[Image above] Credit: Martin Abegglen; Flickr CC BY-SA 2.0

Dealing with zapped batteries is a drain.

Which is one of the many reasons why fuel cells are such an attractive power option—they offer the promise of a longer-lasting yet clean power supply.

Solid oxide fuel cells (SOFCs), which use a solid ceramic electrolyte, operate at high temperatures and have high overall efficiency, making this technology a hot area of research.

But despite significant advancements in SOFC technology, these power sources are still plagued by problems that inhibit their viability for many commercial uses.

One big problem is that many current designs use silicon to support the cell’s internal membranes, but these cells eventually suffer from degradation and instability because of thermal-expansion mismatch between those materials. This instability limits use of SOFCs in devices that require fast switching between on and off (i.e., most electronic devices).

Researchers at Pohang University of Science & Technology (POSTECH) have now developed a micro-sized SOFC that sidesteps silicon’s problems, instead using a much more thermally and mechanically robust support—porous stainless steel, which significantly improves the cell’s thermal and mechanical stability.

“To the best of our knowledge, this is the first demonstration of the ability of the thermal robustness of a micro-SOFC, which has never been attained in many conventional Si-based devices,” the authors write in the open-access Scientific Reports paper describing their work.

The team thinks its development could help usher these bite-sized power sources into consumer electronic devices, including smartphones, laptops, drones, and more.

To build the novel micro-SOFC, the POSTECH scientists deposited a dual layer substrate on porous stainless steel. They first deposited a contact layer made of a combination of (La, Sr)(Ti, Ni)O3 (LSTN) and yttria-stabilized zirconia (YSZ) (LSTN-YSZ) on the stainless steel. Then, on top of the contact layer, the scientists deposited a gas-tight YSZ thin film electrolyte.

0322ctt fuel cell fig2 lo res

(a) Schematic of the thin-film supported on porous STS substrate. (b) Prototype cell. Cross-sectional SEM images of (c) Pt/LSC/YSZ (Inset: magnified view of Pt/LSC); (d) YSZ/Ni-YSZ/LSTN-YSZ; and (e) LSTN-YSZ contact layer. Credit: Kim et al.

But it wasn’t easy—according to first author Kun Joong Kim, depositing the thin-film components on top of the steel posed the biggest challenge in developing the micro-cell.

Although perfecting the recipe was not simple, the team’s efforts paid off. The combination of materials allowed the researchers to fabricate a cell with a total area of just 78 mm2 that exhibited “good mechanical stability, ease of handling, and flexibility,” the authors write in the paper.

And, because they fabricated the cells using tape casting–lamination–cofiring, which is typically used to fabricate larger SOFCs, the scientists say that commercial scale-up should be feasible. “For larger cells, we will either modify the thin-film deposition method or adopt new thick-film cell components and firing processes,” Kim says in an email.

The authors report that their experiments show that the prototype micro-SOFC performed well, exhibiting a peak power of 560 mW/cm2 at 550°C.

But to really put the cell to the test, the authors subjected it to rigorous thermal cycling—between 350°C and 550°C, up to 15°C per minute, for 6 hours. Even those stressful conditions failed to cause measurable degradation or appearance of cracks or delamination, the authors report.

They write in the paper, “We speculate that the porous but ductile [stainless steel] substrate may absorb the thermomechanical stress caused by sealants or the alumina tube during thermal cycles.”

0322ctt micro fuel cell lo res

A graphical representation of the paper’s abstract. A thermally robust micro-SOFC was successfully fabricated on dual-layer substrates consisting of a functional composite oxide and stainless steel (STS). The cell shows peak power density of ~560 mW cm-2 at 550°C and maintains it during rapid thermal cycles. Credit: Kim et al.

The researchers speculate that such micro-SOFCs will be able to power small portable electronic devices that require high-power density and fast thermal cycling. For example, they say the cells could fly drones for more than an hour and power smartphones for a whole week.

But research is never done.

“Although we have fabricated a micro-SOFC that shows good resistance against mechanical and thermal shocks, we are looking for an improved cell design that has further stability against redox-shock,” Kim says in an email. “A new cell design will include a new anode that is stable against redox cycle and also can be used as a substrate for thin-film cell deposition.”

The open-access paper, published in Scientific Reports, is “Micro solid oxide fuel cell fabricated on porous stainless steel: a new strategy for enhanced thermal cycling ability” (DOI: 10.1038/srep22443).