Sub-nanometric materials for hydrogen evolution reaction

Abstract

With the development of renewable and clean energy, hydrogen energy is booming as an alternative to fossil energy. Hydrogen energy has the highest mass-energy density of all fuels and is versatile enough to be considered as the ultimate energy source for the planet. Among them, electrolytic hydrogen production technology is considered as one of the most promising strategies for large-scale hydrogen production. Among the current electrocatalytic hydrogen evolution catalysts, sub-nanometric materials (SNMs) have become the focus of research in recent years due to their atomic size and different compositions. In contrast to nanometric materials (NMs), SNMs have essentially fully exposed atoms, in addition to having different active sites for different reaction intermediates, which facilitate their enhanced electrocatalytic performance. In this review, firstly, we affirm the absolute advantages of SNM catalysts in the hydrogen evolution reaction (HER), and then outline the mechanism of the HER and the methods to evaluate the catalyst performance, and introduce the strategies to enhance the catalyst performance, such as synergistic effects, support anchoring, doping and vacancy defects. Next, the application of high-performance noble metal-based Pt, Pd and Ru-based SNMs in the HER is illustrated, and finally the prospects and challenges of SNMs development as HER electrocatalysts are foreseen.

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Yaodong Yu

Yaodong Yu is currently a PhD student under the supervision of Prof. Jianping Lai and Lei Wang in Qingdao University of Science and Technology. He received his Bachelor of Engineering degree from the School of Chemical and Molecular Engineering, Qingdao University of Science and Technology in 2020, where he pursued his PhD degree in 2022. His current research interests are in the synthesis of nanomaterials and their applications in electrocatalysis.

Ruixin Liu

Ruixin Liu is currently pursuing her Bachelor's degree in the College of Chemical and Molecular Engineering at Qingdao University of Science and Technology. Her major is Molecular Science and Engineering.

Yuyao Sun

Yuyao Sun is a PhD candidate under the supervision of Prof. Jianping Lai and Prof. Lei Wang at Qingdao University of Science and Technology. He received his Master degree from the College of Chemical and Molecular Engineering, Qingdao University of Science and Technology in 2022. His current research interests include the synthesis of micro- and nanostructured materials and their applications in electrocatalysis.

Ziyi Liu

Ziyi Liu is currently studying for her master's degree at Qingdao University of Science and Technology under the supervision of professors Jianping Lai and Wang Lei. The current research direction is clean energy electrocatalytic materials.

Xinrong Shi

Xinrong Shi is currently an undergraduate at Qingdao University of Science and Technology. The tutor is Professor Jianping Lai. Entered School of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, in 2021.

Jianping Lai

Jianping Lai is currently a professor at Qingdao University of Science and Technology. He received a postdoctoral at Peking University. He received his BS degree from College of Chemistry in Fuzhou University (2011) and PhD degree from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (2017). His research interests focus on the synthesis and electrocatalytic applications of nanomaterials.

Lei Wang

Lei Wang is currently a professor at Qingdao University of Science and Technology. He obtained his PhD degree from the Jilin University in 2006 under the direction of Prof. Shouhua Feng. And then, he went to Shandong University to work as a postdoctoral. His main research interests are focused on energy storage and conversion materials, electrocatalysis, photocatalysis, supramolecular chemistry, metal–organic coordination polymers and organic–inorganic hybrid materials.

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1. Introduction

As the global population grows every year, the demand for fossil fuels continues to grow rapidly.^1,2 The consumption of fossil fuels today brings satisfaction to mankind, but also poses a great threat to the planet on which mankind depends: global warming, atmospheric pollution, etc.^3–5 Therefore, we urgently need to develop clean and sustainable energy sources to gradually reduce the use of fossil energy.^6–9 Hydrogen energy has been considered as one of the cleanest and most promising modes of energy utilization, since the only combustion product is water, which can be recycled.^10–13 However, unlike energy sources such as oil and natural gas, which are already present on the earth, hydrogen energy is not naturally occurring and therefore it needs to be produced or converted from other sources to be used.^14,15 There are many ways to obtain hydrogen energy, such as: electrochemical hydrocracking, hydrogen production from water gas, and hydrogen production from decomposing water from renewable energy sources (e.g. solar, wind, tidal and geothermal).^16–19 Among them, Electrocatalytic HER is considered to be a clean, efficient and sustainable alternative strategy for hydrogen production from fossil fuels.^20–22 The process of hydrogen production from electrolytic water, like other chemical reactions, requires overcoming the energy barriers from reactants to activation products for the reaction to proceed smoothly.^23–25 Development of high performance HER electrocatalysts has become an important strategy for hydrogen production.^26,27

Today, NMs have become a hot topic in chemistry because of their unique size effect.^28–31 NMs are materials with at least one dimension on the scale of 1–100 nm.^32,33 SNMs are located in the transition zone between NMs and molecules, with dimensions around or even less than 1 nm, leading to significant changes in many properties.^34–36 Compared to NMs, SNMs exhibit different properties, with their atoms almost completely exposed due to the size of the atomic scale. In addition, different reaction intermediates have different active sites, so the construction of sub-nanometre-scale nanoclusters helps to enhance their HER electrocatalytic performance.^34,37 The current widely used ones are noble metal-based SNMs, but the high cost of noble metals limits their large-scale application.^38–41 Optimization of the SNMs by strategies such as support anchoring and doping, their activity is comparable to that of Pt/C catalysts with high stability, which also results in lower loading of precious metals Pt, Pd, and Ru in the catalysts, which greatly improved their utilization.^{16,39,42–44}

In this review, we focus on strategies to improve the catalyst performance of SNMs and their application in HER. Firstly, we summarize the HER mechanism and the methods to evaluate the performance of HER catalysts, and explain in detail the instrumental strategies to optimize the catalyst performance. In the next section, we present the progress of Pt, Pd and Ru-based SNMs for HER applications by elemental species. Finally, the challenges and trends in the future development of sub-nanoscale HER catalysts are envisioned. It is hoped that this review will provide new perspectives and insights into the application of SNMs in electrocatalysis.

