NixSy Nanowalls/Nitrogen-Doped Graphene Foam Is an Efficient Trifunctional Catalyst for Unassisted Artificial Photosynthesis

Solar-to-hydrogen (STH) conversion through unassisted artificial photosynthesis (APS) devices is one of the promising and environmentally friendly strategies for sustainable development. However, the practical large-scale application of the unassisted APS devices is impeded by the need for expensive noble metal-based catalysts in photovoltaics and/or electrolyzers. Herein, well-aligned 2D Ni x S y nanowalls (2D Ni x S y NWs) on a 3D nitrogen-doped graphene foam (3D NGF) are synthesized and further employed it in unassisted APS. Due to the positive synergistic effect between the highly electrocatalytic activity of Ni x S y NW and excellent conductivity of NGF, this low cost material of (2D/3D) Ni x S y NW/NGF is highly efficient as a multifunctional catalyst in various applications: a counterelectrode for dye-sensitized solar cell (DSSC) and a “bifunctional” electrocatalyst for oxygen and hydrogen evolution for electrocatalytic overall water splitting. Furthermore, three Ni x S y NW/NGF-based DSSCs as a tandem cell for unassisted solar-driven overall water splitting is connected, using Ni x S y NW/NGF itself on nickel foams as the anode and cathode. Impressively, such integrated photovoltaic-electrolyzer APS device can achieve an STH efficiency of 3.2% with an excellent stability and low cost. This work opens an avenue to advanced multifunctional materials for the low-cost and unassisted solar-driven overall water splitting.