While this catalyst combination has been used previously in aerobic oxidations, we didn't know if it would be a good fuel cell catalyst. It turns out that it is the most effective molecular catalyst system ever reported.

—UW-Madison chemistry Professor Shannon Stahl

Fuel cells are based on two complementary half-reactions: fuel (such as H₂) oxidation and reduction of O₂ to water. One of the gating factors for fuel cell efficiency is the slow kinetics of oxygen reduction; overcoming this requires a high-efficiency electrocatalyst. The predominant material at this point is platinum.

While molecular ORR catalysts—such as metalloporphyrins and related macrocyclic metal complexes—have been explored as alternatives to expensive platinum, these catalysts typically operate at potentials far from the thermodynamic limit and often generate hydrogen peroxide. Other aerobic oxidation reactions face similar issues.

Use of nitrogen oxide (NO_x) co-catalysts provides a potential opportunity to overcome the limitations noted above. The reaction of nitric oxide (NO) with oxygen is kinetically facile and thermodynamically favorable, and it results in direct cleavage of the O−O bond of O₂ to afford nitrogen dioxide (NO₂) without forming H₂O₂ as an intermediate or byproduct.

Moreover, the proton-coupled steps for reduction of NO₂ to NO exhibit standard potentials close to the thermodynamic potential for O₂ reduction to water. The use of NO_x-based mediators to achieve high-potential ORR, however, is limited by poor direct electrochemical reduction of NO_x species. … An ideal mediator would exhibit facile kinetics at the electrode, in addition to undergoing rapid reaction with NO_x species derived from O₂ reduction.

The above considerations drew our attention to catalytic aerobic alcohol oxidation reactions that employ 2,2,6,6- tetramethylpiperidinyl-N-oxyl (TEMPO) or other organic nitroxyls in combination with NO_x-based co-catalysts.

—Gerken and Stahl

The researchers speculated that the alcohol substrate could be replaced with an electrode to provide the basis for nitroxyl/NO_x-mediated electrocatalytic O₂ reduction.

The results presented in their paper validated this concept and showed that the nitroxyl/NO_x co-catalysts enable O₂ reduction at overpotentials at least 200 mV lower than those previously attained with molecular ORR electrocatalysts.

Because the approach involves chemical reactions between gases, liquids and solids, moving from concept to demonstration was no small feat. Gerken spent months studying and optimizing each component of the setup they had envisioned before testing everything in a model system.

This work shows for the first time that molecular catalysts can approach the efficiency of platinum. And the advantage of molecules is that you can continue to modify their structure to climb further up the mountain to achieve even better efficiency.

—James Gerken

The work was supported by the US Department of Energy through the Center for Molecular Electrocatalysis, an Energy Frontiers Research Center. Stahl and Gerken credit the center for promoting cross-pollination among various chemistry disciplines to open the door for future advances in this area.

Resources

• James B. Gerken and Shannon S. Stahl (2015) “High-Potential Electrocatalytic O₂ Reduction with Nitroxyl/NO_x Mediators: Implications for Fuel Cells and Aerobic Oxidation Catalysis” ACS Central Science doi: 10.1021/acscentsci.5b00163