A porous carbon-based monolithic chainmail electrode, namely Co2P@CSA, is fabricated via direct carbonization of Co2+-cross-linked-starch aerogel (Co2+-SA) followed by low-temperature vapor phosphorization. During successive carbonization-phosphorization, the SA framework is formulated into 3D hierarchically porous carbon membrane matrix comprising hollow open carbon microspheres while the cross-linked Co species are converted into uniformly distributed carbon-encapsulated Co2P nanoparticles on carbon microspheres. Thanks to the high porosity, excellent electrolyte wettability, unique chainmail structure, and good mechanical strength, the monolithic Co2P@CSA can be directly used as a binder-free bifunctional electrocatalyst for alkaline water splitting, and it can afford a high current density of 100 mA cm−2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at low overpotentials of 140.0 and 305.5 mV, respectively, with outstanding stability at 50 mA cm-2 for >30 h. More significantly, an alkaline electrolyzer assembled using Co2P@CSA achieves a current density of 100 mA cm-2 for overall water splitting (OWS) at a cell voltage of 1.94 V with unit Faradaic efficiency and provides a high solar-to-hydrogen (STH) conversion efficiency of 13.4 % when driven by a commercial silicon solar cell. This work offers an effective strategy towards cost-effective fabrication of high-performance carbon-based monolithic chainmail electrocatalysts for energy conversion reactions.