SURFACE AND INTERFACIAL ENGINEERING IN REVERSIBLE PROTONIC CERAMIC CELLS FOR ELECTROCHEMICAL ENERGY CONVERSION

Abstract

AbstractProtonic ceramic electrochemical cells (PCCs)—operating reversibly as protonic ceramic fuel cells (PCFCs) for power generation and protonic ceramic electrolysis cells (PCECs) for hydrogen production—offer an attractive pathway for efficient, solid-state energy conversion in the intermediate-temperature regime (≈400–600 °C). By transporting protons through a dense ceramic electrolyte, PCCs provide a unique coupling between electricity and hydrogen that can enable reversible “power-to-gas” and “gas-to-power” operation within a single device platform. This reversibility is especially compelling for renewable energy integration and long-duration storage, where the ability to switch efficiently between electrical output and fuel generation can mitigate intermittency and improve system flexibility. Compared with conventional oxide-ion solid oxide cells, PCCs can reduce thermomechanical stress and expand materials compatibility by operating at lower temperatures; compared with low-temperature electrochemical devices, they can benefit from enhanced reaction kinetics and broadened oxide-based catalyst options that become viable at intermediate temperatures. Despite this promise, practical PCC deployment remains constrained by oxygen-side polarization and durability challenges under steam-rich and high-current conditions—conditions that are intrinsic to electrolysis and to reversible cycling. In protonic systems, oxygen-electrode reactions are intrinsically proton-coupled: in fuel-cell mode, oxygen is reduced and reacts with protons to form water predominantly at the oxygen electrode; in electrolysis mode, steam is oxidized at the oxygen electrode to evolve oxygen while generating protons that must be transported through the electrolyte. Consequently, oxygen-electrode performance is governed by the coordinated overlap of (i) gas delivery and product removal through porous microstructures, (ii) electron transport through percolated solid networks, and (iii) proton accessibility to the reaction zone. At intermediate temperatures, where surface exchange and charge-transfer kinetics become more prominent, the oxygen electrode frequently dominates the polarization resistance. Moreover, oxygen-electrode durability is challenged by steam-induced surface reconstruction, nanoparticle coarsening, cation segregation, and defect-driven chemical expansion, all of which can accelerate under high current density and reversible cycling. Compounding these issues, the buried electrode–electrolyte interface often becomes the practical bottleneck: conventional porous electrodes typically contact the dense electrolyte through discontinuous point contacts, creating current constriction, high local overpotential hot spots, and vulnerability to interfacial reaction-layer growth and delamination. These coupled transport–kinetic–interfacial limitations produce a familiar pattern: impressive initial performance can be achieved by optimizing a single factor, but improvements quickly plateau as the limiting step shifts, and long-term operation reveals degradation pathways that erode the initial gains. This doctoral dissertation addresses these limitations by developing an integrated oxygen-side design framework that unifies three elements: (1) microstructure engineering and multiscale transport architecture, (2) defect- and redox-enabled catalytic modulation, and (3) electrode–electrolyte interfacial engineering. The central thesis is that high reversible performance and durability cannot be achieved through isolated optimization of composition, porosity, or electrolyte thickness alone, because PCC operation is governed by coupled processes whose limiting step depends on temperature, steam activity, current density, and operating direction. Instead, durable low-polarization operation emerges when gas access, electronic/protonic percolation, surface exchange activity, and interfacial integrity are engineered as a coordinated multiscale system. Within this framework, “activity” is not defined only by intrinsic catalytic chemistry; it is equally defined by accessibility of reaction sites and stability of the defect landscape and interface under steam-rich reversible conditions. Four studies constitute the technical core of this dissertation, each addressing a different leverage point within the integrated framework while building toward a coherent multiscale strategy. The first study focuses on transport and reaction-zone engineering through a nano-architectured oxygen electrode design that combines high porosity with triple-conducting functionality. In intermediate-temperature PCECs, it is often insufficient to increase intrinsic catalytic activity if gas transport remains limiting or if proton accessibility is confined to a narrow zone