Combining theory and experiment in Pt-based catalysts design for energy conversion

Author(s): Huang, Jin | Advisor(s): Huang, Yu | Abstract: Energy crisis and climate change are the imminent challenges faced by mankind that demand instant solutions in replacing fossil fuels with alternative clean energy sources. To meet this demand, the hydrogen fuel cell industry has witnessed tremendous growth within the past decade. However, the broad dissemination of proton-exchange membrane fuel cells (PEMFCs) is still limited by the high cost originated from the high loading of platinum-group metals (PGM) based catalysts to accelerate the sluggish oxygen reduction reaction (ORR) at the cathode. Therefore, it is central to design high-performance ORR catalysts and validate their performance in the membrane electrode assembly (MEA). In the first chapter of my dissertation, by combining theoretical modeling and experimental observations, we developed a binary experimental descriptor (BED) that directly correlates with the calculated oxygen binding energy ∆E O on Pt-alloy catalyst surface. The BED captures both the strain and Pt-transition metal coupling contributions based on experimental parameters extracted from X-ray absorption spectroscopy. This leads to an experimentally validated Sabatier plot wherein the BED can be used to predict not only the catalytic activity but also the stability of a wide range of Pt-alloy ORR catalysts. Based on the BED, we further designed an ORR catalyst wherein high activity and stability are simultaneously achieved. The second chapter is an extension of the first chapter, in which I demonstrated that tetrahedral PtCuNi catalysts, as an efficient multifunctional catalyst, did not only showed excellent ORR performance but also exhibited high methanol/ethanol oxidation reaction (MOR/EOR) performance, which can be potentially used in the direct methanol/ethanol fuel cells (DMFCs/DEFCs). By tailoring the surface composition, the optimal catalyst with a composition of Pt56Cu28Ni16 showed a MOR and EOR specific activity (SA) of 14.0  1.0 mA/cm2 and 11.2  1.0 mA/cm2, respectively; and mass activity (MA) of 7.0  0.5A/mgPt and 5.6  0.6 A/mgPt for the MOR and EOR, respectively. In the third chapter, I applied some highly promising ORR catalysts in MEA. In specific, I developed an ultralow Pt loading (total loading of 0.072 mgPt/cm2) and high-performance MEA using ultrathin platinum-cobalt nanowires (PtCoNWs) as cathode catalysts. The PtCoNWs showed a high ECSA of 73.2 m2/gPt and achieved an unprecedented MA of 1.06 � 0.14 A/mgPt [0.9 ViR-free] at the beginning of life (BOL) stage in MEA. This MA is 3.3 times that of the commercial Pt/C (0.32 A/mgPt) and far surpasses the Department of Energy (DOE) 2020 target (0.44 A/mgPGM). The PtCoNWs reached a peak power density of 1016 mW/cm2, outperforming the PtNWs (830 mW/cm2) and Pt/C (773 mW/cm2) with comparable Pt loading. After the AST, the PtCoNWs showed a respectable end of life (EOL) MA of 0.45 A/mgPt, remaining above the DOE 2020 BOL target. In the last chapter, I tried to tailor the interfacial properties to further enhance the surface microkinetic. In brief, I developed a facile and controllable molecular surface modification approach using dimethylformamide (DMF) to successfully improve the ORR performance of Pt-based catalysts. Significantly, our molecular dynamics (MD) simulations elucidated that DMF can disrupt interfacial water hydrogen-bonding networks, therefore allowing accelerated water exchange kinetics, facile O2 transport towards Pt surface, increased interfacial oxygen concentration, and adsorption time (around twice compared to pure Pt(111) surface), justifying enhanced ORR activity. We further applied this approach to a model Pt-alloy catalysts (PtCuNi), which achieved an unprecedented SA of 21.8 � 2.1 mA/cm2 at 0.9 V versus the reversible hydrogen electrode (RHE), about 2.65 times improvement comparing to original PtCuNi catalysts, and nearly double previously reported the best value, leading to an ultrahigh MA of 10.7 � 1.1 A/mgPt. Importantly, after 20,000 cycles of accelerated degradation tests (ADT), surface-modified PtCuNi showed even better SA and MA than the initial performance of original PtCuNi, suggesting the surface modification can also considerably extend the lifetime of the catalyst.