the Ni-MoO₂/RGO sample gradually shrinks with the increase of temperature, indicating that the R_ct gradually decreases. Compared with low temperature, high temperature can reduce the charge transfer resistance of Ni-MoO₂/RGO and greatly improve its charge transfer rate, which makes Ni-MoO₂/RGO show more excellent catalytic activity at high temperature.

The application of temperature field in the process of hydrogen evolution from water electrolysis can not only promote the rapid removal of hydrogen bubbles on the electrode surface and reduce the charge transfer resistance of the system, but also increase the apparent activation energy. The relationship between temperature and apparent activation energy has been summarized by relevant formulas^[57].

d (log (i 0)) d (1 / T) = - Δ H 0 # 2.3 R

The above formula is called the Arrhenius formula. Where i₀ is the AC current density, T is the reaction temperature, ΔH^0# is the apparent activation energy, and R is the universal gas constant. It can be seen from the formula that under certain conditions, the apparent activation energy is positively correlated with the reaction temperature, and with the increase of temperature, the apparent activation energy will increase to a certain extent.

In a word, the application of temperature field in the process of hydrogen evolution from water electrolysis can enhance the mass transfer in the electrolyte. Promoting the release of bubbles on the surface of the catalyst; Reduce the charge transfer resistance; And that apparent activation energy is improve, so that the reaction is accelerated, the reaction overpotential is reduce, and the hydrogen evolution efficiency of the electrolytic water is finally improved. Temperature field treatment is one of the most commonly used experimental means, and it is usually easy to obtain a controllable temperature field, and it is convenient and simple to use the temperature field in the process of hydrogen evolution by electrolysis of water. Various industrial waste heat, solar energy, light, heat and other temperature fields can be effectively utilized in the process of electrolyzing water to separate hydrogen, thereby meeting the requirement of economy^[59]. However, the application of temperature field in the process of hydrogen evolution by electrolysis of water has the problems of being easily affected by the environment and having a long relaxation time. At present, there are few studies on the enhancement of hydrogen evolution by temperature field, and more temperature fields will be used in the field of electrocatalysis in the future. With the development and deepening of the technology, the rational use of temperature field will become one of the effective strategies to improve the performance of hydrogen evolution from electrolyzed water.

3.2 Magnetic field strengthening

Adding magnetic field in the process of hydrogen evolution from water electrolysis can effectively reduce the critical size of H₂ bubbles on the electrode surface, reduce the bubble coverage on the electrode surface, and improve the efficiency of hydrogen evolution from water electrolysis^[60]. Studies have shown that the introduction of an external magnetic field can enhance the mass transfer in the electrolytic cell and reduce the ohmic voltage drop between the electrodes in the process of water electrolysis, especially in alkaline solution and at high current density^[61,62]. In fact, this change is caused by the electrolyte convection driven by Lorentz force. The interaction between current and magnetic field produces directional Lorentz force, which promotes the vortex around the bubble and produces magnetohydrodynamic (MHD) convection^[63]. This additional convection on the electrolyte can effectively reduce the harmful effects of bubble accumulation on the electrode surface, thus promoting hydrogen evolution from electrolyzed water.

The Lorentz force acting on a charged particle moving in an electric field is expressed as:^[64]

F = qE + qv × B

Where F is the Lorentz force, Q is the charge of the particle, E is the electric field strength, B is the density of the magnetic field, and V is the velocity of the particle. The magnetic and electric equivalent lines are orthogonal, and the charged particles will be forced to move in a direction perpendicular to the magnetic field lines. Under the influence of Lorentz force, the direction of motion of particles will also change. Reasonable design of magnetic circuit and electric circuit will produce uniform flow field and appropriate ion distribution between electrodes, thus improving the efficiency of hydrogen evolution from water electrolysis.

The Lorentz force acts directly on the fluid (electrolyte) and is expressed as:

F = B × j

Where F is the Lorentz force, B is the density of the magnetic field, and J is the current density. Because the bubble acts as an electrical insulator, it produces a non-uniform current density distribution in its vicinity, as shown in Figure 3A. Under the action of the magnetic field, the Lorentz force is generated in the azimuthal direction, which drives the rotating flow around the bubble. Since the sign of the curvature of the electric field lines changes below, the Lorentz force will act in opposite directions above and below the equator of the bubble. In addition, the curvature of electric field lines is larger in the upper part of the bubble, and the Lorentz force in the upper part is stronger than that in the bottom of the bubble. Therefore, the rotational flow driven by Lorentz force is usually faster above the equator of the bubble than below it, thus forming a region of lower pressure above the bubble.This can explain the decrease of the critical size of the bubble observed in the magnetic field normal to the electrode^[65].

