Black etched electroless Ni–P coatings for enhanced efficiency towards alkaline water splitting

Requirements for wider use of water electrolysis for hydrogen production include the development of highly efficient, cheap, and easily made electrodes. Promising properties can be achieved with Ni–P materials. Herein, nitric acid etching (being used for blackening of electroless Ni–P coatings) resulted in enhanced efficiency towards hydrogen and oxygen evolution reactions (HER and OER) in 1.0 M KOH. For etched Ni-7.6 wt%P coating, the potential of HER at −10 mA cm −2 was about −0.20 V vs. RHE, and the potential of OER at 10 mA cm −2 was about 1.67 V, whereas for etched Ni they were −0.40 V and 1.94 V, respectively. Surface layers of etched coatings contained bundles of microrods and were enriched in O and P. Impedance measurements showed that etching caused about a 60-fold increase of CPE which can be ascribed to the development of surface area and pseudocapacitance of redox reactions, involving Ni ⇄ NiO. It is proposed that the electroactivity of Ni–NiO composite is to a large extent associated with the oxide reduction effects, involving high activity of newborn (pristine) Ni nanoparticles, acidification at the metal surface and disclosure of bare metal. Etched Ni–P coatings can be promising for the activation of electrodes for water electrolysis. • Etching of electroless Ni–P strongly increased efficiency towards water splitting. • High efficiency is ascribed to large surface area and to oxides in etched surface. • Catalysis by Ni–NiO can be due to formation of fresh Ni particles. • Medium-P coatings can be promising for activation of used electrodes.