Developing highly efficient and stable electrocatalysts is the key to realize hydrogen production from industrial electrolytic water. In this study, we constructed Co(OH) 2 and CoP ultrathin nanosheet arrays on titanium mesh using electrodeposition and phosphating processes. In alkaline conditions, the Co(OH) 2 /Ti-2.0 needed overpotentials of 414 and 457 mV to achieve 500 and 1000 mA cm −2 for oxygen evolution reaction. Mechanism research showed that CoOOH formed by preoxidation of Co(OH) 2 was the actual active substance. After low-temperature phosphorization of Co(OH) 2 , CoP nanosheets generated abundant defects and increased reactive sites, and CoP/ Ti-2.0 exhibited high activity in the all-pH hydrogen evolution reaction (overpotentials of 106, 116, and 131 mV in acidic, alkaline, and neutral solutions at 10 mA cm −2 , respectively). Density functional theory calculations showed the free energy of hydrogen adsorption of CoP. As efficient electrode materials, the Co(OH) 2 and CoP ultrathin nanosheet arrays on Ti mesh can be assembled to an alkaline electrolyzer, which required only 1.530 V to drive 50 mA cm −2 for overall water splitting with strong durability.