Alkaline Water-Assisted Polyol Synthesis of Pt/Vulcan PEMFC Cathode Electrocatalysts

Abstract

Sluggish oxygen reduction reaction (ORR) kinetics motivates great focus on PEMFC cathode electrocatalyst development.1 Electrocatalysts based on Pt are often prepared via the so-called polyol process, a solvothermal process that allows controlling of the structure, morphology, and chemical nature of various metal nanoparticles.2 Several parameters have been well-studied and thoroughly discussed in literature over the years, varying from surfactant removal or water addition to pH or NaOH/Pt molar ratio effect on Pt particle size. 3,4 Here, we present a slight modification of the well-established surfactant-free alkaline one-pot polyol synthesis of Pt-nanoparticles on Vulcan XC 72R carbon support and the subsequent PEMFC characterization tests with respect to initial electrochemical performance and standardized accelerated stress tests (AST). In this approach, the solvent composition is systematically changed by the substitution of ethylene glycol to water and by increasing the Pt precursor concentration per mass of solvent to ultimately minimize the amount of ethylene glycol used.The so-synthesized electrocatalyst powders, with Pt particle sizes ranging from 2 to 6 nm, were physico-chemically analyzed by powder X-ray diffraction, thermogravimetric analysis, and transmission/scanning electron microscopy. The initial electrochemical performance as well as the stability were evaluated by implementing membrane electrode assemblies (MEAs), prepared from a series of synthesized electrocatalysts in a differential flow single cell setup (12 cm2). The electrochemical performance of the prepared MEAs was evaluated and compared to a commercial reference catalyst coated membrane (CCM), as well as to an internal reference CCM, that was prepared using a cathode Pt/C catalyst synthesized via pure ethylene glycol-based polyol route.Beginning-of-life MEA data indicate the possibility of reducing the required ethylene glycol amount per mass of synthesized catalyst by up to 98%. Taking catalyst durability as an additional optimization criterion into account, a Pt/C electrocatalyst synthesized with 45 vol% H2O at a Pt precursor concentration of 32 mM, ultimately leading to an ethylene glycol reduction of 90%, showed a decent initial electrochemical performance (716 mV at 1 A cm-2), as well as an AST stability similar to the internal reference (Pt/C synthesized using pure EG and Pt precursor concentration of 16 mM).In summary, we conclude that water content and Pt precursor concentration have a crucial impact on the characteristics of Pt/C electrocatalysts in the modified polyol route. At optimum parameter values, catalysts with optimized electrochemical performance in PEMFC applications can be synthesized, with substantially lowered synthesis cost due to solvent substitution.