Pt-based electrocatalysts for energy conversion reactions: an electrochemical <em>In situ</em> X-ray absorption spectroscopy approach in alkaline medium

Abstract

It is well established that energy use via fossil fuels can be related to global warming; this renders higher atmospheric carbon dioxide levels. The migration to renewable energy technologies has proven to be an alternative to mitigate the contamination associated with fossil fuels. However, an overpotential is associated with the energy conversion reactions in renewable technologies (e.g., batteries and fuel cells (FC)). This overpotential can be minimized by incorporating effective electrocatalysts considering cost, abundance, and durability. The Oxygen reduction reaction (ORR) plays a crucial role in FCs; it has been demonstrated that at a low pH (Proton exchange membrane fuel cells (PEMFCs)), the ORR has sluggish kinetics. However, higher pH levels have been demonstrated to enhance the ORR (Alkaline exchange membrane fuel cells (AEMFCs))&mdash;opening the opportunity to explore different electrocatalysts as low-loading Platinum group metals (PGM) alloyed with first transition metals, metal oxides and non-precious metals.<br /> <br /> In this thesis, we presented the synthesis, characterization, and performance measurements of PGM electrocatalysts alloyed to first-row transition metals for the electroreduction of oxygen in an alkaline medium. A new approach was taken to synthesizing PtNi-Nanowires (NWs) supported on Vulcan XC-72R (V) by hydrothermal synthesis and spontaneous galvanic displacement (SGD). The rotating disk-slurry electrodeposition (RoDSE) technique was used for the first time to synthesize Ni/V, Co/V and Cu/V nanoparticles and by the SGD PtNi/V, PtCo/ V and PtCu/V nanoparticles were obtained. The catalysts were submitted to durability experiments showing similar behavior to Pt/C commercial catalyst after 10,000 and 30,000 cycles. <em>In situ</em> electrochemical X-ray absorption spectroscopy (XAS) was used to test these nanowires and nanoparticles, confirming their testing capability in an alkaline medium for the ORR and providing evidence for alloy interaction.<br /> <br /> The Pb electroless (e-less) deposition method was explored with Au, Nb and Cu catalysts as monocrystals and nanoparticles. The PdCu (100) catalysts were obtained by the Pb e-less method. The Pd monolayer (ML) formation on Cu (100) was proven by charge density calculations on the Pb underpotential deposition (UPD) region. The PdCu (100) were submitted to ORR testing and showed better performance than Pd (hkl) electrocatalyst in an alkaline medium. These results suggest that the synthesis of low PGM loadings electrocatalyst can drive the ORR in an alkaline medium with enhanced performance compared to commercial Pt/C catalysts.