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DOI: 10.1039/D4TA02337D (Review Article) J. Mater. Chem. A, 2024, 12, 23297-23314

Recent advances of ruthenium-based materials for acidic oxygen evolution reaction: from catalyst design to proton exchange membrane water electrolysers

Lin-Lin Wang , Zi-You Yu * and Tong-Bu Lu
MOE International Joint Laboratory of Materials Microstructure, Institute for New Energy Materials and Low Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China. E-mail: yuziyou@email.tjut.edu.cn

Received 5th April 2024 , Accepted 29th July 2024

First published on 30th July 2024

Abstract

Harvesting renewable energy to split water offers an ideal approach to the production of clean hydrogen energy. Among various water electrolysis devices, the proton exchange membrane water electrolyser (PEMWE) with a high current density, quick response operation, and compact design has attracted much attention. The anodic oxygen evolution reaction (OER) in an acidic electrolyte seriously relies on iridium-based catalysts, but their use is limited owing to their scarcity and high cost. Ruthenium (Ru)-based catalysts have been considered as the most promising candidates to replace Ir in acidic OER, due to their low cost and high activity. Nevertheless, there is still much room to enhance the OER activity and durability of Ru-based catalysts for the practical application in PEMWEs. Herein, we first give a brief introduction of the main configuration and operating factors of PEMWEs. Then we discuss three OER mechanisms and reasons for the degradation of Ru-based catalysts in acid. Afterwards, the performance improvement strategies of Ru-based acidic OER catalysts are emphatically summarized. We further spotlight some typical examples of PEMWEs using Ru-based OER catalysts as anodes. Finally, further challenges and directions in the development of high-performance Ru-based OER catalysts in PEMWEs are offered and speculated.

image file: d4ta02337d-p1.tif

Zi-You Yu

Dr Zi-You Yu received his PhD from the University of Science and Technology of China (USTC) under Prof. Shu-Hong Yu in 2017. Afterwards he worked as a postdoc and associate research fellow at USTC between 2017 and 2021. He is currently a professor at Tianjin University of Technology. His research interests focus on the design and development of high-performance electrocatalysts for water splitting and CO2 reduction reaction. As the first or corresponding author, he has published many papers in high-impact journals, including Nat. Commun., J. Am. Chem. Soc., Angew. Chem. Int. Ed., Adv. Mater., Energy Environ. Sci., J. Mater. Chem. A, Chem. Sci., etc. All his publications have attracted over 6000 citations.

1. Introduction

With the fossil energy crisis and increasing air pollution, it is very urgent to develop green and sustainable energy.1–4 In the 1970s, John Bockris proposed the term ‘hydrogen economy’, and afterwards hydrogen energy as one of the cleanest energies has attracted much attention from academic and industrial researchers all over the world.5 Due to the inexistence of hydrogen energy in nature, we need to produce it from other resources before we can utilize it.6,7 An ideal approach for hydrogen production is harvesting the renewable wind, solar, tidal, and geothermal energies as power sources to split water.8,9 The overall process is very green with zero-carbon emission.10–13

According to the different water-splitting technologies, there are three main low-temperature water electrolytic devices, including alkaline water electrolysers, anion exchange membrane water electrolysers, and proton exchange membrane water electrolysers (PEMWEs). Among them, PEMWEs offer many remarkable advantages, such as high current density, high hydrogen purity, compact design, and quick response.14,15 Meanwhile, the transfer rate of H+ in the proton exchange membrane (PEM) is much faster than that of OH− in the anion exchange membrane, leading to a low ohmic loss and high energy efficiency.16 Based on these advantages, PEMWEs have been recognized as an attractive water-splitting technology, and nowadays megawatt-level PEMWEs have been established in many water-splitting devices.17 However, the main drawback of PEMWEs is their high cost owing to the use of noble metal-based electrocatalysts.8

The water-splitting process involves two half reactions, which are the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER).18 Compared with the HER with a two-electron transfer, the OER with a complex four-electron transfer and sluggish kinetics requires a higher overpotential for the formation of an O–O bond, making it the decisive step for the water-splitting reaction.19,20 Additionally, many active Fe, Co, and Ni-based alkaline OER catalysts are easily corroded and dissolved in the harsh acidic and strong oxidation environments.21 A few OER catalysts may undergo surface reconfiguration during the reaction process, and it is a challenge to explain the reaction mechanism and the origin of activity.22,23 Currently, iridium (Ir)-based materials are the only OER catalysts for commercial application in PEMWEs, due to their good corrosion resistance and moderate OER activity in acid. However, Ir metal (US $60[thin space (1/6-em)]670 per kg) is very scarce and expensive, with an annual production of <10 tons, which can hardly satisfy the requirements for large-scale hydrogen production. Therefore, the development of low-cost and high-performance acidic OER catalysts is highly required but extremely challenging.

Ruthenium (Ru)-based catalysts have been considered as a promising alternative to replace Ir for use in acidic OER, because of two major advantages: the price of Ru is 1/6 that of Ir and the high OER activity of Ru-based catalysts. However, the main problem in the application of Ru-based catalysts is their unsatisfactory stability.24 It has been reported that RuO2 is gradually over-oxidized to RuO4 or soluble RuO52− in the potential range of the OER in acidic media.25 To inhibit the excessive oxidation and dissolution of Ru, the following two strategies are often adopted. The first one is improving the OER activity of Ru-based catalysts to lower the OER overpotential. The second is the design of more stable Ru-based structures to reduce the demetallation and collapse of surface Ru atoms. In most cases, these strategies are unitized to work together to improve the activity and stability of Ru-based catalysts. Over the past few years, we have witnessed the rapid development of acidic Ru-based OER catalysts, as evidenced by the increased studies of the application of Ru-based catalysts in PEMWEs with good water-splitting performance.

We should point out that several excellent review articles about acidic OER have been published. For example, Strasser et al.26 and Huang et al.16 reviewed several kinds of acidic OER catalysts and the corresponding performance improvement strategies. Chen et al.27 reviewed Ru-based catalysts for the HER and OER in both acidic and alkaline electrolytes. Mei et al.,28 Ge et al.,29 and Hou et al.30 focused on the different design strategies for Ru-based OER catalysts in acidic media. Sun et al.31 mainly discussed the doping strategy for Ru-based acidic OER catalysts. However, a comprehensive summary and discussion of Ru-based acidic OER catalysts are still lacking. In particular, the application of Ru-based OER catalysts in PEMWEs has never been reviewed.

In this review article, we begin with a brief introduction of the electrolyser configuration and PEMWE operating factors. Then we discuss the reaction mechanism and reasons for the degradation of Ru-based OER catalysts in acid. We subsequently emphatically summarize several important strategies for the performance improvement of Ru-based acidic OER catalysts. After that, the applications of Ru-based OER catalysts in PEMWEs are also discussed. Finally, we give an outlook on further challenges and perspectives for the development of Ru-based catalysts in the practical PEMWE application.

2. Introduction of PEMWEs

PEMWE is a mature commercial electrolyser to produce hydrogen fuels with a low resistance, fast response, and high purity. The performance of PEMWEs is not only affected by the catalysts used, but also seriously relies on the electrolyser component, membrane electrode assembly (MEA) technology, and some operating factors of PEMWEs. Optimization of the electrolyser assembly method and suitable operating conditions can reduce contact resistance, strengthen water and gas management, and improve the utilization rate of catalysts, which are very vital for the actual evaluation of PEMWE performance. In this section, we will give a brief introduction of MEA technology and some operating factors of PEMWEs.

