Severe Plastic Deformation for Advanced Electrocatalysts for Electrocatalytic Hydrogen Production

Xiying Jian; Jian Li; Liqing He; Hai-Wen Li; Meng Zhang; Peng Zhang; Huai-Jun Lin

doi:10.2320/matertrans.MT-MF2022011

Abstract

Developing electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of water splitting is very important for electrocatalytic hydrogen production. Very recently, emerging study have demonstrated that materials show some superior HER and OER performances of water splitting after processed by severe plastic deformation (SPD). In this work, recent advances on electrochemical hydrogen production performances of materials by SPD are summarized. Some basic principles of electrocatalytic water splitting are briefly described, and then the development of SPD technology and recent advances on SPDed materials with enhanced hydrogen production properties are comprehensively reviewed. Moreover, future direction on SPDed materials for hydrogen production is also prospected.

1. Introduction

As the continuous impact of geopolitical conflicts, global energy market continues to usher in great changes, and the world is facing the risk of energy crisis. Traditional fossil energy sources include coal, oil, natural gas, etc., which are deposited from the fossils of ancient organisms. When burned, fossil fuels emit greenhouse gases and damage the environment. Current energy systems may not be able to meet green and low-carbon energy requirements. Therefore, countries around the world are trying to reduce their dependence on traditional fossil fuels and seek energy transformation. In this context, the development of renewable energy sources has become more and more important.¹⁾ Nuclear power is one of the major pillars of the world’s supply of green energy. In some countries such as France and Belgium, nuclear energy has become the major source of electricity. Nuclear power provides continuous and sufficient power delivery, but its safety is difficult to guarantee. Once nuclear leakage occurs, it will affect the safety of the whole area residents.²^,³⁾ Hydrogen energy is another promising alternative to traditional energy sources.⁴^–⁶⁾

Hydrogen energy has some obvious advantages as follows.⁷^–¹¹⁾ First, only clean water is produced after hydrogen combustion, which will not cause pollution to the environment. Second, the energy density of hydrogen is about 3 times that of gasoline, 3.9 times that of alcohol and 4.5 times that of coke. Hydrogen fuel engine has the characteristics of strong power and endurance. Hydrogen fuel is also a good choice for tanks, ships and transport vehicles. Third, the calorific value of hydrogen is high. Except for nuclear fuel, the calorific value of hydrogen is the highest among all fossil fuels, chemical fuels and biofuels. What’s more, hydrogen can be used advantageously as a chemical feedstock in many fields like petrochemical, microelectronics, chemical and polymer synthesis.

In the past few decades, various hydrogen production technologies have made significant progress, and traditional fossil fuel hydrogen production technology has developed maturely. At present, hydrogen production from fossil fuel is still the major processes in which steam reforming of natural gas is one of the most economic processes. But in these processes, a large amount of CO₂ will be released, which is not in line with the policy of low carbon emission reduction.¹²^,¹³⁾ Photocatalytic water splitting and electrocatalytic water splitting are two cutting-edge ways of hydrogen production. Unlike other methods of hydrogen production, they have the advantages of high purity of hydrogen produced, does not emit greenhouse gases and provides an excellent adaptability to the environment and efficiently produce hydrogen through photochemical and electrochemical energy devices.¹⁴⁾ TiO₂ is the most representative photocatalytic catalyst for water splitting. TiO₂ photocatalysts have been widely studied since Fujishima discovered the photocatalytic splitting of water on TiO₂ electrodes in 1972.¹⁵⁾ Nowadays, many metal oxides are used to photocatalytic water splitting, such as TiO₂, ZnO, ZrO₂, and Fe₂O₃ and so on.¹⁶^–²⁰⁾

The research on water electrolysis catalysts can be mainly divided into two directions: noble metal catalysts and non-noble metal catalysts. Specific examples of these two kinds of catalysts are discussed in detail in the body below. Although the two catalysts have many differences, they both follow the same principle of improvement: increasing the active sites of the catalyst. The researchers used to increase the active site of the catalyst through surface modification, monatomization and other methods. However, the disadvantages of technological difficulties of preparation and high price hinder the development of water electrolysis catalysts.

