Quaternary Iron Nickel Cobalt Selenide as an Efficient Electrocatalyst for Both Quasi‐Solid‐State Dye‐Sensitized Solar Cells and Water Splitting

: Iron nickel cobalt selenides are synthesized through a one-step hydrothermal method. Quaternary Fe0.37 Ni0.17 Co0.36 Se demonstrates multifunctionality and shows high electrocatalytic activity for quasi-solid-state dye-sensitized solar cells with a power conversion efficiency of 8.42 %, the hydrogen evolution reaction, the oxygen evolution reaction, and water splitting. The electric power output from tandem quasi-solid-state dye-sensitized solar cells under one-sun illumination is sufficient to split water and exhibits a solar-to-hydrogen conversion efficiency of 5.58 % with Fe0.37 Ni0.17 Co0.36 Se as the electrocatalyst in this integrated system. Owing to a remarkable synergistic effect, quaternary Fe0.37 Ni0.17 Co0.36 Se is proven to be superior to ternary nickel cobalt selenide in terms of conductivity, electrocatalytic activity, and photovoltaic performance.