Many studies have been carried out on the use of magnetic field to enhance hydrogen evolution from water electrolysis. Diao et al. Used electrochemical noise analysis to quantitatively analyze the hydrogen evolution reaction of Pt working electrode in 0.5 M Na₂SO₄ electrolyte, and found that adding 1.5 T magnetic field during electrolysis reduced the overpotential of hydrogen formation by about 10%^[33]. Liu et al studied the effect of uniform magnetic field on hydrogen bubble separation behavior during water electrolysis in 4.24 M KOH solution using copper foam electrode as electrode material^[35]. They found that under the same porosity of copper foam electrode and the same current density, there was a significant difference in hydrogen evolution from electrolytic water with or without magnetic field enhancement. Compared with the case without magnetic field, the number of large H₂ bubbles in the process of hydrogen evolution from electrolytic water enhanced by magnetic field was significantly reduced, and their critical size was also significantly reduced (Fig. 3 B and C). As shown in Figure 3e, they divided the bubble size into two groups with diameters d ≥ 0.9 mm and d < 0.9 mm, and the number of bubbles of both diameters increased with increasing current density. However, under the conditions of the same porosity of the foam electrode and the same current density J (1000 A/m²), there is a significant difference between the experimental results with and without a magnetic field. The number of bubbles with diameter d ≥ 0.9 mm was significantly reduced, while the number of bubbles with diameter d < 0.9 mm was significantly increased under the external magnetic field of 0. 9 T. In the process of electrolyzing water, all ions in the electrolyte move between the two electrodes, and under the action of an external magnetic field, the Lorentz force drives the electrolyte to circularly flow on both sides of the electrode through a pore channel of the foam electrode,The bubbles generated inside the foam electrode are carried away before the maximum volume is reached, while fresh electrolyte is brought to the electrode, as shown in fig. 3D. Therefore, the external magnetic field can reduce the bubble coverage on the surface of the foam electrode, increase the active specific surface area in the foam electrode, and reduce the solubility of dissolved gases, thereby reducing the reaction overpotential and reducing the energy consumption of the system. Liu et al. Also found that the combination of porous electrodes and magnetic fields could promote the removal of gas production and further reduce the cell voltage^[34]. They used a porous nickel plate electrode as a catalyst and added a 0.9 T magnetic field during the catalytic process, and found that the cell voltage was reduced by about 2.5%. In addition, the applied magnetic field has the effect of reducing the cell voltage in the current range of the 130~670 A/m². If a nickel electrode is used, the voltage drop can be as high as 2.5% at a current density of 130 A/m². As shown in Figure 3F, with the increase of current, the potential difference decreases with the increase of magnetic field because the influence of external magnetic field on the hydrogen evolution performance of electrolytic water becomes weaker. In addition, although the nickel electrode and the copper electrode have the same porosity, the influence of the magnetic field on the nickel electrode is more obvious than that on the copper electrode. This is because the nickel electrode is a ferromagnetic material and the copper electrode is a diamagnetic material. It is found that with the same applied magnetic field, the actual magnetic field between the ferromagnetic electrodes is different from that around the diamagnetic electrodes. Compared with diamagnetic materials and paramagnetic materials, ferromagnetic materials can induce larger Lorentz forces under the same magnetic field on the surface of the electrode. Therefore, as a ferromagnetic material, nickel electrode has better performance than diamagnetic and paramagnetic electrodes under the action of magnetic field^[61].

In the process of hydrogen evolution from water electrolysis enhanced by magnetic field, the magnetohydrodynamic convection generated by Lorentz force can significantly promote the removal of bubbles and reduce the bubble coverage on the electrode surface, which leads to the significant reduction of ohmic pressure drop and reaction overpotential, and greatly improves the performance of hydrogen evolution from water electrolysis. In the process of hydrogen evolution from water electrolysis enhanced by magnetic field, the physical properties of the catalyst itself need to be reasonably considered, for example, compared with diamagnetic and paramagnetic materials, magnetic field has a better promotion effect on ferromagnetic materials. The reasonable arrangement of the external magnetic field can effectively enhance the hydrogen evolution in water electrolysis. However, in the process of hydrogen evolution from water electrolysis, there are some problems such as high energy consumption and difficult to control the process. At present, most of the studies on hydrogen evolution from water electrolysis under the action of magnetic field are presented by macroscopic observation. The electrocatalytic process under the influence of magnetic field still needs more in-depth and systematic microscopic observation and electrochemical study.

3.3 Ultrasonic field strengthening

In the process of hydrogen evolution from water electrolysis, ultrasonic field (USF) is also an effective strategy to improve the bubble coverage and dispersion on the electrode surface. Under the action of ultrasonic field, hydrogen bubbles can be quickly separated from the surface of the electrode and the electrolyte, and then the electrolyte will re-submerge the nucleation site and react to form new bubbles. As the hydrogen bubbles covering the nucleation sites on the surface of the electrode are rapidly removed, both the coverage of the bubbles on the surface of the electrode and the porosity of the electrolyte are significantly reduced.The use of ultrasonic field to enhance hydrogen evolution from water electrolysis is conducive to improving the production of hydrogen and achieving rapid and efficient hydrogen evolution from water electrolysis.Many people have focused on how to skillfully use USF to achieve efficient promotion of hydrogen precipitation from water electrolysis^[36,37]. Li et al., using the experimental apparatus shown in Figures 4A and 4B, carried out water electrolysis reactions in sodium hydroxide solutions of different concentrations using ultrasonic fields with RuO₂ and IrO₂ coated with Ti as working electrodes, and found that ultrasonic fields could significantly promote water electrolysis^[36]. The results shown in Figure 4C show that when an ultrasonic field is applied to the system, the cell voltage with the ultrasonic field is significantly lower than the cell voltage without the ultrasonic field, and this result is more evident in a low concentration of NaOH electrolyte. This is because the ultrasonic field enhances the forced convection of the electrolyte, thus increasing the mass transfer rate of the electrolyte. At the same time, they studied the effect of ultrasonic field on hydrogen production in sodium hydroxide electrolyte at 303 K by measuring the efficiency of hydrogen production with and without ultrasonic field at different current densities. Fig. 4D and fig. 4E respectively show the hydrogen evolution efficiency of electrolyzed water without ultrasonic field and with ultrasonic field. By comparison, it can be seen that the generation efficiency of H₂ can be improved by using ultrasonic field. When the current density is greater than 80 mA·cm^-2, the hydrogen production efficiency is 80% ~ 85% in the presence of ultrasonic field, which is significantly higher than 60% ~ 75% in the absence of ultrasonic field. It can be seen that the existence of ultrasonic field can greatly improve the generation of H₂. It was found that the bubbles covering the nucleation site of the electrode were quickly removed from the electrode under the action of ultrasonic field, and then the nucleation site was re-immersed by the electrolyte and reacted to form new bubbles.Electrolysis of water in ultrasonic field was beneficial to improve the production of hydrogen. The results show that when the current density is greater than the 80 mA·cm^-2, the hydrogen production efficiency can be increased by 5% ~ 18%. In addition, Hung et al. tested the effect of ultrasound on the efficiency of hydrogen evolution during electrolysis and showed that the production efficiency of cells operated with USF was significantly higher than that of cells without USF (5% to 16%)^[37].

