A team from Georgia Tech, with colleagues at the university of Kansas, has designed a high-performance solid-oxide fuel cell that operates directly on nearly dry (only ~3.5 vol% H₂O) methane at 500 °C, demonstrating a peak power density of 0.37 W cm⁻² while maintaining excellent durability (no evidence of coking after ~550 h of continuous operation). A paper on their work is published in Nature Energy.

Structure and performance of an intermediate-temperature fuel cell. a, Schematics of a single cell (the yellow and grey grains represent the BZCYYb and the Ni phase, respectively). b, A scanning electron micrograph of a single cell, including a fiber cathode, a dense electrolyte, an AFL and an ASL. c, A cross-sectional view of the ASL and the ARL. d, A top-down view of the ARL. e, I–V and I–P curves of a single cell when operated at 500 °C using H₂ (black lines) or methane (red lines) (with ~3.5 % H₂O) as the fuel and ambient air as the oxidant. f, Current density of a single cell at a constant cell voltage of 0.75 V when methane (with ~3.5% H₂O) or H₂ was used as the fuel and ambient air was used as the oxidant. Chen et al.

Solid oxide fuel cells (SOFCs) are potentially the most efficient technology for direct conversion of hydrocarbons to electricity. While their commercial viability is greatest at operating temperatures of 300–500 °C, it is extremely difficult to run SOFCs on methane at these temperatures, where oxygen reduction and C–H activation are notoriously sluggish. Here we report a robust SOFC that enabled direct utilization of nearly dry methane (with ~3.5% H₂O) at 500 °C (achieving a peak power density of 0.37 W cm⁻²) with no evidence of coking after ~550 h operation.

The cell consists of a PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ nanofibre-based cathode and a BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3–δ-based multifunctional anode coated with Ce_0.90Ni_0.05Ru_0.05O₂ (CNR) catalyst for reforming of CH₄ to H₂ and CO. The high activity and coking resistance of the CNR is attributed to a synergistic effect of cationic Ni and Ru sites anchored on the CNR surface, as confirmed by in situ/operando experiments and computations.

—Chen et al.

Methane fuel cells usually require temperatures of 750 to 1,000 degrees Celsius to run. The new one needs only about 500 degrees—a notch cooler than automobile combustion engines, which run at around 600 degrees Celsius.

The lower temperature could trigger cascading cost savings in the ancillary technology needed to operate a fuel cell, potentially pushing the new cell to commercial viability. The researchers feel confident that engineers can design electric power units around this fuel cell with reasonable effort, something that has eluded previous methane fuel cells.

Our cell could make for a straightforward, robust overall system that uses cheap stainless steel to make interconnectors. Above 750 degrees Celsius, no metal would withstand the temperature without oxidation, so you’d have a lot of trouble getting materials, and they would be extremely expensive and fragile, and contaminate the cell.

—Meilin Liu, who led the study and is a Regents’ Professor in Georgia Tech’s School of Material Science and Engineering

Interconnectors are parts that help bring together many fuel cells into a stack, or functional unit.

Lowering the temperature to 500 degrees Celsius is a sensation in our world. Very few people have even tried it. When you get that low, it makes the job of the engineer designing the stack and connected technologies much easier.

—Ben deGlee, co-author

The new cell also eliminates the need for a major ancillary device called a steam reformer, which is normally needed to convert methane and water into hydrogen fuel.

The work was funded by the US Department of Energy (DOE) Office of Basic Energy Sciences and Advanced Research Projects Agency-Energy (ARPA-E); it was also funded by the National Science Foundation’s Division of Chemistry.

If it goes to market, though the new cell might not power automobiles for a while, it could land sooner in basements as part of a more decentralized, cleaner, cheaper electrical power grid. The fuel cell stack itself would be about the size of a shoebox, plus ancillary technology to make it run.

The new catalyst, CNR, manufactured by research collaborators at the University of Kansas, is the outer layer of the anode side of the cell and doubles as a protectant against decay, extending the life of the cell. CNR has strong cohort catalysts in inner layers and on the other side of the cell, the cathode.

On the cathode end, oxygen’s reaction and movement through the system are usually notoriously slow, but Liu’s lab has recently sped it up to raise the electricity output by using what’s called nanofiber cathodes, which Liu’s lab developed in a prior study.

The structures of these various catalysts, as well as the nanofiber cathodes, all together allowed us to drop the operating temperature.

—Yu Chen

Resources

• Yu Chen, Ben deGlee, Yu Tang, Ziyun Wang, Bote Zhao, Yuechang Wei, Lei Zhang, Seonyoung Yoo, Kai Pei, Jun Hyuk Kim, Yong Ding, P. Hu, Franklin Feng Tao & Meilin Liu (2018) “A robust fuel cell operated on nearly dry methane at 500 °C enabled by synergistic thermal catalysis and electrocatalysis” Nature Energy doi: 10.1038/s41560-018-0262-5