Tiny Doses of Many Metals Supercharge Fuel Cell Electrodes That Run Both Ways

A single device that can turn hydrogen into electricity and then flip itself around to turn electricity back into hydrogen sounds like the kind of technology that belongs in a futuristic energy blueprint. Yet reversible protonic ceramic electrochemical cells, known as R-PCECs, are edging closer to practical reality, and a new study published in Advanced Science reports a deceptively simple materials trick that could remove one of their most stubborn bottlenecks. Instead of redesigning the entire electrode, researchers at Khalifa University and the University of Utah sprinkled minute quantities of three extra metals into the crystal lattice of a benchmark air electrode and watched its performance climb in both directions of operation.

The air electrode is the hardest-working component in these cells. In fuel cell mode it must catalyze the oxygen reduction reaction, pulling oxygen from the air and shuttling it into the electrochemical circuit. In electrolysis mode the very same electrode must run in reverse, driving the oxygen evolution reaction under steam-rich, highly oxidizing conditions. Few materials can perform both jobs efficiently for thousands of hours. Cobalt- and strontium-containing perovskites, including the layered double perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ, or PBSCF, have long been considered benchmarks because of their high mixed conductivity and strong reversible activity. But they suffer from surface strontium segregation and chemical instability in carbon dioxide- and water-containing atmospheres, which erodes durability exactly where it matters most.

Recent years have seen a wave of enthusiasm for high-entropy perovskite oxides, in which five or more principal cations share a crystallographic sublattice in near-equal proportions. The resulting configurational disorder can stabilize phases and tune defect chemistry. Several high-entropy air electrodes have posted impressive numbers, including power densities above 1.6 watts per square centimeter and electrolysis currents approaching 4.4 amperes per square centimeter at 1.3 volts. The new study, however, takes a deliberately different path. Rather than maximizing entropy, the team adopted what it calls multi-element micro-doping, distributing a small total dopant concentration among several minor cations to create spatially distributed perturbations in local bonding, cation valence, lattice strain, and defect energetics while preserving the catalytically active host framework.

The researchers started from PBSCF and introduced manganese, nickel, and molybdenum onto the B-site of the perovskite, the sublattice occupied by the transition metals that orchestrate oxygen chemistry. Two compositions were prepared: PBSCFMNM01, in which manganese and nickel each occupy 5 percent of B-sites and molybdenum 0.5 percent, and PBSCFMNM03, where the molybdenum fraction rises to 1.5 percent at the expense of iron. Together the three modifiers account for roughly 10.5 to 11.5 percent of the B-site population, leaving cobalt at 75 percent. Crucially, the site-resolved configurational entropy on the B-sublattice remains below the conventional high-entropy threshold, so the authors describe these as micro-doped derivatives rather than high-entropy oxides, a distinction that keeps the mechanistic interpretation honest.

Structural analysis confirmed that the doping strategy worked as intended. X-ray diffraction showed sharp, well-defined peaks indexable to the layered double perovskite family, with no detectable secondary phases of manganese, nickel, or molybdenum. The main diffraction peak shifted slightly to higher angles, and Rietveld refinement revealed a small but systematic lattice contraction as molybdenum content increased, from a unit cell volume of 115.025 cubic angstroms for PBSCFMNM01 down to 114.864 for PBSCFMNM03. The contraction makes physical sense: the dopant cations carry smaller ionic radii than the iron they partially replace. High-resolution transmission electron microscopy showed continuous lattice fringes and a d-spacing that shrank from 0.275 to 0.272 nanometers, while scanning transmission electron microscopy with energy-dispersive spectroscopy confirmed that all nine elements, including the dilute molybdenum, were uniformly co-distributed with no nanoscale clustering.

The defect chemistry is where the story becomes genuinely exciting. Thermogravimetric analysis revealed that the higher-molybdenum composition released substantially more lattice oxygen, with an oxygen-related weight loss of 1.173 percent versus 0.817 percent for the lower-molybdenum sample, indicating greater oxygen nonstoichiometry and more mobile lattice oxygen. When the atmosphere was switched from dry to wet air, PBSCFMNM03 absorbed far more water, a mass gain of 0.687 percent against 0.261 percent, demonstrating markedly enhanced hydration. In protonic ceramic cells, hydration of oxygen vacancies near the electrode surface generates protonic defects, so this responsiveness matters directly for the reactions the electrode must perform. Oxygen temperature-programmed desorption reinforced the picture, showing stronger high-temperature lattice oxygen release from the molybdenum-rich sample.