Severe plastic deformation (SPD) is an efficient way to produce bulk ultrafine-grains.²¹^,²²⁾ Large strains can be easily introduced during the deformation process to refine the grains. Compared with the conventional plastic deformation method, the SPD method produces a larger amount of deformation through strain accumulation, which can effectively obtain sub-micron or even nano-sized grains. Furthermore, crystal defects and oxygen vacancies can be introduced into the material through SPD method. These defects and oxygen vacancies can serve as the reaction zone of HER. In other words, SPD treatment increases the number of active sites in the catalysts. Therefore, SPD has a deep application prospect in the modification of electrocatalytic water splitting catalyst materials. Recently, there are some emerging studies on improving electrocatalytic hydrogen production properties of materials by SPD, particularly those in amorphous structures. In this review, recent advances on electrocatalytic hydrogen production properties of materials by SPD are summarized. First, some basic principles of electrocatalytic water splitting are described. Second, development of SPD technology is briefly reviewed. Finally, recent advances on SPDed materials with enhanced hydrogen production properties are summarized. Moreover, future direction on SPDed materials for electrocatalytic hydrogen production reactions is prospected.

2. Mechanism of Electrocatalytic Water Splitting

Electrocatalytic water splitting is an effective way to produce high-purity hydrogen on a large scale in a short period of time without causing too much burden on the environment, and is an excellent way to convert energy. However, the resistance of pure water is very large, which up to 18 MΩ cm so it is very hard to split. Tap water has a resistance of only 5 Ω cm, so it can theoretically be splitted with a low energy supply. But in reality, electrolysis of tap are not energy-efficient as they contain many redox active impurity ions that would take up extra energy for side reactions.²³⁾ In order to inhibit the side reactions, extremely acidic electrolytes or extremely basic electrolytes such as KOH and NaOH are used in electrocatalytic water splitting. Electrocatalytic water splitting is mainly composed with two of half reactions. The oxygen evolution reaction (OER) occurs at the anode to produce O₂, and the hydrogen evolution reaction (HER) occurs at the cathode to produce H₂. Equation (1) is the reaction of water splitting which can be separated into two half-cell reactions (eq. (2) and eq. (3)).

\begin{equation} H_{2}O \to H_{2} + \frac{1}{2}O_{2}\quad E^{\circ} = 1.23\,\text{V}\ \text{vs.}\ \text{SHE} \end{equation}

(1)

\begin{equation} 2H^{+} + 2e^{-} \to H_{2}\quad E^{\circ} = 0\,\text{V} \end{equation}

(2)

\begin{equation} 2H_{2}O \to O_{2} + 4H^{+} + 4e^{-}\quad E^{\circ} = 1.23\,\text{V} \end{equation}

(3)

Electrolyzed water requires an electrolytic cell to provide the stable working environment. According to the above equations, the minimum cell voltage required to split water into hydrogen and oxygen is 1.23 V. Electrolyzed water cells operating at two sides of extreme pH have their advantages and disadvantages. For example, there are many protons that can be used under acidic conditions, and it is easier to form intermediates and release H₂, so the HER electrocatalytic efficiency at the cathode is high. However, the anode extreme oxygen evolution reaction exhibits very slow kinetic characteristics under acidic conditions. Similarly, because the electrolyte with 14 pH cannot quickly provide protons to the cathode for water electrolysis, the efficiency of hydrogen production under alkaline conditions is very low.²⁴⁾

2.1 Mechanism of hydrogen evolution reaction

The HER in acid medium proceeds through the following elementary steps: First, mass transfer such as convection, diffusion or electromigration is used to transport H⁺ from solution to electrode surface. At the beginning of HER, H⁺ is adsorbed on the surface of the electrode to form adsorbed hydrogen atoms, which is known as the Volmer reaction. Adsorbed hydrogen atoms created on the electrode surface may generate hydrogen molecules in two different ways (Heyrovsky step and Tafel step) and desorb from the electrode surface.

