UC Santa Barbara

The rapidly developing world poses a high energy demand. Hydrogen is recognized as one of the most important renewable energy sources with some great features including cleanness, and high gravimetric energy density. Hydrogen is predominantly produced by steam-methane reforming process at present, which is an energy intensive and CO2 emission intensive process. Alternatively, the hydrogen evolution reaction (HER) by electrochemical water splitting is part of a sustainable and scalable process to produce H2. Currently, precious metal platinum is required to catalyze this reaction which limits its industrial application. Hence, the design of a highly efficient and low-cost electrocatalyst is in demand. Recently, metal phosphides were developed as a good substitute of platinum for HER catalysis. Some examples are nickel phosphide, cobalt phosphide, iron phosphide. All of them have shown excellent HER activity and stability. Among them, iron phosphide is particularly attractive because of the abundance of iron on earth. Traditionally, the synthesis of iron phosphide is based on high temperature process that generates highly corrosive and flammable phosphine byproduct. Thus, greener and more straightforward synthetic method is desired. Herein, an easy and cost-effective method is developed to electrodeposit iron phosphide (FeP) film on a copper electrode at room temperature. The pulsed electrodeposition method was used to electrochemically etch co-deposited metallic iron which improved the film’s stability. In addition, formic acid and pH are crucial to promoting FeP deposition and it was well explained by experiments of cyclic voltammetry, capacitance studies, electron microscopy, and inductively coupled plasma mass spectrometry.

The as-deposited FeP film was shown to be a highly active HER catalyst in all-pH conditions. The FeP modified electrode produced current densities of-10 mA/cm2 at a low overpotential of 66 mV in 0.5 M sulfuric acid (H2SO4, pH=0.3), 131 mV in 1.0 M phosphate buffer (PB, pH=6.4), and 110 mV in 1.0 M potassium hydroxide (KOH, pH=14.0) solution, as well as a Tafel slope of 55 mV/dec, 108 mV/dec, 60 mV/dec, respectively. Electrochemical impedance studies revealed the low charge transfer resistance and fast HER kinetics of FeP electrocatalyst. More importantly, the film has excellent stability during HER catalysis under acidic and neutral conditions, as shown by long-term chronoamperometry, supported further with electron microscopy, and X-ray photoelectron spectroscopy. Specifically, the overpotentials increased diminutively 7 mV after 24 hours of electrocatalysis at a current density of -10 mA/cm2 in acidic solution. Similarly, only a 10 mV increase in overpotential was observed under neutral conditions. Overall, in this work, I was able to successfully demonstrate that a carefully controlled electrodeposition process provided a simple yet powerful method to synthesize heterogeneous iron phosphide catalyst that is highly efficient towards hydrogen evolution reaction. Via potentiostatic tuning of the deposition rate proceeding by electrochemical etching of phosphide reach inactive alloy, we were able to achieve a 2-D film that could find applications in large scale hydrogen production under wide range of industrial conditions.