An efficient Ag-P-i oxygen-evolving catalyst was fabricated in situ in a phosphate electrolyte solution (pH 12.4). The catalyst approaches an amorphous structure, unlike the reported Ag-based oxygen-evolving catalysts. The catalytic activity for water oxidation of the Ag-P-i catalyst is 11.1 mu mol h(-1) cm(-2) and it has a modest oxygen-evolution overpotential of 457.0 mV at 1 mA cm(-2) in 0.1 M K3PO4 electrolyte. The effects of different electrolytes on the formation of the Ag-P-i catalyst were investigated. The active component of the Ag-P-i oxygen-evolving catalyst as a water-splitting catalyst is the AgO phase as determined by Xray diffractometry and X-ray photoelectron spectroscopy analysis. A Ag-P-i mechanism that includes the Ag(I) and Ag(III) cycles is proposed to explain the electrocatalytic oxygen evolution. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.