\begin{equation} \text{Volmer}\quad H^{+} + e^{-} + * \to H_{ad} \end{equation}

(4)

\begin{equation} \text{Tafel}\quad 2H_{ad} \to H_{2} + 2* \end{equation}

(5)

\begin{equation} \text{Heyrovsky}\quad H_{ad} + H^{+} + e^{-} \to H_{2} + * \end{equation}

(6)

where $*$ represent active site in these equations. In alkaline medium, Volmer step and Heyrovsky step will be different because of water dissociation.

\begin{equation} \text{Volmer}\quad H_{2}O + e^{-} + * \to H_{ad} + OH^{-} \end{equation}

(7)

\begin{equation} \text{Heyrovsky}\quad H_{ad} + H_{2}O + e^{-} \to H_{2} + OH^{-} + * \end{equation}

(8)

At the beginning of HER reaction, the catalyst generates H_ad through the Volmer step. Then, Heyrovsky step or Tafel step occurs according to the adsorption capacity of the electrode surface for hydrogen ions. In the whole reaction process, hydrogen atoms adsorbed on the electrode surface is particularly important. The ability of the electrode surface to adsorb hydrogen atoms is called Gibbs adsorption energy (ΔGH^*), which can reflect the activity of the electrode used in hydrogen evolution reaction. According to Sabatier’s principle, HER kinetics depends on the ΔGH^* of the catalyst.²⁵^–²⁸⁾ When ΔGH^* < 0, The smaller the value, the weaker the metal binds to hydrogen atoms and is less able to form stable H^*. While ΔGH^* > 0, The larger the value, the stronger the binding force between the metal and the hydrogen atom, which is not conducive to the release of hydrogen. As a result, the closer the metal’s ΔGH^* is to zero, the better it’s HER performance. Based on these conclusions, Trasatti constructed the first volcanic curve for the HER catalyst, as shown in Fig. 1.²⁹⁾

Fig. 1

Volcano plot of exchange current density vs. ΔGH^*.²⁵⁾

2.2 Mechanism of oxygen evolution reaction

OER is an extremely complex electrochemical process, which involves multiple interfaces like solid, liquid, and gas phase containing oxygen diffusion, bond cleavage, reactant and intermediate adsorption, product desorption, and so on.³⁰⁾ Therefore, the OER process is not as well understood as the HER process. Due to the following reasons, many difficulties have been brought to the research of OER mechanisms.

1) OER is a multi-electron electrochemical process with 4 electrons participating in the reaction. There are various intermediate steps and intermediate products in the electrode process.
2) The reversibility of the reaction is very low. OER is always accompanied by high overpotential, so it is almost impossible to study the dynamics of OER near the thermodynamic equilibrium potential, and it is even difficult to measure the accurate equilibrium potential experimentally.
3) Many metals are thermodynamically unstable until the required overpotential for OER is reached, where dissolution and oxidation of metals occur, or metal dissolution and oxidation occur simultaneously with OER. This brings many difficulties to the study of OER mechanism.

Figure 2 shows the possible OER mechanism under acid (black line) and alkaline (grey line) conditions.²⁷^,³¹⁾ The proposed OER mechanism under alkaline conditions are as follows:

\begin{equation} * + O_{2} + H_{2}O + e^{-} \to OOH^{*} + OH^{-} \end{equation}

(9)

\begin{equation} OOH^{*} + e^{-} + \to O^{*} + OH^{-} \end{equation}

(10)

\begin{equation} O^{*} + H_{2}O + e^{-}\to OH^{*} + OH^{-} \end{equation}

(11)

\begin{equation} OH^{*} + e^{-} \to OH^{-} + * \end{equation}

(12)

Fig. 2

The OER mechanism for acid and alkaline conditions.²⁷⁾

In acidic conditions, the proposed OER mechanism change to follows:

\begin{equation} * + H_{2}O + \to OH^{*} + H^{+} + e^{-} \end{equation}

(13)

\begin{equation} OH^{*} + OH^{-} \to O^{*} + H_{2}O + e^{-} \end{equation}

(14)

\begin{equation} 2O^{*} \to 2* + O_{2} \end{equation}

(15)

\begin{equation} O^{*} + H_{2}O \to OOH^{*} + H^{+} + e^{-} \end{equation}

(16)

\begin{equation} OOH^{*} + H_{2}O \to * + O_{2} + H^{+} + e^{-} \end{equation}

(17)

