Understanding of the structure changes of the catalysts during hydrogen (HER) and oxygen (OER) evolution reactions is of paramount importance for design of catalysts with enhanced performance. This work reports the structure change investigations of phosphide-based catalysts in both HER and OER. Specifically, nickel foam supported Co2P-Ni2P (Co2P-Ni2P/NF) is synthesized by in-situ growth of Co2P-Ni2P on the current collector, i.e. nickel foam, and then used for the investigation of the structure changes during HER and OER. This method avoids the use of polymer binders, allowing for the systematic analysis of the structure changes of the catalysts after HER and OER. The obtained Co2P-Ni2P/NIF shows an excellent catalytic bifunctionality for HER and OER in alkaline solutions. It only needs an overpotential of 90 mV vs RHE to offer a current density of 10 mA cm(-2) for HER and needs an overpotential of 230 mV vs RHE to offer a current density of 50 mA cm(-2) for OER. Both HER and OER activities of Co2P-Ni2P/NF are higher than those of most bifunctional catalysts reported. Post-analysis shows the occurrence of the surface roughening and the formation of M-P-O (M=Co and Ni) at the surface in its initial period of the HER, and the surface roughening and the formation of a thin layer of amorphous MOOH (Mo and Ni) at the surface in its initial period of OER. The surface roughening and formation of M-P-O and amorphous hydroxide layer can be attributed to the main reason responsible for its high performance for HER and OER. Additionally, the Co2P-Ni2P/NF is also usable as both the cathode and anode for an electrolyzer for overall water splitting and shows high performance. (C) 2019 Elsevier Inc. All rights reserved.