Surface-sensitive X-ray photoelectron spectroscopy added a further layer of evidence. The ratio of adsorbed to lattice oxygen rose from 0.72 to 0.75 with the higher molybdenum level, signaling a larger population of surface oxygen-vacancy-related active sites. Perhaps most revealing, the charge compensation for the extra high-valence molybdenum did not flow through a single redox couple. Instead, the cobalt valence shifted downward while the fractions of tetravalent praseodymium, iron, and manganese all increased, indicating a multichannel redistribution of oxidation states across the multication lattice. Importantly, the surface-to-lattice strontium ratio actually decreased slightly in the molybdenum-rich sample, suggesting that the improved defect chemistry did not come at the price of accelerated strontium segregation, one of the classic degradation pathways for cobaltite electrodes.

Stability and mechanical compatibility, often the casualties of aggressive doping, survived intact. Both compositions showed smooth thermal expansion behavior with no phase-transition anomalies, and the coefficients of thermal expansion were remarkably low for cobalt-rich electrodes, approximately 11.5 and 10.8 millionths per kelvin for PBSCFMNM01 and PBSCFMNM03 respectively, a favorable match for proton-conducting electrolytes. After 100 hours in 3 percent steam at 600 degrees Celsius, X-ray diffraction patterns retained the perovskite reflections with no impurity peaks. Electrochemical impedance spectroscopy on both proton-conducting BZCYYb and oxygen-ion-conducting GDC supported symmetric cells then showed that PBSCFMNM03 consistently delivered lower polarization resistance across 500 to 650 degrees Celsius, with distribution-of-relaxation-times analysis pointing to suppressed contributions from surface exchange, bulk diffusion, and interfacial charge transfer alike. The apparent activation energy for oxygen reduction dropped slightly, from about 0.733 to 0.722 electron volts.

The full-cell results translated these kinetic gains into device-level performance. A reversible cell with the PBSCFMNM03 air electrode, a dense BZCYYb electrolyte, and a porous nickel-based fuel electrode delivered peak power densities of 1.10, 0.81, 0.60, and 0.42 watts per square centimeter at 650, 600, 550, and 500 degrees Celsius, compared with 0.91, 0.72, 0.54, and 0.38 for the lower-molybdenum counterpart. In electrolysis mode at 1.3 volts, the molybdenum-rich cell produced current densities of 1.88, 1.37, 0.82, and 0.47 amperes per square centimeter at the same temperatures, against 1.82, 1.08, 0.55, and 0.22, with the advantage growing most pronounced at the lower temperatures where protonic cells are meant to shine. When cycled repeatedly between fuel cell and electrolysis conditions at 600 degrees Celsius for 240 hours, alternating between 0.8 and 1.2 volts, the current response remained periodic and stable with no sign of collapse.

What makes this work resonate beyond its impressive numbers is the elegance of the design philosophy. The performance enhancement is attributed not to a meaningful entropy change but to molybdenum-mediated regulation within a fixed manganese- and nickel-containing framework: modified local metal-oxygen bonding, redistributed cation valences, increased oxygen vacancy availability, and improved hydration response, all while the layered perovskite backbone and its cobalt-dominated activity remain untouched. For a field chasing ever more complex multicomponent compositions, the message is that a few percent of well-chosen dopants, cheaply incorporated through a standard sol-gel route, can deliver reversible activity and durability that rival far more elaborate high-entropy designs. As renewable-heavy grids demand devices that can store surplus electricity as hydrogen and return it on demand, electrodes engineered at this level of chemical finesse may prove exactly what reversible protonic ceramic cells need to leave the laboratory behind.

Subject of Research: Multi-element B-site micro-doping of layered double perovskite air electrodes for reversible protonic ceramic electrochemical cells

Article Title: B‐Site Multi‐Element Micro‐Doping Engineered Air Electrodes With Fast Oxygen Kinetics for Reversible Protonic Ceramic Electrochemical Cells

Article References: Yang, W., Bao, Y., Aworinde, T., Yu, L., Abdullah, S., Mathur, L., Wang, D., Vega, L. F., Duan, C., & Sengodan, S. (2026). B‐Site Multi‐Element Micro‐Doping Engineered Air Electrodes With Fast Oxygen Kinetics for Reversible Protonic Ceramic Electrochemical Cells. Advanced Science, Article e78109. https://doi.org/10.1002/advs.78109

Image Credits: AI Generated

DOI: 10.1002/advs.78109

Keywords: protonic ceramic electrochemical cells, air electrode, perovskite, micro-doping, oxygen reduction reaction, oxygen evolution reaction, oxygen vacancies, hydrogen production, fuel cell, electrolysis, PBSCF, molybdenum doping