2. HER mechanism and catalyst evaluation methods

2.1 HER mechanism

Depending on the reaction medium, the HER mechanism (Fig. 1(a)) can be divided into two categories: acidic and alkaline. In the acidic medium there is a large amount of free proton H⁺, and H⁺ acquires an electron on the electrode material to form a hydrogen intermediate in the adsorbed state (H*), this step is called electrochemical adsorption (Volmer reaction), and the electrochemical desorption step (Heyrovsky step) is the combination of H⁺ and the resulting adsorbed state H* on the electrode material to acquire an electron reaction to form H₂; the combination of H* and H* to form H₂ is the chemical resolution step (Tafel step).^45,46 In alkaline medium, there is no large amount of free H⁺, so water becomes the source of supply of protons in the reaction, and H₂O gains an electron to dissociate at the electrode material to produce H* and OH⁻, this step is called Volmer reaction, Heyrovsky step is H* and H₂O combine one more electron to form H₂ and OH⁻; the produced H* combines itself to produce H₂, which is called Tafel step.^16,47–49

Fig. 1 (a) The mechanism of HER in acidic (green route) and alkaline (blue route) electrolytes. (b) The volcano plot of acidic HER.⁵¹

Regardless of the mechanism by which the HER reaction proceeds, the reaction rate and efficiency are determined by the formation and desorption processes of H*.⁵⁰ From a physicochemical point of view, the adsorption and desorption of H* can be evaluated by measuring the free energy of hydrogen adsorption (ΔG_H*) during HER.^50,51 According to Sabatier principle, the interaction between the catalyst and the active intermediate should be moderate. If the interaction is too weak, the amount of active intermediate bound to the catalyst is too small and the reaction rate is slow, while if the interaction is too strong, the reaction product cannot be desorbed from the catalyst and its occupies more active sites and the reaction is terminated.⁵² A “volcano-type” diagram was created by relating the exchange current density j₀. A larger ΔG_H* value indicates weaker adsorption of H* and difficulty in H* production; a smaller ΔG_H* value indicate stronger adsorption of H* and difficulty in H₂ desorption.⁵³ According to Fig. 1(b), the platinum group metals are located near the top of the volcano curve and have better HER reaction activity. Meanwhile, we can observe that the noble metals such as Pt, Pd and Rh are still some distance away from the top of the volcano-type curve, which indicates that the hydrogen adsorption energy of this type of catalyst can be regulated to achieve the purpose of enhancing the HER reaction activity.⁵³

2.2 Catalyst performance evaluation methods

2.2.1 Overpotential. Overpotential is the part of an electrochemical reaction where the potential observed in actual experiments exceeds the thermodynamically determined equilibrium potential.⁴² Depending on the cause of overpotential generation, overpotential can be classified into activation overpotential, concentration overpotential and resistance overpotential, of which only activation overpotential is directly related to catalytic activity of the catalyst, while concentration overpotential and resistance overpotential are caused by concentration polarization and series resistance of solutions, wires, respectively. Therefore, the activation overpotential is an important parameter to evaluate the catalytic performance. In order to obtain the accurate activation overpotential of a catalyst, a rotating electrode system is usually used to reduce the concentration overpotential and IRs compensation is used to eliminate the resistance overpotential.^54–56 Where I is the current flowing through the circuit and Rs is the series resistance, the magnitude of this value can be obtained from the electrochemical impedance spectrum. Currently, the overpotential corresponding to a current density of 10 mA cm⁻² is used as a benchmark for a better assessment of the catalyst activity level.

2.2.2 Tafel slope and conversion current density. The Tafel slope is an inherent property of catalysts. It visualizes the kinetic mechanism of the catalytic decomposition reactions occurring in the electrode material. The simplified Tafel equation is η = b log j + a, where η, b, j and a are the overpotential,⁵⁷ Tafel slope, current density and regulation constant, respectively. The unit is mV dec⁻¹, which indicates the amount of change required in overpotential for a tenfold change in current. It is seen that the smaller the value of b, the faster the reaction kinetics. Importantly, the Tafel slope values provide insight into the reaction mechanism of HER at the catalyst surface. The theoretical Tafel slopes for the Volmer, Heyrovsky and Tafel steps in the HER reaction mechanism are 120, 40 and 30 mV dec⁻¹, respectively.⁵⁸

2.2.3 Turnover frequency. The turnover frequency (TOF) refers to the total number of hydrogen molecules released from each active site per unit time, and it is a method to evaluate the catalytic rate of a catalyst. The common formula for calculating the TOF value of HER catalysts is , where A is the area of the working electrode and n is the number of moles of active material calculated from the electrochemically active area of the catalyst used.^42,59 The TOF can usually be obtained by cyclic voltammetry (CV) of neutral solutions, “current–voltage” absolute area integration, and related equations. Obviously, the higher the TOF, the higher the catalytic activity of the catalyst itself.^60–62

2.2.4 Electrochemically active area. The electrochemical surface area is the effective area of the catalyst exposed to the active component at the electrode surface, not simply the specific surface area of the material, which determines the kinetic parameters of the reaction. Since it is proportional to the double-layer capacitance (C_dl) in the non-Faraday interval, the electrochemically active area is usually estimated from the C_dl of the catalytic surface, which in turn reveals the number of catalyst active sites.^63,64

2.2.5 Stability. Stability is an important parameter for evaluating the catalytic performance of electrocatalysts, which is usually expressed in terms of catalyst lifetime, and refers to the usage time of a catalyst to maintain a certain reaction activity and selectivity under certain reaction conditions. Usually, electrocatalyst stability is tested by two types of methods: cyclic voltammetry and constant current methods. Cyclic voltammetry method is usually not less than 1000 times. The constant current method is usually a false current density of not less than 10 mA cm⁻² and a duration of not less than 10 h. A higher number of cycles or a longer duration with no change in potential or current of the electrocatalyst indicates a better catalyst stability.^59,65

3. Strategies to enhance catalyst performance

To make nanomaterial catalysts with better catalytic activity in HER, some strategies are usually used for modulation, such as heterometallic synergies, anchoring in supports, heteroatomic doping, vacancies and defects (Fig. 2).³⁹

Fig. 2 Strategies to enhance catalyst performance.

