Phase-Controlled Synthesis of Nickel-Iron Nitride Nanocrystals Armored with Amorphous N-Doped Carbon Nanoparticles Nanocubes for Enhanced Overall Water Splitting

Well-defined Ni3N nanoparticles armored in hollow carbon nanotube shell for high-efficiency bifunctional hydrogen electrocatalysis

Alkaline H2-O2 fuel cells and water electrolysis are crucial for hydrogen energy recycling. However, the sluggish kinetics of the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) in an alkaline medium pose significant obstacles. Thus, it is imperative but challenging to develop highly efficient and stable non-precious metal electrocatalysts for alkaline HOR and HER. Here, we present the intriguing synthesis of well-defined Ni3N nanoparticles armored within an N-doped hollow carbon nanotube shell (Ni3N@NC) via the conversion of a hydrogen-bonded organic framework (HOF) to metal-organic framework (MOF), followed by high-temperature pyrolysis. As-developed Ni3N@NC demonstrates exceptional bifunctionality in alkaline HOR/HER electrocatalysis, with a high HOR limiting current density of 2.67 mA cm-2 comparable to the benchmark 20 wt% Pt/C, while achieving a lead in overpotential of 145 mV and stronger CO-tolerance. Additionally, it achieves a low overpotential of 21 mV to attain a HER current density of 10 mA cm-2 with long-term stability up to 340 h, both exceeding those of Pt/C. Structural analyses and electrochemical studies reveal that the remarkable bifunctional hydrogen electrocatalytic performance of Ni3N@NC can be ascribed to the synergistic coupling among the well-dispersed small-sized Ni3N nanoparticles, chain-mail structure, and optimized electronic structure enabled by strong metal-support interaction. Furthermore, theoretical calculations indicate that the high-efficiency HOR/HER observed in Ni3N@NC is attributed to the strong OH- affinity, moderate H adsorption, and enhanced water formation/dissociation ability of the Ni3N active sites. This work underscores the significance of rational structural design in enhancing performance and inspires further development of advanced nanostructures for efficient hydrogen electrocatalysis.

An oxygen vacancy-modulated bifunctional S-NiMoO4 electrocatalyst for efficient alkaline overall water splitting

S-doped nickel molybdate nanorods grown on nickel foam (S-NiMoO4/NF) were fabricated by a two-step hydrothermal method. The resultant S-NiMoO4/NF exhibited remarkable bifunctional electrocatalytic activity, with overpotentials of 235 mV for the hydrogen evolution reaction and 150 mV for the oxygen evolution reaction at a current density of 50 mA cm-2. Assembled into the two-electrode S-NiMoO4/NF electrolyzer in alkaline electrolytes for overall water splitting, it required only low cell voltages of 1.55 V and 1.63 V to drive 50 mA cm-2 and 100 mA cm-2, respectively. No significant performance degradation occurred during the water electrolysis process. The experimental results confirmed that S-doping induced the increase of the oxygen vacancies, accelerating the reaction kinetics and thus improving the electrocatalytic performance. Meanwhile, more active sites exposure on the surface of S-NiMoO4/NF enhanced the reactivity. This work may guide the development of efficient bifunctional catalysts in alkaline electrolysis through oxygen vacancy regulation.

Bimetallic Phosphide Heterostructure Coupled with Ultrathin Carbon Layer Boosting Overall Alkaline Water and Seawater Splitting

Seawater electrolysis is promising for green hydrogen production but hindered by the sluggish reaction kinetics of both cathode and anode, as well as the detrimental chlorine chemistry environment. Herein, a self-supported bimetallic phosphide heterostructure electrode strongly coupled with an ultrathin carbon layer on Fe foam (C@CoP-FeP/FF) is constructed. When used as an electrode for the hydrogen and oxygen evolution reactions (HER/OER) in simulated seawater, the C@CoP-FeP/FF electrode shows overpotentials of 192 mV and 297 mV at 100 mA cm^-2 , respectively. Moreover, the C@CoP-FeP/FF electrode enables the overall simulated seawater splitting at the cell voltage of 1.73 V to achieve 100 mA cm^-2 , and operate stably during 100 h. The superior overall water and seawater splitting properties can be ascribed to the integrated architecture of CoP-FeP heterostructure, strongly coupled carbon protective layer, and self-supported porous current collector. The unique composites can not only provide enriched active sites, ensure prominent intrinsic activity, but also accelerate the electron transfer and mass diffusion. This work confirms the feasibility of an integration strategy for the manufacturing of a promising bifunctional electrode for water and seawater splitting.