Badreldin, Ahmed Sherif El Sayed Mohamed (2023-05). Direct Seawater Electrolysis Towards Near-Neutral pH Green Hydrogen Production. Doctoral Dissertation.
Thesis
Hydrogen (H2) is central into key industrial processes and is widely used as a feedstock in the production of various chemicals. In addition, its compelling properties as an energy carrier make H2 a promising candidate for energy storage and transportation. Water electrolysis using electricity generated from renewable sources offers an attractive approach to produce H2 through a near-zero carbon route. It is also a promising approach for storing intermittent renewable energy such as solar or wind. Contemporary electrolyzers require purified water, which is usually obtained from freshwater resources. However, freshwater is not sufficiently available in many countries that face exacerbated water stresses from population growth and real-time effects of climate change. Further, minor upstream upsets in feed water quality remains a bottleneck and one of the primary root causes for stack failure in commercial electrolyzers. Seawater, if it can be used directly as a water source and electrolyte, is of interest as water supply for H2 production. Herein, ultra-active and stable cathodic and anodic electrocatalysts were developed towards the hydrogen (HER) and oxygen evolution reactions (OER), respectively, for operation under a wide range of relevant, and conventionally challenging, pH environments. Moreover, the resilient electrocatalysts developed also serve towards effective operation under harsh near-neutral pH seawater conditions. This is pivotal towards utilization of these inexpensive, earth-abundant, and versatile electrocatalysts within commercial electrolyzers operating under milder conditions. Throughout developing these electrocatalysts, this research investigated the manipulation of catalytic surface chemistries, electrochemically active surface areas, conductivities, intrinsic affinities to rate-limiting reaction steps, hydrophilicity/aerophobicity, and surface interface engineering towards maximizing activity and stability of electrocatalysts. Further, the presence of chloride with high concentration in seawater results in an undesired and kinetically favorable anodic chlorine evolution reaction (CER) and corrosion of electrolyzer's metallic components. Therefore, selectivity for anodic electrocatalysts towards OER, and stability for cathodic electrocatalysts from chloride-induced deactivation, was controlled via surface electrostatic shielding strategies. The best performing anodic Co-(NiFe)N@NiSx@NF and cathodic NiVN@NF exhibit superior activities and stabilities compared to benchmark platinum-group metal catalysts IrO2/C@NF and Pt/C@NF, respectively, under the same operating conditions. Unconventional near-neutral pH operation of the developed materials showcased comparable performance to benchmark conventional water electrolysis in kinetically favorable extreme pH regimes. To that end, inexpensive membraneless electrolyzers were developed and tested using the aforementioned in-house developed electrocatalysts for near-neutral pH seawater electrolysis. Novel asymmetric feeding schemes under low near-neutral pH buffer concentrations of synthetic seawater to the membraneless electrolyzer exhibited stable performance. This paradigm shift in utilizing near-neutral pH water and impaired water electrolysis is expected to bring forth advancements in the decentralization of sustainable energy systems and presents opportunities for intrinsically near-neutral pH electrochemical applications.