The hydrogen bubbles generated on the surface of the electrode can be quickly removed by adding an ultrasonic field in the process of hydrogen evolution by electrolysis of water, so that the hydrogen bubbles are quickly released, and the efficiency of hydrogen evolution by electrolysis of water is improved. It is very cost-effective to use ultrasonic field to improve the efficiency of water electrolysis. For example, in a 100 kA electrolytic cell, the overpotential generated by bubbles is about 0.3 V. Compared with the 30 kW conservation energy required to eliminate this overpotential with a magnetic field, the ultrasonic field only needs about 0.05 kW^[64]. However, the realization of ultrasonic field is more complex and not easy to control. At present, the research of hydrogen evolution by electrolysis of water under the action of ultrasonic field still stays at a relatively simple level, and deeper microscopic observation and modeling analysis are needed. As an economical and efficient strategy to enhance hydrogen evolution in water electrolysis, ultrasonic field will show more and more significant performance in the process of enhancing hydrogen evolution in water electrolysis.

3.4 Electric Field Strengthening

In the process of water electrolysis, electric field enhancement is also an effective means to promote, and the use of electric field will make the catalyst have better activity for hydrogen evolution reaction. The electric field enhancement strategy is to add an electric field perpendicular to the catalyst on the basis of the original experimental device, and effectively adjust the Schottky barrier type and barrier height through the vertical electric field, which is conducive to the carrier transport of the catalyst and promotes the electrons to reach the activation center (Figure 5A)^[38,66]. At the same time, as a promising strategy, the electric field effect can also reduce the contact resistance and the sheet resistance of the catalyst, increase the channel conductance of the catalyst, and obtain a faster electron transport rate, that is, to improve the catalytic performance of the catalyst under the conditions of lower channel resistance and faster charge transfer^[66,67]. Ling et al studied the effect of electric field on the electronic structure of MoS₂/Au interface by using density functional theory, focusing on the charge transfer and Schottky barrier properties^[38]. They found that the applied electric field can effectively adjust the Schottky barrier type and barrier height, which is conducive to the carrier transport of the catalyst. More interestingly, further studies have found that the addition of electric field makes the difference of charge density around hydrogen atoms change with the change of electric field, thus optimizing the adsorption energy of atomic hydrogen on MoS₂/Au and making it more controllable, which is of great benefit to HER. Wang et al. designed a circuit of MoS₂ back-gate two-dimensional electrode under field-effect tuning (Fig. 5B) to explore the performance of hydrogen evolution from water electrolysis enhanced by electric field^[68]. In a sense, the field-effect modification of the two-dimensional working electrode is similar to chemical doping (such as n-type doping or p-type doping), that is, the field-effect modification can adjust the shift between the Fermi level (Fermi level,E_F) and the band edge. However, there are obvious differences between field effect and chemical doping: (1) the former is electronically controlled, continuous variable and easily reversible; (2) In the former, it is the band edges rather than the E_F that change with the external field, which means that the positions of these edges relative to the redox pairs in solution change, which in turn means that the heterogeneous electron transfer rate constant can be adjusted. They found that the overpotential of hydrogen evolution at 50 mA/cm² current density could be reduced by 140 mV by applying a voltage to the back gate electrode during the electrochemical potential sweep of MoS₂ in a 0.5 M H₂SO₄. Tafel analysis shows that the exchange current density increases by a factor of four to 0.1 mA/cm² with increasing gate voltage. At the same time, the density functional theory calculation shows that the electric field of the gate electrode will induce the increase of the charge on the Mo metal center (active site) near the S vacancy (Fig. 5C), which will lead to the increase of the Mo — H bond strength and promote the hydrogen evolution reaction of water electrolysis. In addition, inspired by the field effect transistor-based electrochemical liquid gate research, Yan et al applied a back-gate voltage to the VSe₂ nanosheet, which induced the redistribution of ions at the interface of the electrolyte -VSe₂ the nanosheet, thus optimizing the kinetics of the adsorption process and accelerating the discharge step^[39]. As shown in fig. 5d, the Tafel slope and the onset overpotential of the hydrogen evolution process of the VSe₂ nanosheet gradually decrease as the back-gate voltage negatively increases from 0 to − 0.8 V, the onset overpotential decreases from 126 mV to 70 mV, and the Tafel slope decreases from 70 mV/dec to 59 mV/dec; At the same time, it can be found that when the back gate voltage reaches -1 V, the cell performance does not improve any more, and even decreases to some extent. Further study found that as the back-gate voltage decreased from 0 V to -1 V, the corresponding charge transfer resistance R_ct decreased from 1.03 MΩ to 0.15 MΩ, and the electrical desorption or recombination reaction resistance R_p decreased from 2.38 MΩ to 1.78 MΩ, which indicated that the back-gate voltage could accelerate the interface charge transfer. Similarly, when the back-grid voltage decreased to -1 V, the R_ct did not decrease (Figure 5e). They used continuous simulation method to study the change of ion distribution under different back-gate voltages. Fig. 5F shows the H⁺ concentration for different back-gate potential deviations along the centerline. Obviously, the back-gate voltage will cause the ions to redistribute in the electrolyte, and the H⁺ concentration on the surface of the SiO₂ will increase. Based on the above discussion, it can be concluded that the back-gate voltage can induce the directional distribution of ions at the electrolyte-electrode material interface, and the concentration of H₃O⁺ on the surface of the electrode material leads to an increase in the coverage of H, thereby reducing the heat of adsorption and accelerating the charge transfer process, thereby improving the cell performance. If the ions are excessively concentrated locally in the dilute electrolyte, the conductivity of the electrolyte will decrease, which will cause the cell performance to no longer improve with the decrease of the back gate voltage^[69].

