General Strategy for Controlled Synthesis of Ni x P y /Carbon and Its Evaluation as a Counter Electrode Material in Dye-Sensitized Solar Cells.

Hydrothermal treatment of nickel acetate and phosphoric acid aqueous solution followed with a carbothermal reduction assisted phosphorization process using sucrose as the carbon source for the controlled synthesis of Nix Py /C was successfully realized for the first time. The critical synthesis factors, including reduction temperature, phosphorus/nickel ratio, pH, and sucrose amount were systematically investigated. Remarkably, the carbon serves as a reducer and plays a determinative role in the transformation of Ni2 P2 O7 into Ni2 P/C. The synthesis strategy is divided into four distinguishable stages: (1) hydrothermal preparation of Ni3 (PO4 )2 ·8H2 O precursor for stabilizing P sources; (2) dimerization of Ni3 (PO4 )2 ·8H2 O into more thermal stable Ni2 P2 O7 amorphous phase along with the generation of NiO; (3) carbothermal reduction and phosphidation of NiO into Nix Py (0 ≤ y/x ≤ 0.5); and (4) further phosphidation of mixed-phase Nix Py and carbothermal reduction of Ni2 P2 O7 into single-phase Ni2 P. The resultant Ni2 P, the highly active phase in electrocatalysis, was applied as counter electrode in a dye-sensitized solar cell (DSSC). The DSSC based on Ni2 P with 10.4 wt.% carbon delivers a power conversion efficiency of 9.57%, superior to that of state-of-the-art Pt-based cell (8.12%). The abundant Niδ+ and Pδ- active sites and the metal-like conductivity account for its outstanding catalytic performance.