Fast Degradation for High Activity: Oxygen- and Nitrogen-Functionalised Carbon Nanotubes in Solid-Acid Fuel-Cell Electrodes

Hydrogen Production by Steam Electrolysis in Solid Acid Electrolysis Cells

Hydrogen production by steam electrolysis at intermediate temperatures has potential for both the high energy conversion efficiency and the flexible operability suitable for the utilization of renewable energy resources. Employment of proton-conducting solid acid electrolytes at around 200 °C is considered promising but has rarely been investigated. Here, steam electrolysis was performed at 160-220 °C using a solid acid electrolysis cell (SAEC) composed of a CsH₂ PO₄ /SiP₂ O₇ composite electrolyte and Pt/C electrodes. Hydrogen production was successfully demonstrated with Faraday efficiencies around 80 %. Key factors affecting the SAEC stability were investigated in detail for the first time. It was revealed that a certain part of the electrolyte migrated into the porous anode structure during the operation. The migrated electrolyte prevented the gas diffusion and flooded the Pt/C catalyst layer. It was also found that carbonaceous materials in the anode was oxidized, leading to the decrease in the number of electrochemically active sites. Based on the findings, Pt mesh was employed as an alternative anode. The SAEC with the Pt mesh anode showed superior stability, demonstrating the importance of the anode design. The present work provides a comprehensive view of the stability issues, which is essential for the development of durable and practical SAECs.

The stability limits of highly active nitrogen doped carbon ORR nano-catalysts: a mechanistic study of degradation reactions

A new approach in electrode catalysis bearing immense potential for electrochemical technologies is the prospect of carbon-based electrodes. Pristine carbon nanostructures are relatively inert and modifications like nitrogen doping are known for their beneficial effects on the electrochemical activity of carbon nanomaterials in both alkaline and acidic media. However, the long-term stability of these materials, especially in an acidic environment, is rarely mentioned. Here, we evaluate the stability and long-term degradation of nitrogen doped graphene flakes as an oxygen reduction electrocatalyst with theoretical and experimental techniques. We assume that nitrogen dopants in the graphene sheet interact with e- and H+ at the electrode-electrolyte interface, leading to NH3 scission and continuous catalyst deactivation. With Density Functional Theory calculations, NH3 scission pathways of pyridinic, graphitic and pyrrolic nitrogen species were analyzed and compared under different operating conditions which are relevant for low and intermediate temperature fuel cells. The computational results are correlated with electrochemical measurements in solid acid fuel cells in a humidified oxygen environment at 240 °C.