Mohammad Gholami,
Majid Sedighi - Northern Research Center for Science & Technology, Malek Ashtar University of Technology, Iran
Vahid Kord Firouzjaei

Measurement of an electrochemical active surface area (ECSA) of stacks is not possible by directly using of potentiodynamic methods such as cyclic voltammetry. In-stack electrode potentiometry is another electrochemical technique that can be used to assess the electrochemically active surface areas of the electrodes of a PEMFC stack. In this work we calibrate the potentiometry against voltammetry by calculating ECSA of a single cell PEMFC.Quantifying of an electrochemical surface area of fuel cell's electrodes creates a useful parameter for monitoring of performance, degradation, and state of health of polymer electrolyte membrane fuel cells (PEMFCs). However, conventional potentiodynamic methods for measuring ECSA require potentiostatic control of the cell, which is not feasible in the fuel cell stack. In-stack electrode potentiometry which requires galvanostatic control of the cell is an alternative to measure the electrochemically active surface areas of the electrodes of a PEMFC stack [۱]. Before routinely using the potentiometry method to determine ECSAs in the stack, it should be calibrated against in-cell voltammetry, i.e., find the constant current that will give the same ECSAs as those resulting from voltammetry [۱]. In this work, we calibrate potentiometry against voltammetry by calculating ECSA of a single cell PEMFC.The single cell temperature was maintained at ۲۵ °C. Under fully humidified hydrogen (۰.۳ slpm, anode) and nitrogen (۰.۳ slpm, cathode), CV was performed between ۰.۰۸ and ۱.۲V at scan rates of ۱۰۰ mV/s. Then, galvanostatic measurements were performed at constant currents between ۰.۱-۱ A with initial and final cutoff voltages of ۰.۰۸V and ۱.۲V, respectively. For the potentiometry measurement, the stack is first fed with air on the studied side and H۲ on the counter side under open circuit. Subsequently, the air is switched to N۲ on the studied side and the constant current is applied to the cell.The obtained voltammogram is depicted in Figure ۱. By integrating the shaded areas in Figure ۱, the hydrogen adsorption charge can be extracted from voltammogram. Then, the electrode’s ECSA can be determined from the calculated hydrogen adsorption charge. The calculated hydrogen adsorption charges and ECSA values of the electrode from the CV measurements are ۱۷.۸۷ C and ۴۲.۵۴ m۲/g, respectively.For potentiometry measurements, a constant current is applied to the cell and voltage response is recorded. The time interval corresponding to the change in cell voltage from ۰.۴ to ۰.۱ V is measured. The product of the current and time is taken as total charges that include hydrogen adsorption charges, the charge for double-layer charging, and the charge for oxidation of the crossovered hydrogen. For applying anodic current, it's possible to calculate the charge for double-layer charging and the charge for oxidation of the crossovered hydrogen but for cathodic current it's not possible [۲]. For cathodic current, it's recommended to perform potentiometry with different constant current to find a current that gives the same ECSA as that of CV. As it can be seen from figure ۲, potentiometry with a constant current near ۰.۴ A has less difference with that of the CV. In this study, we measure ECSA of single cell PEMFC by CV and potentiometry. Based on our result,performing potentiometry with constant current of ۰.۴ A give the same results as CV

Single cell PEMFC, Electrochemical active surface area, Potentiometry