(145b) Electrochemical Ammonia Decomposition over Nickel and Cobalt Nanostructures on Nickel Foam for Sustainable Hydrogen Production

The utilization of fossil fuels leads to the emission of greenhouse gases, contributing significantly to climate change and global warming. Coupled with the rising demand for energy and limited global reserves of these fossil fuels, hydrogen emerges as a sustainable alternative. As a hydrogen carrier, ammonia offers a unique advantage, addressing challenges associated with hydrogen storage and transportation. The electrochemical decomposition of ammonia into hydrogen is particularly appealing as it requires lower energy input compared to alternative methods, presenting a viable pathway for clean and efficient energy production.

In this study, we investigate the electrochemical decomposition of ammonia for hydrogen production using nickel foam (NF) as a catalytic substrate. Nickel (Ni) and cobalt (Co) nanostructures were synthesized on NF via a hydrothermal method to fabricate Ni/NF and Co/NF electrodes. The bare NF and modified NF-based electrodes underwent physical characterization. Scanning Electron Microscopy (SEM) revealed uniform nanostructured growth of Ni and Co on the smooth NF surface, while Atomic Force Microscopy (AFM) demonstrated a marked increase in surface roughness from 0.796 nm for bare NF to 28.5 nm and 12.6 nm for Ni/NF and Co/NF, respectively. X-Ray Diffraction (XRD) confirmed the presence of metallic, oxide, and hydroxide phases of Ni and Co on the NF, which was further corroborated by Raman Spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR). X-Ray Photoelectron Spectroscopy (XPS) provided additional insights into the chemical states of the electrode materials.

The ammonia oxidation reaction (AOR) activity of these electrodes was evaluated in an alkaline electrolysis cell using a solution of 0.5 M NH₄OH and 1.0 M KOH at ambient conditions. Electrochemical tests, including Linear Sweep Voltammetry (LSV) and Cyclic Voltammetry (CV), demonstrated superior activity for Ni/NF and Co/NF electrodes compared to bare NF. Specifically, the bare NF exhibited an onset potential of 0.38 V vs. Ag/AgCl and a peak current density of 646 mA/cm². Both Ni/NF and Co/NF electrodes showed a negative shift in onset potential to 0.36 V vs. Ag/AgCl, with maximum current densities of 776 mA/cm² and 819 mA/cm², respectively. Investigations into electrooxidation cycles and scan rate effects provided insights into the apparent and intrinsic catalytic activity of each electrode. The electrochemical surface area (ECSA) values were determined to be 1.87 mF/cm² for Ni/NF and 1.61 mF/cm² for Co/NF, surpassing the bare NF value of 1.01 mF/cm². Furthermore, Tafel analysis revealed electron transfer rates, with slope of 34.43 mV/dec for bare NF, 25.25 mV/dec for Ni/NF, and 23.82 mV/dec for Co/NF, while Electrochemical Impedance Spectroscopy (EIS) indicated higher resistance in bare NF, affirming the enhanced conductivity of the modified electrodes.

The performance was further evaluated through controlled potential coulometry in ammonia solution at an applied potential of 0.8 V. Within 100 minutes, ammonia conversion reached 573.3 ppm, 464.1 ppm, and 313.9 ppm with Co/NF, Ni/NF, and bare NF electrode, respectively, with hydrogen flow rates and volumes decreasing in the same order. Chronopotentiometric tests conducted at a current density of 50 mA/cm² showed that Co/NF, Ni/NF, and bare NF required cell voltages of 0.834 V, 0.895 V, and 1.0 V, leading to power consumptions of 4.08 mWhg^-1_NH3, 4.38 mWhg^-1_NH3, and 4.89 mWhg^-1_NH3, with corresponding cell efficiencies of 65%, 63%, and 59%.

Additionally, Density Functional Theory (DFT) was employed to provide an atomistic perspective on catalyst surface interactions and reaction mechanisms. Utilizing a modified computational hydrogen electrode (CHE) approach, we determined an energy span of 2.27 eV for Co/NF, which is lower than the 2.45 eV observed for Ni/NF. Results indicate that ammonia adsorption is more favorable on Ni surface, while nitrogen desorption is more favorable on Co surface, elucidating the roles of each electrode in facilitating AOR.

In summary, this research utilizes cost-effective, abundant non-noble metals to demonstrate efficient ammonia decomposition, paving the way for a scalable approach to hydrogen production. This work offers new perspectives and research pathways for overcoming current challenges and advancing catalyst development, thus driving progress toward industrial applications in sustainable hydrogen production.