The change of reactant concentration induced by electric field can promote the catalytic reaction, and the electric field effect can adjust the change of electron transport properties and the potential barrier of charge transfer process. This further coordinates the adsorption kinetics and is the fundamental reason for enhancing its performance.

In the process of hydrogen evolution by electrolysis of water, the distribution of H⁺ ions in the electrolytic cell can be improved under the action of an external electric field, so that the atomic hydrogen adsorption energy is optimized and reduced, and the rapid generation of hydrogen bubbles is realized. At the same time, the addition of electric field can reduce the contact resistance and the sheet resistance of the catalyst through the electric field effect, improve the channel conductance of the catalyst, obtain a faster electron transport rate, and significantly improve the efficiency of hydrogen evolution from water electrolysis. As one of the strategies to effectively reduce the channel resistance and promote the charge transfer, the electric field is fast and easy to control. However, at present, the related research on the performance of hydrogen evolution from water electrolysis enhanced by electric field is less and not deep enough. In the future, electric field enhancement will show more and more excellent performance in the field of electrocatalysis.

3.5 Super-gravity field strengthening

In the process of hydrogen evolution by electrolysis of water, buoyancy is the actual driving force for hydrogen bubbles to break away. After being released, the hydrogen bubbles move along the direction of buoyancy, so when external force acts on the bubbles along the direction of buoyancy, the bubbles can be more effectively promoted to be separated, and the bubble coverage rate on the surface of the catalyst is reduced, thereby strengthening the hydrogen evolution process of electrolytic water. As an important factor affecting the cell voltage in the electrocatalytic hydrogen evolution process, the bubble coverage and the bubble dispersion on the catalyst surface can be effectively improved by the high gravity field^[70,71]. Supergravity field (also known as centrifugal force field) is used to enhance hydrogen evolution from electrolyzed water, that is, the experimental device is placed in a centrifuge to obtain a supergravity field while hydrogen evolution from electrolyzed water. Through the unique flow behavior of the two-phase flow system produced by the high gravity field, the relative velocity and mutual contact between the phases can be enhanced, and the efficient mass transfer and chemical reaction process can be realized. In the gas/liquid system, the heavy phase (electrolyte) moves in the direction of gravity acceleration, and the light phase (bubble) moves in the opposite direction (buoyancy) under the action of supergravity field^[64]. Therefore, high gravity acceleration can increase the slip velocity between phases and the strength of gas-liquid separation^[53]. At the same time, under the action of supergravity field, the critical radius of the bubble nucleus formed on the surface of the catalyst is much smaller, so under the action of enhanced buoyancy,Hydrogen bubbles are easier to form and detach from the electrode surface, and the gas coverage on the electrode surface is significantly reduced, which reduces the reaction overpotential and ohmic voltage drop, thus reducing the cell voltage and system energy consumption.

Wang et al. Have studied in detail the effect of high gravity field on the process of water electrolysis^[40]. They used polycrystalline platinum electrodes as working electrodes and centrifuges to obtain supergravity fields of different sizes (G = 1 ~ 256), and found that in the process of water electrolysis under supergravity fields, the critical volume of bubbles decreased and the coverage of bubbles on the electrode surface decreased significantly. Under normal gravity conditions, the hydrogen bubbles move upward along the electrode surface (Fig. 6a), and the bubbles detach from the electrode surface through line contact with the electrode surface. However, in the supergravity field, the bubble moves upward in an arc due to inertia, and the hydrogen bubble detaches from the electrode surface through point contact (Fig. 6B). It is shown that the contact time between the bubble and the electrode surface is shorter and the bubble coverage on the electrode surface is significantly reduced in the process of bubble detachment under the action of hypergravity field.

In another study, macroscopic observations also found a significant role of hypergravity field in the process of hydrogen evolution from electrolyzed water^[41]. Under normal gravity conditions, due to the low removal rate of hydrogen bubbles in the electrolysis system, hydrogen bubbles are easy to combine with surfactants and electrolytes to form a large number of hydrogen bubbles. As shown in Fig. 6C, a layer of high-density hydrogen foam covers the electrolyte surface under normal gravity (G = 1) after chronoamperometric measurements. This phenomenon means that a large number of hydrogen bubbles can not be quickly separated from the electrolyte during the HER process, resulting in an increase in ohmic resistance. With the increase of G value, the electrolyte can be seen clearly in the central area, and only hydrogen bubbles can be seen at the edge of the cell. Under the action of supergravity field, the speed of hydrogen bubbles breaking away from the electrolysis system is much faster. At the same time, it is found that the ohmic resistance (R_G) during water electrolysis decreases significantly under different gravity coefficients G, and the ohmic resistance decreases approximately linearly with the increase of G value (Fig. 6d). Lao et al. Also used Ni mesh electrode materials and found that under normal cell operating conditions (7.7 mol/L KOH, ~ 348 K) and when the acceleration was less than the 16 g(g=9.81 m·s^-2), the cell voltage could be reduced by more than 80% by using a high gravity field^[42].

The high gravity field can effectively promote the generation and release of hydrogen bubbles in the process of hydrogen evolution from water electrolysis by increasing the slip velocity between phases and the intensity of gas-liquid separation, and reduce the bubble coverage on the surface of the catalyst, thus enhancing the process of hydrogen evolution from water electrolysis. The hypergravity field is usually generated by the operation of a centrifuge, and the essence of the hypergravity field is to provide an external force to accelerate the movement of hydrogen bubbles on the surface of an electrode, thereby enhancing the mass transfer in an electrolyte,In order to improve the efficiency of hydrogen evolution in the system, the strategy of mechanical stirring and vibration in the process of hydrogen evolution in water electrolysis can also achieve the purpose of efficient hydrogen evolution through this mechanism, which can replace the high gravity field to a certain extent. At present, there is a lack of more in-depth microscopic observation and electrochemical testing in the study of electrocatalytic process enhanced by hypergravity field, and the effect of hypergravity field on electrocatalytic process can also be explored more deeply through modeling analysis. The realization of high gravity field consumes a lot of energy, if it is used in the field of electrocatalysis, its economy and controllability should be considered comprehensively, so as to achieve the best performance of promoting electrocatalysis.

