It is highly desired but still remains a key challenge to develop iron-based large-surface-area arrays as heterogeneous water oxidation catalysts that perform efficiently and durably under mild pH conditions for solar-to-hydrogen conversion. In this work, we report the in situ. derivation of an iron phosphate-borate nanosheet array on carbon cloth (Fe-Pi-Bi/CC) from an iron phosphide nanosheet array via oxidative polarization in a potassium borate (KBi) solution. As a 3D catalyst electrode for water oxidation at mild pH, such a Fe-Pi-Bi/CC shows high activity and strong long-term electrochemical durability, and it only demands an overpotential of 434 mV to drive a geometrical catalytic current density of 10 mA cm(-2) with maintenance of its activity for at least 20 h in 0.1 M KBi. This study offers an attractive earth abundant catalyst material,in water-splitting devices toward the large-scale production of hydrogen fuels under benign conditions for application.