Potential Electrocatalysts for Water Splitting Devices : A Journey Through the Opportunities and Challenges of Catalyst Classes

Abstract: In this thesis work, different classes of catalysts and their suitability for integration into an electrolyzer cell has been investigated.Ruthenium based molecular catalysts have shown high activities and stabilities towards water oxidation in neutral pH. Especially the oligomeric catalysts exhibited a superior performance. The electrical conductivity of the electrode and the low loading of catalyst might impose limitations on reaching high current densities at reasonable potentials.Among the tested transition metal single atom catalysts, synthesized by pyrolyzing transition metal doped ZIF-8 structures, cobalt has shown the highest activity towards hydrogen evolution and a stable behaviour in acidic pH. The enhanced stability of single atomic sites compared to the corresponding nanoparticles was proposed. However, also for this class of catalyst, the low number of active sites seems to present a difficulty need to be overcome.With the novel method presented to fabricate a membrane electrode assembly, the usage of commonly used expensive membranes could possibly be avoided.Both nickel molybdate hydrate nanoparticle shapes have been proposed to transform in an electrochemical activation step into γ-NiOOH as active phase for the oxygen evolution reaction in alkaline pH. With the removal of molybdenum, a high electrochemical surface area with a large number of exposed nickel sites was indicated to be the origin behind the high catalytic activity of the nanoparticles. Molybdenum was suggested to only serve as structure and pore forming agent. Preliminary results indicated a higher activity for the rod structure towards the oxygen evolution reaction. An essential outcome is that it is uncertain if rods can be isolated synthesized on a nickel foam and hence the absence of the sheet structure should be verified, which could be done for example by selective molybdenum leaching combined with Raman spectroscopy. Furthermore, the two nanostructures are fundamentally different materials and characterized by various techniques.Among all different classes of catalysts investigated, the nanoparticle catalysts seem to be the most promising for a successful integration in a large scale electrolyzer cell for widespread use.