Engineering electrode and catalyst environments for enhanced water electrolysis

A sustainable energy infrastructure is possible through the generation and storage of renewable energy. However, the intermittent nature of renewable energy sources underscores the importance of reliable, economical, and large-scale energy storage. Water electrolyzers provide a pathway for the conversion of electricity to chemical energy. Specifically, water electrolyzers can be used to produce this carbon-neutral hydrogen fuel and provide for long term renewable energy storage. Unfortunately, electrolyzers in acidic conditions commonly employ precious metal catalysts (iridium/platinum) and ones in acidic/alkaline conditions suffer from kinetic limitations of the oxygen electrochemical reaction taking place at the device anode. Such limitations require for the development of new catalysts structures that can overcome the high expense of precious metals while achieving high activity. In this dissertation, we explore the use of phosphides and layered phosphonates as catalysts for the hydrogen evolution and oxygen evolution reaction (HER and OER, respectively). Specifically, we engineer the electrode and catalyst environments for cobalt phosphide and zirconium phosphate systems for enhanced catalytic activity, respectively. First, we explore surface modifications to engineer the local environment (oxygen content) on the surface of carbon electrodes for the HER in acidic conditions. These modifications produce a variety of O/C ratios on the surface which directly impacting the morphology of the CoP catalyst. The improved hydrophilicity, stemming from introduced oxyl-groups on the carbon electrode, creates an electrode surface that yields a well-distributed growth of cobalt electrodeposits and thus a well-dispersed catalyst layer with high surface area upon phosphidation. This work demonstrates the high‐performance achievable by CoP at low loadings which facilitates further cost reduction, an important part of enabling the large-scale commercialization of non-platinum group metal catalysts. The fabrication strategies described herein offer a pathway to lower catalyst loading while achieving high efficiency and promising stability on a 3D electrode. We then explore the use of zirconium phosphate as a host structure to study the effect of confined environments for the OER in alkaline conditions. By intercalating single metal transition metal cations into zirconium phosphate, we find that we can successfully produce materials that are catalytically active for the OER. Control studies showcase that the activity of intercalated species is similar to that of confined ones where the activity is hindered by electron transport through the layers. To overcome the low activity of single-metal intercalated systems, we then create a suite of catalysts that consists of the co-intercalation of Ni and Fe cations within the sheets of zirconium phosphate. We find that the co-intercalation of Ni and Fe cations produces a water-rich interlayer environment that is shown to correlate strongly to the enhanced activity over surface-based controls. Additionally, we discuss the role of water and other intercalants in the interlayer environment and showcase its impact on catalytic activity. We anticipate that property tuning within confined systems via interlayer engineering could be a promising strategy towards further enhancing performance for water oxidation, as well as other reactions of interest. This dissertation explores the field of electrochemical water-splitting at multiple levels. We dive into the activity challenges facing the field and present detailed and novel strategies for overcoming them for both OER and HER catalysts. We also put our results into a greater context, revealing important field-wide trends, and discussing the challenges that must be solved to enable this sustainable technology to change how we produce and consume energy

Keywords: Chemical Engineering.
Advisor: Jaramillo Thomas Francisco
Cargnello Matteo
Tarpeh William