UCLA

Environmental issues have attracted more and more public attention. Fuel cell, which is an energy conversion device, consumes renewable fuel (hydrogen, methanol etc.) and oxygen to produce electricity. It holds broad application potential for future automobile vehicle and portable device. Electrochemistry catalysis plays a key role in fuel cell operation as all energy conversion reactions are based on electrochemistry catalysis.

In present hydrogen based proton exchange membrane fuel cell, cathode oxygen reduction reaction (ORR) demands significant larger amount of catalyst than hydrogen oxidation reaction (HOR) at anode because the cathode ORR is six orders magnitude slower than anode HOR. Now days, Platinum is used as catalyst for ORR, the scarcity and precious feature of platinum contributes to the high cost of fuel cell system which is the biggest obstacle for fuel cell broad application. Non-platinum catalyst is under developing but far away from practical requirement. Thus, intensive research is focused on developing new Pt based catalyst with lower Pt loading, higher activity and longer life time. Inspired by ORR study on Pt75Ni25(111) single crystal electrode, my work is focused on developing octahedral PtNi, PtNiCo, and PtNiCu nano structures with exposed {111} facet. I developed a simple method for direct growth of PtNi octahedral nanostructure on carbon support. The advantage of prepared octahedral PtNi catalysts is the good dispersity on carbon support as well as bulk surfactant free surface. In ORR test, the prepared octahedral PtNi/C catalyst demonstrates at least 7.9 times mass activity (ORR activity normalized by Pt mass loading) compared to commercial Pt/C catalyst. The method can also be extended for synthesis of PtNiCo and PtNiCu ternary alloy catalyst. By introducing Co2CO8 as precursor, which can release metallic cobalt during decomposition, cobalt co-reducing challenge during PtNi synthesis can be overcome. Thus, I developed a method for direct growth of PtNiCo ternary alloy catalyst with uniform elemental distribution on carbon support. The ORR test result demonstrates that ORR activity can be optimized by composition tuning for PtNiCo ternary catalyst. It is noted that alloying Pt with transition metal can improve the ORR activity of Pt. The transition metal retention is important for catalyst stability because transition metal content is critical for ORR activity. I find with the present of Cu, octahedral PtNiCu ternary nanostructure can retain more transition metal than octahedral PtNi, leading to the improved activity after catalyst activation and better stability. The PtNiCu shows at least 13.2 times mass activity compared to commercial Pt/C. Furthermore, octahedral PtNiCu shows significantly improved stability (mass activity retention) compared to commercial Pt/C.

In addition to ORR catalysis, hydrogen evolution reaction (HER) is also important for hydrogen fuel cell broad application. HER is a potentially carbon dioxide emission free route for hydrogen mass production, holds the environmental advantage especially compared to current main industrial hydrogen production routes using natural gas as source. Thus, HER also attracts broad research interests as an electrochemistry catalysis. In my work, octahedral PtNiCu shows significantly improved both activity and stability compared to octahedral PtNi as well as commercial Pt/C for HER catalysis in alkaline electrolyte. The phenomenon can be explained as the addition Cu modified surface d-band structure thus optimized HO binding on catalyst surface, which is an important reaction intermediate for HER in alkaline media.

Direct methanol fuel cell holds advantage for powering future portable device due to its high theoretical potential and energy density. However, it is currently limited by the slow kinetics of the anode methanol oxidation reaction (MOR), which is also an electrochemistry catalysis. In my work, stable palladium hydride nanomaterials is developed, which showed large Pd-Pd distance compared to palladium alone. The increased Pd-Pd distance can weaken the binding for carbon monoxide molecules on surface Pd atoms, which is predicted by previous theoretical study and demonstrated experimentally in this work. As a result, palladium hydride nanomaterials show better MOR activity than palladium nanomaterials with same morphology. It is also first time the catalytic characteristic of palladium hydride nanomaterials is reported.