No abstract

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.

Stable and high electrode performance is paired with low catalyst loading, achieved by using a novel electrode architecture. Finely dispersed, 2–3 nm Pt particles on CNTs are obtained via metal-organic chemical vapor deposition, forming an interconnected catalyst network on solid acid microparticles.

Similar to polymer electrolyte membrane fuel cells, the widespread application of solid acid fuel cells (SAFCs) has been hindered partly by the necessity of the use of the precious-metal catalyst Pt in the electrodes. Here we investigate multi-walled carbon nanotubes (MWCNTs) for their potential catalytic activity by using symmetric cell measurements of solid-acid-based electrochemical cells in a cathodic environment. For all measurements, the carbon nanotubes were Pt free and subject to either nitrogen or oxygen plasma treatment. AC impedance spectroscopy of the electrochemical cells, with and without a DC bias, was performed and showed significantly lower initial impedances for oxygen-plasma-treated MWCNTs compared to those treated with a nitrogen plasma. In symmetric cell measurements with a DC bias, the current declines quickly for oxygen-plasma-treated MWCNTs and more slowly, over 12 days, for nitrogen-plasma-treated MWCNTs. To elucidate the degradation mechanisms of the oxygen-plasma-treated MWCNTs under SAFC operating conditions, theoretical calculations were performed using DFT. The results indicate that several degradation mechanisms are likely to occur in parallel through the reduction of the surface oxygen groups that were introduced by the plasma treatment. This finally leads to an inert MWCNT surface and a very low electrode performance. Nitrogen-plasma-treated MWCNTs appear to have a higher stability and may be worthwhile for future investigations.