Ni2–xMnxP Nanoparticles as Earth-Abundant Precatalysts for Electrochemical Water Oxidation

Establishing efficient catalysts based on earth-abundant elements is critical for large-scale adoption of electrocatalytic water splitting. In this work, the synthesis of Ni2–xMnxP nanoparticles with different ratios of Ni to Mn is described and these phases are evaluated as precatalysts for the electrocatalytic oxygen evolution reaction (OER) process. Using arrested precipitation techniques, it was possible to incorporate up to 75% Mn into Ni2P (x ≤ 1.5), which represents a greater solubility for Mn relative to previously studied Fe2–xMnxP (x ≤ 0.9) and Co2–xMnxP (x ≤ 1.4) systems. Electrocatalytic OER activity assessment of the Ni2–xMnxP system (pH = 14) as a function of x revealed maximum activity for x = 1. Faradaic efficiency was calculated as 96.6%, indicating high selectivity of the NiMnP-derived catalyst toward the OER process. After an initial drop in the current density (ca. 20% over 1 h) during controlled potential electrolysis (CPE) measurements, the current density remains constant over the remainder of the 15 h test, suggesting reasonable stability. The X-ray photoelectron spectroscopy data collected on samples before and after catalysis indicate that the NiMnP precatalyst is becoming oxidized during the OER process, losing phosphate, and forming the Ni–Mn oxide/hydroxide presumed catalyst in situ. Compared to the most active compositions in Fe2–xMnxP (x ≤ 0.9) and Co2–xMnxP (x ≤ 1.4), the NiMnP precatalyst resulted in the highest OER activity with a geometric overpotential of 280.0 mV at 10 mA/cm2 relative to 302.5 mV for CoMnP and 350.0 mV for Fe1.1Mn0.9P. The relative activity can be correlated to electronegativity differences between Mn and M (M = Fe, Co, or Ni), which governs the extent of oxo-mediated charge transfer between Mn and M, and hence, the activity of the catalyst.