Two-electron oxygen reduction reaction (2e-ORR) provides an environmentally friendly direction for the on-site production of hydrogen peroxide (H₂O₂). Central to this technology is the exploitation of efficient, economical, and safe 2e-ORR electrocatalysts. This overview starts with the fundamental chemistry of ORR to highlight the decisive role of adsorbing intermediates on the reaction pathway and activity, followed by a comprehensive survey of the tuning strategies to favor 2e-ORR on traditional precious metals. The latest achievements in designing efficient and selective precious-metal-based single-atom catalysts (SACs) and metal-nitrogen-carbon (M-N_x/C) catalysts, from the aspects of material synthesis, theoretical calculations, and mass transport promotion, are systematically summarized. Brief introductions on the evaluation metrics for 2e-ORR catalysts and the primary reactor designs for cathodic H₂O₂ synthesis are also included. We conclude this review with an outlook on the challenges and direction of efforts to advance electrocatalytic 2e-ORR into realistic H₂O₂ production.

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.