The Effects of Ethanol Dehydrogenation By-Products on a PEM Fuel Cell Performance: A Multi-Technique Analysis

Abstract

The catalytic dehydrogenation of ethanol on a Cu / ZrO2 catalyst at 250 ° C has been recently proposed as a suitable approach for the production of CO-free hydrogen[1]. In this way, the hydrogen-rich gas stream generated in this system had been directed to the anode of a PEM fuel cell, that presented 70% of the efficiency obtained when pure hydrogen was used. The reason for this considerable loss in performance was not clear and this is the main motivation of this work. For this investigation, we have used a system in which hydrogen was contaminated with the by-products of the ethanol dehydrogenation reaction, which were unreacted ethanol, acetaldehyde and ethyl acetate. The influence of these last substances on the performance of the PEMFC was studied through various techniques, including electrochemical impedance spectroscopy, cyclic voltammetry, linear sweep voltammetry, cell polarization response, high performance liquid chromatography (HPLC) and mass spectrometry. The results had confirmed that the impurities coming from the ethanol dehydrogenation reactor are the responsible for the loss of the fuel cell efficiency. Polarization curves showed that the presence of unreacted ethanol in the system is the main cause of this effect, wherein the acetaldehyde and ethyl acetate have smaller contribution. Studies involving online mass spectrometry indicated that there is no formation of CO or CO2 as by-products, as detected in the anode outlet of the cell. Analysis of the liquid effluent of the anode by HPLC, at several cell potentials ranging from 900 to 100 mV, revealed the presence of acetic acid as oxidation product on the PEM fuel cell electrode, with contents particularly high at the higher cell potentials (closer to 900mV). These studies showed that ethanol is the major contaminant of the electrode, either when directly administrated to the cell, or when it is formed by degradation of other contaminants at the PEMFC anode. Electrochemical impedance spectroscopy was utilized to evaluate the influence of the above-mentioned contaminants on the oxygen reduction reaction, since occurrence of their crossover from the anode to the cathode of the MEA was detected. The influence of each contaminant in the PEMFC performance was analysed in operando at low overpotentials (low currents - 72 mA to 500 mA). It can be inferred that at high potentials, the losses in the PEMFC performance is due to the poisoning of the oxygen electrode, when both anode and cathode catalyst were Pt/C 30 wt.%. From these data, it is shown that the proton transport resistance (membrane and cathode catalyst layer) and electrical contacts are not affected by the contaminants at these potentials. Studies on the increase of the Pt load on the anode, as well on the replacement of the Pt/C catalyst on the cathode by bimetallic Pt-based catalysts, such as PtCo and PtCr, proved ways to minimize the effects of the contaminants on the oxygen cathode. [1] A. G. Sato, G. C. D., Silva, V. A. Paganin, E. A. Ticianelli. ECS Transactions, 64 (2014)999-1005.