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RESEARCH PRODUCT

Optimisation of metallic interconnects for hydrogen production by high temperature water vapour electrolysis (HTVE)

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The high temperature water vapour electrolysis offers a promising method for highlyefficient hydrogen production. It works as an inverse solid oxide fuel cell, using water vapourand electricity in order to produce hydrogen. A major technical difficulty related to hightemperature water vapour electrolysis (HTVE) is the development of interconnects workingefficiently on a long period. From the electrical point of view, the interconnect must have alow contact resistance with the electrodes. Indeed, it directly affects the electrochemicalconversion efficiency (water into hydrogen) and it can penalize the process. The interconnectmust present a slow oxidation kinetics and form as less as possible electrical insulatingoxides. From the chemical point of view, the interconnect has to be resistant against oxidationin an oxygen rich atmosphere (anode side) and water vapour rich atmosphere (cathode side).Moreover, the problem of the volatility of chromium oxide species, which might migrate andpoison the electrodes, leading to a decrease in their electrochemical activity and degradationof stack performance, over long-term operation, needs to be reduced. The operatingtemperature between 700°C and 900°C allows the use of metallic interconnects, which havehigher electrical and thermal conductivities, easier shaping and lower cost, with respect to theceramic materials.In this study, two materials were tested as interconnects for the HTVE systems: a ferriticchromia-forming alloy, the K41X, and a Fe-Ni-Co alloy, which does not contain chromium.High temperature corrosion behaviour and electrical conductivity were tested in both anode(95%O2-5%H2O) and cathode (10%H2-90%H2O) atmospheres at 800°C. Moreover, for theK41X alloy, the effect of the initial surface state of the samples on the chemical nature of theoxides formed at 800°C in H2-H2O atmosphere was evaluated, by comparing as received andmirror polished surfaces. The effect of a short-term air preoxidation at 800°C on the hightemperature behaviour of the K41X as received sample in H2-H2O atmosphere was tested.The most original part of this study consisted in the investigation of the oxidation mechanismsof both as received and mirror polished K41X samples at 800°C in H2-H2O atmosphere bymeans of marking experiments using Au and isotopes (H216O-H218O mixture). Moreover,marking tests using H2-D2O and D2-H2O were carried out, in order to further investigate therole of hydrogen and water vapour in the oxidation mechanism