3.1 Heterometallic synergies

Synergistic effect refers to the effect of two or more components added or mixed together, which is greater than the sum of the effects of each component when applied individually. It is due to the synergistic effect of polymetallic catalysts that they exhibit excellent catalytic reaction performance. Polymetallic catalysts have attracted much attention in recent years and become a hot research topic.^66,67

Heterogeneous metals have synergistic effects with each other to enhance the catalytic activity of catalysts. Ultrasmall high-entropy alloy (us-HEA) nanoparticles (NPs) with five NiCoFePtRh elements have been synthesized by an ultra-fast and powerful chemical co-reduction method (Fig. 3(a)).⁶⁸ The average particle size of US-HEA is 1.68 nm, which is the smallest hydrogen evolution high-entropy alloy catalyst reported so far (Fig. 3(b)). At a current density of 10 mA cm⁻², us-HEA exhibits a small overpotential of 27 mV and can maintain its stability for 100 h (Fig. 3(c) and (f)). Based on DFT theory calculations, it was found that the five elements of us-HEA exhibit synergistic effects among themselves, where Pt/Rh atoms are the main and direct active sites for HER, and Fe/Co/Ni atoms can effectively modulate the active sites (Fig. 3(d) and (e)). The five elements exhibit extremely superior performance and stability in acidic media through synergistic interaction.

Fig. 3 (a) Representative HAADF-STEM image of us-HEA NPs. (b) Size distribution histogram of us-HEA NPs. (c) Linear sweep voltammetry (LSV) curves of us-HEA/C, the precursor, C, commercial Rh/C, and commercial Pt/C (0.5 M H₂SO₄). (d) Schematic illustration of the binding strength between H and the Rh site, Fe/Co/Ni site, and Pt site of us-HEA, respectively. (e) Calculated hydrogen-adsorption free energy (ΔG_H*) profiles for three representative sites on the us-HEA (111) surface and Pt site on pure Pt (111) surface. (f) Constant applied potential for 100 h.⁶⁸ Reprinted with permission from J. Am. Chem. Soc., 2021, 143, 17117–17127. Copyright 2021, American Chemical Society. (g) Schematic diagram of the synthesis of sub-nanometer PtCo clusters confined into the small pores of ZIF-8 derived N-doped NMs. (h) Corresponding mass activity at 20 mV vs. RHE. (i) Calculated free-energy diagram of the HER on Pt₆Co₆, Pt₉Co₄, Pt₁₂Co₁ and Pt₁₃.⁶⁹ Reproduced with permission. Copyright 2021, The Royal Society of Chemistry.

Recently, SNMs PtCo clusters have been prepared in small holes of nitrogen-doped NMs derived from ZIF-8 by electrogenic substitution (Fig. 3(g)).⁶⁹ The synthesis of SNMs-PtCo clusters in small pore channels and the synergistic interaction between Pt and Co, the catalysts showed excellent catalytic activity at low loading (1.2% of Pt). Its highest mass activity at 20 mV overpotential was 13.83 A mg⁻¹ (Fig. 3(h)). The DFT theoretical calculations show that the enhanced d-orbital electronic coupling between Pt and Co, charge redistribution around Pt. Lower d-band center of Pt reduces the Gibbs free energy of H* adsorption, thus improving HER kinetics and activity (Fig. 3(i)).

3.2 Anchoring in supports

Some metal nanocatalysts are prone to aggregation during synthesis due to their high surface free energy. By using support anchoring as an optimization strategy, highly dispersed metal nanoclusters can be obtained to ensure more active sites for the catalysts, thus greatly enhancing the catalytic performance. The essence of support anchoring is through strong metal-support interaction (SMSI), thus significantly improving the catalytic activity and stability.^70–72

By confining SNM-Pt clusters in hollow mesoporous carbon spheres (Pt₅/HMCS), active and stable Pt-based hydrogen precipitation electrocatalysts were synthesized (Fig. 4(a)).⁷³ Subsequent analysis showed that the Pt clusters were anchored in the mesoporous channels of the carbon supports. With the removal of ligand and electrochemical hydrogen production, the Pt cluster was effectively stabilized, resulting in excellent electrocatalytic performance for HER in both acidic and alkaline solutions. The overpotential was only 20.7 mV (acidic) and 46.2 mV (alkaline) at 10 mA cm⁻² current density for the same Pt loading (Fig. 4(b) and (c)).

Fig. 4 (a) Schematic illustration of the symthetic procedure of Pt₅/HMCS. (b), (c) LSV curves of Pt₅/HMCS-5.08% and commercial Pt/C (5 and 20 wt%) catalysts based on geometric area of the working electrode in: (b) 0.5 M H₂SO₄ and (c) 1.0 M KOH.⁷³ Reproduced with permission. Copyright 2019, Wiley VCH. (d) Scheme illustration of the preparation process of Pd–CeO_2−x-NC. (e) The HRTEM images of 0.1% Pd–CeO_2−x-NC. (f) LSV curves of 1% Pd–CeO_2−x-NC. (g) LSV of NC, CeO_2−x-NC, 20% Pt/C and x% Pd-CeO_2−x-NC.⁷⁴ Reproduced with permission. Copyright 2021, Elsevier.

In addition, sub-nanometer Pd–CeO_2−x-NC catalysts were prepared by polymerization and photodeposition (Fig. 4(d)).⁷⁴ The catalyst made full use of SMSI effect to accurately anchor Pd and CeO_2−x, resulting in a higher degree of uniform dispersion of Pd and CeO_2−x, preventing the self-aggregation of small particles, and significantly improving the stability of the catalyst (Fig. 4(e) and (f)). It is noteworthy that the SMSI effect leads to the electron transfer from Pd to CeO_2−x, forming the Pdⁿ⁺ in the highly oxidized state of the dominant active species. The HER performance of Pd–CeO_2−x-NC is closely related to the percentage of Pdⁿ⁺, and only 1% Pd–CeO_2−x-NC (overpotential and Tafel slope of 115 mV and 58 mV dec⁻¹, respectively) exhibited HER activity comparable to 20 wt% Pt/C (overpotential and Tafel slope of 72 mV and 65 mV dec⁻¹, respectively) (Fig. 4(g)).