2.3 Catalytic materials for electrocatalytic water splitting

Noble metal catalysts, such as Pt- and Pd-based compounds own a close-to-zero Gibbs free energy (ΔGH^*), which facilitates the adsorption and desorption of H₂ and achieving a high exchange current density to showing an excellent electrocatalytic water splitting catalytic performance. Single atom noble metals are widely used in HER catalysts due to the virtues of their atomic dispersion, maximized noble metal utilization, unsaturated coordination and its capacity of fine control of the electronic structure. In single atom catalysts, metal nanoparticles are immobilized on a support and minimizes the structure of metal components, which provides the well-defined active sites and reactive species. Yi et al.³²⁾ found that the fine control of oxidation states of single-atom Pt catalysts can modulates the catalytic activities of HER, achieving a lower overpotential an lower mass activities than commercial Pt/C catalyst at current density of 10 mA cm⁻². Sun et al.³³⁾ used an atomic layer deposition method to load single-atom Pt clusters on nitrogen-doped graphene nanosheet substrates (ALD Pt/NGN) as HER catalyst, achieving a lower overpotential than commercial Pt/C catalyst and its Tafel slope is only 29 mV dec⁻¹. However, the high price of noble metal limits their commercial use.³⁴⁾

Another hotspot of the water splitting catalysts is high-entropy alloys (HEAs). The HEAs contains five or more elements which usually have similar atomic ratios, and the content of each component is relatively high concentrations (5–35 at%). The presence of multiple components is conducive to promoting the formation of the solid solution phase and inhibiting the movement of dislocations.³⁵^–³⁹⁾ The superior catalytic efficiency of high-entropy alloy catalysts is likely to be attributed to the synergistic catalytic interaction between different components.⁴⁰⁾ Li et al.⁴¹⁾ used a low-temperature oil phase strategy to synthesize ultrasmall (∼3.4 nm) HEAs Pt₁₈Ni₂₆Fe₁₅Co₁₄Cu₂₇ nanoparticles achieving ultrasmall overpotential of 11 mV at the current density of 10 mA cm⁻².

Metallic glasses (MGs), which are usually prepared by rapid solidification, can serve as self-supporting catalysts for electrocatalytic water splitting. The metastable characteristics endow better properties than those of the crystalline materials, including catalytic and chemical properties MGs is rather homogeneous at sub-nanometer scale.⁴²^–⁵⁰⁾ Wang et al.⁴³⁾ developed a noble-metal-free FeCoMoPB amorphous nanoplate for alkaline water oxidation with reliable stability of 48 h. Li et al.⁴⁴⁾ created an order/disorder interfaces Fe–Ni–B metallic glass matrix, exhibiting a low oxygen-evolving overpotential of 214 mV at 10 mA cm⁻², a small Tafel slope of 32.4 mV dec⁻¹, and good stability in alkaline media.

3. Severe Plastic Deformation (SPD) Technology

3.1 Development of severe plastic deformation (SPD)

The principle of obtaining high strength and superior performance in metal alloys through severe plastic deformation can be traced back to the Han Dynasty more than 2,000 years ago.⁵¹⁾ Since the pioneering work on the preparation of ultrafine nanocrystals using severe plastic deformation (SPD).⁵²^,⁵³⁾ There are several SPD technologies and they can be found in some previous review works.⁵⁴^,⁵⁵⁾ In this study, we only focus on the two SPD technologies, including equal channel angular pressing (ECAP) and high-pressure torsion (HPT), which have attracted attention to improve the electrocatalytic performances of materials.

