Electrocatalytic Ammonia Oxidation Coupled with the Production of Hydrogen for Wastewater Remediation

Abstract

Ammonia is a pollutant present in wastewater, but it also is a carbon-free energy carrier. Recently, technological solutions have been introduced to strip and concentrate ammonia from wastewater. The recovered ammonia can be electrochemically oxidised to benign nitrogen in an electrolyser at low cell voltage, offering a cost-efficient solution to couple wastewater remediation with the production of a valuable by-product, hydrogen. Consequently, the development of inexpensive, stable, efficient catalytic material is imperative to reduce costs associated with ammonia destruction and electrochemical hydrogen production. The first part of the thesis focused on the development of an efficient and robust thermally decomposed IrO2 catalyst-coated substrate for ammonia oxidation reaction (AOR). First, the electrochemical surface area of the electrode was improved by optimisation of the Ti substrate preparation procedure and selecting an optimal decomposition temperature. The electrode was characterised in a three-electrode cell and showed promising results for ammonia oxidation in a pH 14 electrolyte. A high current density of 193 mA cm-2 (at 1.3 V vs Hg/HgO) was achieved at 20 °C. At 60 °C, a current density of 615 mA cm-2 was achieved, one of the highest recorded for AOR. In a two-electrode bulk electrolysis cell, the electrode exhibited excellent short-term stability with no changes in composition observed by X-ray photoelectron spectroscopy (XPS). However, when the catalyst-coated substrate was used as an anode in a divided anion exchange membrane electrolyser, the electrode showed low Faradaic efficiency and poor selectivity towards N2. It was determined that the electrode was unsuitable for large-scale ammonia electrolysis. The second part of the thesis focused on a non-noble NiCu electrocatalyst. First, the catalyst was optimised by enhancing the electrochemical surface area by electrodeposition onto a high surface area nickel felt substrate to develop a catalyst-coated substrate. Cyclic voltammetry analysis has shown that nickel-copper catalysts can successfully oxidise ammonia to nitrogen at room temperature, reaching a current density of 250 mA cm-2 at 0.8 V vs Hg/HgO. In alkaline anion exchange membrane electrolyser tests, gas chromatography of the off-gases confirmed that both, nitrogen at the anode and hydrogen at the cathode, were generated with high Faradaic efficiency for ammonia oxidation (88%) and hydrogen production (99%). To confirm the practicability of the electrolyser, it was operated for > 150 h with the NiCu/NF as an anode and Pt/C as a cathode. The cell voltage remained low (1.5 V) throughout the experiment with a specific energy demand of 40.0 kWh kg-1 H2.

Date of Award	18 Sept 2023
Original language	English
Awarding Institution	Coventry University
Sponsors	Severn Trent Plc & Seedcorn Access scheme
Supervisor	Liang Wu (Supervisor), Rong Lan (Supervisor) & Peter Vale (Supervisor)