3.3 Heteroatomic doping

Catalyst doping refers to the alteration of the catalyst structure by adding new substances to the catalyst, thus creating new active sites to change the catalytic performance of the catalyst. Currently, a range of metal/non-metal elements are used as dopants for catalyst materials to modulate their HER catalytic activity.^75,76

In electrocatalytic reactions, doping modulation is not only considered as an effective method to modulate the active sites of catalysts, but also seems to be an important means to obtain high performance catalysts. In 2021, Wang et al.⁷⁷ found that through the guidance of highly dispersed Cu atoms derived from MOF, Cu–Pt alloy nanoparticles (C-ZIF-CuPt) were prepared on N-doped porous carbon (Fig. 5(a)). The overpotential was only 46 mV at a current density of 10 mA cm⁻², and the stability of C-ZIF-CuPt was found to be significantly better than that of commercial Pt/C after 1000 cycles of CV accelerated durability tests, and the strong interaction between the carbon support and the CuPt alloy enhanced the stability of the catalyst (Fig. 5(b) and (c)). The DFT calculations showed that the Cu doping enhanced the d-band center of Pt and lowered the activation energy barrier of the molecule, thus improving the HER catalytic performance of the material (Fig. 5(d)).

Fig. 5 (a) C-ZIF-CuPt catalyst preparation process. (b) LSV curves for samples in N₂-saturated 0.1 M KOH at 25 °C and 1600 rpm at a scan rate of 5 mV s⁻¹. (c) The HER polarization curves of C-ZIF-CuPt and Pt/C before and after the ADT test by 1000 cycles. (d) The activation energy barriers of H₂O molecules in the HER of three model structures.⁷⁷ Reproduced with permission. Copyright 2021, Elsevier. (e)Schematic illustration of the formation of Pt/Ni_xFe LDHs. (f) LSV curves for the HER of Pt/Ni_xFe LDHs, Pt/NiFe LDHs, Ni_xFe LDHs, NiFe LDHs, commercial 20% Pt/C, RuO₂, and 20% Ir/C catalysts in 1 M KOH solution. (g) Optimized model structures of intermediates adsorbed on Pt/Ni_xFe LDHs and Pt/NiZnFe LDHs surfaces.⁸⁰ Reproduced with permission. Copyright 2021, American Chemical Society.

3.4 Vacancies and defects

Surface defects are usually formed during the crystal growth process. Adding surface defects to electrocatalysts is one of the effective ways to enhance the performance, including vacancy defects, gap filling defects, etc.^78,79

Sub-nanometer Pt clusters (Pt-NCs) were prepared by simple electrodeposition and strong alkaline etching. And they were immobilized on defect-rich NiFe layered double hydroxide (Ni_xFe-LDHs) nanosheets (Fig. 5(e)).⁸⁰ The Ni_xFe-LDHs have the best HER activity relative to the other samples in the 1 M KOH electrolyte (Fig. 5(f)). The DFT calculations showed that after selective etching of Zn sites on LDHs by strong alkaline solutions, the generation of M(II) vacancies effectively changed the surface electronic structure around the active sites, providing abundant ligand-unsaturated active sites. Their absorption energies on Pt/Ni_xFe LDHs are lower than those on pristine Pt/NiZnFe LDHs (Fig. 5(g)), leading to more rapid H_ad and hydroxyl adsorption and reduction/oxidation processes, suggesting that the H₂ generated on the surface of Pt/Ni_xFe LDHs is more readily desorbed to avoid accumulation, thus promoting the exposure of active sites and further accelerating the hydrolysis process.

4. Electrocatalysis of SNMs in HER

Novel SNMs can be extended to the quantum level due to their extremely small particle size. Thus, they have unique properties and functions. The noble metals Pt, Pd, and Ru exhibit excellent catalytic activity in HER due to their unique electronic structures. However, the widespread use of noble metals is limited due to their small reserves and high costs. Therefore, related researchers have designed and prepared sub-nanometer noble metal HER catalysts by associating noble metals with sub-nanometer scales, which not only greatly reduces the loading of noble metals, but also exposes the active sites on the catalyst surface more fully and enhances the catalytic performance of the catalysts. In the following, we will discuss in detail the recent developments of precious metal Pt, Pd and Ru-based SNMs in HER according to their applications in different electrolytes.

4.1 Acidic HER

4.1.1 Pt-Based SNMs in acidic HER. Compared to other metals, Pt is considered to be the most catalytically active and efficient metal for HER, but its high cost and low reserves have limited its widespread use.^81–84

Rational design and fabrication of low Pt content and high utilization Pt-based SNMs catalysts using synergistic effects is a feasible route. It has been reported that the synergistic effect of sub-nanoparticles is greater than that of nanoparticles.⁶⁷ The atomic scale mixing phase of sub-nanometer size is observed to be higher than that of nanometer size by annulardark-field scanning transmission electron microscopy (ADF-STEM), The enhancement of mixing phase improves the synergistic effect between different elements (Fig. 6(a) and (b)). Among them, Pt and Zr have the largest synergistic effect, especially Pt₄Zr₂ (67% atomic percentage of Pt), which has the largest synergistic effect between elements and exhibits the highest exchange current (Fig. 6(c)–(f)). The results indicate that SNMs catalysts are an attractive class of materials for applications.

Fig. 6 (a) ADF-STEM images of Pt₄Zr₂ SNPs. (b) ADF-STEM images of Pt₁₃Zr₇ NPs. (c), (d) Original linear sweep voltammogram (LSV) curves of PtZr alloy SNPs and NPs in the aqueous argon-saturated electrolyte (0.05 M H₂SO₄). (e), (f) Exchange currents (i₀) of HER derived by the Tafel plots.⁶⁷ Reproduced with permission. Copyright 2022, Wiley VCH. (g) High annular dark field (HAADF)-STEM images of PtN_x clusters (bright spots) in the PtN_x/TiO₂ specimen with 3 nm scale bars. (h) HER polarization curves for PtN_x/TiO₂, PtO_x/TiO₂, Pt/C and TiO₂ in H₂-saturated 0.5 M H₂SO₄ acquired at a scan rate of 2 mV s⁻¹. (i) Free-energy diagram for different H* adsorption sites on the PtN_x/TiO₂ structure.⁸⁵ Reproduced with permission. Copyright 2020, Elsevier.

