Although Sn-based catalysts have recently achieved considerable improvement in selective electro-catalyzing CO₂ into HCOOH, the role of various valence Sn species is not fully understood due to the complexity and uncertainty of their evolution during the reaction process. Here, inspired by the theoretical simulations that the concomitant multivalent Sn (Sn⁰, Sn^II and Sn^IV) can significantly motivate the intrinsic activity of Sn-based catalyst, the Sn/SnO/SnO₂ nanosheets were proposed to experimentally verify the synergistic effect of multivalent Sn species on the CO₂-into-HCOOH conversion. During CO₂ reduction reaction, the Sn/SnO/SnO₂ nanosheets, which are prepared by the sequential hydrothermal reaction, calcined crystallization and low-temperature H₂ treatment, exhibit a high FE_HCOOH of 89.6% at -0.9 V_RHE as well as a large cathodic current density. Systematic experimental and theoretical results corroborate that multivalent Sn species synergistically energize the CO₂ activation, the HCOO* adsorption, and the electron transfer, which make Sn/SnO/SnO₂ favour the conversion from CO₂ into HCOOH in both thermodynamics and kinetics. This proof-of-concept study establishes a relationship between the enhanced performance and the multivalent Sn species, and also provides a practicable and scalable avenue for rational engineering high-powered electrocatalysts.

Developing cost-efficient electrocatalysts for oxygen evolution is vital for the viability of H₂ energy generated via electrolytic water. Engineering favorable defects on the electrocatalysts to provide accessible active sites can boost the sluggish reaction thermodynamics or kinetics. Herein, Co_1-xS nanosheets were designed and grown on reduced graphene oxide (rGO) by controlling the successive two-step hydrothermal reaction. A belt-like cobalt-based precursor was first formed with the assistance of ammonia and rGO, which were then sulfurized into Co_1-xS by L-cysteine at a higher hydrothermal temperature. Because of the non-stoichiometric defects and ultrathin sheet-like structure, additional cobalt vacancies (V'_Co) were formed/exposed on the catalyst surface, which expedited the charge diffusion and increased the electroactive surface in contact with the electrolyte. The resulting Co_1-xS/rGO hybrids exhibited an overpotential as low as 310 mV at 10 mA·cm^-2 in an alkaline electrolyte for the oxygen evolution reaction (OER). Density functional theory calculations indicated that the V'_Co on the Co_1-xS/rGO hybrid functioned as catalytic sites for enhanced OER. They also reduced the energy barrier for the transformation of intermediate oxygenated species, promoting the OER thermodynamics.

Highly sensitive, selective, and stable hydrogen peroxide (H₂O₂) detection using nanozyme-based catalysts are desirable for practical applications. Herein, vertical α-FeOOH nanowires were successfully grown on the surface of carbon fiber paper (CFP) via a low-temperature hydrothermal procedure. The formation of vertical α-FeOOH nanowires is ascribed to the structure-directing role of sodium dodecyl sulfate. The resulting free-standing electrode with one-dimensional (1D) nanowires offers oriented channels for fast charge transfer, excellent electrical contact between the electrocatalyst and the current collector, and good mechanical stability and reproducibility. Thus, it can serve as an efficient electrocatalyst for the reduction and sensitive detection of H₂O₂. The relation of the oxidation current of H₂O₂ with the concentration is linear from 0.05 to 0.5 mM with a sensitivity of -0.194 mA/(mM·cm²) and a low detection limit of 18 μM. Furthermore, the portability in the geometric tailor and easy device fabrication allow extending the general applicability of this free-standing electrode to chemical and biological sensors.

A cost-efficient and stable oxygen evolution electrocatalyst is essential for improving energy storage and conversion efficiencies. Herein, 2D nanosheets with randomly cross-linked CoNi layered double hydroxide (LDH) and small CoO nanocrystals were designed and synthesized via in situ reduction and interfacedirected assembly in air. The formation of CoNi LDH/CoO nanosheets was attributed to the strong extrusion of hydrated metal–oxide clusters driven by the interfacial tension. The obtained loose and porous nanosheets exhibited low crystallinity due to the presence of numerous defects. Owing to the orbital hybridization between metal 3d and O 2p orbitals, and electron transfer between metal atoms through Ni–O–Co, a number of Co and Ni atoms in the CoNi LDH present a high +3 valency. These unique characteristics result in a high density of oxygen evolution reaction (OER) active sites, improving the affinity between OH^– and catalyst, and resulting in a large accessible surface area and permeable channels for ion adsorption and transport. Therefore, the resulting nanosheets exhibited high catalytic activity towards the OER. The CoNi LDH/CoO featured a low onset potential of 1.48 V in alkaline medium, and required an overpotential of only 300 mV at a current density of 10 mA·cm^–2, while displaying good stability in accelerated durability tests.

Enzymeless hydrogen peroxide (H₂O₂) detection with high sensitivity and excellent selectivity is desirable for clinical diagnosis. Herein, one-dimensional Co₃O₄ nanowires have been successfully constructed on reduced graphene oxide (rGO) via a simple hydrothermal procedure and subsequent thermal treatment. These Co₃O₄ nanowires, assembled by small nanoparticles, are interlaced with one another and make a spider web-like structure on rGO. The formation of Co₃O₄-rGO hybrids is attributed to the structure-directing and anchoring roles of DDA and GO, respectively. The resulting structure possesses abundant active sites, the oriented transmission of electrons, and unimpeded pathways for matter diffusion, which endows the Co₃O₄-rGO hybrids with excellent electrocatalytic performance. As a result, the obtained Co₃O₄-rGO hybrids can serve as an efficient electrochemical catalyst for H₂O₂ oxidation and high sensitivity detection. Under physiological conditions, the oxidation current of H₂O₂ varies linearly with respect to its concentration from 0.015 to 0.675 mM with a sensitivity of 1.14 mA·mM^-1·cm^-2 and a low detection limit of 2.4 μM. Furthermore, the low potential (-0.19 V) and the good selectivity make Co₃O₄-rGO hybrids suitable for monitoring H₂O₂ generated by liver cancer HepG2 cells. Therefore, it is promising as a non-enzymatic sensor to achieve real-time quantitative detection of H₂O₂ in biological applications.