Collateral hydrogenation over proton-conducting Ni/BaZr0.85Y0.15O3−δ catalysts for promoting CO2 methanation

Electrocatalysis in Solid Oxide Fuel Cells and Electrolyzers

Interest in energy-to-X and X-to-energy (where X represents green hydrogen, carbon-based fuels, or ammonia) technologies has expanded the field of electrochemical conversion and storage. Solid oxide electrochemical cells (SOCs) are among the most promising technologies for these processes. Their unmatched conversion efficiencies result from favorable thermodynamics and kinetics at elevated operating temperatures (400-900 °C). These solid-state electrochemical systems exhibit flexibility in reversible operation between fuel cell and electrolysis modes and can efficiently utilize a variety of fuels. However, electrocatalytic materials at SOC electrodes remain nonoptimal for facilitating reversible operation and fuel flexibility. In this Review, we explore the diverse range of electrocatalytic materials utilized in oxygen-ion-conducting SOCs (O-SOCs) and proton-conducting SOCs (H-SOCs). We examine their electrochemical activity as a function of composition and structure across different electrochemical reactions to highlight characteristics that lead to optimal catalytic performance. Catalyst deactivation mechanisms under different operating conditions are discussed to assess the bottlenecks in performance. We conclude by providing guidelines for evaluating the electrochemical performance of electrode catalysts in SOCs and for designing effective catalysts to achieve flexibility in fuel usage and mode of operation.

Protonic Ceramic Electrochemical Cell for Efficient Separation of Hydrogen

Advancement of a hydrogen economy requires establishment of a whole supply chain including hydrogen production, purification, storage, utilization, and recovery. Nevertheless, it remains challenging to selectively purify hydrogen out of H₂-containing streams, especially at low concentrations. Herein, a novel protonic ceramic electrochemical cell is reported that can sustainably separate pure H₂ out of H₂-diluted streams over the temperature regime of 350-500 °C by mildly controlling the electric voltage. With the Faraday's efficiency above 96%, the measured H₂ separation rate at 0.51 V and 500 °C is 3.3 mL cm^-2 min^-1 out of 10% H₂ - 90% N₂, or 2.4 mL cm^-2 min^-1 out of 10% H₂ - 90% CH₄ taken as an example of renewable hydrogen blended in the natural gas pipelines. Such high hydrogen separation capability at reduced temperatures is enabled by the nanoporous nickel catalysts and well-bonded electrochemical interfaces as produced from well-controlled in situ slow reduction of nickel oxides. These results demonstrate technical feasibility of onsite purification of hydrogen prior to their practical applications such as fuels for fuel cell electric vehicles.

Pulse-Driven Nonvolatile Perovskite Memory with Photovoltaic Read-Out Characteristics

This paper presents a unique GdFe_0.8Ni_0.2O₃ perovskite thin film for use in pulse-controlled nonvolatile memory devices (combined with a SrTiO₃ (STO) substrate) without the need for an electrical-stressing read-out process. The use of pulse voltage imposes permanent downward/upward polarization states on GFNO, which enables greater energy density and higher energy efficiency than the unpoled state for memory. The two polarization states produce carrier migrations in opposing directions across the GFNO/STO interface, which alter the depletion region of the device, as reflected in photovoltaic short-circuit current density (J_sc) values. Modulating the duration (varying the number of sequential pulses but fixing the pulse width and delay time) and direction of continuous pulse voltage is an effective method for controlling J_sc, thereby allowing the fabrication of nondestructive, light-tunable, nonvolatile memory devices. In experiments, J_sc in the downward polarized state was approximately 6 times greater than that in the upward polarized state. It is promising that more memory states can be enabled by the proposed heterostructure by selecting appropriate pulse trains. Real-time interfacial changes (relative to the nonvolatile characteristics of the device) were obtained by applying synchrotron X-ray techniques simultaneously with pulse characterization. This made it possible to separately probe the electronic and chemical states of the GFNO (a p-type-like semiconductor) and STO (an n-type-like semiconductor) while varying the pulse direction, thereby making it possible to identify the mechanisms underlying the observed phenomena.