In addition, the control of the central atomic coordination environment can also greatly improve the catalytic performance. In 2020 Cheng et al.⁸⁵ synthesized sub-nano raft-like PtN_x clusters on TiO₂ supports (PtN_x/TiO₂) (Fig. 6(g)). The Pt–N coordination structure was analyzed and found to have a significant enhancement of HER performance. At a current density of 10 mA cm⁻², the overpotential was only 67 mV (0.5 M H₂SO₄), which was much lower than that of PtO_x/TiO₂ (Fig. 6(h)). The DFT calculations show that when hydrogen is adsorbed on the nitrogen atom, the charge is transferred from the nitrogen atom to the adjacent platinum atom. The charge redistribution in the Pt–N coordination structure leads to a reduction in the H* free energy of the activated N atom (Fig. 6(i)). Thus, the catalyst has a higher HER activity.

Support anchoring is an effective means to modulate the properties of the active phase, which is essential to obtain optimal performance in multiphase catalysis.^86–88 Recently, Huang et al.⁸⁹ reported for the first time heat-treated tannic acid (TA) fragments, which are stably anchored to the carbon surface to form highly heat-resistant Pt_c/C catalysts (Fig. 7(a)). The TA monolayer functionalizes the nanoscale carbon surface through non-covalent interactions, thereby maintaining high electrical conductivity. This promotes the dispersion of nanoscale carbon. In addition, controlled adsorption of PtCl₆²⁻ monolayer on the nanoscale carbon during impregnation. This resulted in a high transition frequency (TOF) of 154.5 A mg⁻¹/156.1 s⁻¹ for Pt_c/C at 60 mV (Fig. 7(b) and (c)), indicating an excellent activity and high thermal stability.

Fig. 7 (a) Illustration of the TA monolayer adsorbed on carbon, the PtCl₄²⁻ monolayer adsorption on TA_i/C,and thermal-treated TA fragments stabilizing Pt_cs anchored on the carbon. (b) TOF/η₁₀ and (c) j_mPt/D_ee of Pt_c(T)/C vs. thermal treatment temperature (red and blue parallel straight lines present the corresponding parameters of Pt₂₀/C).⁸⁹ Reproduced with permission. Copyright 2021, The Royal Society of Chemistry. (d) Schematic illustration for the preparation of Pt/MXene. (e) Mass activity of Pt/MXene and Pt/C. (f) The calculated free energy diagram for hydrogen evolution on Pt/TiCO, Pt (111), TiCO, and Pt/TiCO-F.⁹⁰ Reproduced with permission. Copyright 2022, Wiley VCH. (g) Negative sweep of the voltammograms for 0.03 ML Pt_n/ITO (n = 1, 4, 7, and 8) after subtracting the capacitive background current from ITO, ic. The subtracted currents were integrated to calculate the total charge transferred in the H_upd potential region, assumed to extend to −0.036 V. (h) Network of various elementary reactions on Pt₇H_x/ITO where energetically accessible structures of the Pt₇H_x cluster are connected by curved arrows representing two-step Volmer–Heyrovsky reactions, and pairs of straight arrows represent three-step Volmer–Tafel reactions. (i) Bars give the size-dependent numbers of H atoms per Pt atom adsorbed in the UPD region, estimated from the integrated H_upd currents from (i).⁹¹ Reproduced with permission. Copyright 2023, American Chemical Society.

In addition, three-dimensional folded Ti₃C₂T_x MXene containing sub-nanometer Pt clusters (Pt/MXene) was prepared by a fast continuous spray drying (Fig. 7(d)).⁹⁰ The analysis revealed a strong interaction between the active component Pt and the three-dimensional folded MXene, which led to the anchoring of Pt clusters on the substrate. Even with 2.9 wt% Pt loading, the prepared Pt/MXene showed a high mass activity of 1847 mA mg_Pt⁻¹ at an overpotential of 50 mV, which was much better than commercial Pt/C catalysts with a Pt loading of 20 wt% (Fig. 7(e)). DFT calculations demonstrated that the increased Pt/MXene activity could be attributed to the charge transfer from Pt clusters to MXene, which facilitated hydrogen desorption (Fig. 7(f)).

Sautet et al.⁹¹ combined density functional theory (DFT), and experiments with atom-size-selective Pt_n clusters deposited on indium tin oxide (ITO) electrodes, for examining the effect of applied potential and Pt_n size on the electrocatalytic HER activity of Pt_n (n = 1, 4, 7 and 8) (Fig. 7(g)). The rapid increase with increasing Pt_n size resulted in an approximate doubling of the activity per Pt atom for Pt₇/ITO compared to atoms in the polycrystalline Pt surface layer (Fig. 7(h)). Both DFT and experiments revealed that hydrogen underpotential deposition (H_upd) resulted in the adsorption of ∼2H atoms/Pt atom by Pt_n/ITO (n = 4, 7 and 8) at the HER threshold potential. This corresponds to approximately twice the H_upd of Pt blocks or nanoparticles (Fig. 7(i)).

Furthermore, in 2022, Wu et al.⁹² developed a solvent-free microwave synthesis method for subnanoclusters (SNCs) by designing the anchor point and reaction time (Fig. 8(a)). This method is fast, green, and most importantly, the prepared catalysts are highly loaded and have excellent stability. The SMSI was used to enable the SNCs to be firmly anchored on the support, which enhanced the stability of the catalyst. The HER catalyst Pt_0.6/KB-O_S5-40 was synthesized by this method with an overpotential as low as 22 mV at a current density of 10 mA cm⁻² (0.5 M H₂SO₄), which is significantly better than that of commercial Pt/C (36 mV) (Fig. 8(b)). It is worth mentioning that after 10 000 CV cycles, the overpotential did not change significantly (Fig. 8(c)).

