Lanthanide Nickel Phosphides as Electrocatalysts for Direct Seawater Electrolysis

• Jimmy Fu, Chemistry, University of Delaware

• Emil Hernandez-Pagan, Chemistry, University of Delaware

Direct seawater electrolysis is a scalable route to green hydrogen without using scarce freshwater. However, seawater’s high chloride content drives competing chlorine evolution, which can outpace oxygen evolution and corrode the catalyst. Catalysts in this need to remain active and stable in both OER and HER reactions. Prior studies have shown that nickel-based materials can catalyze seawater electrolysis. Lanthanide nickel phosphides were synthesized and tested to see if they could act as viable catalysts. Lanthanide-nickel phosphides were synthesized via a molten tin flux method. Reactions were sealed under vacuum in quartz tubes and heated at 900°C for about 1 week. Resulting crystals were isolated by centrifugation and acid treatment, with phase formation confirmed by using powder X-ray diffraction. The product was then ball milled, turned into a catalyst ink with carbon black and Nafion, and drop cast onto a carbon paper electrode for electrochemical testing. Pr4Ni19P12 and Ce4Ni19P12 showed comparable HER and OER activity, with only small differences in the potentials required to reach the selected current densities. Ce4Ni19P12 performed slightly better for HER, while Pr4Ni19P12 performed slightly better for OER. These results suggest lanthanide-nickel phosphides are viable catalysts for seawater electrolysis, with lanthanide identity being a possible way for tuning HER/OER selectivity. Testing will extend to other already synthesized lanthanide-nickel phosphides: Nd2Ni12P7, LaNi5P3, PrNi5P3, and Pr2Ni12P7. Future work will replace nickel with iron in this phosphide system, since iron is far more earth-abundant and lower-cost than nickel.