For vehicles using polymer electrolyte fuel cells (PEFCs) to be commercially competitive with conventional internal combustion engine vehicles, three major challenges must be resolved, i.e., cost, performance and durability (1). The durability of a PEFC system can be largely affected by hydrogen starvation. During the startup/shut down process (2) of the fuel cell system, or when the flow channel is blocked by water or ice, local hydrogen starvation arises within one or many of the cells in the fuel cell stack. When this happens, hydrogen oxidation reaction (HOR) on the anode side cannot support the current imposed by the other cells. In that case, the anode half-cell potential increases relative to the cathode potential, and the cell voltage reverses. The high anode half-cell potential during cell reversal leads to water electrolysis and carbon corrosion reaction, the latter of which causes damage to the carbon support in the catalyst layer and leads to severe cell performance degradation. Hydrogen starvation in normal operating condition has been extensively investigated (3-5). However, little research has been conducted on cell reversal at sub-zero temperatures.

In past work on studying cell reversal, the anode potential and the corresponding reactions are estimated from the fuel cell voltage assuming a stable cathode potential. However, at sub-zero temperatures, ice formation affects both the anode and cathode potentials, which makes it difficult to evaluate the cell reversal origin and the anode half-cell potential. To better understand hydrogen starvation and cell reversal behavior of PEFCs sub-zero temperatures, a specially designed, easy-handling reference electrode has been developed and integrated with the membrane electrode assembly (MEA) in order to separate anode and cathode potentials, as shown in Fig. 1. The external hydrogen reference electrode is attached to the MEA through a thin polymer electrolyte bridge. With the reference electrode installed, the cell reversal behavior is investigated for both the typical operating temperatures (65-80 ^oC) and the sub-zero temperatures (e.g., -15 ^oC). At 65 ^oC, the cell voltage change during hydrogen starvation is mainly affected by anode potential change, which shows the potential plateau for water electrolysis and carbon corrosion. In contrast, the cell voltage experiences several transient states at -15 ^oC. Using the reference electrode, we identify that only one of the potential plateaus in caused by changes in the anode potential and the remainder are a result of phenomena related to ice formation in the cathode. Herein, we present several unique aspects related to hydrogen starvation cell reversal at sub-zero temperatures that are identified using the reference electrode.