Nanostructured metallic catalysts for electrochemical energy conversion

Abstract

Tremendous research is being carried out in the development of electrochemical energy technologies such as fuel cells and water splitting for conversion and storage of renewable energy resources. Oxygen reduction reaction (ORR) is crucial to the development of proton exchange membrane fuel cells (PEMFCs) which are promising devices due to their high energy density, high conversion efficiency and low environmental impact. Electrochemical water splitting has also the capability to store the electricity obtained from renewable energy resources in the form of hydrogen as an energy carrier. However, the sluggish kinetics of the anodic oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode electrode in water splitting remain the major challenges of this technology. Catalysts can play a key role in increasing the ORR, OER, and HER kinetics and thus improving the overall performance of the system. However, the most efficient electrocatalysts are based on noble metals which their scarcity and exorbitant costs hinder the large-scale application of such electrochemical energy devices. Therefore, development of nanostructured non-precious metal-based electrocatalysts is highly required. This thesis aims to design, synthesise, and develop economical, efficient, and stable catalysts for ORR, OER, and HER. To this end, electrochemistry techniques are used as the main method for synthesising the catalysts. Specifically, i) electrodeposited Pd thin films from ionic liquids show better ORR catalysis performance than Pd electrodeposited from aqueous solution, ii) Pd nanoparticles are achieved by electrodeposition at high negative potentials thanks to the evolution of hydrogen bubbles during the electrodeposition from protic ionic liquids, iii) oxygen vacancy-rich Ni-Fe composite was fabricated via a facile and straightforward method. The electrode showed an excellent OER activity, iv) a Ni-Fe electrode is made by one-step electrochemically roughening of nickel foam and simultaneously Fe electrodeposition to produce an active OER electrode, v) 3D porous NiCu is electrodeposited through hydrogen dynamic templates following a phosphorous doping process to obtain a stable HER electrode in wide range of pH. All synthesised catalysts are characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results highlight that well-designed transition metal-based materials with suitable structure and porosity are highly promising for electrochemical energy conversion and storage technologies.