(66e) Hierarchically Structured NiO Nanocubes with Atomically Dispersed Pt for Water Splitting

Hydrogen has been considered as one of the most promising candidates as the clean energy carrier in the future. However, current hydrogen production is mainly from the fossil fuel such as methane reforming and coal gasification. On the other side, renewable energy production (e.g., solar and wind power) capacities are growing rapidly in the past decade. Water electrolysis therefore is rejuvenating since it can provide an attractive route to convert the ‘green’ electricity into hydrogen fuels, which can be stored and transported in a fashion commensurate with use. One critical issue in the electrolysis cell is the over-potential loss. Especially for the anode, substantial over-potentials typically in excess of 450 mV are required to drive the sluggish four-electron oxygen evolution reaction (OER).

The well-known volcano plot suggests that Pt is one of the most active element effectively catalyzing the OER. Practical deployment of Pt based electrocatalysts for oxygen redox reactions in the industry scale is required to meet four major criteria: cost, productivity, performance and durability. Despite a number of reports to date have achieved successful synthesis of Pt based nanocatalysts with superior performance and long term stability, few of them resolve the challenges in terms of cost and productivity. Here, we reported one simple surfactant-free and cost-effective approach to prepare hierarchically structured NiO nanocubes with atomically dispersed Pt via impregnation & calcination of their metal nitrites with irregular porous SiO₂ microcubes as the template. With a minimal 0.5 w% loading of Pt, it demonstrated overwhelming advantages over NiO nanocubes with higher Pt loading and commercial Pt/C catalysts. It afforded a small Tafel slope of ~35 mV/decade, an overpotential of a mere 0.37 V at the current density of 10 mA/cm², which is comparable with the performance of state-of-art electrocatalysts. It also presented outstanding durability under harsh OER conditions, no degradation was observed and the current density even slightly increased after 500 electrochemical cycles. A single batch preparation of 200 g Pt-NiO nanocubes was demonstrated, while precisely preserving its nanoscale feature. Through high resolution XPS analysis and impedance spectroscopy, we unveiled the remarkable changes of the corresponding physiochemical properties induced by the atomic synergy between Ni and Pt. Further density functional theory (DFT) calculations revealed the significant changes of the electronic property. The bandgap of NiO reduced from 1.49 to 0.35 eV after Pt incorporation, suggesting that Pt can greatly enhance the charge transfer rate. Our approach highlighted that the old-fashioned impregnation & calcination method can also fabricate hierarchically structured electrocatalysts with both excellent activity and stability, and more importantly, ready for scaling-up.