2.1 MEA technology

The main components of a PEMWE include bipolar plates (BPPs), porous transport layers (PTLs), PEM, and catalysts (Fig. 1a). The typical operation principles of a PEMWE are summarized as follows: deionized water is circulated at the anode side of the PEMWE, which flows past the channels of BPP and PTL, and finally arrives at the anodic catalyst layer, where pure water is employed as the electrolyte, avoiding the use of strong acid or alkali and the subsequent handling of the corrosive electrolytes. Then dissociation of the water molecule into oxygen and proton is catalyzed by the OER catalyst . The generated protons driven by the electric field pass through PEM to the cathodic HER catalyst to produce H2. As the most core component of PEMWE, a MEA consists of two PTLs and two catalyst layers sandwiched by PEM. A typical triple-phase boundary including the catalyst, electrolyte, and gas bubble is formed at the MEA (Fig. 1b).32 Because the MEA is responsible for the catalytic reaction, optimization of the MEA structure has a decisive effect on the catalytic activity and the long-term stability of PEMWEs. A good MEA configuration should satisfy some essential requirements, such as excellent mechanical and chemical stability, abundant contact areas of PTL, PEM, and catalysts, and a stable triple-phase interface.34
image file: d4ta02337d-f1.tif
Fig. 1 A brief introduction of PEMWEs. (a) Main components of a PEMWE. (b) The formed triple-phase interface in the MEA. Reproduced with permission.32 Copyright 2022, Elsevier Ltd. (c) Two different MEA configurations, reproduced with permission.33 Copyright 2019, The Royal Society of Chemistry.

Currently, two common MEA configurations have been developed, namely catalyst-coated membrane (CCM)-type MEA configuration and porous transport electrode (PTE)-type MEA configuration (Fig. 1c).33 For the CCM-type MEA, the catalysts with ionomer solutions are coated on the membrane by a direct spraying method or through a decal transfer process. The former method can simplify the operation process and reduce the catalyst loss, while the latter method can relieve some problems such as the membrane swelling and catalyst layer fracture.35 The CCM-type configuration can offer full contact between the PEM and catalyst with a zero gap. In the PTE-type MEA technology, the catalyst ink is first deposited on the conductive PTL and then combined with the membrane via the hot pressing treatment. The preparation process can weaken the deformation and damage of the membrane, which is very efficient for the employment of a thin PEM.36 However, the main drawback of the PTE-type MEA is the poor contact between the catalysts and PEM. Zenyuk and co-workers observed the structural difference between the two MEA configurations using operando X-ray computed tomography.37 To achieve a current density of 1 A cm−2, the PEMWE based on the PTE-type MEA requires a higher cell voltage of about 200 mV than that of CCM-type MEA, showing the worse water-splitting performance for PTE-type PEMWEs. The X-ray computed tomography imaging revealed the inhomogeneous distribution of catalysts on PTL and the poor cohesion between catalyst particles with the membrane in PTE-type MEA, thus leading to the high contact resistance and low utilization of catalysts.

In addition to the conventional MEA technology, some novel MEA methods have been reported recently. For example, the roll-to-roll fabrication method offers a continuous and high-throughput coating process,38 which can enhance the coating efficiency with a full automatic operation and prolong the interaction time between the catalyst ink and membrane to obtain a high-quality MEA. In addition, reactive spray deposition technology has been reported as a flame-based method. This strategy can allow the catalyst synthesis by the heat decomposition of precursors and then direct deposition of obtained nanoparticles on the PEM or PTL substrate in one step.39 Nevertheless, more efforts and attempts are still needed to develop more advanced MEA techniques.

2.2 Factors affecting PEMWE performance

2.2.1 Effect of PEM and ionomer. As a solid polymer electrolyte, PEM is used to achieve proton transport and avoid the crossover of hydrogen and oxygen gases.40 As a typical perfluorosulfonic acid-based membrane, Nafion membrane with a Teflon backbone and hydrophilic sulfonate has been widely utilized in PEMWEs, due to the small electrical resistance, high proton conductivity, and good thermal and chemical stability.41 The thickness of the Nafion membrane has a prominent impact on the performance of PEMWEs. Ma and co-workers reported that the activity of PEMWEs increases with the decrease of Nafion membrane thickness (Fig. 2a).42 At a cell voltage of 2.0 V, a PEMWE using Nafion 112 (50 μm) can deliver a current density of 1.4 A cm−2 at 30 °C, exceeding that of Nafion 1135 (88 μm) and Nafion 115 (125 μm) by 1.33 and 2.05 times, respectively. The thin membrane shows enhanced current density with small ohmic resistances, but easily causes increased risk of gas crossover. For instance, H2 crossover through Nafion 115 with 125 μm thickness is 2-fold higher than that through Nafion 117 (175 μm).46 Therefore, the choice of a suitable PEM not only depends on its effect on the PEMWE performance, but also greatly on the system pressure.
image file: d4ta02337d-f2.tif
Fig. 2 Factors affecting PEMWE performance. (a) The effect of Nafion membrane. Reproduced with permission.42 Copyright 2009, Elsevier Ltd. (b) The effect of ionomer content. Reproduced with permission.43 Copyright 2010, Elsevier Ltd. (c) The effect of temperature. Reproduced with permission.44 Copyright 2020, Elsevier Ltd. (d) The effect of flow rate. Reproduced with permission.45 Copyright 2021, Multidisciplinary Digital Publishing Institute (MDP).

The ionomer has a multi-functional role in the catalyst layer, including as a binder to form a stable triple-phase interface, as a proton conductor to provide fast proton transport, and as a hydrophilic medium to maintain the moisture on the catalyst surface.43 Currently, Nafion is the most used ionomer. The optimization of ionomer content is very vital, because a low content usually causes insufficient contact between the proton and catalyst, thus leading to poor catalyst utilization, whereas a high content might block the gas transport channels and increase mass transfer resistance. For example, Scott et al. found that an appropriate Nafion content of 25 wt% in the anode exhibits the best PEMWE performance (Fig. 2b),43 due to the remarkable electrochemical active area. It should be noted that the optimal content does not maintain the constant value, which varies with the surface properties, specific surface area, and mass loading of the catalyst.

2.2.2 Effect of temperature, pressure, and electrolyte flow rate. In general, the overall performance of PEMWEs could be improved at an elevated operating temperature, due to the decreased thermodynamic potential and ohmic loss, accelerated reaction kinetics, and fast mass transport. For example, Bazylak et al. observed reduced cell voltage with the operating temperature increasing from 40 to 80 °C (Fig. 2c).44 To reach a current density of 2 A cm−2, the required cell voltage at 80 °C is merely 1.78 V, superior to that of 1.84 V and 1.92 V operated at 60 °C and 40 °C, respectively. Nevertheless, the increased temperature would bring a higher requirement of PEM and PTL. When the temperature rises above 80 °C, the Nafion membrane undergoes a dehydration process, resulting in a decline in proton transport capacity. Meanwhile, the corrosion and oxidation of PTL becomes serious, which will impair the long-term durability of the PEMWE.47 Therefore, a commercial PEMWE usually operates at a moderate temperature of 60–80 °C.

The operating pressure also affects the PEMWE performance. Although a high pressure requires extra energy and specific equipment, it is beneficial for the direct storage or utilization of high-pressure hydrogen.40 Meanwhile, the high-pressure condition can efficiently promote the transport of liquid and gas through the electrolyser. Commonly, the in situ generated bubbles easily block the contact between catalysts and electrolytes by decreasing the electrochemical active surfaces, which could be relieved by elevating the system pressure. However, the high pressure increases the crossover risk of hydrogen and oxygen gases due to the elevated gas solubility and mobility.48,49

In addition, the electrolyte flow rate affects the charge transfer resistance of the catalyst layer and the proton transfer resistance of PEM. Choi et al. reported that at a cell voltage of 2 V, the current density of the electrolyser at 25 sccm is 14.8% higher than that at 75 sccm, indicating the low flow rate possessing a better water-splitting activity (Fig. 2d).45 A high flow rate can cause an obvious temperature gradient from the anode inlet to the outlet, which reduces the proton conductivity and increases the area-specific impedance. Moreover, a high flow rate can accelerate the electrolyte's arrival at the catalyst surface and may rinse some catalyst particles from the MEA, leading to the degradation of catalysts. However, if the electrolyte flow rate is too low, the mass transfer is insufficient. The flow rate should be properly optimized according to the structure of PTL and MEA.

