Promoting High-Oxidation-State Metal Active Sites in a Hollow Ternary Metal Fluoride Nanoflake Array for Urea Electrolysis.

The adsorption ability of hydrogen, hydroxide, and oxygenic intermediates plays a crucial role in electrochemical water splitting. Electron-deficient metal-active sites can prompt electrocatalytic activity by improving the adsorption ability of intermediates. However, it remains a significant challenge to synthesize highly abundant and stable electron-deficient metal-active site electrocatalysts. Herein, we present a general approach to synthesizing a hollow ternary metal fluoride (FeCoNiF2) nanoflake array as an efficient and robust bifunctional electrocatalyst for the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). We find that the F anion withdraws electrons from the metal centers, inducing an electron-deficient metal center catalyst. The rationally designed hollow nanoflake array exhibits the overpotential of 30 mV for HER and 130 mV for UOR at a current density of 10 mA cm-2 and superior stability without decay events over 150 h at a large current density of up to 100 mA cm-2. Remarkably, the assembled urea electrolyzer using a bifunctional hollow FeCoNiF2 nanoflake array catalyst requires cell voltages of only 1.352 and 1.703 V to afford current densities of 10 and 100 mA cm-2, respectively, which are 116 mV less compared with that required for overall water splitting.

Dopant-Induced Charge Redistribution on the 3D Sponge-like Hierarchical Structure of Quaternary Metal Phosphides Nanosheet Arrays Derived from Metal-Organic Frameworks for Natural Seawater Splitting.

Dopant-induced electron redistribution on transition metal-based materials has long been considered an emerging new electrocatalyst that is expected to replace noble-metal-based electrocatalysts in natural seawater electrolysis; however, their practical applications remain extremely daunting due to their sluggish kinetics in natural seawater. In this work, we developed a facile strategy to synthesize the 3D sponge-like hierarchical structure of Ru-doped NiCoFeP nanosheet arrays derived from metal-organic frameworks with remarkable hydrogen evolution reaction (HER) performance in natural seawater. Based on experimental results and density functional theory calculations, Ru-doping-induced charge redistribution on the surface of metal active sites has been found, which can significantly enhance the HER activity. As a result, the 3D sponge-like hierarchical structure of Ru-NiCoFeP nanosheet arrays achieves low overpotentials of 52, 149, and 216 mV at 10, 100, and 500 mA cm-2 in freshwater alkaline, respectively. Notably, the electrocatalytic activity of the Ru-NiCoFeP electrocatalyst in simulated alkaline seawater and natural alkaline seawater is nearly the same as that in freshwater alkaline. This electrocatalyst exhibits superior catalytic properties with outstanding stability under a high current density of 85 mA cm-2 for more than 100 h in natural seawater, which outperforms state-of-the-art 20% Pt/C at high current density. Our work provides valuable guidelines for developing a low-cost and high-efficiency electrocatalyst for natural seawater splitting.

Boosting Urea-Assisted Natural Seawater Electrolysis in 3D Leaf-Like Metal-Organic Framework Nanosheet Arrays Using Metal Node Engineering.

Metal node engineering, which can optimize the electronic structure and modulate the composition of poor electrically conductive metal-organic frameworks, is of great interest for electrochemical natural seawater splitting. However, the mechanism underlying the influence of mixed-metal nodes on electrocatalytic activities is still ambiguous. Herein, a strategic design is comprehensively demonstrated in which mixed Ni and Co metal redox-active centers are uniformly distributed within NH2-Fe-MIL-101 to obtain a synergistic effect for the overall enhancement of electrocatalytic activities. Three-dimensional mixed metallic MOF nanosheet arrays, consisting of three different metal nodes, were in situ grown on Ni foam as a highly active and stable bifunctional catalyst for urea-assisted natural seawater splitting. A well-defined NH2-NiCoFe-MIL-101 reaches 1.5 A cm-2 at 360 mV for the oxygen evolution reaction (OER) and 0.6 A cm-2 at 295 mV for the hydrogen evolution reaction (HER) in freshwater, substantially higher than its bimetallic and monometallic counterparts. Moreover, the bifunctional NH2-NiCoFe-MIL-101 electrode exhibits eminent catalytic activity and stability in natural seawater-based electrolytes. Impressively, the two-electrode urea-assisted alkaline natural seawater electrolysis cell based on NH2-NiCoFe-MIL-101 needs only 1.56 mV to yield 100 mA cm-2, much lower than 1.78 V for alkaline natural seawater electrolysis cells and exhibits superior long-term stability at a current density of 80 mA cm-2 for 80 h.

Highly Efficient and Stable Hydrogen Evolution from Natural Seawater by Boron-Doped Three-Dimensional Ni2P-MoO2 Heterostructure Microrod Arrays.

The rational design of highly active and stable electrocatalysts toward the hydrogen evolution reaction (HER) is highly desirable but challenging in seawater electrolysis. Herein we propose a strategy of boron-doped three-dimensional Ni2P-MoO2 heterostructure microrod arrays that exhibit excellent catalytic activity for hydrogen evolution in both alkaline freshwater and seawater electrolytes. The incorporation of boron into Ni2P-MoO2 heterostructure microrod arrays could modulate the electronic properties, thereby accelerating the HER. Consequently, the B-Ni2P-MoO2 heterostructure microrod array electrocatalyst exhibits a superior catalyst activity for HER with low overpotentials of 155, 155, and 157 mV at a current density of 500 mA cm-2 in 1 M KOH, 1 M KOH + NaCl, and 1 M KOH + seawater, respectively. It also exhibits exceptional performance for HER in natural seawater with a low overpotential of 248 mV at 10 mA cm-2 and a long-lasting lifetime of over 100 h.