3.6 Light Field Enhancement

It is well known that water is a dielectric and can be polarized when an electric field is applied. As shown in Fig. 7A (1), when the electrolyte is electrified, the electric charges near the electrode, including the electric dipoles of anions, cations and water, will be polarized to generate an electric field induced by ions and dipoles. These two electric fields are antiparallel to the electric field formed by the applied direct current. The vector sum of the three electric fields is called the residue electric field (REF)^[43]. Since the applied DC electric field is opposite to the electric field induced by ions and dipoles, the residual electric field is significantly weaker than that generated by the applied DC, so the efficiency of hydrogen evolution from water electrolysis is reduced. In the process of polarization, the addition of an external electric field is helpful to the orderly arrangement of random non-directional dipoles. A laser is an electromagnetic wave that carries both an electric field and a magnetic field. As mentioned above, the presence of an applied magnetic field and an electric field can affect the production of hydrogen. The electric field generated by the laser is enough to polarize the water molecules, and the newly generated electric field and the electric field generated by the applied direct current are in the same direction, so that the electric fields generated by original ions and dipoles are counteracted, and the residual electric field is increased, thereby promoting hydrogen evolution^[72,73].

Based on this, Bidin et al. set up an experimental bench as shown in Fig. 7B to study the effect of collimated solar field on hydrogen production by electrolysis of water in NaCl solution^[43]. As shown in Figure 7A (2), under the condition of collimated solar field, there are two external fields in the electrolytic chamber, one is the electrolysis process provided by the DC power supply, and the other is the collimated solar field. Near the electrode, the direction of the electric field caused by the electric dipole of the ionic compound and water is opposite to that of the DC power supply, and the residual electric field becomes weak. However, under the collimated solar field, the electric dipole field of many ionic compounds and water tends to align along the direction of the electric field induced by the collimated solar field, which is the same as the electric field generated by the applied direct current, increasing the residual electric field and enhancing the hydrogen evolution reaction of water electrolysis. As shown in Figure 7c, as the laser power increases, the conductivity of the electrolyte in the beam path region also increases, which facilitates charge transfer during electrolysis. The hydrogen bonds in a single water molecule are stretched, and this stretching further separates the positive and negative charges of the water molecule, which increases the electric strength of the water dipole. Therefore, if the dipole electric field direction is parallel to the external electric field (caused by the collimated beam), the electric field will also increase. In this paper, the charge separation is quantitatively studied by using the Wenner method to measure the conductivity, and the data obtained show that the greater the laser power, the greater the electric field strength, the more charge separation, and the higher the conductivity (Fig. 7c). In addition, they also found that the surface tension of the electrolyte increases with the increase of laser power, which indicates that the hydrogen bonds in water molecules are stretched in the optical field, which is conducive to the decomposition of water to produce hydrogen and oxygen. It is worth mentioning that different wavelengths of light fields have significant differences in enhancing hydrogen evolution from electrolytic water. Bidin et al. Used three types of lasers, 450 nm, 532 nm and 650 nm, to test the change of hydrogen production with time under different optical fields at room temperature of 26 ℃ and constant power of 120 mW, as shown in Fig. 7d^[73]. As can be seen from the figure, compared with the other two light sources, 532 nm laser (green light) shows the highest hydrogen production rate, about 583 mL/min, which is about 7 times higher than that of normal electrolysis. This is because 532 nm laser will not be absorbed by water, so it is very suitable for the light field of hydrogen evolution by electrolysis of water.

When the light field is added in the hydrogen evolution process of water electrolysis, the water molecules can be polarized, and the electric dipole fields of the anions, the cations and the water tend to be arranged along the direction of the electric field induced by the light field, so that the residual electric field is increased; and meanwhile, hydrogen bonds in the water molecules are stretched, so that hydrogen and oxygen are generated by water decomposition, and the hydrogen evolution efficiency of water electrolysis is improved. Sunlight is a natural and endless source of energy that does not degrade like other energy sources, is environmentally friendly, wasteful and cost-effective. It is a simple and economical technology to use light field to enhance hydrogen evolution from water electrolysis, but the stability of sunlight is insufficient and the controllability is low. At present, the research of hydrogen evolution from water electrolysis in the light field is more and more abundant, and people gradually focus on the actual performance of electrocatalysis enhanced by different intensities and wavelengths of light field.

3.7 Compound field strengthening

The enhancement of hydrogen evolution in electrolysis of water by composite field, that is, adding two or more external fields simultaneously in the process of hydrogen evolution in electrolysis of water, can achieve much higher enhancement effect than a single field. Photothermal field is one of the most commonly used composite fields to enhance hydrogen evolution from water electrolysis. Adding light field in the process of hydrogen evolution from water electrolysis can not only directly promote the polarization of water molecules by the action of light field, but also produce photothermal effect, that is, an additional temperature field is generated in the water electrolysis system, so as to achieve the effect of efficient hydrogen evolution. The photothermal effect comes from the interaction between the photon energy provided by the light field and the lattice, which intensifies the vibration between molecules and leads to the increase of temperature. Gu et al. Added optical field in the process of hydrogen evolution from water electrolysis using Ni/RGO composite materials, and found that the overpotential was 49 mV lower than that without optical field at 10 mA/cm² current density, and the photocatalytic activity was significantly improved^[74]. Fig. 8A is a schematic diagram of hydrogen evolution on the catalyst surface under light and non-light conditions. Irradiating light on the sample, the temperature of the Ni/RGO rapidly reached 50 ° C within 260 s and finally remained at 53 ° C (Fig. 8 B), indicating that the Ni/RGO composite material has a good photothermal conversion capability.

