Protonated phosphonic acid electrodes for high power heavy-duty vehicle fuel cells

Hydrogen fuel cells are attractive devices for automotive applicationswith benefits such as extended driving range, swiftrefuelling time and clean exhausts1. Although passenger fuelcell electric vehicles have been successfully launched, further technologicalinnovations are needed for the next-generation fuel cellplatform to evolve for heavy-duty vehicles (HDVs)2–4. One of thedifficulties encountered in HDV operation is adequate heat rejection.As current low-temperature polymer electrolyte membranefuel cells (LT-PEMFCs) operate at ~80 °C, the waste heat needs tobe rejected across a 40 °C temperature difference5. Heat rejection inHDVs is facilitated by high cell voltage at rated power (≥0.76 V)6,which translates to lower power densities (<0.45 W cm–2). Toachieve high power without the heat rejection issue, the operatingtemperature of the fuel cell stacks must increase to the enginecoolant temperature (100 °C) and ideally up to 160 °C. At thehigher operating temperature, the cost of fuel cell systems can bereduced by downsizing the fuel cell cooling system and eliminatinga large humidifier or complex temperature/humidity controllerunit7. Furthermore, enhanced CO tolerance would permit theuse of reformed methanol–water mixtures that typically contain 2%CO (ref. 8), which would further simplify the fuelling process andmitigate the complicated and energy-intensive liquefaction of purehydrogen transportation. If pure hydrogen fuel is replaced by methanol,the volume of the fuel, fuel tanks and reformer used to storeand convert the methanol into hydrogen becomes approximately30% of the volume-equivalent of compressed hydrogen, substantialincreasing the payload space of HDVs9.However, increasing the operating temperature for LT-PEMFCshas overwhelming challenges because perfluorosulfonic acid (PFSA)electrolytes require adequate hydration, which is difficult whenthe cell operates at >100 °C due to high water partial vapour pressure10.Therefore, extensive research efforts to develop polymerelectrolytes for high-temperature polymer electrolyte membranefuel cells (HT-PEMFCs) have been undertaken over the pastdecade. HT-PEMFCs typically use phosphoric acid-doped polybenzimidazole(PA-PBI)11–13. However, PA-PBI-based fuel cells arechallenging to operate below 140 °C or during frequent startup/shutdown14 without suffering a loss of phosphoric acid, which makesHT-PEMFCs unfavourable for automotive applications. We previouslyreported that HT-PEMFCs based on quaternary ammoniumbiphosphate ion-pair coordination (ion-pair HT-PEMFCs) exhibitedexcellent phosphoric acid retention in the temperature range80–200 °C, but the performance of the ion-pair HT-PEMFCs waspoor15–17. Therefore, further performance improvement is requiredfor the ion-pair HT-PEMFCs to be commercially viable for HDVapplications18 (Table 1).Here we report on a protonated phosphonated ionomer thatincreases proton conductivity more than an order of magnitudecompared with a non-protonated phosphonated ionomer. We showexperimental and theoretical evidence of the protonation of phosphonicacids that is distinct from the hydrogen bonding of phosphonicacids18. Based on this concept, we designed protonatedphosphonic acid electrodes that enable remarkable power densityand are well suited for HDV fuel cells.