3. Mechanistic studies on Ru-based acidic OER catalysts

In-depth understanding of the OER mechanism can help to design and develop more active Ru-based OER catalysts. Nowadays, three mechanisms, e.g., adsorbate evolution mechanism (AEM), lattice oxygen mechanism (LOM), and oxide path mechanism (OPM) have been well recognized. The conventional AEM involves the surface adsorption of intermediates (*OH, *O and *OOH) on active centers,50 the lattice oxygen participates in the overall oxygen cycle in the LOM pathway,30 and OPM allows the direct *O–*O coupling on two metal sites.51 Different catalyst structures can cause different reaction mechanisms, which greatly affect their OER performances including the activity and stability. In addition, the limitation of Ru-based catalysts in the application of PEMWEs is the poor stability in acidic electrolytes. Therefore, it is highly imperative to investigate its deactivation mechanism. In this section, we will give an overview of the three OER mechanisms, as well as the degradation mechanism of Ru-based catalysts in acidic media.

3.1 AEM

The AEM consists of four concerted proton–electron transfer steps with a typical adsorption process between the oxygen-containing intermediates and metal sites at each step.52 The combination of intermediates and Ru site forms Ru–OH, Ru–O, and Ru–OOH in the acidic AEM (Fig. 3a).30 In detail, H2O is first adsorbed on the Ru active site to produce Ru–OH, which is deprotonated to form Ru–O (reaction (1) and (2), Fig. 3a). The following O–O bond formation step involves the adsorption of another H2O on the Ru–O surface to yield Ru–OOH (reaction (3), Fig. 3a). Finally, the deprotonation of Ru–OOH can release O2 and reform the active Ru site through reaction (4) (Fig. 3a).
image file: d4ta02337d-f3.tif
Fig. 3 The adsorbate evolution mechanism (AEM) for the OER. (a) The typical AEM pathway. Reproduced with permission.30 Copyright 2023, The Royal Society of Chemistry. (b) Adsorption energy of *OOH plotted against the adsorption energy of *OH on different oxides. (c) A volcano-type plot between OER activity and the descriptor. (b and c) Reproduced with permission.50 Copyright 2011, Wiley-VCH.

As shown in Fig. 3b, the binding energy between *OOH and *OH on the surface of various oxides has a good linear scaling relationship with a slope of 1 and an intercept of about 3.2 ± 0.2 eV, indicating a strong correlation between *OOH and *OH intermediates.50 Therefore, only two free energies affect the OER activity and we can broadly describe the theoretical overpotential of the studied OER catalyst from the difference between ΔG*O and ΔG*OH (ΔG*O − ΔG*OH).53 The OER activity as a function of the descriptor of (ΔG*O − ΔG*OH) exhibits a volcano-type trend (Fig. 3c). According to the Sabatier principle, an ideal catalyst requires that the adsorption strength of the key intermediates be neither too strong nor too weak. Among these catalysts, RuO2 with an optimal binding is located at the top of the volcano plot, demonstrating the low theoretical overpotential and high OER activity. Due to the existing scaling relationship between *OOH and *OH, the smallest theoretical overpotential of the OER could be determined to be about 0.37 V.54

3.2 LOM

As the AEM cannot fully explain the pH-dependent OER activity on the reversible hydrogen electrode (RHE) scale, some researchers proposed the possible LOM pathway.55–57 In the LOM process, the first two steps are similar to those of the AEM (Fig. 4a).30 Subsequently, the combination of the adsorbed oxygen and lattice oxygen through O–O coupling can release the O2 to leave oxygen vacancy in the crystal structure of the catalyst (reaction (3), Fig. 4a). Then the oxygen vacancy can be repaired through the adsorption of *OH and deprotonation process (reaction (4) and (5), Fig. 4a).
image file: d4ta02337d-f4.tif
Fig. 4 The lattice oxygen mechanism (LOM) for the OER. (a) The typical LOM pathway. Reproduced with permission.30 Copyright 2023, The Royal Society of Chemistry. (b) Schematic representation of the switch of active center. Reproduced with permission.58 Copyright 2016, Springer Nature.

The activation of lattice oxygen usually requires some specific conditions. When the metal d band is above the oxygen p band, the active center is controlled by the metal alone through the AEM (the left scenario in Fig. 4b).58 By increasing the metal–oxygen covalency, the metal d band enters the oxygen p band and the redox center of the catalyst is no longer only located by the metal, which switches the active center from the cationic site to the anionic site (the middle case in Fig. 4b). When the anionic redox center is activated, oxygen is produced through the oxidation of lattice oxygen at a potential higher than 1.23 V versus RHE (the right case in Fig. 4b). According to the theoretical calculation studies, Liu and co-workers demonstrated that the OER mechanism of rutile RuO2 could be regulated by controlling the doping amount of transition metal.59 At a low doping concentration of transition metal, the production of O2 is mainly caused by the AEM, whereas a higher doping concentration can trigger the LOM. The distribution of metal d band and oxygen p band is the main factor that affects the conversion of AEM to LOM.

Obviously, LOM does not involve the formation of *OOH intermediates, which can thus break the scaling relationship and bypass this theoretical ceiling in the AEM. Some prior studies have demonstrated that a LOM-involved catalyst exhibits better OER activity than an AEM-involved catalyst.60,61 Unfortunately, the participation of lattice oxygen in LOM easily causes the structural collapse and catalyst reconstruction, leading to decreased stability.8 Therefore, careful consideration is needed for the balance of activity and stability by the regulation of AEM and LOM in acidic OER.

3.3 OPM

The recently proposed OPM provides a new perspective for the development of OER catalysts,63,64 owing to the symmetry and special bimetallic sites with appropriate metal–metal distance. OPM allows direct *O radical coupling (*O–*O) without lattice O sacrificed and extra reaction intermediates (such as *OOH) (Fig. 5a). In detail, two *OH are successively adsorbed on two metal sites and are converted into *O by deprotonation (reactions (1) and (2), Fig. 5a), which are then directly coupled by O–O to release O2 (reactions (3) and (4), Fig. 5a).
image file: d4ta02337d-f5.tif
Fig. 5 The oxide path mechanism (OPM) for the OER. (a) The OPM pathway. (b) Crystalline structure models of RuO2 Ru atoms substituting for the Mn at surface sites Reproduced with permission.62 Copyright 2021, Springer Nature.

For example, Li et al. reported Ru atom arrays on a MnO2 catalyst with a preferential OPM pathway.62 The Ru–Ru distance of 2.9 Å for Ru/MnO2 is shorter than that of pure RuO2 (3.1 Å), enabling the effective *O–*O coupling (Fig. 5b). Density functional theory (DFT) calculation revealed a lower theoretical overpotential for the OPM pathway compared with the AEM pathway (0.26 V versus 0.31 V). Operando synchrotron infrared spectroscopy and operando differential electrochemical mass spectrometry successfully detected the reaction intermediates and isotope-labelled products through the OPM process on the Ru/MnO2 catalyst not on RuO2 catalyst.