Some people also try to use the optical magnetic field to enhance the hydrogen evolution of electrolyzed water. As mentioned above, adding the optical field to the electrolyte can promote the polarization of water molecules and increase the rate of hydrogen evolution. Then, the Lorentz force generated by the magnetic field drives the convection to promote the vortex around the bubbles and promote the production and release of hydrogen bubbles. Through the combination of optical field and magnetic field, the process of hydrogen evolution from electrolytic water is greatly enhanced. Bidin et al. Found that in the same water electrolysis system (Fig. 8C), the addition of optical field can increase the hydrogen production by more than one time, and the addition of magnetic field can obtain nearly seven times the hydrogen production^[73]. By combining optical and magnetic fields, more than 9 times more hydrogen can be produced than by conventional electrolysis of water (Fig. 8d).

Compound field is a feasible and efficient strategy to enhance hydrogen evolution from water electrolysis, and a reasonable combination of different external fields can achieve the desired catalytic effect. At present, there are many studies on the enhancement of electrocatalytic process by different combinations of composite fields, and the combination of different external fields can play an excellent synergistic role^[73,74]. However, the principle is still to be put forward, so more research on the enhancement of hydrogen evolution by electrolysis of water in composite fields is urgently needed. In the future, there will be more complex fields such as electromagnetic field, thermoelectric field and other technologies to enhance hydrogen evolution from water electrolysis.

4 Conclusions and outlook

In this paper, the research progress on the enhancement of hydrogen evolution by electrolysis of water with external fields at home and abroad in recent years is reviewed, and the basic principles of the enhancement of hydrogen evolution by electrolysis of water with different external fields are preliminarily pointed out. It is found that the temperature field can effectively reduce the charge transfer resistance, increase the apparent activation energy, promote the release of bubbles on the surface of the catalyst, and enhance the mass transfer in the electrolyte, so that the rate of hydrogen evolution from electrolytic water is significantly increased, and the efficiency of hydrogen evolution from electrolytic water is greatly improved. The external electric field can improve the ion distribution in the electrolytic cell, realize the optimization of atomic hydrogen adsorption energy, realize the rapid generation of hydrogen bubbles,At the same time, through the electric field effect, the contact resistance and the sheet resistance of the catalyst are simultaneously reduced, the channel conductance of the catalyst is improved, a faster electron transmission rate is obtained, and the hydrogen evolution efficiency of water electrolysis is significantly improved. When the light field is added in the hydrogen evolution process of water electrolysis, water molecules can be polarized, and the electric dipole fields of ionic compounds and water tend to be arranged along the direction of the electric field induced by the light field, so that the residual electric field is increased; and meanwhile, hydrogen bonds in water molecules are stretched, so that hydrogen and oxygen are generated by water decomposition, and the hydrogen evolution efficiency of water electrolysis is improved. The magnetic field generates magnetohydrodynamic convection in the electrolyte, so that the bubble coverage on the surface of the electrode and the void fraction of dispersed bubbles in the electrolyte are obviously reduced, thereby obviously reducing the ohmic voltage drop and the reaction overpotential and greatly improving the hydrogen evolution performance of water electrolysis. The ultrasonic field can quickly remove the hydrogen bubbles generated on the surface of the electrode, so that the hydrogen bubbles are quickly released, the charge transfer is accelerated, and the efficiency of hydrogen evolution from water electrolysis is improved. The high gravity field can effectively promote the generation and release of hydrogen bubbles in the process of hydrogen evolution from water electrolysis by increasing the slip velocity between phases and the intensity of gas-liquid separation, and reduce the bubble coverage on the surface of the catalyst, thus enhancing the process of hydrogen evolution from water electrolysis. At the same time, based on the review work of this paper, some suggestions for future research and research paths in this field are put forward. Under the condition of comprehensive system energy consumption reduction and additional energy consumption brought by the addition of external field, the reasonable external field is selected to strengthen different electrolytic water systems to achieve the best hydrogen evolution effect. With the development of research, there will be more and more studies on the enhancement of hydrogen evolution from water electrolysis by external fields in the future, and the principles of hydrogen evolution from water electrolysis by different external fields are also worthy of further study and interpretation.

References

Publishing order | Descend order by publishing year | Descend order by cited within

[1]	Li T F, Luo G, Liu K H, Li X, Sun D M, Xu L, Li Y F, Tang Y W. Adv. Funct. Mater., 2018, 28(51): 1805828.

[2]	Pu Z, Wei S, Chen Z, Mu S. Appl. Catal. B, 2016, 196:193.

[3]	Wei R B, Huang Z L, Gu G H, Wang Z, Zeng L X, Chen Y B, Liu Z Q. Appl. Catal. B: Environ., 2018, 231: 101.

[4]	Yu L, Zhou H Q, Sun J Y, Qin F, Yu F, Bao J M, Yu Y, Chen S, Ren Z F. Energy Environ. Sci., 2017, 10(8): 1820.

[5]	Luo X, Ji P X, Wang P Y, Cheng R L, Chen D, Lin C, Zhang J N, He J W, Shi Z H, Li N, Xiao S Q, Mu S C. Adv. Energy Mater., 2020, 10(17): 1903891.

[6]	Zhao Y, Hoivik N, Wang K Y. Nano Energy, 2016, 30: 728.

[7]	Zou X X, Zhang Y. Chem. Soc. Rev., 2015, 44(15): 5148.

[8]	Zhang Y L, Luo M C, Yang Y, Li Y J, Guo S J. ACS Energy Lett., 2019, 4(7): 1672.

