UCLA

Carbon neutrality has been the most popular topic of the twenty-first century. Substitutingnon-sustainable fossil fuels with the cleaner energy source hydrogen is a viable strategy for reducing total carbon footprints, but the conventional method of hydrogen production is energy-intensive and polluting. Water electrolysis stands out among all hydrogen production methods due to its low instrumentation requirements and high efficiency. However, water electrolysis costs have yet to be reduced. By engineering the catalysts used at the cathode and anode, the focus of this thesis will be to improve the water electrolysis efficiency and material durability. In addition, corresponding theory is studied in order to reveal the structureperformance relationship of the catalyst, which provides perspective and theoretical support for the design of future catalysts.

In the first chapter, I will briefly describe the current status of global carbon production.The rationale for selecting water electrolysis is then presented, along with an overview of water electrolysis devices.

In the second chapter, I will describe our work (J. Am. Chem. Soc. 2018, 140, 29, 9046–9050) on improving the performance of the hydrogen evolution reaction (HER) by applyingsurface engineering to PtNi alloy. Hydrogen holds the potential of replacing nonrenewable fossil fuel. Improving the efficiency of hydrogen evolution reaction (HER) is critical for environmental friendly hydrogen generation through electrochemical or photoelectrochemical water splitting. Here we report the surface-engineered PtNi-O nanoparticles with enriched NiO/PtNi interface on surface. Notably, PtNi-O/C showed a mass activity of 7.23 mA/μg at an overpotential of 70 mV, which is 7.9 times higher compared to that of the commercial Pt/C, representing the highest reported mass activity for HER in alkaline conditions. The HER overpotential can be lowered to 39.8 mV at 10 mA/cm2 when platinum loading was only 5.1 μgPt/cm2, showing exceptional HER efficiency. The performance improvement could be attributed to the successful creation of Ni(OH)2/Pt(111) interface. Ni(OH)2 facilitated H2O molecule to be adsorbed on the surface as the first step of HER, and then recombination on the Pt(111) surface happened. Thus, the overall potential needed was decreased. Meanwhile, the prepared PtNi-O/C nanostructures demonstrated significantly improved stability as well as high current performance which are well over those of the commercial Pt/C and demonstrated capability of scaled hydrogen generation.

In the third chapter, I will demonstrate continuation of the last work, which is furtherimproving the alkaline HER performance on Pt-based alloy. Lattice tuning is one of the effective ways to optimize the HER performance on Pt-based alloy. Here, we report a facile lattice tuning method on Pt-based alloy using Cu addition to control the lattice parameter for optimal HER performacne. During the performance evaluation, PtCuNi/C and PtCuNi- O/C showed an average overpotential of 38.8 mV and 31.3 mV at 10 mA/cm2, respectively. The overpotential of PtCuNi-O/C is dramatically smaller than that of commercial Pt/C (115.2 mV). At an overpotential of 70 mV (-70 mV vs. RHE), the octahedral PtCuNi/C presents a mass activity (MA) of 4.9 mA/μgPt, while the PtCuNi-O/C demonstrates a MA of 8.7 mA/μgPt, which is nearly 9.5 folds to that of the commercial Pt/C (0.92 mA/μgPt). Also, the PtCuNi-O/C can reach a current density of 114 mA/cm2 at -0.2 V vs. RHE without iR compensation, which is well above that of Pt/C (22.4 mA/cm2), indicating a promising potential for the industrial scale hydrogen production. For the stability test, in contrast to the 160.1 mV potential drop for the Pt/C, there was only 55.5 mV, 48.2 mV potential drop for octahedral PtCuNi/C, PtCuNi-O/C, correspondingly, showing a significantly improved durability. Moreover, the dealloyed nanocatalysts showed the best performance when the lattice parameter is in the range of 0.3825-0.3835 nm for both PtNi-O/C and PtCuNi-O/C. In the fourth chapter, the main focus will be on the anode side featuring oxygen evolution reaction (OER) in acidic media. Developing durable non-precious catalysts for the acidic OER is crucial for the hydrogen production industry. In this regard, we report a facile strategy to synthesize the cobalt-based spinel oxide for the acidic OER with ultrahigh activity and outstanding durability. Specifically, the as-prepared NiCo2O4 delivered a low overpotential of 407 mV vs. reversible hydrogen electrode at 100 mA/cm2 and only a 68.9 mV increase at 10 mA/cm2 after 20 hours of chronopotentiometry test. Ex situ x-ray absorption spectroscopy studies revealed that Ni mainly occupies the octahedral site. In situ x-ray absorption spectroscopy studies demonstrated that adding Ni helped minimize the structure change during the OER, leading to NiCo2O4’s outstanding durability. Density functional theory calculations demonstrated that the OER overpotential is lowered by 90 mV on the NiCo2O4 surface compared to that of Co3O4. The acidic OER on the NiCo2O4 spinel structure undergoes a kinetically more favorable direct O-O coupling mechanism rather than the adsorbate evolution mechanism, which was seldom reported regarding acidic OER on non-precious metal oxides. We showcase an ideal way to produce cobalt-based spinel oxides following direct O-O coupling mechanism design rules, achieving promising acidic OER performance cost-effectively.

The last chapter generally conclude the content covered in the thesis and provided someperspective on future catalyst design and scale-up applications.