2026 Theses Doctoral

Design and In Situ Investigation of Electrocatalysts for Enhanced Performance in Water and Seawater Electrolysis

Bushiri, Daniela Asifiwe

Meeting the global imperative to decarbonize energy-intensive industries and transition to renewable energy systems requires scalable technologies for producing clean fuels and chemical feedstocks. Among these, electrochemical water splitting has emerged as a leading platform for sustainable hydrogen generation. Proton exchange membrane (PEM) electrolysis, in particular, offers high efficiency and compatibility with intermittent renewable energy sources, but its operation under strongly acidic conditions imposes stringent demands on the oxygen evolution reaction (OER) electrocatalyst. Catalysts must sustain high activity under oxidative potentials while resisting dissolution and degradation. The reliance on scarce and expensive noble metals, such as iridium (Ir) and ruthenium (Ru), further motivates the development of improved catalyst architectures that enhance performance while minimizing material usage.

Direct seawater electrolysis offers an equally compelling route to decarbonization, enabling hydrogen production at coastal and offshore sites without requiring desalination. Yet, chloride ions introduce a competing chlorine evolution reaction (CER), reducing OER selectivity and accelerating catalyst degradation. Local acidification at the anode surface further lowers the interfacial pH, promoting chlorine production relative to OER and imposing acid-like corrosive conditions on the catalyst. This suggests that durability in seawater inherently demands stability under acidic conditions as well. Addressing these challenges across both environments requires advanced catalyst design strategies and characterization tools capable of probing the electrocatalytic interface under realistic operating conditions. This thesis addresses these challenges by combining the development of mechanistic understanding, catalyst design strategies, and advanced in situ characterization methodologies.

First, in Chapter 3, a mechanistic framework for OER and CER competition was established using iridium oxide (IrO₂) as a model system. Electrochemical measurements were integrated with in situ surface-enhanced Raman spectroscopy (SERS), grand canonical density functional theory (GC-DFT), and kinetic Monte Carlo (KMC) simulations to probe the evolution of surface intermediates under reaction conditions. KMC simulations reproduced experimental trends and revealed the evolution of *OH, *O, and *OCl surface intermediates as a function of potential and pH. This was directly validated by in situ SERS, using a uniquely developed substrate for the detection of characteristic ν(Cl–Cl), ν(O–Cl), and ν(Ir–OH) vibrational modes. The *OH- to-*OCl surface coverage ratio was identified as the primary descriptor of OER/CER selectivity, revealing a fundamental tension: high pH favors OER selectivity but increases overpotential and reduces activity, while low pH affords superior activity but promotes CER. This intrinsic limitation defines the central catalyst design challenge addressed throughout the remainder of the thesis. To overcome this trade-off, two complementary catalyst design strategies were developed. In Chapter 4, a support-engineering strategy based on bimetallic transition-metal nitrides (TMNs) was explored. IrOₓ dispersed on bimetallic TiTaN and TiZrN supports benefited from tensile strain at the support interface, tuning OER intermediate adsorption energetics consistent with DFT calculations, and yielding higher intrinsic activity and lower overpotentials than monometallic TiN-supported and commercial IrO₂ references. Stability testing confirmed durable operation with suppressed Ir dissolution, and under seawater-relevant conditions, the promotional effects persisted, delivering higher mass activity, improved longevity, higher OER Faradaic efficiencies, and reduced chlorine-containing product formation. In situ SERS provided direct spectroscopic evidence for improved selectivity through delayed and attenuated ν(Cl–Cl) and ν(O–Cl) features, attributed by DFT to a strain-induced thermodynamic preference for OER over CER intermediates, establishing bimetallic nitride support engineering as a viable strategy for advancing activity, selectivity, and durability across both electrolysis environments.

A second complementary approach was also investigated in Chapter 5, in which selectivity was achieved through oxide encapsulation of the electrocatalyst (OEC). Ruthenium (Ru) and RuOₓ thin films encapsulated with ultrathin SiOₓ or TiOₓ overlayers deposited via a room-temperature photochemical process were evaluated in pH-neutral synthetic seawater. SiOₓ overlayers proved particularly effective at enhancing OER selectivity in unbuffered conditions, with in situ Raman measurements of the ν(O–H) stretching mode providing evidence that these overlayers suppress chloride transport to the buried catalytic interface. SiOₓ outperformed TiOₓ owing to lower chloride permeability and higher electronic resistivity, and encapsulated RuOₓ emerged as a promising candidate due to higher stability and selectivity, with increasing overlayer thickness and Ru⁴⁺ atomic fraction providing compositional handles for further optimization. The oxide-encapsulation architecture established a broadly applicable strategy for controlling ion transport to the catalytic interface, enabling selectivity in complex environments while highlighting the need to balance activity and stability.

Chapter 6 presents a methodological advance underpinning the in situ characterization throughout this thesis. Conventional static Raman cells accumulate gas bubbles during operation, restricting measurements to current densities far below practical relevance. A custom flow cell dramatically reduced bubble surface coverage, enabling direct in situ Raman access at current densities approximately two orders of magnitude higher than previously accessible. Applied to graphene-modified Au and Pt electrodes during the hydrogen evolution reaction (HER) in acidic conditions, this platform enabled quantification of carrier densities and interfacial electric field strengths, revealing substrate-dependent doping regimes governed by the interplay of metal work functions, proton adsorption, and electrostatic gating, and establishing a generalizable tool for interrogating electrocatalytic interfaces under realistic conditions.

Taken together, this thesis reframes harsh electrochemical environments not as barriers to implementation but as opportunities to reveal the fundamental principles that govern catalytic performance. A mechanistic evaluation of OER and CER selectivity informed two complementary design strategies, support engineering and oxide encapsulation, both of which highlighted trade- offs among activity, selectivity, and stability that suggest promising directions for future development. The in situ platforms developed alongside these strategies offer a broadly applicable foundation for probing reaction mechanisms and connecting mechanistic insight to catalyst design across electrochemical energy conversion.