The increasing world population and the need to improve quality of life for a large percentage of human beings are the driving forces for the search for sustainable energy production systems, alternative to fossil fuel combustion. Among the various types of alternative energy production technologies, solid oxide fuel cells (SOFCs) operating at intermediate temperatures (400–700 °C) show the advantage of possible use both for stationary and mobile energy production. To reach the goal of reducing the SOFC operating temperature, proton-conducting oxides are gaining wide interest as electrolyte materials. This critical review provides a broad overview of the most recent progresses obtained tailoring the properties of proton-conducting oxides for fuel cell applications, analyzing and comparing the different strategies proposed to match high-proton conductivity with good chemical stability (170 references).

Materials challenges toward proton-conducting oxide fuel cells: a critical review

The need for reducing the solid oxide fuel cell (SOFC) operating temperature below 600 °C is imposed by cost reduction, which is essential for widespread SOFC use, but might also disclose new applications. To this aim, high-temperature proton-conducting (HTPC) oxides have gained widespread interest as electrolyte materials alternative to oxygen-ion conductors. This Progress Report describes recent developments in electrolyte, anode, and cathode materials for protonic SOFCs, addressing the issue of chemical stability, processability, and good power performance below 600 °C. Different fabrication methods are reported for anode-supported SOFCs, obtained using state-of-the-art, chemically stable proton-conducting electrolyte films. Recent findings show significant improvements in the power density output of cells based on doped barium zirconate electrolytes, pointing out towards the feasibility of the next generation of protonic SOFCs, including a good potential for the development of miniaturized SOFCs as portable power supplies.

Towards the next generation of solid oxide fuel cells operating below 600 °c with chemically stable proton-conducting electrolytes.

Energy crisis and environmental problems caused by the conventional combustion of fossil fuels boost the development of renewable and sustainable energies. H2 is regarded as a clean fuel for many applications and it also serves as an energy carrier for many renewable energy sources, such as solar and wind power. Among all the technologies for H2 production, steam electrolysis by solid oxide electrolysis cells (SOECs) has attracted much attention due to its high efficiency and low environmental impact, provided that the needed electrical power is generated from renewable sources. However, the deployment of SOECs based on conventional oxygen-ion conductors is limited by several issues, such as high operating temperature, hydrogen purification from water, and electrode stability. To avoid these problems, proton-conducting oxides are proposed as electrolyte materials for SOECs. This review paper provides a broad overview of the research progresses made for proton-conducting SOECs, summarizing the past work and finding the problems for the development of proton-conducting SOECs, as well as pointing out potential development directions.

Steam electrolysis by solid oxide electrolysis cells (SOECs) with proton-conducting oxides

BaCeO3: Materials development, properties and application

Copper-containing cerium oxide materials have received considerable attention both in catalysis and electro-catalysis fields due to their unique physicochemical characteristics in conjunction to their lower cost compared to noble metals (NMs)-based catalysts. Nowadays, it is well documented that the complex Copper–Ceria interactions (either geometric or electronic) have a key role on the catalytic performance. Hence, considerable efforts have been devoted on the understanding and the fine-tuning of metal–oxide interactions. Despite the growing progress in the field, several crucial issues related to the influence of: i) particle’s shape and size, ii) active site’s chemical state, iii) charge transfer between interfacial sites, and iv) intrinsic defects (e.g., surface oxygen vacancies) on the interfacial activity are still under investigation. This survey summarizes the recent advances in the last 10 years on the fundamental origin of Copper–Ceria interactions and their implications on the catalytic activity. The insights lately obtained by means of: i) ex situ advanced characterization techniques, ii) in situ sophisticated studies (e.g., operando techniques), iii) theoretical analysis (e.g., DFT calculations), and iv) innovative probing approaches (such as the inverse CeO2/CuO model system) are provided. The state-of-the-art catalytic applications of CuO/CeO2 binary oxides (water gas shift (WGS) reaction, preferential oxidation (PROX) of CO, CO2 hydrogenation, selective catalytic reduction (SCR), N2O decomposition, etc.) in relation to the aforementioned aspects are discussed. Some guidelines towards the fine-tuning of the surface chemistry of CuO/CeO2 catalysts for real life energy and environmental application are provided.

Juri Melnik

Papers

ZnO-doped BaZr0.85Y0.15O3−δ proton-conducting electrolytes: Characterization and fabrication of thin films

BaZr0.8Y0.2O3-δ electrolyte with and without ZnO sintering aid: Preparation and characterization

Surface modified Ni foam as current collector for syngas solid oxide fuel cells with perovskite anode catalyst

Copper coated nickel foam as current collector for H2S-containing syngas solid oxide fuel cells

Ceria and copper/ceria functional coatings for electrochemical applications: Materials preparation and characterization