3.4 Degradation mechanism

There are two possible factors that affect the degradation of Ru-based OER catalysts in acid. One is the overoxidation of Ru at a high OER potential. Accompanied by the OER cycle, the dissolution of RuO2 is related to the attack of nucleophilic water, which leads to the oxidation of Ru4+ to a high valence Ru, and finally to the formation of unstable RuO4 or RuO52− (Fig. 6).65 Although a part of RuO2 also participates in the OER process, the gradual loss of Ru-based active sites leads to deteriorated durability. Another possible factor for Ru degradation is the unstable structure caused by the generated oxygen vacancy in the LOM as discussed above. The Ru–O bond becomes weak after the formation of oxygen vacancy, leading to the easy demetallation of coordinatively unsaturated Ru atoms.66
image file: d4ta02337d-f6.tif
Fig. 6 The degradation process of Ru-based catalysts in acid. Reproduced with permission.65 Copyright 2019, American Chemical Society.

To further improve the structural stability of Ru-based OER catalysts, several possible strategies have been proposed, such as introducing the electron donor components to decrease the Ru oxidation state,67,68 optimizing the Ru coordination environment to enhance the strength of Ru–O bond,69,70 regulating the reaction mechanism to suppress the lattice oxygen oxidation,71 improving the redeposition rate of dissolved Ru species,62 and promoting the regeneration of oxygen vacancy to recharge the missing lattice oxygen.72,73 Nevertheless, the exploration of more efficient approaches to relieving the degradation of Ru-based catalysts is still underway.

4. Recent developments of Ru-based acidic OER catalysts

Compared with Ir-based materials, Ru-based catalysts involve low cost and have high OER activity. However, the unsatisfactory long-term stability limits their industrial application in PEMWEs. To settle this challenge, many previous studies have developed various strategies to further improve the activity and stability of Ru-based OER catalysts in acid. In this part, we will review and summarize these design strategies and some research progress, including the tailoring of pure Ru-based catalysts, Ru-based catalysts with heteroatom doping, and Ru-based catalysts with support coupling.

4.1 The tailoring of pure Ru-based catalysts

The morphologies and surface structures of Ru-based catalysts are closely related to the accessible active site and electronic structure, which greatly affect the OER performance. Therefore, it is an effective method to improve the acidic OER performance by directly tailoring the morphology and electronic structure of pure Ru-based compounds, without the introduction of other components. For example, the control of catalyst morphology and size can alter the number of active sites, or the utilization of crystal phase engineering as well as defect engineering can optimize the intrinsic OER activity (Table 1).
Table 1 Summary for the tailoring of pure Ru-based catalysts
Catalysts Electrolytes η 10 (mV) Tafel (mV dec−1) Stability (h) Tailoring strategy References
RuO2 nanowires 0.1 M HClO4 224 61.5 12 h at 10 mA cm−2 Morphology engineering 74
NaRuO2 0.1 M HClO4 255 38 6 h at 10 mA cm−2 Morphology engineering 75
a-RuTe2 PNRs 0.5 M H2SO4 245 47 24 h at 1.52 V Crystal phase engineering 76
a/c-RuO2 0.1 M HClO4 205 48.6 60 h at 10 mA cm−2 Crystal phase engineering 77
RuO2 nanosheets 0.5 M H2SO4 199 38.2 6 h at 10 mA cm−2 Defect engineering 78
RuO2-NS/CF 0.5 M H2SO4 212 78 30 h at 10 mA cm−2 Defect engineering 79

4.1.1 Morphology engineering. To expose more active sites of Ru-based catalysts, the most direct strategy is nanostructure engineering. For example, Krtil et al. reported that the OER activity of RuO2 could be moderately improved by reducing the particle size from 27 nm to 14 nm.80 Vaidhyanathan and co-workers further synthesized ultrasmall RuO2 nanoparticles of about 3–4 nm by a covalent organic framework modified method, which delivered a low overpotential of 210 mV at 10 mA cm−2 and a small Tafel slope of 65 mV dec−1.81

In addition to size control, Ru-based catalysts with various morphologies such as one-dimensional nanorods and nanowires,82–84 two-dimensional nanosheets,85,86 and three-dimensional assembly structures87,88 have been well designed and synthesized to regulate the OER activity. The synthesis of different morphological RuO2 catalysts using a similar method is beneficial to compare their OER activities, but still remains largely underexplored. By adjusting the growth thermodynamics and kinetics, Liang et al. developed a gas controlled strategy to synthesize three Ru-based nanoparticles, nanowires, and nanosheets in Ar, H2, and O2 atmospheres, respectively (Fig. 7a–d).74 To obtain a current density of 10 mA cm−2, the overpotentials of different samples followed an order of RuO2 nanowires (224 mV) < RuO2 nanosheets (236 mV) < RuO2 nanoparticles (255 mV) < commercial RuO2 (295 mV) < commercial IrO2 (390 mV), demonstrating that RuO2 nanowires possessed the best OER activity in acid (Fig. 7e). The specific activity further confirmed the excellent OER performance of RuO2 nanowires (Fig. 7f), which could be due to the combination of high surface area and high conductivity of this catalyst.


image file: d4ta02337d-f7.tif
Fig. 7 Morphology engineering of Ru-based compounds. (a) Schematic illustration of the synthetic routes. (b–d) Transmission electron microscopy (TEM) images of different catalysts. (e) OER polarization curves of different catalysts in 0.1 M HClO4. (f) The comparison of specific activities at a potential of 1.53 V versus RHE. Reproduced with permission.74 Copyright 2021, Wiley-VCH.

In addition, ultrathin two-dimensional Ru-based nanosheets also attracted much attention, because of the high specific area and numerous unsaturated sites. Lotsch et al. synthesized ultrathin RuO2 nanosheets with a thickness of around 1 nm by the chemical exfoliation of proton exchanged NaRuO2 precursor.75 Electrochemical tests revealed that RuO2 nanosheets had mere 255 mV overpotential at 10 mA cm−2 in acidic electrolytes. More interestingly, the morphology and oxidation state of RuO2 nanosheets remain unchanged after the OER test, indicating the good long-term stability. Theoretical calculations showed that the terminal O species at the under-coordinated Ru edge atoms can stabilize the intermediate and reduce the activation energy of the reaction, thus leading to the high activity and stability. Although remarkable results have been achieved in improving the acidic OER performance through morphology manipulation, the strategy of simply increasing the specific surface area can easily reach the active platform due to the limitation of the intrinsic OER activity of Ru-based catalysts.

4.1.2 Crystal phase engineering. To date, most studies of OER catalysts have been conducted on crystalline structures,89,90 because crystalline catalysts have good electrical conductivity and can effectively maintain the structural stability during the reaction. However, recent studies showed that although the initial crystallinity is ordered, some catalysts might go through surface reconstruction to form amorphous structures during the catalytic reaction.91,92 The disordered and loose structure inside the amorphous phase can yield a large number of unsaturated coordination sites and defects, thus having more active sites.93–95 In addition, the unique electronic structure and isotropic properties endow the amorphous materials with corrosion-resistance under acidic conditions, providing new insights into the search for stable OER catalysts.96 Therefore, researchers proposed the direct synthesis of amorphous acidic OER catalysts. For example, Huang and co-workers developed amorphous RuTe2 porous nanorods as an efficient OER catalyst in acid.76 Theoretical calculations revealed that the amorphous RuTe2 with a local distorted Ru–Te lattice increases the inter-d-orbital electron-transfer ability among Ru sites and thus promotes the electron-lattice coupling effect, much better than the crystalline RuTe2 catalyst with limited d–d transfer (Fig. 8a). To verify the high catalytic ability of the amorphous structure, they prepared amorphous and crystalline RuTe2 under similar conditions but with different annealing atmospheres (Fig. 8b and c). Further electrochemical tests (Fig. 8d) showed that amorphous RuTe2 had excellent OER activity with a low overpotential of 245 mV at 10 mA cm−2 in 0.5 M H2SO4 solution, much superior to that of crystalline RuTe2 (442 mV) and Ir/C (323 mV). Post-mortem characterization revealed that the structure defect in amorphous RuTe2 is easily replaced by oxygen atoms to form active RuOxHy sites for the boosted OER activity.
image file: d4ta02337d-f8.tif
Fig. 8 Crystal phase engineering of Ru-based compounds. (a) Schematic diagram of the electronic structures of amorphous and crystalline catalysts. (b and c) Electron microscopy characterization of (b) crystalline RuTe2 and (c) amorphous RuTe2. (d) OER polarization curves of different catalysts in 0.5 M H2SO4. (a–d) Reproduced with permission.76 Copyright 2019, Springer Nature.

