Boron-Activating Ruthenium from Organic-Free Synthesis Process: A Hydrogen Oxidation Reaction Catalyst for Anion Exchange Membrane Fuel Cells

Abstract

Using fossil fuels, power generation is always accompanied by environmental pollution. However, hydrogen-oxygen fuel cells, using H2 generated from renewable energy, can provide electricity with zero emissions, meeting the significant demands. Proton exchange membrane fuel cells (PEMFCs), though commercialized, are hindered by the high cost due to relying on expensive Pt. By contrast, as the potential alternative, anion exchange membrane fuel cells (AEMFCs), have attracted more and more attention as they can provide the benefit of using low-cost cathode catalysts while still having high power density. Although the oxygen reduction reaction (ORR) on the cathode can be well achieved by platinum group metal (PGM)-free catalysts in an alkaline environment, the high loading of Pt for the anode is still needed for high power output caused by the sluggish hydrogen oxidation reaction (HOR) kinetics. As the cheapest PGM, Ru with lower HOR activity is often added as an activity promoter to reduce the Pt loading. Recently, more efforts have been put into using exclusively Ru as the main catalysis active site, but the synthesis methods are often complex and involve toxic organics.In this project, boron-activated Ru supported on carbon was developed for HOR catalysis in alkaline media. The B-Ru/C was synthesized by a simple organic-free method, making it possibly attractive for industrial application. This method adopted water as the only solvent and boric acid as the boron source to avoid the toxic problem and the tedious removal step of organics. It produced uniformly dispersed Ru nanoparticles in smaller particle sizes with a boron-rich environment verified by TEM and STEM images. Compared to Ru/C or Ru/BC, B-Ru/C not only has a higher electrochemical active surface area (ECSA) but also reached a much higher ECSA-normalized exchange current density which means boron does promote the activity of Ru. From the XPS spectra, the more significant peak shifts of Ru3p and B1s in B-Ru/C relative to Ru/C and BC indicate that the boron-ruthenium interaction is more robust in B-Ru/C than in Ru/BC. A theoretical simulation was conducted further to understand the mechanism of the HOR activity enhancement. From an electrochemical perspective, the B-Ru/C had a very high exchange current density in ex-situ tests and was also studied as an anode in operating AEMFCs. We believe this cost-effective Pt-free HOR catalyst with excellent performance prepared through an organic-free process can contribute to the industrial application of AEMFCs in the future.AcknowledgmentsThis work is supported by the Research Grant Council (HKUST C6011-20G).