Fig. 8 (a) The solvent-free microwave method, which is a scalable synthesis of high loading and stable metal SNCs. (b) LSV curves (the inset is a comparison of η₁₀ values) of Pt_0.4/KB-O_S5-40, Pt_0.6/KB-O_S5-40, Pt_0.8/KB-O_S5-40 and commercial Pt/C. (c) LSV of HER of Pt_0.6/KB-O_S5-40 before and after 10 000 CV cycles.⁹² Reproduced with permission. Copyright 2022, The Royal Society of Chemistry. (d) The charge density difference Δρ of the Pd₄@CeO₂ model for the slice cut along the Pd2–Pd3 bond. (e) Schematic illustration of the preparation of CeO₂ porous nanorods and the subsequent confinement of Pd clusters for HER. (f) Polarization curves of HER on the Pd NCs@CeO₂, commercial Pt/C, and Pd/C. (g) The calculated free-energy diagram of HER for the Pd₄@CeO₂ and pure Pd₄ cluster at the equilibrium potential.⁹⁵ Reproduced with permission. Copyright 2023, American Chemical Society. (h) Schematic illustration of the transformation from cubic Pd to the final Pd–Pt tripleshelled nanocube via layer-by-layer epitaxial approach. (i) HER polarization curves of the bilayer and monolayer nanocages in comparison with a commercial Pt/C catalyst. (j) Mass activities (at −0.05 V_RHE) of these three catalysts before and after durability test.⁹⁶ Reproduced with permission. Copyright 2019, Springer International Publishing.

4.1.2 Pd-Based SNMs in acidic HER. In addition to Pt-based nano-HER catalysts, Pd-based NMs also have excellent HER electrocatalytic properties.^93,94

In recent years, Gao et al.⁹⁵ successfully subnanometer-sized palladium clusters (PdNCs@CeO₂) accommodated in porous cerium dioxide (Fig. 8(d)). In 0.5 M H₂SO₄, HER showed an onset potential at −0.036 V vs. RHE (Fig. 8(e)). DFT calculations showed that the catalyst activity was centered on the subunits of the Pd clusters, and there was a synergistic effect between the Pd and CeO₂ supports in Pd NCs@CeO₂ compared to the displaced Pd nanoclusters, allowing efficient electron transfer between the Pd atoms to the O atoms, facilitating H* with a suitable adsorption strength at the Pd site, which results in excellent HER activity and stability of Pd NCs@CeO₂ (Fig. 8(f) and (g)).

It is worth noting that in 2019, Wang et al.⁹⁶ prepared bimolecular Pd–Pt alloy nanocages catalyst by regulating various reagent components (Fig. 8(h)). This method effectively avoids the problem of spontaneous nucleation of metals. The successive formation of Pd and Pt shells, which fully expose the active sites and the existence of synergistic effects between Pd and Pt, greatly enhanced the HER performance and stability of the catalysts (Fig. 8(d)). The prepared catalysts exhibited more than twice the electrocatalytic activity compared with the monolayer nanocages. And it only decreased to 1.93 A mg_Pt⁻¹ after 10 000 cycles, while the other two catalysts both had a greater loss of mass activity (Fig. 8(j)).

4.1.3 Ru-Based SNMs in acidic HER. Recently, by a one-pot hydrothermal method, Pi et al. synthesized SNM Ru cluster HER catalysts (Ru SNC/W₁₈O₄₉ NWs) loaded with W₁₈O₄₉ nanowires (Fig. 9(a)).⁹⁷ It was characterized and theoretically calculated that charge transfer between the Ru clusters and the support W₁₈O₄₉ would take place to make Ru positively charged, thus optimizing the binding strength of H* to Ru and ensuring excellent HER activity and durability of the catalyst (Fig. 9(b)). The catalyst overpotential is 21 mV at a current density of 10 mA cm⁻², which is only 2 mV higher than that of commercial Pt/C (19 mV), and remains stable at a current of 10 mA cm⁻² for 20 h continuously (Fig. 9(c) and (d)).

Fig. 9 (a) Schematic illustration for the synthesis of Ru SNC/W₁₈O₄₉ NWs. (b) Comparison of the hydrogen adsorption on different surface models. (c) Polarization curves of Ru SNC/W₁₈O₄₉ NWs, Ru NP/W₁₈O₄₉ NWs, W₁₈O₄₉ NWs, Ru/C, and Pt/C (0.5 M H₂SO₄). (d) Durability test of Ru SNC/W₁₈O₄₉ NWs. The polarization curves were recorded before and after 5000 potential cycles from 0.1 to −0.2 V (versus RHE).⁹⁷ Reproduced with permission. Copyright 2023, Wiley VCH. (e) Schematic illustration of HER/HOR for different catalysts. (f) HER polarization curves (0.1 M KOH). (g) Pt 4f XPS spectra of PtIr DNWs/C, Pt₂Ir DNWs/C, Pt₄Ir DNWs/C, and Pt DNWs/C.¹⁰² Reproduced with permission. Copyright 2022, The Royal Society of Chemistry. (h) LSV curves of various electrodes at a sweep rate of 1 mV s⁻¹. (i) LSV curves of the Pt–NiFe LDH/CC electrode before and after 1000 cycles. (j) Schematic illustration of the full water splitting system assembled with the Pt–NiFe LDH/CC and (Ni_0.77Fe_0.23)Se₂/CC electrodes.¹⁰³ Reproduced with permission. Copyright 2019, The Royal Society of Chemistry.

