Experimental Analysis of Catalyst Layer Operation in a High-Temperature Proton Exchange Membrane Fuel Cell by Electrochemical Impedance Spectroscopy

Abstract

High-temperature proton exchange membrane fuel cells (HT-PEMFC) directly convert hydrogen and oxygen to produce electric power at a temperature significantly higher than conventional low-temperature fuel cells. This achievement is due to the use of a phosphoric acid-doped polybenzimidazole membrane that can safely operate up to 200 °C. PBI-based HT-PEMFCs suffer severe performance limitations, despite the expectation that a higher operating temperature should positively impact both fuel cell efficiency and power density, e.g., improved ORR electrocatalyst activity or absence of liquid water flooding. These limitations must be overcome to comply with the requirements in mobility and stationary applications. In this work a systematic analysis of an HT-PEMFC is performed by means of electrochemical impedance spectroscopy (EIS), aiming to individuate the contributions of components, isolate physical phenomena, and understand the role of the operating conditions. The EIS analysis indicates that increases in both the charge transfer and mass transport impedances in the spectrum are negatively impacted by air humidification and consistently introduce a loss in performance. These findings suggest that water vapor reduces phosphoric acid density, which in turn leads to liquid flooding of the catalyst layers and increases the poisoning of the electrocatalyst by phosphoric acid anions, thus hindering performance.

Challenges and opportunities for characterisation of high-temperature polymer electrolyte membrane fuel cells: a review

“…There are studies that do report the inductive phenomena for HT-PEMFCs and solid acid fuel cells. 266,281,295 Within these studies, the loop is signicantly smaller than in the LT-PEMFC studies. Different conclusions may be drawn from this observation.…”

“…There were expected competing impacts of increased temperature on reaction kinetics and changes in humidication due to water vapour pressure changes. 266 Whereas, the effect of air stoichiometry was clear at 1.0 A cm −2 , decreasing the stoichiometry from 2.0 to 1.5 results in signicantly higher mass transport impedance due, and likely caused inhomogeneous current density distributions. The effect of air humidication is shown in Fig.…”

“…260 A range of 0.1 Hz to 100 kHz has been recommended specically for HT-PEMFCs. 264 The fuel cell should be held at the EIS measurement conditions of interest prior to recording the spectrum to enable stable measurements; values between 5 min and 30 min are reported for HT-PEMFCs, [264][265][266][267] and the necessary time may differ with operating conditions. The resulting spectra then only provides a snapshot of the impedance at this condition (e.g.…”

“…37 So far, the low-frequency inductive phenomena in HT-PEMFCs has been attributed to phosphate poisoning, although Pt oxide growth and relaxation may also contribute. 281,295 EIS has also been used to investigate the effects of different operating conditions on HT-PEMFC impedance, 266 as shown in Fig. 10.…”

“…92 This improvement is offset by increases in charge transfer and mass transport impedance, expected to be caused by increased phosphate anion adsorption and oxygen diffusion resistance, respectively. 266 In summary, EIS is a powerful tool for HT-PEMFC characterisation and modelling. Careful attention must be paid to the validity of the data, this is also true for LT-PEMFCs.…”