Microstructural optimizations of an iron-containing electrode for electrochemical ammonia synthesis on a proton-conducting ceramic membrane

In this study, we address the challenges associated with developing a mechanically and chemically stable electrode featuring iron as the active component for ammonia synthesis. Our investigations reveal that Fe-BCZY721 cermet electrodes induce significant morphological changes in the BCZY721 electrolyte. These changes are influenced by various parameters, such as sintering temperature and the Fe 2 O 3 /BCZY721 ratio, leading to the formation of interlacing Ba-rich and Ce-rich phases. This process is challenging to control and can result in mechanical weakening of the material. As a promising alternative, an iron-filled backbone electrode was proposed, constructed from two consecutively sintered components. In this novel electrode design approach, a porous BCZY721 backbone is first sintered on the electrolyte, followed by incorporation of the catalyst ink into it using a screen-printing technique. This method enables a substantially lower sintering temperature for the catalyst compared to conventional composite electrodes, effectively mitigating morphological alterations while enhancing mechanical stability. Additionally, sintering the scaffold separately from the inserted catalyst provides greater flexibility in adjusting parameters and allows for the introduction of various catalysts as needed. Initial tests demonstrate that the developed electrode design is suitable for ammonia production when compared to existing literature, with ammonia formation rates at -1.2 V against OCV of 1.1 ⋅ 10 −9 mol cm −2 s −1 , 4.9 ⋅ 10 −9 mol cm −2 s −1 and 5.0 ⋅ 10 −9 mol cm −2 s −1 measured at 400 °C, 500 °C and 600 °C respectively. The active cell surface area was 12.57 cm 2 , which is the largest to date in electrochemical ammonia synthesis using proton-conducting ceramic cells.