Catalyst Development for the Electrochemical Oxidation of Isopropanol in LOHC Fuel Cells

Abstract

The transportation sector is certainly one of the largest contributors to energy consumption worldwide and its further growth is still expected. To keep up with this huge energy demand from the mobility, it is inevitable for the whole sector to undergo the transition towards renewable energy resources as soon as possible. There are several approaches that can be considered; as in fuel cell research, hydrogen fuel cells are the most studied, and current focus lies on optimization of different parts starting from the actual catalytic material towards the whole engineering design to increase their efficiency. The current infrastructure for liquid fuels cannot be used for hydrogen and its long-term storage is problematic due to safety concerns. An alternative could be the use of a liquid fuel possessing the benefits of hydrogen oxidation such as high selectivity and comparably fast oxidation, simultaneously tackling the mentioned disadvantages. The electrocatalytic oxidation of isopropanol on PtRu offers a viable solution due to its low overpotential and nearly 100 % selectivity towards acetone under fuel cell operation conditions.1 This opens the possibility of the zero-emission operation of the fuel cell where the formed acetone can be recycled to isopropanol when the fuel cell is coupled to a transfer hydrogenation unit. 1 Still, there are several challenges yet to be overcome : first, restricting only to the catalyst, PtRu-based materials seem to be the most promising due to the oxidation of isopropanol at low overpotentials attributed to Pt-Ru sites.1, 2 However, PtRu itself suffers from surface poisoning by acetone leading to a gradual loss of activity3. Therefore, to overcome these limitations there is an urgent need to develop new materials or explore modified electrocatalyst surfaces that could lead to improved direct isopropanol fuel cell (DIFC) efficiencies.In this work, we present our recent findings on catalyst development for the electrochemical oxidation of isopropanol. Firstly, the electrocatalytic activity towards isopropanol oxidation of PtRu-based, ternary alloys was studied where the Pt:Ru:X (X= Ir, Au, Ni, Cu, Co, Mo, Ti, Pd) ratio was fixed to 45:45:10 at%. Among these, PtRuIr proven to be the best-performing candidate. As a next step, the composition-dependent activity of PtRuIr was mapped by means of cyclic voltammetry using a magnetron sputtered thin-film library. All measurements were performed in a scanning flow cell in acidic conditions (0.1 M HClO4) both in the absence and presence of isopropanol (0.1 M). Besides the assessment of activity, the effect of cycling in a wider potential window on the electrocatalytic behavior was also studied. To see if the observed trends can be translated to systems that are closer to real applications, compositions showing the highest activity were synthesized in the form of carbon-supported nanoparticles. Rotating disk electrode and scanning-flow cell measurements confirmed the behavior experienced in the case of the thin-film library. Looking at cyclic voltammograms, the beneficial role of Ir can be clearly identified: in addition to the observed low onset potential of isopropanol oxidation (0.05 VRHE) for the PtRu catalysts, Ir seems to help keeping the current between two oxidation peaks (Pt-Ru and Pt-Pt sides) constant. In order to see the benefit of Ir in real application, these catalysts are planned to be further tested in DIFC. Khanipour, P.; et. al., ACS Applied Materials & Interfaces 2020, 12 (30), 33670-33678.Mangoufis-Giasin, I.; et. al., Journal of Catalysis 2021, 400, 166-172.Kormanyos, A.; et. al., Journal of Catalysis 2021, 396.