Binary and Ternary Earth-Abundant Transition Metal Phosphides As Electrocatalysts and Photocatalysts for Water Splitting
Authors: Rodene Dylan Daniel , Saraswat Sushil Kumar , Tallapally Venkatesham , Arachchige Indika , Gupta Ram B. 
Published: February 2020

To provide a sustainable carbon free energy system while moving away from fossil fuels and greenhouse gas (CO2) generating energy sources, an inexpensive and reliable process to produce hydrogen has to be implemented. Water splitting by electrolysis has been thought of as the ideal way to produce hydrogen but the current electrolysis of water requires energy or rare materials that make the process neither carbon independent or feasible due to the amount energy gained vs energy input. In this talk there are two methods to perform the electrolysis of water: a) electrocatalytic water-splitting and b) solar photo-electrochemical water-splitting.

Recently, transition metal phosphides (TMP) have shown promising catalytic properties for the water splitting half reactions. Two key aspects to studying TMPs for the electrolysis of water are 1) to generate hydrogen with as little energy input as possible; meaning using materials that can harness solar energy or that have significantly low overpotentials for each water splitting half reactions and 2) to find competitive materials to rival the performance and stability of platinum and other noble metals so that industrial production and manufacturing may be achieved. This talk will focus on binary and ternary transition metal phosphides as electocatalysts and photocatalysts for the hydrogen and oxygen evolution reactions; transition metals of focus primarily include Nickel, Molybdenum, and Cobalt with Platinum and Titanium as reference metals.

Phosphorus (P) atoms have been determined to stabilize the TMP surface structure and thus the concentration of surface P is a key factor in controlling the surface activity. To control size, shape and crystallinity, a colloidal synthesis method was used to synthesize the nano particles. MoxCo2-xP and Ni2-xMoxP ternary TMPs were synthesized owing to their synergistic effects towards electrocatalysis. These two specific ternary phosphides are being considered for the proposed work influenced by commercial hydrotreating ternary catalyst system (sulfided Co-Mo and Ni-Mo). Nickel phosphide is of primary interest resulting from its consistent performance over other TMPs as an electrocatalyst for water splitting.

As electrocatalysts, TMPs show low overpotentials and good stability for the hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER) under relevant current densities (10 mA/cm2) without any sacrificial agent present. The particles show high stability under most conditions, specifically in acidic electrolytes. In our group, work has been done to synthesize, characterize and test TMP nanoparticles as electrocatalysts and photocatalysts using PEC methods for the HER and OER to understand the material’s electrochemical behavior. Electrode preparation methods may influence the behavior, stability and reproducibility of the electrode. Therefore, multiple preparation methods were conducted to determine the true performance of the material. FTO on glass and Titanium foil were used as substrates/current collectors for the catalytic nano-particles. The electrocatalytic performance of the binary and ternary TMPs have been evaluated as well as the photocatalytic performance.

The photocatalytic testing was evaluated under 1 SUN (100mW/cm2) illumination from a 150 W Xe Lamp with and without an AM1.5 filter. The photocatalytic performance was evaluated for the materials tuned to have more semiconductor characteristics and that show signs of photovoltage. Various ratios of metal to phosphide and metal to metal alloys were examined to study the effect of the electrocatalytic and photocatalytic performance and the material’s stability.