Meanwhile, the mixed phase strategy has been reported to enhance the OER activity and stability of Ru-based catalysts. Zhang et al. prepared RuO2 with a mixed amorphous and crystalline phase by a sol–gel method.77 The crystalline part is expected to expose specific crystal faces that promote the OER activity. The flexible structure of the mixed phase can self-regulate and withstand the structural distortions and Ru redox reaction to relieve the dissolution of Ru, resulting in the enhanced stability.

4.1.3 Defect engineering. The defect sites in a catalyst can change the electronic structure, the surface charge distribution, and the coordination environment of active centers, which will affect the adsorption and desorption energies of OER intermediates and further regulate the catalytic performance.97–99 The introduction of some defects in RuO2-based catalysts could provide a suitable platform to regulate the OER activity. For instance, Li and co-workers reported a vacancy-rich RuO2 catalyst, which was prepared by a facile molten salt method.78 The RuO2 catalyst possesses an ultrathin nanosheet structure with a thickness of 1–2 nm (Fig. 9a). Abundant Ru vacancies and grain boundaries were observed on the crystal structure (Fig. 9b). The vacancy-rich RuO2 nanosheets can achieve 10 mA cm−2 current density at a low overpotential of 199 mV and a small Tafel slope of 38.2 mV dec−1, exceeding that of the annealed RuO2 with low vacancy sites and the commercial RuO2 (Fig. 9c). DFT calculation with the AEM process showed that Ru vacancy can significantly weaken the binding energy of *O relative to *OOH, which decreases the energy barrier from *O to *OOH, thus improving the OER activity. Further growth of defect-rich RuO2 nanosheets on a conductive carbon fiber electrode (RuO2NS/CF) could facilitate the electron transfer and gas detachment, which will enhance the OER activity at a high current density (Fig. 9d–f).79 Intriguingly, RuO2-NS/CF can deliver 200 mA cm−2 at merely 310 mV overpotential, better than other control samples. The results showed that the abundant defects are the OER reactive sites and the self-supporting structure can effectively prevent the aggregation of catalysts, which provides a new idea for the design of stable OER catalysts. It is worth noting that the defect engineering will increase the risk of the formation of Ru and oxygen vacancies, leading to structural collapse as well as poor stability.
image file: d4ta02337d-f9.tif
Fig. 9 Defect engineering of Ru-based compounds. (a) TEM image of defect-rich RuO2 nanosheets. (b) Structure model of Ru defects in RuO2. (c) OER polarization curves of different catalysts in 0.5 M H2SO4. (a–c) Reproduced with permission.78 Copyright 2020, The Royal Society of Chemistry. (d) Scanning electron microscopy (SEM) image of RuO2-NS/CF. (e) Structure model of lattice defects in RuO2. (f) OER polarization curves of different catalysts in 0.5 M H2SO4. (d–f) Reproduced with permission.79 Copyright 2021, Elsevier Ltd.

4.2 Ru-based catalysts with heteroatom doping

Because there is still much room to improve the OER performance of Ru-based catalysts in acid, substantial efforts have been devoted to exploring other strategies, such as element doping engineering. Doping RuO2 with heteroatoms is the most common approach to regulate Ru–O bonds and the Ru coordination environment, which will greatly influence the binding energies of OER intermediates (*O, *OH, and *OOH) as well as their OER activity.100
4.2.1 RuO2 with single metal doping. Numerous foreign metal atoms, such as alkali metals (Li101 and Na102), 3d transition metals (Cr,103 Mn,104,105 Fe,106 Co,107 Ni,108 Cu,109 and Zn110), 4d transition metals (Nb,111 Mo,112 and Rh113), 5d transition metals (W114 and Re68), and rare earth metals (La,115 Ce,116 and Y117,118), and p-block elements (In119 and Sn120,121) have been reported to incorporate into the RuO2 lattice to partly replace Ru element.

For example, some metrics have been proposed as the descriptors that affect the OER activity and stability of RuO2 by doping engineering with different dopants. For example, Ge et al. used MRuOx (M = Ce4+, Sn4+, Ru4+, Cr4+) solid solution as the structure model and proposed the Ru charge as the descriptor to describe the activity and stability of the RuO2 catalyst.121 Ru charge can reflect the Ru–O bonding interaction, and the appropriate adsorption of oxygenated intermediates results in the best OER activity. For stability, the increase of Ru charge can result in the strengthened Ru–O bonding and the increased formation energy of Ru vacancy, but it easily causes the formation of oxygen vacancy. Meanwhile, the increase of Ru charge can shift the mechanism from AEM to LOM (Fig. 10a). Therefore, SnRuOx catalyst with moderate Ru charge (1.52) exhibited the best OER activity and stability, which required a low overpotential of 194 mV at 10 mA cm−2 (Fig. 10b), and stably operated at 100 mA cm−2 for 250 h. Most recently, Guo et al. reported Ru–O covalency as the descriptor of OER stability in Ln–RuOx (Ln = Ho, Er, Tm).122 The doped lanthanide element can modulate Ru–O covalence through the shielding effect of 5s/5p orbitals. Due to the optimized Ru–O covalency, Er–RuOx at the top of the volcano plot had the highest formation energies of oxygen vacancy and Ru vacancy (Fig. 10c and d), thus demonstrating the high long-term stability (Fig. 10e).


image file: d4ta02337d-f10.tif
Fig. 10 RuO2 with single metal doping. (a) The variation of apparent overpotential at 10 mA cm−2 with Ru oxidation states. The apparent overpotential experimentally obtained is given as the blue dots scaled with the right Y-axis. The theoretical overpotential is given as the green dots scaled with the left Y-axis. (b) OER polarization curves of different catalysts. (a and b) Reproduced with permission.121 Copyright 2023, Springer Nature. (c) The ΔGO vacancy and (d) ΔGRu vacancy as a function of –ICOHP for Ln–RuOx. The upshift values of ICOHP indicate lower Ru–O covalency. (e) The stability tests of different catalysts in 0.5 M H2SO4. (c–e) Reproduced with permission.122 Copyright 2024, Springer Nature.
4.2.2 RuO2 with multiple metal doping. Besides single metal doping, the incorporation of RuO2 with multiple metals could provide a chance to synergistically tune the RuO2 structure and the active sites. For example, Zhang and co-workers reported the synthesis of W and Er co-doped RuO2 (W0.2Er0.1Ru0.7O2−δ) nanosheets by a simple hydrothermal method.123 Elemental mapping images showed the uniform distribution of Ru, W, Er, and O in the nanosheets (Fig. 11a). DFT calculation indicates that W and Er dual dopants in W0.2Er0.1Ru0.7O2−δ can pull the downshift of O 2p-band centers and increase oxygen vacancy formation energy (Fig. 11b), which thereby reduces the OER adsorption energies and prevents the overoxidation of Ru ions. As a result, the W0.2Er0.1Ru0.7O2−δ catalyst offers a very low overpotential of 168 mV at 10 mA cm−2 (Fig. 11c). W0.2Er0.1Ru0.7O2−δ can maintain an excellent stability of 500 h with very low dissolved amounts of metal elements (Fig. 11d and e). Post-mortem characterization revealed that the partial electrons from W and Er were transferred to Ru atoms through a charge redistribution process, which could suppress the over-oxidation of RuO2 to improve the durability.
image file: d4ta02337d-f11.tif
Fig. 11 RuO2 with multiple metal doping. (a) Elemental mapping images of W0.2Er0.1Ru0.7O2−δ nanosheets. (b) The calculated oxygen vacancy formation energies of different catalysts. (c) OER polarization curves of different catalysts in 0.5 M H2SO4. (d) The stability tests of W0.2Er0.1Ru0.7O2−δ nanosheets in 0.5 M H2SO4. (e) ICP analysis for W0.2Er0.1Ru0.7O2−δ after 500 h operation in an acidic electrolyte. (a–e) Reproduced with permission.123 Copyright 2020, Springer Nature.
4.2.3 Ru-based perovskite catalysts. Because ABO3-type perovskites have the advantage of adjustable valences in two metal sites, the design of Ru-based perovskites allows the flexible regulation of Ru electronic structure, and provides a suitable platform to study the structure–activity–mechanism relationship.57

