UCLA

Oxygen reduction reaction (ORR) and hydrogen evolution (HER) are two critical electro-chemical reactions for proton exchange membrane fuel cell (PEMFC) applications. However, both ORR and HER need catalysts to overcome their respective kinetic barrier, and noble metal platinum has been proved to be the most active element to catalyze both reactions. Due to the extreme sacristy and high price of Pt, promoting the Pt mass activity (activity per given Pt mass) present the key challenge for electro-catalyst design. Improving the Pt mass activity should optimize both the specific activity and the electrochemical active surface area (ECSA) simultaneously. In first part of my dissertation, we show that solution-synthesized Pt/NiO core/shell nanowires can be readily converted into PtNi alloy nanowires through a post-synthesis thermal annealing process, and then transformed into jagged Pt nanowires (J-PtNWs) via an electrochemical dealloying. The jagged nanowires exhibit an ECSA of 118 meter square per gram Pt and a specific activity of 11.5 milliamperes per square centimeter for ORR (at 0.9 Volt versus the reversible hydrogen electrode) for a mass activity of 13.6 ampere per milligram Pt, or nearly doubles previously reported best values. Reactive molecular dynamics simulations suggest that highly stressed, undercoordinated rhombahedral-rich surface configurations of the jagged nanowires enhanced ORR activity versus more relaxed surfaces. In second part of my dissertation, we developed a controlled electro-chemical approach to modify jagged platinum nanowire with nickel hydroxide species [J-PtNWs/Ni(OH)2]. The result materials feature rich surface defects and locally decorated Ni(OH)2 species as bifunctional catalysts for highly efficient electro-catalytic water splitting. Electrocatalytic studies show that J-PtNWs/Ni(OH)2 exhibits extraordinary activity for hydrogen evolution reactions (HER) with a record high mass activity of 11.8 A/mgPt at -70 mV versus reversible hydrogen electrode (RHE) at pH 14, which is 17 times higher than that of Pt/C catalyst and 9 times higher than best HER performance reported to date. Density functional theory calculations demonstrate that defective surface offers an ensemble of highly active sites, and that surface Ni(OH)2 species further tune the hydrogen binding energy towards optimal value. Moreover, we further show such surface modification could also greatly enhance the catalytic activity towards the oxygen evolution reaction (OER) and enable a bifunctional catalyst for highly efficient water splitting with a mass activity of 0.6 A/mgPt at 1.6 V vs. RHE.