Investigation of the ammonia-formation electrodes for alkaline membrane electrosynthesis
Electrochemical synthesis of ammonia has recently attracted enormous interests from research societies, industries, and governments. Such electrochemical approach has demonstrated the great potential to become the next-generation ammonia-manufacturing technology that is scale-flexible, energy-efficient, and environmentally friendly. The research of this dissertation has been primarily conducted on the investigation into the electrodes of ammonia formation in alkaline membrane electrosynthesis under ambient pressure and temperature. In order to address the research needs for understanding electrode functions for ammonia formation, the three research aspects of electrode substrates, nitrogen-reduction catalysts, and water supply have been chosen as the three major focuses. The experimental results have shown that electrode substrates have dramatic impact on faradaic efficiency of ammonia formation through materials hydrophilicity: the higher hydrophilicity leads to higher faradaic efficiency among four typical substrates. The study on metallic catalysts revealed that there are two catalyst groups based on the distinct favorable electrode potential (either -0.20~-0.25 V vs. RHE; or -0.05 V vs. RHE). The catalytic activity of the eight chosen metallic catalysts follows the descending trend in each group: Co, Fe, Ru, and Mo for the first group; and Pd, Rh, Ni, and Pt. for the second group. The study on water supply exhibited the effect of relative humidity on ammonia productivity: 75% of relative humidity on both cathode and anode is the optimum for symmetric water supply; and the combination of 58% relative humidity on cathode and 167% relative humidity on anode provides the highest ammonia productivity for asymmetric water supply. The research findings provide useful guidance for both materials selection and operational optimization, helping the design of the future electrodes for advanced ammonia electrosynthesis.