Zhou et al. directly synthesized the quadruple perovskite CaCu3Ru4O12 by a high-temperature solid-state reaction (Fig. 12a).124 DFT calculation reveals that the Fermi level of CaCu3Ru4O12 decreases in Ru 4d and O 2p bands relative to RuO2 (Fig. 12b), indicating a weaker Ru–O binding strength. It can thus decrease the formation energy of *OOH from *O through the AEM process. Notably, CaCu3Ru4O12 only requires 171 mV to reach 10 mA cm−2 in 0.5 M H2SO4, 145 mV lower than that of RuO2 (Fig. 12c). In addition, Retuerto et al. reported that Na doping of SrRuO3 can shorten the Ru–O distance and regulate the octahedral configuration (Fig. 12d), thus improving its OER stability and activity.125 Compared with the strong binding energy of OER intermediates on SrRuO3, the Sr0.9Na0.1RuO3 catalyst had an optimal binding energy (Fig. 12e). Furthermore, Na doping can increase the oxidation state of Ru with lower surface energy and less distorted RuO6 octahedra, leading to the enhanced stability (Fig. 12f).


image file: d4ta02337d-f12.tif
Fig. 12 Perovskite-type catalysts. (a) Crystal structure of CaCu3Ru4O12. (b) Computed density of states (DOS) of CaCu3Ru4O12. (c) OER polarization curves of different catalysts in 0.5 M H2SO4. (a–c) Reproduced with permission.124 Copyright 2019, Springer Nature. (d) Crystal structure of Sr1−xNaxRu>n+O3. (e) OER volcano-type activity plot. (f) Percentage catalytic activity of Sr1−xNaxRuO3 after 20 cycles with respect to the initial activity, including RSD as shadow bars. (d–f) Reproduced with permission.125 Copyright 2019, Springer Nature.

Although great progress has been made to improve the acidic OER performance of Ru-based catalysts by doping engineering, some challenges still exist in the design strategy. Firstly, some doping metals have a large lattice mismatch with Ru, leading to inhomogeneous doping on the RuO2 lattice. The corresponding characterization excessively relies on the elemental mapping, and it is difficult to verify the bulk doping or surface doping. Secondly, a few dopants, such as Fe, Co, Ni, and Zn, are not very stable in acidic electrolytes. The gradual dissolution of these dopants causes numerous structure defects, which will deteriorate the stability of Ru-based OER catalysts, and further damage the whole durability of PEM electrolysers. Finally, almost all metal dopants can plausibly improve the OER performance of RuO2, but the reason behind it remains unclear. We speculate that the Ru–O distance or Ru–O bonding may be the main factor that controls the activity and durability of Ru-based catalysts. The development of a more efficient doping strategy and theory will be helpful to search more novel and active Ru-based catalysts.

4.3 Ru-based catalysts with support coupling

The OER activity is also associated with the properties of catalyst interfaces. The construction of catalyst–support interfaces can stabilize surface active sites and facilitate the electron transfer, which could modify the valence states and coordination structures of Ru-based catalysts. Based on the species of supported Ru-based catalysts, we classify the catalysts into Ru-based single atoms and nanoparticles on support in this section.
4.3.1 Ru-based single atoms on support. As a new class of emerging catalysts, single atom catalysts containing the atomically dispersed metal atoms as the primary active sites have recently received intensive attention. The distribution of isolated single-atom Ru on a suitable support can achieve high atom utilization efficiency and an adjustable coordination environment of Ru sites, aiming to optimize the OER activity.126 For example, Yao et al. reported a single-atom Ru site anchored on a nitrogen–carbon support (Ru–N–C) as an efficient acidic OER catalyst.127 The high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM) image revealed the homogeneous distribution of Ru atoms with bright spot sizes of about 0.2 nm across the entire N–C framework (Fig. 13a). The Ru K-edge extended X-ray absorption fine structure (EXAFS) curve of the Ru–N–C catalyst showed a dominant peak at 1.5 Å arising from the Ru–N/C bond (Fig. 13b), which was well-fitted into four Ru–N coordination sites in the form of an Ru1–N4 configuration. The lack of any peaks from the Ru–Ru bond excluded the formation of Ru-based nanoparticles in the Ru–N–C catalyst. Ru–N–C exhibits excellent OER activity and stability in 0.5 M H2SO4. To achieve a current density of 10 and 100 mA cm−2, the Ru–N–C catalyst only requires overpotentials of merely 267 mV and 340 mV, respectively, surpassing the RuO2/C catalyst with overpotentials of 300 mV and 515 mV, respectively (Fig. 13c). Moreover, Ru–N–C with a Ru mass fraction of about 1.0 wt% can achieve a mass activity of 14[thin space (1/6-em)]284 A g−1 at 300 mV overpotential, which is 410 times higher than that of the RuO2/C catalyst. Recently, Lee and co-workers developed Ru atom arrays on an α-MnO2 nanowire support with a Ru mass loading of 11.6 wt% (12Ru/MnO2).62 Multiple characterization studies revealed the periodic arrangement of Ru atom arrays in the 12Ru/MnO2 catalyst (Fig. 13d and e), in accord with the theoretical calculation results which reveal that the formation of an ordered Ru structure has a more favorable energy when the Ru mass loading is high. The 12Ru/MnO2 catalyst delivers a high OER activity with a low overpotential of 161 mV at 10 mA cm−2 and a small Tafel slope of 29.4 mV dec−1 in 0.1 M HClO4 electrolyte (Fig. 13f). DFT calculation and experimental results show that the OPM pathway is more favorable than the AEM pathway on the 12Ru/MnO2 catalyst, giving a small theoretical overpotential. In addition, due to the dynamic trapping of the leached Ru ions by the MnO2 substrate through a cation exchange reaction, the OER durability of the 12Ru/MnO2 catalyst could be substantially enhanced, wherein it continuously operated at 10 mA cm−2 for 200 h with small degradation.
image file: d4ta02337d-f13.tif
Fig. 13 Ru-based single atoms on the support. (a) HAADF-STEM image of Ru–N–C. (b) EXAFS spectra of different samples. (c) OER polarization curves of different catalysts in 0.5 M H2SO4. (a–c) Reproduced with permission.127 Copyright 2019, Springer Nature. Copyright 2023, Springer American Chemical Society. (d) HAADF-STEM image of 12Ru/MnO2. (e) EXAFS spectra of different samples. (f) OER polarization curves of different catalysts in 0.1 M HClO4. (d–f) Reproduced with permission.62 Copyright 2021, Springer Nature.
4.3.2 Ru-based nanoparticles on a support. The anchoring of Ru-based nanoparticles on a support can increase the Ru mass loading to enhance the geometrical OER activity. For example, Lv et al. reported the growth of Ru nanoparticles on non-stoichiometric TiOx nanorods (Ru/TiOx) as a binder-free acidic OER catalyst by a one-step method (Fig. 14a).128 Ru nanoparticles with sizes of about 4 nm are homogeneously deposited on the in situ formed TiOx nanorods (Fig. 14b). As a result, the as-prepared Ru/TiOx exhibits low overpotentials of 174, 209, and 265 mV to achieve current densities of 10, 100, and 500 mA cm−2, respectively (Fig. 14c). The durability test showed that Ru/TiOx can retain 98.6% of the initial activity after a continuous operation for 37 days. Ru/TiOx shows pH-independent OER kinetics on the RHE scale, typical of the AEM pathway. DFT calculations revealed that the nonstoichiometric TiOx support can induce the charge accumulation at Ru sites to prevent the overoxidation of Ru (Fig. 14d), thereby yielding excellent long-term stability.
image file: d4ta02337d-f14.tif
Fig. 14 Ru-based nanoparticles on a support. (a) Schematic route for the synthesis of Ru/TiOx. (b) Elemental mapping images of Ru/TiOx. (c) OER polarization curves of different catalysts in 0.5 M H2SO4. (d) The calculated oxygen adsorption energy and Ru–Ru bond length. (a–d) Reproduced with permission.128 Copyright 2023, Springer Nature.

