(4ba) Rational Design of (Electro)Catalytic Systems for Energy Conversion
Electrochemical transformations permit the direct interconversion of electrical energy and the energy of chemical bonds. Such processes can be carried out at high efficiencies and are of particular interest for distributed, mobile, and auxiliary power generation, where energy demands fluctuate rapidly and there is limited opportunity for heat integration. Electrochemical reactions can also be coupled with renewables such as wind and solar energy to efficiently convert low-value feedstocks to fuels. Herein, a multi-faceted approach combining molecular level insights from detailed kinetic analysis, spectroscopic observations of reactive intermediates, and quantum chemical calculations, along with advanced synthetic techniques and thorough characterization is used to design optimal (electro)catalyst structures. This research methodology is discussed in the context of the development of stable and economical electrode catalysts for low-temperature hydrogen fuel cells, which are advantageous in transportation applications but currently suffer from the requirement of costly Pt-based catalysts.
Insights from quantum chemical calculations (Density Functional Theory) and microkinetic modeling were used to target ideal catalyst materials for electrochemical oxygen reduction (O2+4H++4e-=2H2O), which is a major source of lost efficiency in low temperature fuel cells. Materials based on alloying Ag with 3d transition metals (Co,Ni,Fe) were identified, and a novel synthesis technique was developed to force alloying between these ordinarily immiscible metals. The Ag-alloys were demonstrated to exhibit oxygen reduction activity on the same order of magnitude as Pt, making them competitive on a cost basis. A variety of techniques, including cyclic voltammetry, x-ray diffraction, aberration-corrected electron microscopy, and electron energy loss spectroscopy were used to characterize the materials and confirm the presence of alloyed surface structure and its contribution to the activity enhancements.
In contrast to the trial-and-error approach that has historically dominated catalyst research, the discovery of the Ag-alloy electrocatalysts was enabled by a relatively firm understanding of the oxygen reduction reaction mechanism. This mechanistic understanding was derived from the development of a new theoretical formalism for electrochemical kinetics. The relationship between applied electrical potential and observed current was shown to be a predictable fingerprint that can be determined from elementary-step reaction free energies, activation barriers, and the availability of catalytically active sites under reaction conditions. This result was generalized to describe arbitrary electrochemical reactions, permitting analysis of a variety of important energy conversion reactions.
This work illustrates the efficacy of combining theoretical analysis, synthetic chemistry and characterization tools to rationally design (elecro)catalytic materials for energy conversion processes with great potential impact. Future work using this approach in the areas of advanced lithium-air batteries, electrochemical fuel production from CO2, and small-molecule oxidation in fuel cells will also be discussed.