Equal channel angular pressing (ECAP) is an efficient method to introduce large plastic deformation as first proposed by Segal, a former Soviet scientist, in the 1980s in order to obtain pure shear deformation.⁵⁶⁾ ECAP pressed molds have two equal cross-section channels, usually intersecting at an angle of 90° (Fig. 3(a)). It is often used to process materials that are difficult to deform. The sample is placed in an extrusion mold consisting of two intersecting isometric channels, and the punch pressure presses the specimen into the channel at a constant speed, resulting in uniform and violent shear deformation at the corner. Since the cross-sectional area of the specimen before and after extrusion remains unchanged, it can be repeatedly extruded to obtain a larger amount of deformation by accumulating the deformation amount.

Fig. 3

Principles of severe plastic deformation techniques. (a) High-pressure torsion; (b) Equal channel angular pressing.⁸⁵⁾

High-pressure torsion (HPT) is another SPD technique which was first proposed by Bridgman in 1935.⁵⁷⁾ In 1943, when Bridgman published a classic paper in the Journal of Applied Physics, “On Torsion Combined with Compression”, that HPT treatment was recognized. In this early report, Bridgman succinctly set out the basic tenets of this type of testing by stating:⁵⁸⁾ “If a bar is twisted while a longitudinal compressive load is simultaneously applied, it is possible to twist the bar through much greater angles without fracture than is possible without the compressive load. At the same time the magnitude of the torque which the bar can support without fracture is increased”. This fundamental concept formed the basis of a series of experiments conducted by Professor Bridgman during his tenure as the Hollis Professor of Mathematics and Natural Philosophy at Harvard University. All of these experiments were centered primarily on the effects of very high pressures on solids and ultimately, in 1946, Bridgman was awarded the Nobel Prize in Physics.⁵⁹⁾ The principle of HPT is shown in Fig. 3(b), The upper and lower dies apply high pressure P, and the relative torsion between the upper and lower dies introduces strong shear strain.⁶⁰⁾ The upper and lower molds were originally made of chrome steel, which was replaced by WC in 1940⁵⁵⁾ to double the load limit of the mold, and this set of molds has been used to this day.

Early HPT technology was mainly used for grain refinement of elemental metal materials. Alberdi (1984) and Gil Sevillano (1987) reported the formation of ultrafine-grained microstructure in commercially pure Al during severe torsional deformation, as shown in Fig. 4.⁶¹⁾ Zhorin et al.⁶²⁾ used HPT technology to prepare copper powder with a grain size of less than 50 nm. However, Bridgman has found through many experiments that HPT treatment of elemental metals not only brings grain refinement, but also most of them are accompanied by phase transitions (Table 1). In addition, Bridgman has used traditional HPT methods to study binary alloys, intermetallic compounds and other material systems.

Fig. 4

Performance of different elemental metals after HPT treatment.⁶¹⁾

Table 1 Some pure elements and pressures for phase transformations.⁸¹^–⁸⁴⁾

After decades of development, traditional HPT technology has derived many new methods. An important limitation with conventional HPT is that the samples are extremely small. Typically, the processed specimens are in the form of disks having diameters of 1 cm and thicknesses less than 1 mm. In an attempt to overcome this limitation, exploratory tests were undertaken using bulk samples in the form of small cylinders with diameters of 1 cm and well-controlled heights of 8.57 mm (Fig. 5(a)).⁶³⁾ In addition, Bridgman⁶⁴⁾ was also aware of the problem of inhomogeneous strain when HPT processed cylindrical bulk samples, and proposed that thin-walled tubes could be used instead of cylindrical samples to overcome them. In recent years, scholars have popularized this concept and extended it to ring samples (Fig. 5(b)).⁶⁵⁾ A recent report described an alternative two-step processing route for HPT which was termed the 2-HPT procedure.⁶⁶⁾ The principle of this method is shown in Fig. 5(c), where the first torsional strain on the left consists of applying conventional HPT to 10 clockwise to a disc with an initial diameter of 20 mm and applying a pressure of 1.25 GPa. After completing the entire 10 revolutions, the disc is cut into a new disc with a diameter of 10 mm, so that its outer edge passes through the center point of the original disc. Then, under pressure of 5 GPa, this new disc twists and pulls 10 turns, called Route I in a counterclockwise direction, or Path II in the same clockwise direction.⁶⁶⁾

