58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

The largest culprit toward efficiency loss in low temperature polymer electrolyte membrane (PEM) fuels cells is the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode.  Overcoming the ORR barrier has historically required the use of platinum electro-catalysts. High cost of platinum has made this technology unfeasible. We proposed to evaluate a number of silver-based electro-catalysts for the ORR. Our preliminary results obtained using quantum chemical studies of the molecular mechanisms of ORR suggest that these materials should offer a performance similar to Pt at a small fraction of the overall cost. In particular, we proposed to focus on two classes of materials: (i) shaped Ag particles (Ag nanocubes and nanowires) terminated by the Ag(100) surface facet. We note that conventional spherical Ag particles (synthesized using conventional impregnation synthesis) are mainly terminated by the (111) facet which exhibits low electro-chemical activity, and (ii) a number of Ag alloys combining 3d metals (mainly Ni and Co) with Ag. The common feature of these materials is that they can electro-activate molecular O2 (in its reaction with H⁺) with significantly lower activation barriers than conventional spherical Ag particles. Since this elementary step is the main source of over-potential losses on Ag, we anticipated that these nanostructured electro-catalysts should operate with efficiencies approaching those of Pt electro-catalysts. We proposed to synthesize, characterize and evaluate these Ag electro-catalysts. Ultimately, the electro-catalytic ORR performance of the Ag electro-catalysts will be compared to the state-of-the-art Pt materials

.

Findings:

1. In the initial stage of the project, we performed detailed kinetic modeling that allowed us to propose a mechanism of elementary steps for the ORR on Pt and Ag surfaces. These efforts led us to the molecular mechanism that is completely consistent with the experimentally measured ORR reaction kinetics. Among other features, the proposed mechanism allowed us to explain a non-ideal kinetic behavior of ORR on Pt, manifested in the potential dependent Tafel slope. We note that Tafel equation relates the rate of electrochemical reaction to electrical potential. We showed for the first time that shifts in the Tafel slope can be rigorously explained by potential-induced changes in the number of active sites available on the electrode surface.
2. The main mechanistic feature of the proposed mechanism is that there are two elementary steps that affect the rate of ORR on Pt. One step, which controls the kinetic rate of the reaction on the active site, is the attachment of proton (H⁺) to molecular oxygen. The other step which controls the concentration of the active sites under relevant reaction conditions is dissociation of water into adsorbed OH and H⁺. This step is in equilibrium under high electric potential conditions, and the concentration of OH on the surface is very high, preventing a rapid execution of the reaction. Any electro-catalyst that is improved compared to Pt needs to adsorb with lower adsorption energy and be more facile in promoting the attachment of H⁺ to O2. We find that similar mechanistic features govern the ORR on different Ag surfaces, with the exception that on Ag the attachment of proton to O2 controls the rate almost exclusively. This analysis provides us with a molecular handles that will guide us in the design of improved electro-catalyst.
3. Our analysis of elementary step mechanism of ORR on Ag using quantum chemical Density Functional Theory (DFT) calculations showed that there are two classes of Ag-based electro-catalysts that could exhibit improved performance compared to conventional spherical Ag nanoparticles: (I) Shaped Ag nanostructures terminated by more chemically reactive Ag(100) surface such as Ag nanocubes, and (ii) alloys of Ag with 3d metals such as Co, Fe, and Cu.
4. Electrochemical experimental testing of different Ag electrocatalysts showed that Ag-Co alloy exhibits significantly improved performance compared to pure Ag. Its performance in terms of the rate of ORR reaction is ~ 6 times improved compared to pure Ag and it 50 % of the performance of the state-of-the-art Pt electrocatalysts. This performance suggests that this material hits the required targets for moving away forom expensive Pt electrocatalysts.

This grant has been instrumental in training PhD students in the field of electrochemistry and electrocatalysis. Student working on the project are well equipped to model various electrochemical processes from molecular up to macroscopic level. They are also trained in performing detailed experimentation on electrochemical systems.