Oxygen Reduction in Nanoporous Metals Under Proton Diffusion Limited Conditions
Authors: Benn Ellen , Erlebacher Jonah 
Published: February 2020

Ionic liquids (ILs) have been used to control catalyst activity by biasing reactants to and products away from the surface of the catalyst.  Previously, this concept was explored by examining the oxygen reduction reaction (ORR) using nanoporous Ni-Pt electrodes.  These pores, about 3-5nm in diameter, are filled with a hydrophobic IL possessing a high oxygen solubility relative to the aqueous electrolyte, to form a nanoporous metal/ionic liquid composite.  This configuration adds an extra driving force for oxygen to the catalyst surface while expelling product water to improve activity, increasing ORR activity in accordance with Le Chatelier’s Principle.  A recent study explored several ILs with varying properties, such as proticity, viscosity, conductivity, and water solubility, and the impact each had on ORR.  It was found increasing the oxygen solubility while simultaneously decreasing the water solubility leads to a more active composite catalyst for the oxygen reduction reaction [1].

The nanoporous metal + IL electrocatalyst physically influences the oxygen flux to the surface.   Here we examine what happens if the oxygen flux is large compared to the protonflux, even at very high overpotential.  Interestingly, our earlier study found that the intrinsic proticity of the neat IL appeared to matter the least of all physical properties of the IL.  The acidic mechanism for ORR is well established as

                  O2 + 4H+ + 4e- → 2H2O           Eq. 1                                                                                      

which would seem to imply proton transport through the IL would be an important property to consider.  However, water solubility in the ILs was found to be relatively high, of order 1 M.  As the experiments performed were in acidic aqueous electrolytes, protons were carried with the water to through the IL layer, negating any influence proticity might offer.  As a means to control the proton flux at the catalyst surface relative to the oxygen flux, other possibilities are as follows:

Synthesize ILs with lower water solubility. While this appears to be the most straightforward approach, nearly all ILs have some degree of water solubility and therefore is quite difficult.

Alter the pH of the solution to reduce the number of protons in the electrolyte.  This approach allows significant control of the proton flux, permitting one to explore proton diffusion limited conditions of oxygen reduction.  In particular, we expect to see a cross-over in behavior when the pH drops below approximately 3, because the oxygen solubility in the ILs here are of order 1 mM.

The work presented for this conference reports a study in nanoporous NiPt + IL using second approach, specifically exploring the effects of pH on hydrogen evolution reaction (HER) and ORR with np-NiPt, as well as composites made with a protic ([MTBD][beti]) and an aprotic ([bmim][beti]) IL.  We indeed observed cross-over behavior in electrolytes near pH 3.   In de-aearated electrolytes at acidic pHs above 3, we observe diffusion limited hydrogen evolution at moderate overpotentials and water reduction at high overpotential.  By running ORR measurements under proton diffusion limited conditions, we find the following behavior (see Figure 1):  first, at moderate overpotentials (above 0.25 V vs. NHE), ORR progresses via the acidic mechanism (Eq. 1), but with currents associated with the proton flux; second, at high overpotential (below 0.25 V vs. NHE), we suppress hydrogen evolution in favor of oxygen reduction via the “alkaline” ORR mechanism:

                O2 + H2O + 4e- →  4OH-         Eq. 2                                                                                             

Using the strategy of controlling reactant flux to the catalyst surface, we are able to reduce oxygen at potentials at which hydrogen should evolve.  This strategy may be extendable to other electroreduction reactions.

Acknowledgements

This research is funded by the US Department of Energy, under grant DE-SC0008686.

References

[1] E. Benn, H. Uvegi, J. Erlebacher. Journal of The Electrochemical Society, 162 (10) H759-H766 (2015).

Figure 1