adjacent to the electrolyte. The nano-architectured electrode developed in this work is designed to extend the effective reaction zone while maintaining scalable fabrication. By introducing a structure that enhances mass transfer and maintains percolated pathways for charge transport, the oxygen electrode exhibits robust bifunctional performance, achieving a peak power density of 1.50 W cm-2 at 600 °C in fuel-cell mode and a current density of 5.04 A cm-2 at 1.60 V in electrolysis mode, alongside improved stability under transient operation and thermal cycling. This work establishes a foundational message for PCC electrodes: microstructure is not merely supportive; it actively governs apparent kinetics by controlling the overlap of gas access, charge pathways, and proton participation, and it strongly influences durability by moderating gradients and avoiding localized hot spots. The second study advances from architecture to hybrid catalytic modulation, targeting the oxygen electrode’s limited active sites and sluggish kinetics under steam. The work introduces a hybrid oxygen electrode architecture based on a PrNi0.7Co0.3O3-δ (PNC) backbone infused with oxygen-vacancy–rich praseodymium oxide (PrOx) nanoparticles. This design leverages complementary roles: the backbone provides robust transport continuity and structural stability, while the PrOx phase supplies vacancy-rich catalytic sites that enhance oxygen adsorption/activation and accelerate proton-coupled ORR/OER kinetics. Beyond simply increasing surface activity, the hybrid design is intended to couple surface and bulk processes so that catalytic enhancement remains effectively utilized throughout operation. Cells incorporating this hybrid oxygen electrode deliver a peak power density of 1.56 W cm-2 at 600 °C and achieve 2.25 A cm-2 at 1.30 V in electrolysis mode. Importantly, the study emphasizes practical electrolysis metrics: Faradaic efficiency and energy efficiency reach 96.8% and 89.9%, respectively, reflecting effective conversion of charge to hydrogen with minimized parasitic losses. The polarization resistance is substantially reduced (reported as 0.079 Ω cm2), and the electrode exhibits exceptional thermal cycling stability. This work demonstrates that catalytic improvement in PCCs is most impactful when vacancy chemistry and redox flexibility are introduced in a way that is compatible with transport pathways and stability constraints, thereby narrowing the traditional activity–durability tradeoff. The third study addresses a widely observed but often underestimated limitation in reversible PCCs: the buried electrode–electrolyte junction. Even when bulk electrolyte conductivity is high and oxygen-electrode surfaces are active, interfacial polarization and interfacial degradation can dominate the loss budget, particularly under high steam activity in electrolysis mode. Conventional porous electrodes frequently form discontinuous contact networks and chemically reactive interfaces, producing constriction resistance and promoting interphase growth and delamination. To overcome this coupled problem, this dissertation develops a pulsed laser deposition (PLD) enabled interfacial nanostructuring strategy that constructs a dense, nanocrystalline Pr0.5La0.5 BaCo2O5+δ (PLBC) interlayer directly on a chemically robust BaZr0.8Y0.2O3 (BZY) electrolyte. The interlayer forms conformal contact and creates a defect-enriched reaction zone beneath a porous oxygen electrode, thereby reducing interfacial constriction and stabilizing the junction without sacrificing gas accessibility. Mechanically, the engineered interface increases bonding strength by approximately threefold, demonstrating that interfacial design can simultaneously enhance electrochemical performance and thermomechanical integrity. Electrochemically, the interfacial nanostructuring substantially reduces interfacial polarization, enabling peak power densities of 1.16 W cm-2 at 600 °C, and critically supports long-term steam electrolysis stability for ~1000 h under 0.3–0.7 atm steam, maintaining stable operando impedance at 1.30 V with Faradaic efficiency exceeding 92%. This work establishes that controlling the interface is not a secondary refinement but a primary route to durable reversibility, particularly when paired with chemically robust electrolytes such as BZY where long-term stability and interface compatibility are central goals. The fourth study integrates the dissertation’s multiscale logic most fully by coupling hierarchical architecture with a chemically active, redox-buffered interphase, aiming to stabilize oxygen electrocatalysis under steam while maintaining high reversible performance. A three-dimensional, mesh-like PNC scaffold is conformally integrated with a vacancy-rich PrOₓ nanophase, producing both an extended reaction zone and an interphase that enables vacancy–redox coupling between Pr