Stable and efficient single atom catalysts (SACs) are highly desirable yet challenging in catalyzing acidic oxygen evolution reaction (OER). Herein, we report a novel iridium single atom catalyst structure, with atomic Ir doped in tetragonal PdO matrix (Ir_SAs-PdO) via a lattice-confined strategy. The optimized Ir_SAs-PdO-0.10 exhibited remarkable OER activity with an overpotential of 277 mV at 10 mA·cm⁻² and long-term stability of 1000 h in 0.5 M H₂SO₄. Furthermore, the turnover frequency attains 1.6 s⁻¹ at an overpotential of 300 mV with a 24-fold increase in the intrinsic activity. The high activity originates from isolated iridium sites with low valence states and decreased Ir–O bonding covalency, and the excellent stability is a result of the effective confinement of iridium sites by Ir–O–Pd motifs. Moreover, we demonstrated for the first time that SACs have great potential in realizing ultralow loading of iridium (as low as microgram per square center meter level) in a practical water electrolyzer.

The reactant concentration at the catalytic interface holds the key to the activity of electrocatalytic hydrogen evolution reaction (HER), mainly referring to the capacity of adsorbing hydrogen and electron accessibility. With hydrogen adsorption free energy (ΔG_H) as a reactivity descriptor, the volcano curve based on Sabatier principle is established to evaluate the hydrogen evolution activity of catalysts. However, the role of electron as reactant received insufficient attention, especially for noble metal-free compound catalysts with poor conductivity, leading to cognitive gap between electronic conductivity and apparent catalytic activity. Herein we successfully construct a series of catalyst models with gradient conductivities by regulating molybdenum disulfide (MoS₂) electronic bandgap via a simple solvothermal method. We demonstrate that the conductivity of catalysts greatly affects the overall catalytic activity. We further elucidate the key role of intrinsic conductivity of catalyst towards water electrolysis, mainly concentrating on the electron transport from electrode to catalyst, the electron accumulation process at the catalyst layer, and the charge transfer progress from catalyst to reactant. Theoretical and experimental evidence demonstrates that, with the enhancement in electron accessibility at the catalytic interface, the dominant parameter governing overall HER activity gradually converts from electron accessibility to combination of electron accessibility and hydrogen adsorbing energy. Our results provide the insight from various perspective for developing noble metal-free catalysts in electrocatalysis beyond HER.

The scale-up deployment of ruthenium (Ru)-based oxygen evolution reaction (OER) electrocatalysts in proton exchange membrane water electrolysis (PEMWE) is greatly restricted by the poor stability. As the lattice-oxygen-mediated mechanism (LOM) has been identified as the major contributor to the fast performance degradation, impeding lattice oxygen diffusion to inhibit lattice oxygen participation is imperative, yet remains challenging due to the lack of efficient approaches. Herein, we strategically regulate the bonding nature of Ru–O towards suppressed LOM via Ru-based high-entropy oxide (HEO) construction. The lattice disorder in HEOs is believed to increase migration energy barrier of lattice oxygen. As a result, the screened Ti₂₃Nb₉Hf₁₃W₁₂Ru₄₃O_x exhibits 11.7 times slower lattice oxygen diffusion rate, 84% reduction in LOM ratio, and 29 times lifespan extension compared with the state-of-the-art RuO₂ catalyst. Our work opens up a feasible avenue to constructing stabilized Ru-based OER catalysts towards scalable application.

Developing high-performance and low-cost electrocatalysts for oxygen evolution reaction (OER) is the key to implementing polymer electrolyte membrane water electrolyzer (PEMWE) for hydrogen production. To date, iridium (Ir) is the state-of-the-art OER catalyst, but still suffers from the insufficient activity and scarce earth abundance, which results in high cost both in stack and electricity. Design low-Ir catalysts with enhanced activity and stability that can match the requirements of high current and long-term operation in PEMWE is thus highly desired, which necessitate a deep understanding of acidic OER mechanisms, unique insights of material design strategies, and reliable performance evaluation norm, especially for durability. With these demand in mind, we in this review firstly performed a systematic summary on the currently recognized acidic OER mechanism on both activity expression (i.e. the adsorbate evolution mechanism, the lattice oxygen mediated mechanism and the multi-active center mechanism) and inactivation (i.e. active species dissolution, evolution of crystal phase and morphology, as well as catalyst shedding and active site blocking), which can provide guidance for material structural engineering towards higher performance in PEMWE devices. Subsequently, we critically reviewed several types of low-Ir OER catalysts recently reported, i.e. multimetallic alloy oxide, supported, spatially structured and single site catalysts, focusing on how the performance has been regulated and the underlying structure-performance relationship. Lastly, the commonly used indicators for catalyst stability evaluation, wide accepted deactivation characterization techniques and the lifetime probing methods mimicking the practical operation condition of PEMWE are introduced, hoping to provide a basis for catalyst screening. In the end, few suggestions on exploring future low-Ir OER catalysts that can be applied in the PEMWE system are proposed.

Single atom catalysts (SACs) were reported to demonstrate exciting catalytic features for a number of reactions, including hydrogen evolution reaction (HER). However, the true role of these single atom sites in catalysts remains elusive, particularly for those prepared via pyrolysis, where the formation of active nanoparticle counterparts is often unavoidable. Here we report a Ru based catalyst (Ru embedded in N doped carbon spheres (Ru/NPCS)) comprising of both Ru nanoclusters and Ru single sites, who demonstrates activity exceeding Pt catalyst and mass activity among the best of the Ru based catalysts under acidic conditions. The integration of proton exchange membrane water electrolysis with Ru/NPCS as a cathode exhibited an excellent hydrogen generation activity and extraordinary stability (during 120 h of electrolysis) with a 1/48 Ru loading (16.5 µg_Ru·cm⁻²) of a commercial 20% Pt/C catalyst. Through precisely tailoring the dispersion status of the catalysts, we reveal that while ruthenium nanoclusters actively catalyze HER via Volmer–Tafel mechanism, the Ru SACs barely catalyze HER, with H* adsorption difficult to occur. Moreover, no synergy between Ru SACs and Ru cluster is revealed, meaning the Ru SACs act as a spectator rather than active species during H₂ evolution.

Fe/N/C material is the most competitive alternative to precious-metal catalysts for oxygen reduction. In view of the present consensus on active centers, further effort is directed at maximizing the density of single Fe atoms. Here, the imperfections in commonly used doping strategy of Fe for the synthesis of zeolitic imidazolateframework (ZIF)-derived Fe/N/C catalysts are revealed. More importantly, a strikingly improved catalyst is obtained by a 'second pyrolysis’ method and delivers a half-wave potential of 0.825 V (vs. RHE) in acidic media. The strong confinement effect of carbonaceous host accounts for the formation of dense single-atom sites and thus the high activity. Our findings will potentially facilitate future improvement of M/N/C catalysts.