Development and application of lanthanum tungstate series proton conductors

Abstract

<p indent=0mm>As one of the most important clean energy and commodity chemicals, hydrogen is widely used in human life and industrial production. In the present decade, three main technologies for hydrogen recovery have been developed, namely pressure swing adsorption (PSA), cryogenic separation, and membrane separation. Among these three different hydrogen recovery technologies, the membrane separation method is favored for its high efficiency. Benefiting from its absolute selectivity of hydrogen and the low cost, mixed protonic-electronic conductors (MPEC) have become an important research object in the field of hydrogen separation membrane materials, which enable a significant approach for the purification and utilization of hydrogen energy. For the traditional proton-conducting perovskite oxides doped with Ce and Zr, although they have high proton conductivity, their poor chemical stability under the atmosphere in the presence of CO₂ and H₂O hinders the commercial application. Rare-earth metal compound lanthanum tungstate material (LWO) is a kind of crucial proton conductor, which has good chemical stability in CO₂ atmosphere. However, the hydrogen permeability of LWO is not satisfactory because of the low electronic conductivity. A variety of effective methods to improve the hydrogen permeability of lanthanum tungstate film are summarized in this review. This review introduces the basic electronic conductivity of LWO-based single-phase materials, with the emphasis on improving the electronic conductivity of LWO by adjusting La/W ratio, cation doping strategies, and preparation methods. Furthermore, it reviews breakthroughs in their application in proton-conducting solid oxide fuel cells (SOFC) as well as hydrogen separation membranes. The problems now faced by LWO oxides are analyzed and their future development is predicted. Firstly, the proton conductivity of LWO is positively correlated with the La/W ratio. Adjusting the La/W ratio improves proton conductivity of LWO. For cation doping, the substitution of W rather than La more effectively improves the proton conductivity of LWO. Secondly, compositing LWO and the electronic conductors effectively enhance the performance of the hydrogen permeation membranes. Thirdly, in terms of the SOFC applications, an additional oxide layer between the LWO electrolyte layer and the electrode layer avoids the phase reaction on the interface, which prevents the chemical incompatibility between the LWO and electrode materials. Many studies show that the maximum utilization of LWO can be realized by optimizing crystal structure, phase composition, and the SOFC structure. In view of the problems existing in the research of LWO materials, future research can be focused on the following aspects: (1) Further exploring the electronic-ionic conducting pathway and catalytic mechanism of LWO series materials, and then optimizing the electrochemical properties under the guidance. (2) Developing the electrode materials with good catalytic performance and good chemical compatibility with LWO which is significant to the stable operation of SOFC. Also, it is necessary to improve the cell preparation process in which the unbeneficial interface reaction between the electrode and the LWO can be avoided. (3) For hydrogen permeation membrane, future research can be developed towards optimization with cation doping, surface engineering, and composite materials preparation.