Compared to other design strategies, the coupling of Ru-based sites with a support offers a route to disperse small-sized Ru-based species and simultaneously construct the catalyst–support interface. The choice of an appropriate support plays a crucial role in regulating the OER activity and durability of Ru-based catalysts.129 For example, some carbon-based materials were frequently used as substrates for their good conductivity and high surface area, but they were oxidized to CO2 under the acidic OER conditions, leading to a big issue with long-term stability. Some stable supports such as MnO2,62,130 TiO2,131,132 and Ta2O5133 would be better choices. Furthermore, although advanced characterization techniques and theoretical calculations have investigated the catalyst–support interaction, the study of dynamic atomic migration at the interface under the OER conditions is still lacking.

5. Recent developments of Ru-based OER catalysts

The development of high-performance catalysts especially for the energy-consuming OER is extremely vital to improve the energy efficiency and reduce the cost of PEMWEs. Ru-based catalysts have exhibited excellent OER activity in the three-electrode tests (Section 4), such as the small overpotential of <200 mV at 10 mA cm−2, but the application of Ru-based catalysts in practical PEMWEs still remains a big challenge, because Ru-based OER catalysts usually have poor stability under industrial water-splitting conditions, such as high current density (1–3 A cm−2) and elevated temperature (60–80 °C). In the past few years, we have witnessed the fast progress of the application of Ru-based OER catalysts in PEMWEs. In this part, we will summarize and discuss the important progress in terms of the activity and stability of Ru-based catalysts in the application of PEMWEs.

To evaluate the performance of PEMWEs, different electrochemical measurements have been performed. For example, the water-splitting activity can be achieved from the relationship between the cell voltage and current density by the cyclic voltammetry or linear sweep voltammetry test. A large-current electrochemical workstation or direct-current power is utilized to provide wide current/voltage ranges. The long-term stability is evaluated by a chronoamperometry or chronopotentiometry test at a fixed cell voltage or current density. Electrochemical impedance spectroscopy is performed to record the charge transfer resistance and mass transfer loss under the operating conditions. In addition, some post-mortem characterization studies, such as electron microscopy, X-ray diffraction, and inductively coupled plasma atomic emission spectroscopy, are applied to examine the change of morphology, phase, and Ru dissolution content. Combining the above electrochemical tests and characterization techniques, we can compare the activity and durability of Ru-based OER catalysts in PEMWEs.

As shown in Table 2, the typical examples of PEMWEs using Ru-based materials as the anode catalysts, the tested parameters, and the water-splitting performance are included. Commercial Pt/C was used as the cathode catalyst. Excitingly, a few Ru-based PEMWEs exhibit excellent water-splitting activity, and require cell voltages of merely 1.55–1.65 V to reach a current density of 1 A cm−2,71,131,136,142,144 which are comparable with the state-of-the-art Ir-based PEMWEs but with a lower cost. For example, Huang et al. reported that the a PEMWE using defective RuO2 nanosheets as the anode can produce 1 A cm−2 at a voltage of 1.56 V.144 However, the stability test at 1 A cm−2 only runs for about 10 h. Himabindu et al. further extended the stability time of PEMWEs to about 100 h at 1 A cm−2, which was achieved by the employment of the designed Ru0.8Pd0.2O2 based OER catalyst.139

Table 2 Summary of the reported performances of PEMWEs using different Ru-based OER catalysts as the anodes
Anode catalyst Temperature Membrane Ru mass loading (mg cm−2) Cell voltage at current density Stability References
RuCoOx R. T. Nafion 115 0.0856 1.5 V at 0.2 A cm−2 10 h at 0.1 A cm−2 134
W0.2Er0.1Ru0.7O2−δ R. T. Nafion 117 0.19 N/A 120 h at 0.1 A cm−2 123
Ni–RuO2 R. T. Nafion 117 2.35 1.95 V at 1 A cm−2 1000 h at 0.2 A cm−2 108
2.32 V at 2 A cm−2
Y1.85Ba0.15Ru2O7 R. T. Nafion 212 1.58 1.68 V at 1 A cm−2 13 h at 0.15 A cm−2 135
SnRuOx 50 °C Nafion 212 1.46 1.565 V at 1 A cm−2 1300 h at 1 A cm−2 121
1.655 V at 2 A cm−2
1.735 V at 3 A cm−2
Y1.75Ca0.25Ru2O7 60 °C Nafion 212 1.73 1.62 V at 1 A cm−2 16 h at 0.2 A cm−2 136
RuFe@CF 60 °C Nafion 115 0.16 1.90 V at 1 A cm−2 250 h at 0.2 A cm−2 137
2.08 V at 1.5 A cm−2
RuO2/BNNS 80 °C N/A N/A 1.5 V at 0.2 A cm−2 200 h at 0.2 A cm−2 67
Ba0.3(SO4)δW0.2Ru0.5O2−δ 80 °C Nafion 115 0.675 1.68 V at 1 A cm−2 300 h at 0.5 A cm−2 138
1.91 V at 2 A cm−2
RuO2/D-TiO2 80 °C Nafion 115 1.5 1.64 V at 1 A cm−2 6 h at 1 A cm−2 131
1.84 V at 2 A cm−2
Ru0.8Pd0.2O2 80 °C Nafion 115 1.81 2.03 V at 1 A cm−2 100 h at 1 A cm−2 139
RuO2/SnO2 80 °C Nafion 115 1.14 1.723 V at 1 A cm−2 240 h at 0.25 A cm−2 140
Nb0.1Ru0.9O2 80 °C Nafion 117 1.38 1.69 V at 1 A cm−2 100 h at 0.3 A cm−2 111
1.84 V at 2 A cm−2
2.18 V at 3 A cm−2
RuO2 80 °C Nafion 117 0.911 1.68 V at 1 A cm−2 300 h at 1 A cm−2 141
1.90 V at 2 A cm−2
PtCo–RuO2/C 80 °C Nafion 212 0.021 1.61 V at 1 A cm−2 24 h at 1 A cm−2 142
1.71 V at 2 A cm−2
1.82 V at 3 A cm−2
Y2MnRuO7 80 °C Nafion 212 0.2 1.75 V at 1 A cm−2 24 h at 0.2 A cm−2 143
2.08 V at 2 A cm−2
SrRuIr 80 °C Nafion 212 0.53 1.5 V at 1 A cm−2 150 h at 1 A cm−2 71
RuO2-NS/CF 90 °C Nafion 212 1.2 1.56 V at 1 A cm−2 10 h at 1 A cm−2 144
1.65 V at 2 A cm−2

