Nanocrystalline surface layer of WO₃ for enhanced proton transport during fuel cell operation

High ionic conductivity in low-cost semiconductor oxides is essential to develop electrochemical energy devices for practical applications. These materials exhibit fast protonic or oxygen-ion transport in oxide materials by structural doping, but their application to solid oxide fuel cells (SOFCs) has remained a significant challenge. In this work, we have successfully synthesized nanostructured monoclinic WO₃ through three steps: co-precipitation, hydrothermal, and dry freezing methods. The resulting WO₃ exhibited good ionic conductivity of 6.12 × 10⁻² S cm⁻¹ and reached an excellent power density of 418 mW cm⁻² at 550^◦C using as an electrolyte in SOFC. To achieve such a high ionic conductivity and fuel cell performance without any doping contents was surprising, as there should not be any possibility of oxygen vacancies through the bulk structure for the ionic transport. Therefore, laterally we found that the surface layer of WO₃ is reduced to oxygen-deficient when exposed to a reducing atmosphere and form WO_3−δ/WO₃ heterostructure, which reveals a unique ionic transport mechanism. Different microscopic and spectroscopic methods such as HR-TEM, SEM, EIS, Raman, UV-visible, XPS, and ESR spectroscopy were applied to investigate the structural, morphological, and electrochemical properties of WO₃ electrolyte. The structural stability of the WO₃ is explained by less dispersion between the valence and conduction bands of WO_3−δ/WO₃, which in turn could prevent current leakage in the fuel cell that is essential to reach high performance. This work provides some new insights for designing high-ion conducting electrolyte materials for energy storage and conversion devices.