Researchers at Georgia Tech, with colleagues in China and Saudi Arabia, have developed a rationally designed, multi-phase catalyst that significantly enhances the kinetics of oxygen reduction of the state-of-the-art solid oxide fuel cell cathode.

The catalyst is also readily applicable to other energy storage and conversion systems, including metal-air batteries, supercapacitors, electrolyzers, dye-sensitized solar cells, and photocatalysis. An open-access paper on their work appears in the journal Joule.

Oxygen reduction reaction (ORR) is an important but sluggish step in many chemical and energy transformation processes; energy loss due to ORR is still significant in the most advanced fuel cells, more so at lower operating temperatures. To make energy conversion and storage devices economically competitive and commercially viable, however, several materials challenges must be overcome, one of which is the creation of durable, low-cost materials and nanostructures of high electrocatalytic activity for ORR at operating temperatures.

… This work demonstrates that a multi-phase catalyst coating (∼30 nm thick), composed of BaCoO_3−x (BCO) and PrCoO_3−x (PCO) nanoparticles (NPs) and a conformal PrBa_0.8Ca_0.2Co₂O_5+δ (PBCC) thin film, has dramatically enhanced the rate of oxygen reduction reaction (ORR).

When applied to a state-of-the-art La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathode in a solid oxide fuel cell (SOFC), the catalyst coating reduced the cathodic polarization resistance from 2.57 to 0.312 Ω cm² at 600 °C. Oxygen molecules adsorb and dissociate rapidly on the NPs due to enriched surface oxygen vacancies and then quickly transport through the PBCC film, as confirmed by density functional theory-based computations. The synergistic combination of the distinctive properties of the two separate phases dramatically enhances the ORR kinetics, which is attractive not only for intermediate-temperature SOFCs but also for other types of energy conversion and storage systems, including electrolysis cells and membrane reactors for synthesis of clean fuels.

—Chen et al.

Schematic representation of a proposed reaction mechanism for the ORR on BCO NP/PBCC coatings on the LSCF cathode. Arrows represent a bulk diffusion of oxygen vacancies. “V” and “VC” are oxygen vacancies on PBCC and LSCF, respectively. Chen et al. Click to enlarge.

The catalyst is applied as a sheer coating only about two dozen nanometers thick and comprises two connected nanotechnology solutions.

First, nanoparticles highly attractive to oxygen grab the O₂ molecule and let inflowing electrons quickly jump onto it, easily reducing it and tearing it into two separate oxygen ions (each one an O₂^-). Then a series of chemical gaps called oxygen vacancies that are built into the nanoparticles’ structures suck up the oxygen ions like chains of vacuum cleaners passing the ions hand to hand to the second phase of the catalyst.

The second phase is a coating that is full of oxygen vacancies that can pass the O₂^- even more rapidly toward its final destination.

The oxygen moves quickly through the channels and enters the fuel cell, where it meets with the ionized hydrogen or another electron donor such as methane or natural gas. The ions meet to make water, which exits the fuel cell. In the case of methane fuel, pure CO₂ is also emitted, which can be captured and recycled back into fuel.

In the first stage, there are two different types of nanoparticle at work. Both have cobalt, but one contains barium and the other praseodymium.

High operating temperatures in existing fuel cells require expensive protective casings and cooling materials. The researchers believe the catalyst could help lower the temperatures by reducing electrical resistance inherent in current fuel cell chemistry. That could, in turn, reduce overall material costs.

The second stage of the catalyst is a lattice that contains praseodymium and barium, as well as calcium and cobalt (PBCC). In addition to its catalytic function, the PBCC coating protects the cathode from degradation that can limit the lifetime of fuel cells and similar devices.

The underlying original cathode material, which contains the metals lanthanum, strontium, cobalt, and iron (LSCF), has become an industry standard but comes with a caveat.

It’s very conductive, very good, but the problem is that strontium undergoes a diminishment called segregation in the material. One component of our catalyst, PBCC, acts as a coating and keeps the LSCF a lot more stable.

—Meilin Liu, corresponding author

LSCF manufacturing is already well-established, and adding the catalyst coating to production could be likely reasonably achieved. Liu also is considering replacing the LSCF cathode completely with the new catalyst material, and his lab is developing a yet another catalyst to boost fuel oxidation reactions at the fuel cell’s anode.

Resources

• Yu Chen, YongMan Choi, Seonyoung Yoo, Yong Ding, Ruiqiang Yan, Kai Pei, Chong Qu, Lei Zhang, Ikwhang Chang, Bote Zhao, Yanxiang Zhang, Huijun Chen, Yan Chen, Chenghao Yang, Ben deGlee, Ryan Murphy, Jiang Liu, Meilin Liu (2018) “A Highly Efficient Multi-phase Catalyst Dramatically Enhances the Rate of Oxygen Reduction,” Joule doi: 10.1016/j.joule.2018.02.008