Very recently, Ge and co-workers pushed the durability of Ru-based PEMWEs to the climax (Fig. 15).121 They screened SnRuOx as an active and stable OER catalyst through the studies of reaction route and Ru charge regulation. A PEMWE using SnRuOx as the anode and Nafion 212 as the membrane required cell voltages of merely 1.565, 1.655, and 1.735 V to achieve current densities of 1, 2, and 3 A cm−2 at 50 °C, respectively (Fig. 15a). The SnRuOx catalyst has a low cost of $0.0194 per cm2, and high energy efficiencies of 78.7% and 74.3% at 1 A cm−2 and 2 A cm−2, respectively, surpassing other reported cost-efficient Ru-based and Ir-based OER catalysts (Fig. 15b). In particular, the SnRuOx-based electrolyser can run stably at 1 A cm−2 for 1300 h with a very low degradation rate of 53 μV h−1 (Fig. 15c), indicating the excellent stability. The characterization of spent catalysts revealed that the morphology and structure of SnRuOx remained unchanged, and the Ru dissolution amount was only 5.5% after the 1300 h operation.


image file: d4ta02337d-f15.tif
Fig. 15 Performance of the PEMWE using SnRuOx as the anode catalyst. (a) Polarization curve of the PEMWE. (b) Comparison of the cell voltage and energy efficiency of different catalysts. (c) Chronopotentiometry curve of the PEMWE at 1 A cm−2. Reproduced with permission.121 Copyright 2023, Springer Nature.

In three-electrode tests at room temperature and at a low current density of 10 mA cm−2, the over-oxidation of RuO2 is not serious.145 Many in situ and ex situ characterization studies have been performed under mild conditions to investigate the dissolution mechanism. Nevertheless, at present, the application of Ru-based OER catalysts in PEMWEs is still in the initial stage. The dissolution of RuO2 under harsh PEMWE conditions is more serious and few Ru-based catalysts could stably operate for more than 500 h. The study of the dissolution behavior of Ru after PEMWE tests is highly lacking but highly required. We suggest that more future research should focus on the application of Ru-based catalysts in PEMWEs and the study of the degradation mechanism under these conditions, which will be very helpful to develop more practical Ru-based OER catalysts.

6. Conclusions and perspectives

Ru-based catalysts are a promising choice to replace Ir-based catalysts towards acidic OER, due to their high activity and low cost. Here, we first introduced the MEA preparation technology and operating factors of PEMWEd. Then we summarized three OER mechanisms and discussed the reasons for the degradation of Ru-based OER catalysts. We further surveyed several important strategies to improve the performance of Ru-based OER catalysts, including morphology engineering, crystal phase engineering, defect engineering, doping engineering, and interface engineering. We subsequently highlighted some typical examples of PEMWEs using Ru-based OER catalysts as the anode. Although great advances have been made in the development of high-performance Ru-based OER catalysts, there is a long way to go with regard to their practical application in commercial PEMWEs. In order to accelerate the development of this field, more research is still needed in the following aspects.

6.1 Standardized evaluation and development of high-stability Ru-based catalysts

Previous studies have designed and developed various strategies to enhance the OER activity of Ru-based catalysts in acid. Some Ru-based catalysts have been observed to exhibit excellent activity and require merely 150–200 mV overpotential to reach a current density of 10 mA cm−2 in a standard three-electrode test, about 100 mV lower than that of Ir-based OER catalysts. However, some Ru-based catalysts with similar compositions usually exhibit varied OER activity from different groups, probably due to the different test conditions, including the catalyst amount, electrolyte, electrode preparation method, catalyst substrate, and so on. The standardized evaluation of the catalyst is very vital to fairly compare different Ru-based catalysts.

Compared with the activity improvement, the stability improvement of Ru-based OER catalysts remains a bigger challenge. Although some developed Ru-based catalysts can efficiently catalyze the OER, they might be gradually deactivated and degraded under the long-term stability operation. Doping engineering is a common strategy to enhance the OER activity of Ru-based catalysts. However, some Fe, Co, Ni, and Zn-based doping elements are very unstable in a harsh acidic environment. The dissolution of doping elements decreases the Ru–O bond strength and thus accelerates the loss of active Ru sites. Using theoretical calculations, Nørskov et al. predicted that Sb, Ti, Sn, Ge, Mo, and W-based oxides have a preference for being stable in strong acid.146,147 The introduction of these corrosion-resistant elements into RuO2 to form mixed oxides or solid solutions can effectively maintain high stability. In addition, the ionic radius of the dopant has a large effect on the structural stability of RuO2, because the large lattice mismatch easily causes Ru and oxygen vacancies, and subsequent structural collapse during the OER.

6.2 Catalytic mechanisms and advanced characterization techniques

Understanding the real active sites and catalytic mechanism is very vital for the design of better-performing catalysts. However, some catalysts suffer from the serious surface reconstruction process. Meanwhile, the degradation process is very complex. The Ru over-oxidation and dissolution vary very significantly from the different catalysts, as well as the different electrochemical test conditions. Moreover, the study of catalytic mechanisms under PEMWE conditions is very meaningful to design Ru-based catalysts for the practical application. The characterization of catalysts before and after catalytic reactions could not reflect the real change of material structure and coordination environment under the OER conditions, leading to difficulty in uncovering the structure–performance relationship.

Therefore, the application of operando characterization techniques is a requisite to provide experimental evidence for the real catalytic sites and key intermediates. For example, Yao and co-workers identified O–Ru1–N4 as the active site for the Ru–N–C catalyst during acidic OER by the combination of operando Fourier transform infrared spectroscopy and X-ray absorption spectroscopy.127 To further uncover which OER mechanism is preferential for the studied catalyst, operando differential electrochemical mass spectroscopy analysis is usually conducted by labeling the catalyst surface with the isotope 18O. A large amount of the generated 34O2/32O2 can reflect the participation of lattice oxygen during the OER through the LOM pathway. More advanced characterization techniques are still required to elucidate the reaction mechanism.

6.3 The application of Ru-based OER catalysts in PEMWEs

The three-electrode test is a simple and fast electrochemical test method to preliminarily screen out the promising catalyst materials. Many previous research studies reported only the performance of Ru-based catalysts under these conditions and then claimed that the catalyst could be used in PEMWEs. However, the catalytic performance under mild laboratory conditions does not well reflect the actual OER performance in practical PEMWEs. We encourage more studies to assemble the developed Ru-based catalysts into PEMWEs and evaluate the performance under industrial water-splitting conditions.

Although some recent PEMWEs using Ru-based catalysts as anodes have exhibited excellent activity, there is still much room for the performance improvement of Ru-based catalysts in PEMWEs. For example, for the stability tests, most of the Ru-based PEMWEs usually operate at a low current density (<1 A cm−2) for a short time (<500 h), and thus it is hard to meet the industrial requirement (current density of 1–3 A cm−2 and stability time of >5000 h). Moreover, the degradation rate of Ru-based catalysts is relatively high and its evaluation should be carefully done, because a high degradation rate not only decreases the water-splitting activity, but also deteriorates the proton transport in the PEM caused by the dissolved Ru ions. Continued efforts are still needed to accelerate the application of Ru-based catalysts in PEMWEs.

Data availability

The data that support the findings of this study are available within the paper. Source data are provided with this paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (2022YFA1502902), and the National Natural Science Foundation of China (21931007, 22375146, and 22109149).

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