Design and Application of External Reference Electrode for Kinetic Studies at Elevated Temperatures and Pressures

Abstract

We present the design and application of an external reference electrode (RE) system for kinetic studies of the oxygen reduction reaction (ORR), the cathode process in high temperature proton exchange membrane fuel cells (HT-PEMFCs), at elevated temperatures and pressures inside an autoclave. The RE system comprises a solid ion-conducting junction based on Nafion tubing. External RE was a reversible hydrogen electrode (RHE) calibrated reproducibly relative to an internal RHE with an uncertainty <±5 mV over the temperature range of 25-170 • C. Functionality of the RE system was tested by measuring ORR in concentrated H 3 PO 4 relative to a calibrated external RHE. Thus, we are confident that the development is a useful and versatile tool for kinetic studies at elevated temperatures and pressures. Fast and reliable testing of fuel cell catalysts at relevant conditions is of high importance for academia and industry alike. Recently, we introduced an elevated pressure and temperature rotating disk electrode (RDE) system that allows electrocatalytic measurements under well-defined mass transport conditions 1,2 . The setup comprised an internal silver wire RE that enabled a stable determination of the reference potential, but the obtained calibration relative to an internal RHE lacked the required accuracy (< ± 5 mV) for accurate kinetic measurements. We therefore avoided in our previous work the determination of absolute kinetic rates of the ORR at elevated temperature and pressure. An approach to more reliable reference potentials in pressurized cells is to use external RE configurations.3 Recently, external RE systems for fuel cells with solid ion-conducting junctions based on Nafion tubing or the PBI/H 3 PO 4 polymer were published. 4,5Here we present the design and test of an external RE system with solid junction specifically developed for pressurized electrochemical cells. The most meaningful RE to study ORR with respect to fuel cells is the RHE, which is a hydrogen electrode immersed in the same electrolyte and at 1 bar H 2 partial pressure. 6 Thus, we applied an external RHE and calibrated it separately relative to an internal RHE, as its determination in situ 7,8 is not practical in pressurized cells. Application of the calibrated external RE was tested for studying ORR in conc. H 3 PO 4 at elevated temperatures. Such conditions are highly relevant for high temperature proton exchange membrane fuel cells (HT-PEMFC), but usually not accessible to RDE studies due to the low oxygen solubility in the electrolyte. The results demonstrate that our development enables meaningful kinetic studies in pressurized cells and thereby overcomes the significant accuracy issues related to internal REs operated at elevated temperatures. ExperimentalThe electrolyte used in this study was 85 wt% H 3 PO 4 (Suprapur, Merck). The gases used were supplied from gas cylinders (Air Liquide, Denmark) and had a purity of at least 99.995%. The electrochemical cell was placed inside an autoclave as described previously.

Reversible Hydrogen and Pd Hydride Reference Electrodes with Electrochemically Supplied H2 for High Temperature and Pressure Electrochemistry

“…[16][17][18][19] In an external RE configuration, the RE is held in a separate compartment at room temperature and connected to the HTP electrochemical cell by means of a salt bridge. [20][21][22] The major challenges faced by the external REs are: (1) to maintain the pressure of the RE compartment in equilibrium with that of the HTP reactor and (2) to correct the measured potentials for thermal junction potentials, the calculation of which requires the knowledge of the heat transferred between the HTP reactor and the RE. 15 Thus, external REs are useful in cases where a moderate accuracy is acceptable.…”