[9]	Zhou H Q, Yu F, Huang Y F, Sun J Y, Zhu Z, Nielsen R J, He R, Bao J M, Goddard III W A, Chen S, Ren Z F. Nat. Commun., 2016, 7(1): 1.

[10]	Hai X, Zhou W, Wang S Y, Pang H, Chang K, Ichihara F, Ye J H. Nano Energy, 2017, 39: 409.

[11]

Wakisaka T, Kusada K, Wu D S, Yamamoto T, Toriyama T, Matsumura S, Akiba H, Yamamuro O, Ikeda K, Otomo T, Palina N, Chen Y N, Kumara L S R, Song C, Sakata O, Xie W, Koyama M, Kubota Y, Kawaguchi S, Arevalo R L, Aspera S M, Arguelles E F, Nakanishi H, Kitagawa H. J. Am. Chem. Soc., 2020, 142(3): 1247.

[12]	Yang Y Q, Zhang K, Lin H L, Li X, Chan H C, Yang L C, Gao Q S. ACS Catal., 2017, 7(4): 2357.

[13]	Zhao Y G, Mavrokefalos C K, Zhang P, Erni R, Li J G, Triana C A, Patzke G R. Chem. Mater., 2020, 32(4): 1371.

[14]	Liu D, Tong R, Qu Y J, Zhu Q, Zhong X W, Fang M L, Ho Lo K, Zhang F F, Ye Y C, Tang Y X, Chen S, Xing G C, Pan H. Appl. Catal. B: Environ., 2020, 267: 118721.

[15]	Zhao Y Q, Jin B, Zheng Y, Jin H Y, Jiao Y, Qiao S Z. Adv. Energy Mater., 2018, 8(29): 1801926.

[16]	Wang G X, Chen W, Chen G L, Huang J, Song C S, Chen D L, Du Y, Li C R, Ostrikov K K. Nano Energy, 2020, 71: 104637.

[17]	Jiang K, Liu B Y, Luo M, Ning S C, Peng M, Zhao Y, Lu Y R, Chan T S, Groot F M F, Tan Y W. Nat. Commun., 2019, 10(1): 1.

[18]	Wu H, Lu X, Zheng G F, Ho G W. Adv. Energy Mater., 2018, 8(14): 1870064.

[19]	Xia C, Liang H F, Zhu J J, Schwingenschlögl U, Alshareef H N. Adv. Energy Mater., 2017, 7(9): 1602089.

[20]	Wu X Q, Guo Y R, Wang T, Sun B X, Liu Z L, Wu Y, Zhang S J, Zheng J, Li X G. Chem. Commun., 2019, 55(29): 4202.

[21]	Xiao P, Chen W, Wang X. Adv. Energy Mater., 2015, 5(24): 1500985.

[22]	Wang Y, Liu L N, Zhang X, Yan F, Zhu C L, Chen Y J. J. Mater. Chem. A, 2019, 7(39): 22412.

[23]	Ha D H, Han B H, Risch M, Giordano L, Yao K P C, Karayaylali P, Shao-Horn Y. Nano Energy, 2016, 29: 37.

[24]	Xiao P, Ge X M, Wang H B, Liu Z L, Fisher A, Wang X. Adv. Funct. Mater., 2015, 25(10): 1520.

[25]	Li J S, Wang Y, Liu C H, Li S L, Wang Y G, Dong L Z, Dai Z H, Li Y F, Lan Y Q. Nat. Commun., 2016, 7(1): 1.

[26]	Tang C Y, Wang W, Sun A K, Qi C K, Zhang D Z, Wu Z Z, Wang D Z. ACS Catal., 2015, 5(11): 6956.

[27]	Theerthagiri J, Lee S J, Murthy A P, Madhavan J, Choi M Y. Curr. Opin. Solid State Mater. Sci., 2020, 24(1): 100805.

[28]	Zhou P, Xing D N, Liu Y Y, Wang Z Y, Wang P, Zheng Z K, Qin X Y, Zhang X Y, Dai Y, Huang B B. J. Mater. Chem. A, 2019, 7(10): 5513.

[29]	Zhang Y Q, Ouyang B, Xu J, Chen S, Rawat R S, Fan H J. Adv. Energy Mater., 2016, 6(11): 1600221.

[30]	Yu L, Song S W, McElhenny B, Ding F Z, Luo D, Yu Y, Chen S, Ren Z F. J. Mater. Chem. A, 2019, 7(34): 19728.

[31]	Sharma L, Khushwaha H S, Mathur A, Halder A. J. Solid State Chem., 2018, 265: 208.

[32]	Piontek S, Andronescu C, Zaichenko A, Konkena B junge Puring K, Marler B, Antoni H, Sinev I, Muhler M, Mollenhauer D, Roldan Cuenya B, Schuhmann W, Apfel U P. ACS Catal., 2018, 8(2): 987.

[33]	Diao Z, Dunne P A, Zangari G, Coey J M D. Electrochem. Commun., 2009, 11(4): 740.

[34]	Liu H B, Xu H T, Pan L M, Zhong D H, Liu Y. Int. J. Hydrog. Energy, 2019, 44(41): 22780.

[35]	Liu Y, Pan L M, Liu H B, Chen T M, Yin S Y, Liu M M. Int. J. Hydrog. Energy, 2019, 44(3): 1352.

[36]	Li S D, Wang C C, Chen C Y. Electrochimica Acta, 2009, 54(15): 3877.

[37]	Hung C Y, Li S D, Wang C C, Chen C Y. J. Membr. Sci., 2012, 389: 197.

[38]	Ling F L, Zhou T W, Liu X Q, Kang W, Zeng W, Zhang Y X, Fang L, Lu Y, Zhou M. Nanotechnology, 2018, 29(3): 03LT01.

