Control synthesis and phase dependent electrocatalytic and catalytic activity of Ni-P nanoparticle for hydrogen evolution reaction

Design of robust and noble metal free low-cost catalyst for hydrogen evolution reaction (HER) has high importance to decrease the use of fossil fuels. Design of different phases is indeed an important research to understand structure-catalysis relationship. The synthesis of monodispersed and hollow phase pure Ni₅P₄, Ni₂P and Ni₁₂P₅ nanocrystals (NCs) by colloidal technique with variation of Ni:P source ratio has been achieved. The as-developed NCs were used as tri-functional catalyst for electrocatalytic hydrogen evolution both in acid and alkaline medium and for hydrogen production from amino-borane (AB) and sodium borohydride (NaBH₄) hydrolysis. Ni₂P phase was found to exhibit enhanced electrocatalytic activity towards HER in both acid and alkaline medium with low overpotential (126 mV and 180 mV at J= 10 mA/cm² for acid and base respectively) compared to other phases. The catalysts were found to be stable upto 3000 cycles which proves its robustness. The charge transfer between Ni and P generates acid-base pairs in close vicinity within the crystal lattice, facilitating similar activity to hydrogenase, where negatively charged non-metal sites act as H⁺ acceptors and the metal sites act as hydride acceptors. The Turn over frequency (TOF) for H₂ formation from AB hydrolysis was maximum for Ni₁₂P₅ phase with activation energy = 50.5 KJ/mol which is comparable with noble metal nanoparticles. For Aminoborane hydrolysis, Ni₁₂P₅ and Ni₅P₄ both the NCs performed better than Ni₂P for higher charge separation between Ni and P, particularly in Ni₁₂P₅ phase. Moreover, catalytic activity for NaBH₄ hydrolysis to generate H₂ is first time studied by as-synthesized catalysts. The study proves Ni₁₂P₅ NCs should be the best choice for NaBH₄ catalysis. High charge separation between Ni and P was observed for Ni₁₂P₅ and Ni₅P₄ system which results lower activation energy compared to Ni₂P for both AB and NaBH₄ catalysis.