Researchers boost catalytic activity for fuel cell efficiency
A team from Stanford University claims to have discovered a technique to make fuel cells more efficient and increase their power output. Using battery electrode materials to control the lattice strain of a platinum catalyst, the team reportedly boosted the catalytic activity by almost 90%. The technique could also "improve the hydrogen-generation efficiency of water splitters and enhance the production of other fuels and chemicals" said corresponding author Professor Yi Cui, adding "Now, mediocre catalysts can become good, and good catalysts can become excellent."
Modifying the electronic structure of catalysts can improve their performance; lattice strain (either compressive or tensile) modifies the distances between surface atoms and hence modifies catalytic activity. However, the common approach of using metal overlayers to induce strain has some control issues, such as introducing ligand effects.

A team at Stanford University has developed a method for using battery electrode materials directly and continuously to control the lattice strain of a platinum (Pt) catalyst, thereby boosting catalytic activity for the oxygen reduction reaction (ORR) in fuel cells by up to nearly 90%. A paper on their work is published in Science .

Another strategy is to deposit catalysts onto flat substrates that undergo physical transformations as external forces are applied or the temperatures varied. Those flat and tunable substrates present great importance to fundamental analysis, but only a few of them have been successfully demonstrated effective in electrocatalysis. Thus, new methods that can flexibly and effectively control both tensile and compressive lattice strain in catalysts without introducing additional effects are needed.

We report a way to tune catalyst strain by exploiting the widely tunable lattice constant of Li-ion battery electrode materials as the catalyst support. —Wang et al.



Schematic of the lattice constant change of LCO substrates and how the lattice strains are induced to Pt NPs. (A) Pristine LCO shows a Co-O layer spacing of 4.69 Å. By electrochemically extracting half of the Li ions out, the layer spacing is increased to 4.82 Å. (B) LCO is first delithiated into L0.5CO with layers expanded and then coated with Pt NPs. By intercalating back those Li ions, the Co-O layer spacing is decreased so as to induce compression on Pt NPs. The orientation of Pt NP is set to be different from (A) as a suggestion of nonepitaxial ALD growth. Credit: AAAS, wang et al. Click to enlarge.

The basis of the approach is the changes in volume and lattice spacing as Li ions are electrochemically intercalated into or extracted out of electrode materials such as graphite, transition-metal dichalcogenides, silicon (Si), or Li metal oxides. Volume and lattice spacing can change from several percent to severalfold.

Even the small, ~3% volume change of LiCoO 2 during charge and discharge is sufficient to generate strain that can alter catalysis, the researchers said.

For the study, the team deposited small Pt nanoparticles (NPs; ~5 nm) onto the surface of LCO or L 0.5 CO particle supports (~500 nm). By controlling the charging or discharging states of the substrate, they directly observed ~5% compressive and tensile strain on Pt (111) facets.

The researchers were able to tune the ORR catalytic activities of the NPs over a wide range, achieving nearly 90% improvement—i.e., a near doubling—or more than 40% decrease in activity under compressive and tensile strain, respectively.

Our tuning technique could make fuel cells more energy efficient and increase their power output. It could also improve the hydrogen-generation efficiency of water splitters and enhance the production of other fuels and chemicals. Our technology offers a very powerful way to controllably tune catalytic behavior. Now, mediocre catalysts can become good, and good catalysts can become excellent. —Professor Yi Cui, corresponding author

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