[39]	Yan M Y, Pan X L, Wang P Y, Chen F, He L, Jiang G P, Wang J H, Liu J Z, Xu X, Liao X B, Yang J H, Mai L Q. Nano Lett., 2017, 17(7): 4109.

[40]	Wang M Y, Wang Z, Guo Z C. Int. J. Hydrog. Energy, 2009, 34(13): 5311.

[41]	Wang M Y, Wang Z, Guo Z C. Int. J. Hydrog. Energy, 2010, 35(8): 3198.

[42]	Lao L, Ramshaw C, Yeung H. J. Appl. Electrochem., 2011, 41(6): 645.

[43]	Bidin N, Azni S R, Abu Bakar M A, Johari A R, Abdul Munap D H F, Salebi M F, Abd Razak S N, Sahidan N S, Sulaiman S N A. Int. J. Hydrog. Energy, 2017, 42(1): 133.

[44]	Tuomi S M, Guil-Lopez R, Kallio T. J. Catal., 2016, 334: 102.

[45]	Niether C, Faure S, Bordet A, Deseure J, Chatenet M, Carrey J, Chaudret B, Rouet A. Nat. Energy, 2018, 3(6): 476.

[46]	Feng W S, Pang W B, Xu Y, Guo A M, Gao X H, Qiu X Q, Chen W. ChemElectroChem, 2020, 7(1): 31.

[47]	Morales-Guio C G, Stern L A, Hu X L. Chem. Soc. Rev., 2014, 43(18): 6555.

[48]	Strmcnik D, Lopes P P, Genorio B, Stamenkovic V R, Markovic N M. Nano Energy, 2016, 29: 29.

[49]	Ledezma-Yanez I, Wallace W D Z, Sebastián-Pascual P, Climent V, Feliu J M, Koper M T M. Nat. Energy, 2017, 2(4): 1.

[50]	Conway B E, Jerkiewicz G. Electrochimica Acta, 2000, 45(25/26): 4075.

[51]	Ma Q, Ba J Z, Jiang Y X, Li J, Wu W H, Chem. Defenc. Ships, 2007, 6.

[52]	(马强, 巴俊洲, 蒋亚雄, 李军, 吴文宏. 舰船防化, 2007, 6.).

[53]	Aldas K. Appl. Math. Comput., 2004, 154(2): 507.

[54]	Wang M Y, Wang Z, Gong X Z, Guo Z C. Renew. Sustain. Energy Rev., 2014, 29: 573.

[55]	Zhang B B, Daniel Q, Cheng M, Fan L Z, Sun L C. Faraday Discuss., 2017, 198: 169.

[56]	Zou Y J, Xiao Z A, Fei X M. Chin. J. Rare Metal, 2004, 954.

[57]	(邹勇进, 肖作安, 费锡明. 稀有金属, 2004, 954.).

[58]	Padhy G K, Ramasubbu V, Parvathavarthini N, Wu C S, Albert S K. Int. J. Hydrog. Energy, 2015, 40(20): 6714.

[59]	Conway B E, Tilak B V. Electrochimica Acta, 2002, 47(22/23): 3571.

[60]	Li T H, Kang S, Zhang X, Fu X, Feng S L, Hu Z F, Zhu D L, Lu W Q. J. Phys. D: Appl. Phys., 2020, 53(18): 185302.

[61]	Lai Y S,Del Rosario M A J V G, Chen W F, Yen S C, Pan F, Ren Q, Su Y H. ACS Appl. Energy Mater., 2019, 2: 7092.

[62]	Monzon L M A, Coey J M D. Electrochem. Commun., 2014, 42: 38.

[63]	Lin M Y, Hourng L W, Kuo C W. Int. J. Hydrog. Energy, 2012, 37(2): 1311.

[64]	Kaya M F, Demir N, Albawabiji M S, Tas M. Int. J. Hydrog. Energy, 2017, 42(28): 17583.

[65]	Koza J A, Mühlenhoff S, Uhlemann M, Eckert K, Gebert A, Schultz L. Electrochem. Commun., 2009, 11(2): 425.

[66]	Darband G B, Aliofkhazraei M, Shanmugam S. Renew. Sustain. Energy Rev., 2019, 114: 109300.

[67]	Weier T, Baczyzmalski D, Massing J, Landgraf S, Cierpka C. Int. J. Hydrog. Energy, 2017, 42(33): 20923.

[68]	Wang J H, Yan M Y, Zhao K N, Liao X B, Wang P Y, Pan X L, Yang W, Mai L Q. Adv. Mater., 2017, 29(7): 1604464.

[69]	Voiry D, Fullon R,Yang J, de Carvalho Castro e Silva C, Kappera R, Bozkurt I, Kaplan D, Lagos M J, Batson P E, Gupta G, Mohite A D, Dong L, Er D Q, Shenoy V B, Asefa T, Chhowalla M. Nat. Mater., 2016, 15(9): 1003.

[70]	Wang Y, Udyavara S, Neurock M, Frisbie C D. Nano Lett., 2019, 19(9): 6118.

[71]	Pell W G, Conway B E, Marincic N. J. Electroanal. Chem., 2000, 491(1/2): 9.

[72]	Wang M Y, Xing H Q, Wang Z, Guo Z C. Acta Phys.- Chim. Sin., 2008, 24:520.

[73]	Zhao L X, Guo Z C, Wang Z, Wang M Y. Metall. Mater. Trans. B, 2010, 41(3): 505.

[74]	Shahid M, Bt Bidin N, Rehman A. Optoelectron. Adv. Mat., 2010, 4:1670.

[75]	Bidin N, Azni S R, Islam S, Abdullah M, Ahmad M F S, Krishnan G, Johari A R, Bakar M A A, Sahidan N S, Musa N, Salebi M F, Razali N, Sanagi M M. Int. J. Hydrog. Energy, 2017, 42(26): 16325.