4.2 Alkaline HER

Most HER is performed in alkaline electrolytes. This is because acidic media can corrode and damage equipment, which is not compatible with green and sustainable requirements.^98,99 The catalyst activity in alkaline media is 2–3 orders of magnitude lower than in acidic media due to insufficient proton supply. Therefore, the development of alkaline HER catalysts with high activity is an important mission.^100,101

4.2.1 Pt-Based SNMs in alkaline HER. Recently, Wang et al.¹⁰² reported a class of sub-nanoscale Pt–Ir twisted nanowires (Pt–Ir DNWs) (Fig. 9(e)), which have excellent catalytic performance in alkaline media. The overpotential is only 26 mV even at a current density of 10 mA cm⁻² (Fig. 9(f)). Analysis by X-ray photoelectron spectroscopy (XPS) showed that the binding energies of different Pt–Ir DNWs/C showed a gradual increase with increasing Ir content, indicating the transfer of electrons from Ir to Pt (Fig. 9(g)). For alkaline HER, attenuated hydrogen adsorption can accelerate hydrogen dissociation. The catalytic activity is ultimately enhanced due to the change in electronic structure of Ir and the balanced H_ad and OH_ad adsorption energies.

In addition, the design and development of a cost-effective electrolytic water electrolyzer is a major challenge. Yan et al.¹⁰³ designed and developed an efficient and durable electrolytic water system for electrolysis of water in alkaline solution. The cathode material NiFe layered double hydroxide nanosheets (Pt/Ni_xFe LDH) were prepared by hydrothermal synthesis. For the cathode material, Pt sub-nanoclusters were uniformly distributed on the NiFe LDH nanosheets. According to the analysis, Pt atoms react with Ni and Fe atoms by redox reaction of [PtCl₆]²⁻, thus forming strong bonds with the anions of NiFe LDH nanosheets, which results in a good dispersion of Pt clusters in NiFe LDH nanosheets. It ensures sufficient exposure of Pt active sites and prevents Pt agglomeration, so that Pt-NiFe LDH/CC exhibits high activity and stability.

4.2.2 Ru-Based SNMs in alkaline HER. Ru is expected to replace Pt material in alkaline HER as a highly promising HER catalyst material.^104–106 To explore the size-activity relationship at sub-nanometer size of catalysts, in 2022, Hu et al.¹⁰⁷ synthesized a series of ruthenium nanocrystals (Fig. 10(a)), ranging from single atoms and sub-nanoclusters to larger nanoparticles, aiming to investigate the size-dependent activity of HER in alkaline media. DFT calculations revealed that Ru₁₉ clusters are highly active for water dissociation with an energy barrier of only 0.03 eV (Fig. 10(b)). This accelerates the production of H* and speeds up the reaction kinetics of alkaline HER. The particle size of Ru shows a negative correlation trend with the d energy band center energy level, and subnanoclusters of Ru with d-energy band centers closer to the Femi energy level exhibit a stronger water dissociation ability. Therefore, its HER activity is superior to that of single atoms and larger nanoparticles. Comparing the HER activity of Ru-1.0, Ru-2.3, and Ru-3.1, it was learned that Ru-1.0 overpotential was as low as 13 mV at 10 mA cm⁻² generation, which is consistent with the above findings, and the subnanometer Ru-clusters also showed excellent HER durability in 100 h stability test (Fig. 10(c) and (d)).

Fig. 10 (a) TEM images of Ru crystals in size 1.0 nm. (b) Kinetic barriers of water dissociation for different Ru cluster models. (c) LSV curves of Ru-1.0, Ru-2.3, Ru-3.1, and commercial Pt/C for HER in 1 M KOH. (d) Stability test of Ru-1.0 at the current density of 1000 mA cm⁻² for 100 h.¹⁰⁷ Reproduced with permission. Copyright 2022, Springer International Publishing. (e) LSV curves of Ru-1.0, Ru-2.3, Ru-3.1, and commercial Pt/C for HER in 1 M KOH. (f) The formation process of Ru_xCu_yP₂/CNF. (g) Comparison of overpotential at current density of 10 mA cm⁻² and 100 mA cm⁻².¹⁰⁸ Reproduced with permission. Copyright 2021, Elsevier.

Bimetallic phosphide materials have received much attention due to their potentially excellent HER properties. In 2021, Yang et al.¹⁰⁸ prepared sub-nanometer Cu and Ru bimetallic phosphide nanoclusters (Ru_xCu_yP₂) in graphitized carbon nanofibers (CNF) by introducing uniformly dispersed Ru (Fig. 10(f)). The comparative analysis of CNF anchored on different metal clusters showed that the doping of Ru significantly enhanced the HER activity. And the RuCuP₂/CNF catalysts showed the best HER performance (Fig. 10(e)). The overpotential was only 10 mV and 88 mV at current densities of 10 mA cm⁻² and 100 mA cm⁻² (Fig. 10(g)).

4.3 Both acidic and alkaline HER

During the electrolysis of water, hydrogen is continuously produced and the concentration of protons in the electrolyte is constantly changing.¹⁰⁹ Therefore, there is a great trend to design and prepare HER catalysts with wide pH range.^110,111

4.3.1 Pt-Based SNMs in acidic and alkaline HER. An ultrafine (∼1.9 nm) trimetallic nanocatalyst (PtRhNi) was developed using the advantage of synergistic effects, using NiO_x as the support and composed of high-density PtRh nanoalloys.¹¹² It was found that the addition of Ni increased the catalyst electrochemical active sites (Fig. 11(a)), Rh in PtRh nanoalloys could change the Pt d-band electronic structure. Importantly, the Rh and Pt sites synergistically promoted the proton adsorption and desorption steps in HER (Fig. 11(b)). The trimetallic synergy allows PtRhNi to exhibit excellent HER performance in acidic (0.5 M H₂SO₄) and alkaline (1.0 M KOH) media, with an overpotential of only 28 mV for both even at a cathodic current density of 10 mA cm⁻² (Fig. 11(c) and (d)).

Fig. 11 Schematic representation for the nanostructure of (a) PtRh and (b) PtRhNi NCs. (c) LSV of the experimental NCs (0.5 M H₂SO₄). (d) LSV of the experimental NCs (1 M KOH).¹¹² Reproduced with permission. Copyright 2021, The Royal Society of Chemistry. (e) The enhanced mechanism of Ru S/DAs + Ru NCs hybrid nanocatalysts for HER. (f) Polarization curves of Ru S/DAs + Ru NCs, GO, commercial Ru powder and commercial Pt/C. (g) H adsorption energies in Ru DAs and Ru NCs. (h) Energetic diagram for the HER of Ru DAs and Ru NCs in an alkaline environment. (i) The calculated energy barrier for water dissociation in Ru DAs and Ru NCs.¹¹³ Reproduced with permission. Copyright 2021, Wiley VCH.

