(Invited) Brief Overview of Current French Hydrogen Research Activities, Focus on SOFC Materials Development
Authors: Thabet Kawther , Quarez Eric , Le Gal La Salle Annie , Joubert Olivier 
Published: December 2020

Twenty years ago, the French scientific community working in the field of hydrogen, started to federate under the leadership of the CNRS. It took the form of successive Research Grouping (GdR) bringing together experts in solid oxide fuel cell (SOFC), proton exchange polymer membrane (PEMFC) fuel cells, hydrogen storage and systems mainly from CNRS but also from CEA. These GdRs promoted and structured an interdisciplinary field of research with excellent results.
Since January 1st of 2020, the CNRS community has formed the Research network on Hydrogen energy (FRH2) based on the active nucleus of the former GDR laboratories (28 laboratories) with about 90 full-time equivalent positions including researchers and professors, without including technical staff, students, doctoral students and post-docs. A new organization, "from material to system", more visible from industries has been set up around four main technical axes, the production of hydrogen, its storage, and its conversion into electricity for mobile and stationary applications and around 2 transversal axes, education and technological platforms. The first part of the presentation will give a brief overview of this french CNRS research network on hydrogen energy including some highligts for each axes.
The second part will be dedicated to the development of materials used in solid oxide fuel cells (SOFC) and more precisely to the ceramic electrolyte. Electrochemical properties of the materials used in SOFC are governed by the defects within the material and limited by the microstructure and the transport mechanisms of charge carrier at the interfaces. Thus, the optimization of the microstructure of the material via accurate monitoring of the synthesis and temperature treatment steps could enhance the conduction properties of existing materials. The usual objective for electrolyte material is to reach a total conductivity level above 10-2 S/cm for operating temperatures below 600°C. Actually, working at low temperature would prevent premature aging of SOFC performance.
BCZY type proton conducting electrolyte such as BaCe0.8Zr0.1Y0.1O3-δ has gained a great attention as one of the most promising candidate for intermediate temperature solid oxide fuel cells, due to the combination of a high bulk conductivity associated to a chemical stability [1]. However, some of challenges to manufacture a dense BCZY ceramic are the high sintering temperature typically 1500 to 1600°[2], and the long processing duration in conventional method which can damage the material and cause barium evaporation resulting in a decrease of electrochemical properties.
Part of this talk is focused on BaCe0.8Zr0.1Y0.1O3-δ showing properties which are a compromise between the high proton conductivity of BCY and the stability of BZY. The limiting factors are the high sintering temperature (≈1600°C) and intrinsic grain boundaries resistance.
The cold sintering process (CSP) is used to reduce the sintering temperature. In this technic, derived from the hydrothermal method, BCZY powder mixed with a small volume of liquid phase (3-30 wt%), is simultaneously pressed (at 50 to 500 MPa) and heat treated during a short time period (1-60 min) at low temperatures (100-200°C), leading to dense pellets.
This study deals with the influence of synthesis conditions using the auto combustion and CSP on the microstructure and conductivity. With a calcination temperature of 1000°C and an optimized CSP treatment, a relative density above 93% can be obtained at only 1200°C. We will show also the influence of the cold sintering process (CSP) on the conductivity. Compared to conventional sintering process at about 1600°C, high conductivity level of 10-2 S/cm is obtained at 400°C (figure 1) using CSP process combined with a 1200°C annealing step.
[1] K. Thabet, M. Devisse, E. Quarez, O. Joubert, et A. Le Gal La Salle, "Influence of the autocombustion synthesis conditions and the calcination temperature on the microstructure and electrochemical properties of BaCe0.8Zr0.1Y0.1O3−δ electrolyte material", Solid State Ionics, vol. 325, 2018, p. 48-56,
[2] F. Iguchi, T. Yamada, N. Sata, T. Tsurui, et H. Yugami, "The influence of grain structures on the electrical conductivity of a BaZr0.95Y0.05O3 proton conductor", Solid State Ionics, vol. 177, no 26, 2006, p. 2381-2384

Figure 1