Metal/Oxide Heterojunction Boosts Fuel Cell Cathode Reaction at Low Temperatures

Among several types of low‐temperature solid oxide fuel cells, hydrogen‐permeable metal‐supported fuel cells (HMFCs) are devices that can achieve outputs of approximately 1.0 W cm−2 at 400 °C. This work clarifies the mechanism for promoting the cathode reaction on proton‐conducting ceramics at such low temperatures. Combined numerical and electrochemical analyses demonstrate that blocking minor oxide ion conduction at metal/oxide heterojunctions promotes proton transfer at the cathode/electrolyte interfaces, thereby enhancing the turnover frequency of the cathode reaction at the triple‐phase boundary. The electrolyte membrane in HMFCs is forced to gain extra protons to compensate for the charge of oxide ions that accumulate because of the blocking, resulting in an increment of the proton concentration gradients near the cathode/electrolyte interfaces so as to eject the excess amount of proton. The interfacial proton concentration gradient increases and thus the cathode polarization resistance of HMFCs decrease with the cell bias. An HMFC with a highly oxygen‐deficient BaZr0.5Sc0.5O3−δ electrolyte accumulates a large amount of oxide ions, thereby developing large concentration gradients. Thus, it achieves a cathode reaction resistance of 0.54 Ω cm2 at 400 °C with conventional cathode materials, La0.6Sr0.4Co0.2Fe0.8O3−δ. These findings demonstrate that HMFCs can efficiently utilize overpotential. A new mechanism to accelerate fuel cell cathode reactions at low temperatures below 400 °C is presented. Combined simulation and experimental studies elucidate that metal/oxide heterojunctions in hydrogen‐permeable metal‐supported cells accelerate proton diffusion at the cathode interfaces and triggers changes in the cathode reaction pathway, allowing bias‐dependent polarization resistance and significantly reduced activation energies.