4.3.2 Ru-Based SNMs in acidic and alkaline HER. In 2021, Wang et al.¹¹³ synthesized hybrid HER catalysts of Ru single/dual atoms (Ru S/DAs) supported by carbon dots with Ru nanoclusters (Ru NCs) (Fig. 11(e)). The team demonstrated for the first time that Ru S/DAs + Ru NCs exhibit better HER performance than commercial Pt/C catalysts. At a current density of 10 mA cm⁻², the overpotential was 15 mV and 40 mV in alkaline (1 M KOH) and acidic (0.5 M H₂SO₄), respectively, exhibiting excellent HER performance (Fig. 11(f)). This is due to the synergistic interaction between Ru S/DAs and Ru NCs. DFT calculations showed that charge rearrangement occurred in single-atom Ru, Ru double-site promoted the adsorption and complexation of active hydrogen (Fig. 11(g) and (h)). In KOH solution, the HER performance was significantly enhanced due to the increased efficiency of water dissociation (Fig. 11(i)).

5. Conclusion and perspectives

Hydrogen energy will be one of the most desirable energy sources in the future, and the development of high-performance and long-lived catalysts is the key to achieve hydrogen energy substitution for fossil fuels. This paper reviews the recent advances in high-performance noble metal SNMs for HER, focusing on the design synthesis strategies, performance optimization methods, and applications. The activity of SNMs catalysts in HER is listed in Table 1. Overall, the superiority of sub-nano HER catalysts can be attributed to the almost complete exposure of atoms and multiple active sites. The above properties make the synergistic inter-component interaction, doping, defects and other strategies particularly obvious and important for the performance enhancement. While sub-nanomaterials have advantages in the field of HER, they also face many challenges. In order to have a better future of sub-nanomaterials in the field of HER, we need to consider the following challenges reasonably.

Table 1 Comparison of electrocatalytic HER performances of SNMs-based catalysts

Metal SNMs	Electrolyte	Current density (mA cm^−2)	Overpotential (mV)	Ref.
Pt4Zr2	0.05 M H2SO4	—	—	67
NiCoFePtRh	0.5 M H2SO4	10	27	68
1.2% PtCo/NPC	0.5 M H2SO4	10	14	69
Pt5/HMCS	0.5 M H2SO4/1.0 M KOH	10	20.7/46.2	73
1% Pd–CeO2−x-NC	1 M KOH	10	115	74
C-ZIF-CuPt	0.1 M KOH	10	46	77
Pt/NixFe LDHs	1 M KOH	10	240	80
PtNx/TiO2	0.5 M H2SO4	10	67	85
Ptc(250)/C	0.5 M H2SO4	10	37.4	89
Pt/MXene	0.5 M H2SO4	10	34	90
Ptn/ITO	0.1 M HClO4	—	—	91
Pt0.6/KB-OS5-40	0.5 M H2SO4	10	22	92
Pd NCs@CeO2	0.5 M H2SO4	—	—	95
Pd–Pt nanocages	0.5 M H2SO4	10	46	96
Ru SNC/W18O49 NWs	0.5 M H2SO4	10	21	97
Pt2Ir DNWs/C	0.1 M KOH	10	26	102
3D Pt–NiFe LDH/CC	1.0 M KOH	10	28	103
Ru-1.0	1 M KOH	10/100	13/60	107
RuCuP2/CNF	1.0 M NaOH	10/100	10/88	108
PtRhNi-NC	0.5 M H2SO4/1.0 M KOH	10	28/28	112
Ru S/DAs + Ru NCs	0.5 M H2SO4/1.0 M KOH	10	40/15	113

---

1. Although a large number of studies have shown that sub-nano HER electrocatalysts have high catalytic activity, only the performance tolerance of the catalysts under operating conditions has been discussed. Therefore, in order to meet the demand of high current and long time uninterrupted operation work in practical applications, our related researchers should next work on optimizing the activity and stability of sub-nano HER catalysts, and show the direction for developing sub-nano HER catalysts with environmental adaptability and long-term stability.^114–116

2. In the practical application of SNMs, the bulk synthesis of catalysts is also a tricky challenge we face. Although the currently developed SNMs have shown excellent reactivity in HER, most of the catalyst synthesis processes are still limited to the laboratory level, therefore, the development of SNMs with high cost performance and mass production is our primary challenge.^117–121

3. Establishing a unified test and evaluation standard to compare the performance of different groups of SNMs. It is difficult to avoid ambiguous or inaccurate information about catalysts due to the differences in various conditions such as preparation methods, composition and structure of HER catalysts, and the lack of details provided in the discussion of catalytic performance.^122,123 Therefore, in future studies, researchers are encouraged to be able to establish uniform test evaluation criteria, which will provide a strong basis for determining and comparing the activity of newly developed electrocatalysts in the future.

In conclusion, we believe that SNMs are a very promising and challenging direction and strategy for the design of efficient HER catalysts for synthesis. We hope that this review will inform the further development of SNMs in the field of electrocatalysis.

Author contributions

L. W. and J. L. supervised the research. J. L. conceived the review. Y. Y. and R. L. completed the references and wrote the manuscript. Y. S. checked and revised the manuscript. X. S. and Z. L. helped answer some questions. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52272222, 52072197), Outstanding Youth Foundation of Shandong Province, China (ZR2019JQ14), Youth Innovation and Technology Foundation of Shandong Higher Education Institutions, China (2019KJC004), the Natural Science Foundation of Shandong Province, China (ZR2021MB061), Major Scientific and Technological Innovation Project (2019JZZY020405), Taishan Scholar Young Talent Program (tsqn201909114), Major Basic Research Program of Natural Science Foundation of Shandong Province under Grant (ZR2020ZD09).

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Footnote

† The two authors contribute equal to the paper.

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