Our research is focused on efficient, renewable, and environmentally benign catalytic processes for the production of energy, as well as commodity and fine chemicals. We have particular interest in electrochemical routes—i.e. the direct interconversion between electrical energy and the energy of chemical bonds. These processes are particularly suited to utilize power from renewables like wind and solar and can generally operate at higher efficiency and in numerous cases also provide access to different product selectivity than their thermochemical counterparts. Emphasis is placed on fundamental characterization of interactions between molecules and (electro)catalytic surfaces to understand reaction mechanisms for the design and optimization of next-generation catalysts.
We are primarily interested in reactions that may be performed in fuel cells, batteries, electrolyzers, and electrochemical sensors. Broadly, our approach is to employ molecular-level insights from detailed kinetic analysis, quantum chemical calculations, and spectroscopic observations of reactive species and catalyst structures to discern the chemistry and physics relevant to catalyst performance. These insights enable informed, targeted catalyst synthesis strategies to attain ideal structures and compositions that facilitate desirable transformations. Current project areas include:
Upgrading Biomass-Derived Oxygenates to Fuels and Chemicals
A number ofbiomass processing routes yield solutions of highly-functionalized small molecules. The removal of oxygen-rich functional groups (hydroxyl, carboxyl, ketone, aldehyde) and unsaturated bonds from these species is crucial for upgrading to fuels. Likewise, transformations involving partial reductions or oxidations are important in the production of value-added chemicals.Electrocatalysis is a promising avenue for these upgrading processes; benefits include ambient operating conditions, in-situproduction of oxidative or reducing equivalents, and compatibility with acidic, aqueous feedstocks. Current projects in this area include:
- Selective oxidations of furanic compounds (furfural, hydroxymethylfurfural [HMF]) into chemical targets such as furoic acid, maleic acid, and furandicarboxylic acid [FDCA], which have applications in polymers, food additives, pharmaceutical synthesis, and other synthetic outlets
- Selective reduction of bio-derived carboxylic acid compounds to alkane or long-chain alcohol species.
Overarching goals for each chemistry involve characterization of reaction pathways by various spectroscopic and kinetic tools, in turn informing rational catalyst design to improve activity and selectivity toward the various synthetic targets.
Fundamentals of Electro-Oxidation: Bifunctional Catalysis and Direct-Alcohol Fuel Cells
Small organic molecules such as methanol, ethanol, and formic acid are promising for fuel cells because of their high energy density and ease of transport and storage. However, the sluggish oxidation kinetics of these compounds limits cell efficiency. State-of-the-art catalysts for these reactions rely on a “bifunctional” mechanism, in which two components (e.g. in alloy materials or composites) separately serve in the capacities of fuel activation (C-H and C-C bond breaking) and oxygen activation (to facilitate C-O bond formation). At the requisite length scales of mixing (Å to nm), constituent materials modify each other’s intrinsic reactivity, such that the activity cannot be readily predicted from the pure component properties. Additional challenges are presented by the fact that reaction products (including incomplete oxidation products) can be formed by multiple parallel pathways.
To understand the mechanisms and relevant active sites in bifunctional electrocatalysts, we are pursuing approaches to design “modular” bifunctional materials—i.e. materials in which the properties of each catalytic component can be adjusted while having a minimal impact on the other. We have focused on using phase-segregated composites consisting of a core-shell architecture substrate particle serving as the fuel-activating component (the particle core composition can be tuned to impact surface reactivity through strain and ligand effects) while the oxophilic functionality is imparted by small metal-oxide clusters, which are deposited on the substrate particle. To probe the coverage of different active site types, we have developed an electrochemical implementation of steady-state isotope-transient kinetic analysis (SSITKA), which involves using a step-change in the isotopic label of a feed stream to an otherwise-steady-state reaction in order to induce exchange of surface-species and permit measurement of the population and residence time of these species on catalytically active sites.
Solid State Electrochemical Energy Storage Materials and Interfaces:
Solid state batteries are of particular interest as a successor to conventional Li-ion batteries, which have seemingly plateaued in realizable energy density and suffer from additional issues with flammable liquid electrolytes. Here we aim to engineer charge transport characteristics in solid state electrochemical materials and to characterize interfaces between these materials in order to understand intrinsic and degradative mechanisms of efficiency losses during energy storage and extraction. Thus far, we have primarily focused on developing a novel solid-state approach to utilize redox chemistry between Li and O2(2Li++2e-+O2⇔Li2O2), which has a much higher theoretical energy density than Li-ion intercalation systems. The product Li2O2 is generally an electronic insulator, which can ordinarily place limitations on discharge capacity. Operating conditions and cell architecture can be modified in ways that alleviate this issue, but these conditions beget a number of new complications, particularly with regard to compatibility of constituent materials.
A key aspect of this work is a multi-faceted methodology for characterization of ‘buried interfaces’ between interacting solid materials, which are notoriously difficult to access. We utilize a combination of in-situ XPS, impedance spectroscopy, and cross-sectional aberration-corrected electron microscopy to characterize the properties of these interfaces and their evolution as a function of various conditions. A number of the methods and systems studied are transferrable to problems of broader interest in solid state ionics.