Fig. 5

(a) Schematic illustration of an HPT facility and in operation with a pressure P for use with the processing of bulk samples; (b) Schematic illustration of the application of HPT processing to ring samples; (c) The principle of the two-step 2 HPT procedure showing the first step on the left and the second step on the right with the separate processing Routes I and II.⁶⁵^,⁶⁶^,⁸⁶⁾

Recently, HPT technology has expanded into electrocatalytic applications, including photocatalytic CO₂ conversion and H₂ evolution. Severe plastic deformation by high-pressure torsion (HPT) is used to form nanostructures of metallic materials and is used as a novel catalyst in the field of catalysis. In the field of carbon dioxide conversion, HPT can not only improve the recombination rate of electrons and vacancies, but also improve some other optical properties, showing excellent catalytic performance.⁶⁷⁾ In the field of H₂ evolution, HPT forms some nanostructures (such as nanoglass) and nano defects, such as flow units, and increase the surface-active area, thereby exhibiting excellent catalytic performance.

3.2 SPD enhances electrocatalytic water splitting

Metallic glasses (MGs), which are known as amorphous alloys, have showed superior properties for HER and OER because of the long-range disordered atomic structure.⁶⁸^,⁶⁹⁾ Even though the atoms in metallic glasses are uniformly and randomly distributed from a macroscopic perspective, some local domains with inhomogeneous chemical compositions are also existed inside the metallic glasses which is called flow units.⁷⁰⁾ Many thermal or mechanical methods, such as shear stress and isothermal anneal, can change the density of flow units so as to improve the properties of MGs.⁷¹^,⁷²⁾ HPT as one of the methods of SPD combining compression and concurrent torsion is an efficient technique to induce abundant flow units and hereby improve the properties of MGs. Wu et al.⁷³⁾ studied the effect of different HPT turns on HER performance. As shown in Fig. 6(a), amorphous Fe₇₈Si₉B₁₃ ribbons were produced by melt spinning, and then treated by HPT with different parameters: rotations of 1, 2 and 10 turns (marked as HPT-1N, HPT-2N and HPT-10N). Figure 6(b) shows the XRD patterns of ribbon and HPT-treated samples. A broad diffraction peak can be seen in ribbon’s XRD pattern, which indicate the amorphous structure. For the HPT-treated samples, some diffraction peaks for α-Fe, Fe₂B and Fe₃Si existed in the XRD pattern. The intensity of these peaks enhances while the HPT turns increase. Figure 6(c) shows the LSV curves of ribbon and HPT-treated samples. With the HPT turns increase, the overpotential at the current density of 10 mA cm⁻² of the HPT-treated sample decreases from 385 mV to 112 mV. The amperometric i-t method was used to study the catalytic cycling performance of HPT sample, the result is shown in Fig. 6(d). In the continuous 50-hour stability test, the current density of HPT-treated sample increased to a maximum of 128%, and after 50 hours of reaction, the catalytic efficiency can still be maintained at the same level as the initial state, which indicates that HPT-treated sample has excellent catalytic stability and service life.⁷³⁾ More research show that, HPT treatment results in the formation of nanoglasses structures. Nanoglasses is formed by the enrichment of internal amorphous atoms, that is, under the dual action of high pressure and plastic deformation, the originally long-range disordered atoms inside the amorphous alloy are enriched into more ordered atomic clusters to form nano boundaries.⁷⁴^,⁷⁵⁾

Fig. 6

(a) Schematic illustration for the fabrication of Fe-based nanoglasses alloy; (b) XRD patterns of Fe₇₈Si₉B₁₃ ribbons and HPT sample; (c) LSV curves; (d) long-term durability tests.⁷³⁾

