Lab fuel cells reach 0.726 W cm−2 at 400°C

The researchers used a thin ceramic film as the electrolyte in a laboratory-scale solid oxide fuel cell; the film conducted oxide ions between the electrodes. The device built by researchers at the University of Science and Technology Beijing, Hainan University and other institutes then reached a maximum power density of 0.726 W cm −2 at 400°C—a result that points to fuel-cell operation below the temperatures normally required by these systems.

Most solid oxide fuel cells use heat-resistant ceramic electrolytes, solid materials that transport ions between two electrodes. Those electrolytes generally need temperatures above 500°C to move oxide ions rapidly. The heat adds engineering complexity and can speed up damage to the cell’s underlying components.

The new materials belong to the Aurivillius family. Their structure stacks blocks of sodium-bismuth-titanium oxide with regularly positioned bismuth oxide layers. The researchers produced films containing four, five, seven or eight perovskite-like layers, using pulsed laser deposition, a process in which laser pulses release material from a solid target onto a supporting surface.

Small additions of magnesium or strontium let the team adjust the films’ structure and their oxygen vacancies—empty sites through which oxide ions can move. The best-performing films contained 4% magnesium and showed an ionic conductivity of 0.025 S cm−1 at 350°C. Atomic-scale electron imaging and first-principles calculations linked the result to localized lattice stretching and two ion-conduction pathways associated with the bismuth oxide layers.

Concretely, lower-temperature operation could simplify thermal-management systems, reduce degradation and widen the choice of materials used around a fuel cell. The researchers tested the optimized film with hydrogen at the anode and air at the cathode, including a 200-hour test at 350°C. The work remains at laboratory scale: the team says the films could be improved further and incorporated into fuel cells with other designs.