and Co. This coupling is proposed to buffer local oxygen chemical potential swings and stabilize the defect landscape during reversible operation, addressing a central durability driver in steam electrolysis: repeated perturbation of cation valence and oxygen nonstoichiometry at active regions and interfaces. The enhanced activity is associated with vacancy-assisted steam activation and defect-mediated oxygen surface exchange and is described within an electronic-structure/defect framework where modulation of metal–oxygen covalency (often rationalized through an O 2p-band center perspective) influences oxygen exchange kinetics. The electrode achieves 1.75 W cm⁻² in fuel-cell mode and 2.77 A cm-2 at 1.30 V in electrolysis at 600 °C, maintains >92% Faradaic efficiency, and exhibits minimal degradation over 200 h. This work generalizes a key concept emerging across the dissertation: the most durable high-rate operation is achieved when active defect chemistry is not merely maximized but buffered and stabilized through engineered heterostructures and interphases that prevent excessive redox swings and suppress steam-driven reconstruction. Across these four contributions, the dissertation clarifies several governing principles and practical design rules for reversible PCCs: 1. Microstructure controls apparent kinetics by determining the overlap of gas pathways, electronic transport, and proton accessibility. High intrinsic activity cannot be fully realized if the reaction zone is confined or if transport bottlenecks create strong gradients; hierarchical and 3D architectures expand utilization and moderate local extremes that accelerate degradation. 2. Catalytic performance is best understood through defect and bonding descriptors, including oxygen vacancy chemistry, redox flexibility, and metal–oxygen covalency. However, activity gains must be balanced against steam-driven instability, motivating strategies that couple vacancy-rich phases with stable backbones and that buffer redox swings during reversible operation. 3. The buried electrode–electrolyte interface is a dominant determinant of both initial polarization and long-term durability. Constriction at point contacts, chemical incompatibility, interdiffusion, and reaction-layer growth can dominate losses even when bulk properties are favorable; engineered interlayers and nanostructured junctions provide a direct route to low-polarization, mechanically robust interfaces. 4. Integrated multiscale design creates synergy: transport-ready architectures amplify catalytic utilization; engineered interphases stabilize defect landscapes; and stabilized interfaces reduce hot spots and suppress the degradation mechanisms that typically erase activity gains. This synergy is essential for reversible operation, where ORR and OER impose different chemical potential fields and where cycling accelerates defect redistribution and interfacial stress. Beyond the specific demonstrations reported here, this dissertation contributes a broader message for PCC development: electrolyte selection, oxygen-electrode architecture, and interface engineering are deeply interdependent. For example, chemically robust electrolytes such as BZY strengthen long-term stability but raise processing and interface demands; interfacial nanostructuring and compatible interlayers can unlock their potential by ensuring proton transparent contact and strong adhesion. Conversely, higher-conductivity electrolytes such as BZCYYb reduce ohmic loss but still require microstructure and interface strategies to stabilize oxygen-side kinetics and durability under steam. In this sense, PCC performance is ultimately a system property anchored not only in bulk material conductivity, but also in grain-boundary control, interface compatibility, and multiscale electrode design. Overall, this dissertation provides a multiscale, mechanism-based pathway for constructing oxygen electrodes and interfaces that deliver both high kinetics and durability in reversible protonic ceramic electrochemical systems. The work establishes design principles for transport-ready architectures, vacancy/redox-managed catalytic interphases, and structurally integrated, proton transparent interfaces, validated through high-performance reversible operation and long-term steam electrolysis testing. The insights developed here also point to future directions: deeper probing of catalytic/material descriptors under realistic steam conditions, broader interface-engineering strategies that combine chemical compatibility with mechanical compliance and scalability and expanded post-operation and operando-informed diagnosis to connect evolving electrochemical signatures with nanoscale structural and defect-chemical changes. Together, these advances are expected to accelerate PCC progress toward practical deployment in renewable energy storage and conversion applications.