The nanoglasses structure of Fe₇₈Si₉B₁₃ amorphous alloy was introduced by HPT treatment to realize its body-surface synergistic catalysis, which broke through the situation of limited catalytic region and catalyst surface, so that it had excellent HER catalytic activity and stability. However, the low charge transfer efficiency of Fe-based amorphous alloys has not been improved, and the corrosion products (FeOOH) generated during the promotion process are not conducive to HER catalytic activity. Studies have shown that FeOOH has synergistic catalytic and surface stabilizing effects when compounded with certain superior conductors such as carbon nanotubes, nickel foam and iron-based amorphous alloys, showing excellent OER performance. Wu et al.⁷⁶⁾ prepared Fe-based nanoglasses covered by FeOOH layer (FeOOH@NG) by cathodic corrosion. The nanoglasses structure was prepared by HPT treatment. Connecting structure between surface and volume through the abundant ordered boundaries produced by the nanoglasses can well promote the catalytic reactions by using the strong oxidation of hydroxyl radicals (OH^•) to destroy the Fe–Fe bond. Melt spinning Fe₇₈Si₉B₁₃ amorphous alloy ribbons covered by FeOOH layer (FeOOH@MSG) was also prepared for comparison. SEM and EDS mapping are shown in Fig. 7(a)–(f). Different from the homogeneous and tightly distributed FeOOH on the surface of FeOOH@MSG sample, the FeOOH on the FeOOH@NG sample surface is discontinuously dispersed over a nanoscale amorphous matrix, resembling a woodland structure. Figure 7(g)–(h) show that FeOOH@NG sample displays excellent activity and stability for OER, yielding a low overpotential of only 240 mV at a current density of 10 mA cm⁻² and the better stability than both melt-spun glass (MSG) and FeOOH@MSG samples.

Fig. 7

(a)–(b) FESEM images of the MSG sample before and after corrosion; (c) EDS mapping of the FeOOH@MSG sample; (d)–(e) FESEM images of the NG sample before and after corrosion; (f) EDS mapping of the FeOOH@NG sample; (g) LSV curves; (h) stability performance.⁷⁶⁾

SPD treatment of amorphous alloys can not only generate nanoglasses, but also most likely promote catalysis by forming microstructure synergy. Chu et al.⁷⁷⁾ put Fe_73.5Si_13.5B₉Cu₁Nb₃ amorphous alloy with different turns of high-pressure torsion (marked as HPT-1N, HPT-2N, HPT-5N, HPT-10N), and the alloy after HPT treatment has different degrees of change in catalytic performance. Due to energy injection, the amorphous alloy produces Fe₂B and Fe₃Si crystal phases (shown in Fig. 8(a)), but the catalytic performance of the amorphous alloy is improved to varying degrees before and after HPT treatment. Among them, HPT-5N Fe_73.5Si_13.5B₉Cu₁Nb₃ amorphous alloy show the most excellent catalytic activity, decreasing from 418 mV to 174 mV at a current density of 10 mA/cm² (Fig. 8(b)). Compared with the microstructure morphology of HPT treatment under different parameters, HPT-5N Fe_73.5Si_13.5B₉Cu₁Nb₃ amorphous alloy shows the richest defects such as micropores, microcracks and shear bands (shown in Fig. 8(c)–(f)). Therefore, HPT treatment of amorphous alloy to form nano-amorphous glass will cause certain microscopic defects of amorphous alloy, enhancing its catalytic HER performance.

Fig. 8

(a) XRD patterns, (b) polarization curves of the melt-spun and HPT-treated Fe_73.5Si_13.5B₉Cu₁Nb₃ alloys; (c)–(f) SEM images of Fe_73.5Si1_3.5B₉Cu₁Nb₃ ribbon after HPT treatment.⁷⁷⁾

