Keerti M. Naik

Heterogeneous catalytic reactions in porous mixed-conducting solids enable energy conversion technologies from fuel cells to electrolyzers, yet the interplay between molecular-level rate-determining steps and their macroscopic spatial extent remains poorly understood. Protonic ceramic fuel cells (PCFCs), in which the cathodic oxygen reduction reaction (ORR) proceeds in H ⁺ /O ²⁻ /e ⁻ triple-conducting oxides, exemplify this challenge: facile bulk proton diffusion is conventionally assumed to spread the active reaction zone ( L _eff ) throughout the entire porous electrode. Here, we transform this understanding by bridging atomic-scale kinetics with macroscopic spatial utilization. By coupling in-situ modulation-excitation infrared spectroscopy with hydrogen/deuterium isotope-resolved impedance analysis, we identify the vacancy-assisted dissociation of accumulated superoxide (O ₂ ⁻ ) into active O ⁻ species as the molecular-scale rate-determining step. Crucially, integrating these surface kinetics with porous-electrode theory reveals that this O⁻ formation acts as a kinetic sink, consuming protons near the electrolyte interface before they can penetrate the electrode bulk. As a result, the overall ORR reaction is confined to only a 110 nm zone in state-of-the-art PrBa _0.5 Sr _0.5 Co _1.5 Fe _0.5 O _5+δ cathodes at 600 °C, leaving ~98% of the porous scaffold electrochemically inert. Overcoming the interfacial proton exchange barrier to expand L _eff , rather than solely accelerating surface kinetics, defines a design principle for porous mixed-conducting electrodes.

Proton-conducting ceramic fuel cells (PCFCs) enable efficient electrochemical energy conversion, yet further performance improvements are limited by sluggish cathodic oxygen reduction kinetics and the lack of direct identification of the rate-determining step under operating conditions. Here, we experimentally identified that oxygen dissociation from superoxide intermediates (O ₂ ²⁻ ) limits cathodic polarization in PCFC devices with a PrBa _0.5 Sr _0.5 Co _1.5 Fe _0.5 O _5+δ (PBSCF) cathode. Modulation-excitation infrared spectroscopy directly observes that superoxide is the major intermediate for the oxygen reduction reaction. We also isolate the intrinsic cathodic kinetics of PBSCF using hydrogen–deuterium isotope effects combined with impedance spectroscopy on working PCFC devices, which achieves peak power densities of 517-1077 mWcm ⁻² at 550-650 °C under a fuel-oxidant combination of H ₂ -air. The cathodic polarization resistance exhibits an oxygen partial pressure dependence with characteristic exponents above 0.5 and shows no measurable dependence on water vapor pressure. This kinetic signature is uniquely consistent with oxygen dissociation via O⁻ formation from O ₂ ²⁻ intermediates, assisted by oxygen vacancies at the cathode–gas interface. These results provide experimental evidence that oxygen dissociation constitutes the dominant kinetic bottleneck in the PCFC device, establishing a clear design principle: strong electron-donating character coupled with a high density of surface oxygen vacancies is essential to accelerate cathodic kinetics.

Li‐CO ₂ batteries offer a revolutionary energy storage solution, combining high specific energy with the utilization of greenhouse CO ₂ . Although promising for applications on Earth and Mars, the limited cycle life and significant overpotentials from stable discharge products, Li ₂ CO ₃ and amorphous carbon, hinder the practicalization of Li‐CO ₂ batteries. To address these, a unique freestanding core‐shell metal–organic framework‐derived heterojunction catalyst, Fe ₃ O ₄ /Co ₃ O ₄ , embedded in electrospun nitrogen‐doped carbon nanofibers for the Li‐CO _2Mars batteries, is designed. It eliminates the need for insulating binders, toxic solvents and ensures uniformly distributed composite catalysts across the nanofibers. The electrochemical performance of Fe ₃ O ₄ /Co ₃ O ₄ /N‐doped carbon fiber (FCo/NCF)‐based Li‐CO _2Mars coin cells operated in the simulated Mars’ atmosphere achieves a cycling life of 120 cycles at a current density of 50 μA cm ^{− 2} , with a limited discharge/charge capacity of 0.1 mAh cm ^{− 2} , and delivers a full discharge capacity of 6.8 mAh cm ^{− 2} outperforming the conventional catalysts. In situ and first‐principle studies demonstrate that enhanced CO ₂ adsorption and improved reversibility, driven by the bifunctional catalytic activity of FCo/NCF, lead to exceptional electrochemical performance of Li‐CO _2Mars batteries. A prototype of Li‐CO _2Mars pouch cell is also developed, operating with an average efficiency of ≈68%, advancing the scalable development of Li‐CO _2Mars batteries for diverse applications.

Lithium‐CO₂ batteries have been at the forefront of reducing CO₂ emissions and addressing the present energy crisis. As the Martian atmosphere is 95 % CO₂, this battery technology is ideal as a stand‐alone energy storage device on the planet. However, challenges such as low reversibility, high charging voltages, and poor cycling life restrict its practicability. The current study addresses these by increasing the reversibility of the active reaction sites on the cathode via a synergistic interaction between the RuNi nanoparticle catalyst and candle soot carbon (CSC). This helps the system achieve a full discharge capacity of 15207 mAh g⁻¹ at a high current density of 200 mA g⁻¹ with 99.3 % coulombic efficiency and an efficient cycling life of over 100 cycles at a higher current density of 500 mA g⁻¹ for a limited capacity of 500 mAh g⁻¹. Further, combining ex‐situ physical characterizations and density functional theory helps understand the underlying mechanism for improving the bidirectional CO₂ reduction and evolution activities during discharging and charging. The study, thus using a bimetallic catalyst and carbon, provides an approach for a single solution towards addressing the dual challenges of regulating CO₂ emissions on Earth and providing a constant energy source on Mars.

Rechargeable lithium‐CO₂ (Li−CO₂) batteries are an attractive energy storage technology that can reduce fossil fuel usage and limit the adverse environmental impact of CO₂ emissions. However, the high charge overpotential, unstable cycling, and incomplete understanding of the electrochemical process limit its advancement for practical applications. Herein, we develop a Li−CO₂ battery by designing a bimetallic ruthenium‐nickel catalyst onto multi‐walled carbon nanotubes (RuNi/MWCNTs) catalyst as cathode by solvothermal method, which exhibits a lower overpotential of 1.15 V and a discharge capacity of 15,165 mAh g⁻¹ with outstanding coulombic efficiency of 97.4 %. The battery can also operate at high rates and have a stable cycle of more than 80 cycles at a current density of 200 mA g⁻¹ with a fixed 500 mAh g⁻¹ capacity. Furthermore, Mars exploration is made feasible with the Li−CO_{2 Mars} battery composed of the RuNi/MWCNTs as cathode catalyst, which performs very similarly to that of pure CO₂ atmosphere. This approach may simplify the process of developing high‐performance Li−CO₂ batteries to achieve carbon negativity on Earth and for future interplanetary Mars missions.