Abstract
Hydrogen has a central role in future energy storage scenarios. In order to realise high market penetrations for proton exchange membrane water electrolyzers (PEMWE), the investment costs need to be reduced without compromising operational costs [1, 2, 3]. Today only the usage of precious platinum group metals (PGM) as electrocatalysts allows a good electrochemical polarisation behaviour and accordingly minimal operating costs. Platinum is known to be an outstanding catalyst for both hydrogen oxidation and evolution [1, 2, 3, 4]. However, platinum also catalyses the formation of reactive oxygen species when oxygen permeates from anode to cathode. The intermediates of the recombination reaction are sufficiently reactive to attack the polymeric backbone of the electrolyte membrane which leads to a performance loss due to material degradation and eventually to a cell failure [1, 5, 6].For this reason, not only the polarisation behaviour but also the hydrogen-oxygen recombination characteristics need to be evaluated in order to find new cathode catalysts for PEMWE. In this contribution, the polarisation behaviour of five different cathode catalysts was investigated. The ordinary electrochemical characterisation (polarisation curves, electrochemical impedance spectroscopy and linear sweep voltammetry) was extended by measuring the permeated oxygen in the cathodic product gas to draw conclusions on the recombination characteristics of the used catalysts.In general, the studied PGM-free catalysts showed lower activities for the oxygen reduction reaction and higher oxygen in hydrogen volume fractions compared to the PGM catalysts. However, all investigated electrocatalysts still show an activity towards the recombination reaction. Ex situ linear sweep voltammetry measurements have shown that the activity for the recombination reaction of oxygen and hydrogen correlates with the activity for the oxygen reduction reaction. In particular, a commercial PGM-free cathode catalyst based on a Ti suboxide showed a comparably good polarisation behaviour for the hydrogen evolution reaction similar to platinum and the lowest activity for the oxygen reduction reaction at the same time. Further, also the influence of the used PTL material on the measurable oxygen crossover was investigated. The results show that also carbon based PTL materials contain catalytically active sites for the recombination of permeated oxygen with hydrogen.Literature:[1] U. Babic, J. Elec. Soc., 2017, 164.[2] K. Ayers, Annu. Rev. Chem. Biomol. Eng., 2019, 10.[3] M. Carmo, Int. J. Hydrogen Energ., 2013, 38.[4] C. V. Pham, ChemElectroChem, 2018, 5.[5] M. Chandesris, Int. J. Hydrogen Energ., 2015, 3.[6] S.A. Grigoriev, Int. J. Hydrogen Energ., 2014, 35. Figure 1
Highlights
From PEM fuel cells it is known that the permeation of hydrogen gas from the anode to the cathode leads to the formation of reactive oxygen species, such as hydroxyl radicals, on the platinum particles.[11,12,13] In PEM water electrolysis, similar problems occur
The evaluation of cathode catalysts is possible by measuring their polarisation behaviour regarding the hydrogen evolution reaction (HER) and their recombination properties, by means of their activity towards the oxygen reduction reaction (ORR) and the measurable oxygen in hydrogen content
Structural characterisation of the porous transport electrodes.—The surfaces of the PTEs were assessed by scanning electron microscopy (SEM), see Fig. 2
Summary
From PEM fuel cells it is known that the permeation of hydrogen gas from the anode to the cathode leads to the formation of reactive oxygen species, such as hydroxyl radicals, on the platinum particles.[11,12,13] In PEM water electrolysis, similar problems occur. From the increasing differences between the extrapolated fits to higher current densities (dashed lines in Fig. 5) and the data sets, other losses such as mass transport resistances and ohmic losses within the catalyst layer can be read.[20,42] In the state of the art cell setup with IrO2 as anodic catalyst and Pt/C as cathodic catalyst, it is assumed that ηact is dominated by the slow oxygen evolution kinetics, whereas the cathodic voltage losses are assumed to be negligible.
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