Chu et al.⁷⁸⁾ introduced SPD by HPT on a Pd₄₀Cu₃₀Ni₁₀P₂₀ MGs, generating more flow units upon the amorphous matrix and significantly improving the electrocatalytic HER performances in both acidic and alkaline media. Figure 9(a)–(c) shows how the HPT treatment effect the energy state of Pd₄₀Cu₃₀Ni₁₀P₂₀ MGs. The severe plastic deformation caused by the HPT treatment leads to the onset crystallization temperature (T_x) reduced from 377.4°C to 357.6°C and the melting temperature is also approximately decreased about 20°C. The reduction of Tx can be explained from that the HPT treatment led to the easier crystallization behavior. Moreover, an obvious exothermic peak below T_g after HPT shows that the sample was rejuvenated to a higher energy state. The structural relaxation of MGs is accelerated during heating, which is convincing evidence that HPT treatment introduce a large amount of flow units. HER performances are shown in Fig. 9(d)–(e). At the current density of 10 mA cm⁻², the overpotential of the HPT-treated sample decreases from 179 mV to 76 mV in acidic media and from 379 mV to 209 mV in alkaline media.

Fig. 9

(a) and (b) DSC curves of the melt-spun and HPT-treated Pd₄₀Cu₃₀Ni₁₀P₂₀ MGs; (c) schematic diagram of energy state; (d) and (e) LSV curves before and after 1000 CV cycles in acidic and alkaline electrolytes.⁷⁸⁾

Kaveh et al.⁷⁹⁾ demonstrated a HPT-treated TiO₂ electrocatalyst deposited on two sides of Ti foils for electrochemical reductions of water and oxalic acid. TiO₂-II phase is formed on TiO₂ surface by using HPT treatment. The result show that the sample with 18 mass% TiO2-II phase shows the highest activity for water splitting. All mentioned SPD-treated electrocatalysts reported for HER and OER with their activity markers are listed in Table 2.

Table 2 SPD-treated electrocatalysts reported for HER and OER till date with their activity markers.

4. Conclusions

In this review work, recent advances on electrocatalytic hydrogen production of materials processed by SPD have been summarized. The basic principles of electrocatalytic water splitting and the SPD technologies were discussed, followed by introducing recent advances on SPDed materials with enhanced hydrogen production properties. The main conclusions of this work include the following points.

(1) SPD can greatly enhance the HER performances through formation of new nanoglass structures. After HPT treatments, the overpotentials at the current density of 10 mA cm⁻² of the amorphous Fe₇₈Si₉B₁₃ and Fe_73.5Si_13.5B₉Cu₁Nb₃ alloys decrease from 385 mV to 112 mV, and from 418 mV to 174 mV, respectively.
(2) HPT treatment upon a Pd₄₀Cu₃₀Ni₁₀P₂₀ amorphous alloy can generate abundant flow units in the amorphous matrix and significantly improving the electrocatalytic HER performances in both acidic and alkaline media. At the current density of 10 mA cm⁻², the overpotential of the HPT-treated sample decreases from 179 mV to 76 mV in acidic media and from 379 mV to 209 mV in alkaline media.
(3) By a cathodic corrosion treatment, a Fe-based Fe₇₈Si₉B₁₃ nanoglass covered by FeOOH layer (FeOOH@NG) show excellent activity and stability for OER, yielding a low overpotential of only 240 mV at a current density of 10 mA cm⁻² and the better stability than both melt-spun and corroded Fe₇₈Si₉B₁₃ alloys.

It can be clearly seen that materials have shown enhanced electrocatalytic activity and durability upon SPD process. Besides as excellent catalysts for HER and OER of water splitting, SPDed materials also show some enhanced hydrogen absorption/desorption properties than those of the materials not treated with SPD.⁸⁰⁾ It can be predicted that SPD materials will make outstanding contributions in many fields such as urea oxidation reaction (UOR), methanol and glucose oxidation reactions, CO₂ reduction reaction (CO₂RR), NH₃ reduction reaction (NH₃RR), hydrogenation reaction, and waste water treatments and many others.

Acknowledgements

Both authors of Xiying Jian and Jian Li contributed equally to this work.

This work was financially supported by National Natural Science Foundation of China (Nos. 52071157, 52171205 and 52101249), Youth Talent Support Programme of Guangdong Provincial Association for Science and Technology (No. SKXRC202309), Anhui Natural Science Foundation (grant number 2108085QE191) and China Machinery Industry Group Co., Ltd. Youth Science and Technology Fund Project.

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