(Invited) Electrocatalysis of Direct Methanol Fuel Cells with a Special Reference to the Work of Shimshon Gottesfeld
Authors: Adzic Radoslav R. 
Published: February 2020

Fuel cells seem firmly advancing towards a broad application in transportation and other areas to take their role of a future clean energy source. It took more than 50 years that to address most of the challenges of fuel cell electrocatalysis in several research cycles. In late sixties electrode kinetics developed diagnostic criteria that allowed in some cases to determine molecular reaction mechanism based on the current – potential relationships. This inspired a considerable fuel cell oriented applied research that culminated in Troisieme Journees Internationales d’Etude des Piles a Combustible, in Bruxelles, 1968. This big meeting did not bring expected advances and the fuel cell research diminished.

The interest for direct methanol fuel cell regained momentum slowly through the research directed towards understanding of the role of surface structure and identifying surface intermediates. Advances came from of ex situ structural studies and in particular in situ UV-visible and IR reflectance spectroscopies. In situ FTIR opened new level of details in methanol oxidation. In parallel, the work with well-ordered single crystal surfaces clearly showed inter alia pronounced structural effects in oxidation of methanol on Pt. A broad research effort brought clarifications of reaction mechanism on Pt, poisoning effect of CO intermediate, development of bimetallic electrodes and bifunctional PtRu mechanism. After several reflectance spectroscopy studies Shimshon focused on fuel cell electrocatalysis. Most of the breakthrough in directed basic and applied work on direct methanol fuel cell came from Gottesfeld’s group at Los Alamos. Some of these include: i) Developing new structures for the Pt/C catalyst layer that substantially increase the utilization efficiency of the catalyst having hydrophilicity, thinness, uniformity, and the proper ratio of ionomer and supported catalyst. iii) The catalyst layers are cast from solution as thin films with ionomer itself as a binder and hot pressed directly onto the ionomer membranes, and the hydrophobic gas diffusion backings complete the assembly. iv) Activity of dispersed PtRu catalysts was found to depend on catalyst surface area and a number of metal alloy sites of atomic ratio close to 1:1.v) Patent on achieving CO tolerance using air bleed at the anode inlet was obtained. vi) Modeling studies showed great advantage of a thinner membrane in alleviating resistance problem

Recent work brought some breakthroughs very promising for application of DMFC. A consensus on the mechanism of methanol oxidation on a pure Pt electrode includes a parallel pathway mechanism forming a number of reaction intermediates and products (CO2, HCOOH, HCHO); among them adsorbed CO can block Pt sites. Alloying Pt with other oxophilic metals, including Ru, Os, Ir, Rh, and Sn, was broadly studied shown to reduce CO poisoning and increase the catalytic activity. This activity of the Pt alloys can be explained either by the bifunctional model or the electronic effect (also called ligand effect).

Most recent breakthrough in this research involve finding that under tensile strain one Pt monolayer (one atom thick layer) placed on Au(111)) exhibits a factor of 7 activity increase and a new reaction mechanism without CO generation, which can eliminate a major drawback of this reaction. Very important is the potentiality of finetuning the catalytic property of Pt monolayer to fabricate electrocatalysts with enhanced activity and ultralow Pt content. This opened up a broad studies of promising core/shell nanoparticle catalysts that will be discussed at the meeting.