Understanding the Enhanced Methanol Oxidation Reactivity on Sulfur-Organic Compound Decorated Pt Electrode in Alkaline Media

Abstract

Due to the rapid increase in environmental and energy issues, it is required to replace fossil fuel-driven conventional internal combustion engines with an energy conversion system using sustainable fuels. Although fuel cell is one of the possible candidates due to its high efficiency, the storage and transportation of hydrogen fuel hinder its widespread use.1 To overcome this problem, hydrogen careers, hydrogen-rich liquid and solid-phase materials, have been proposed as alternative fuels for fuel cell systems. Among the hydrogen carriers, methanol is one of the promising candidates due to its low production cost and high energy density. However, the slow kinetics of the methanol oxidation reaction (MOR), which proceeds via a complex six-electron pathway, limits the overall efficiency of the direct methanol fuel cell (DMFC).2 Much effort has been put into improving the MOR activity, including the development of electrocatalyst by metal alloying and surface modification of the electrocatalyst by thiol derivatives.3 Here, we focused on using sulfur-organic compound copolymer synthesized from S8 and organic molecules as a surface modifier. We then explored the reaction mechanisms on Pt surface in the alkaline environment by tracking the surface-bound intermediates using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS). We selected styrene as an organic component and synthesized sulfur-organic compound copolymer (poly(s-st)). Synthesized poly(s-st) was decorated on the Pt surface by simply immersing the electrode into the poly(s-st)-contained solution. Cyclic voltammogram and X-ray photoelectron spectroscopy confirm the surface modification of the Pt surface by poly(s-st), which was stable after 5 cycles of potential cycling in 0.1 M KOH electrolyte. The oxidation current at 0.78 V vs. RHE, which is related to the MOR, was more significant than the pristine Pt electrode, which confirms the improvement of the MOR activity by poly(s-st) modification. The degree of activity enhancement became more substantial with the increase in alkyl chain length of alcohol fuels (methanol < propanol < 1-butanol), suggesting the reason of the enhanced activity to be the hydrophobic interaction between phenyl group of a surface modifier and alkyl portion of alcohol fuels which could facilitate the fuel supply on Pt electrode. In order to support our hypothesis, we performed SEIRAS measurement, where we observed a peak corresponds to the site-blocking species at ca. 1500 cm-1 for pristine Pt surface, while no corresponding peak was observed for poly(s-st) modified Pt surface. We will discuss the changes in the surface reaction mechanism based on the direct observation of the reaction intermediates by SEIRAS. It will also be demonstrated that the holistic information about the nature of the adsorbed species can provide a strategy to design the surface modifier to tune the energetics of key reaction intermediates, leading to further improvements in electrocatalytic activity for the methanol oxidation reaction.[1] Z-F. Li et al. Electrochim. Acta. 2017, 228, 351–360.[2] C. Lamy et al. J Appl. Electrochem. 2001, 31, 799–809.[3] H. Zimmermann et al. Chem. Eur. J. 2000, 4, 592–599.