The design and synthesis of nanostructured hydrogen oxidation reaction electrocatalysts for hydroxide exchange membrane fuel cells

Hydrogen fuel cell is a promising green technology that transforms the chemical energy in hydrogen into electricity. Currently, most of the efforts are focused on developing proton exchange membrane fuel cells (PEMFCs), however they are heavily dependent on rare and expensive platinum group metal (PGM) catalysts. In contrast, hydroxide exchange membrane fuel cells (HEMFCs) work in alkaline media, thus allow the use of cheaper and more abundant non-PGM catalysts. However, there is no materials found suitable to catalyze the anodic hydrogen oxidation reaction (HOR). Therefore, developing new, active and robust HOR catalysts in alkaline media is of paramount importance. Hereby, this dissertation presented my recent research works on preparation, characterization, and application of HOR catalysts for HEMFCs.Chapter 2 described the exploration of new earth-abundant HOR catalysts. A series of Ni-based compounds were synthesized and tested for HOR, among which Ni3N had the best activity. With further nano-engineering, the mass activity reached the highest at the time reported and the oxidation resistance of the supported version can be significantly improved. Spectroscopic evidence suggested that the downshift of the metal d band weakened the binding energies of the catalyst toward adsorbates, which accounted for the improved activity and stability of Ni3N/C.Chapter 3 demonstrated how to exploit strain-engineering for HOR catalysis. A series of Ni/C composites were prepared by pyrolyzing a Ni-based metal-organic framework (MOF). The optimal catalyst, Ni-H2-2%, has the highest mass activity (at the time reported) and abnormal high intrinsic activity. Characterizations using synchrotron XRD suggested that it was the distinct but relative optimal strain level of Ni-H2-2% that rendered it high activity.Chapter 4 is a complicated version of Chapter 3. The Ni catalysts were also prepared from the same Ni-based MOF with more complex synthesis conditions. The best catalyst, Ni-H2-NH3, possessed highest intrinsic activity among non-precious metals. Combining UPS, H2 chemisorption, isotopic studies, and electrochemical characterizations, the balanced hydrogen binding energy (HBE) and hydroxide binding energy (OHBE) is likely to be the origin of the high activity of Ni-H2-NH3. In the end, MEA measurements confirmed its excellent performance: for non-PGM MEA, the peak power density could reach 488 mW/cm2, 6.4 times higher than the previous record, which demonstrates the feasibility of efficient PGM-free HEMFCs.Chapter 5 exploited the interaction between PGMs and carbon support to improve their HOR activity. A porous N-doped carbon (pN-C) support was found to have prominent enhancement effect toward PGMs. Especially, the Pt0.25Ru0.75/pN-C had the up-to-date highest mass activity and intrinsic activity toward HOR. Using characterizations including XPS, UPS, H2 chemisorption, CO chemisorption, in situ surface-enhanced infrared adsorption spectroscopy (SEIRAS), and electrochemical characterizations, we concluded the superior HOR activity of Pt0.25Ru0.75/pN-C was a combination of interactions between Pt, Ru and the pN-C, with modulated the HBE and strengthened the H2O binding strength. The HEMFC using Pt0.25Ru0.75/pN-C also possessed great performance that far exceeded the U.S. Department of Energy (DOE) 2022 target for HEMFCs, even though the testing condition was harsher. This work demonstrated the feasibility of HEMFC replacing PEMFC.