Harry Atwater, "Artificial Photosynthesis: Progress, Science Prospects and Technology Outlook"
Abstract: The design of highly efficient, non-biological energy conversion system that generate fuels directly from sunlight, water and carbon dioxide is both a formidable challenge and an opportunity, that if realized, could have a revolutionary impact on our energy system. In the past five years, considerable progress has been made in scientific discovery of key materials and mechanisms to realize artificial solar fuels generators, and advances in modeling and design have generated now quite widely embraced conceptual designs for a water splitting solar fuels generator. I will describe recent progress at the Joint Center for Artificial Photosynthesis in realization of efficient, stable solar fuels generators for hydrogen production and will discuss new directions in heterogeneous catalysis for carbon dioxide reduction, and schemes for solar fuels generators for carbon dioxide reduction.
Alexis Bell, "Design of Catalysts for the Selective Electrochemical Reduction of CO2 to C2 Hydrocarbons and Oxygenates"
Abstract: The promise of using solar energy to promote the electrochemical reduction of CO2 to transportation fuels has motivated the research aimed at identifying highly active and selective catalysts that can achieve high current densities at low overpotentials, high catalyst selectivity for the reduction of CO2 to products other than CO, and low catalyst selectivity for the evolution of H2. Since Cu is the only metal known to produce significant quantities of hydrocarbons (e.g., CH4 and C2H4) and oxygenates (e.g., HCOOH, C2H5OH) we have focused on our efforts on maximizing the formation of these products and, in particular, C2H4 and C2H5OH. We have found that high yields of C2 vs C1 products can be achieved by electrochemical restructuring of polycrystalline Cu to enhance the formation of Cu(100) surface and by the use of CsHCO3 as the electrolyte. Theoretical analysis has revealed that Cu(100) surfaces are are particularly effective for forming C-C bonds via the coupling of adsorbed CO or CO and HCO species. The role of using Cs+ cation, rather than Na+ or K+ cations, in the electrolyte is ascribed to the lower pH at the cathode surface, particularly as overpotential is raised, which results in a higher local concentration of dissolved CO2. Further enhancement in the formation of C2 products is achieved by creating Cu bimetallic catalyst containing nano islands of Cu in a background of a metal that exhibits a high selectivity for CO2 reduction to CO. This talk will illustrate how it is possible to design electrocatalysts that exhibit faradaic efficiencies of 60% for C2 products, 30% for C1 products, and only 10% for H2.
Martin Head-Gordon, "Electronic Structure Theory Applied to Modeling Catalysis of the CO2-Reduction Reaction for Artificial Light Harvesting"
Abstract: Of the many challenges facing the development of artificial light harvesting, one of the most significant barriers is the development of selective, energy efficient, and economically viable catalysts that can facililate the electrochemical reduction of CO2 to higher energy content molecules, particularly those that are useful fuels. The first part of this talk will discuss development and implementation of an appropriate electronic structure approach for modeling electrocatalysis under applied bias, using a continuum description of solvent and salt, which we have implemented as a modification to a plane wave density functional theory program package. The second part will describe the application of this approach to model critical steps in the electrocatalysis of CO2 on a copper surface, whose activity for CO2 reduction is already experimentally well established. The use of potential-dependent electronic structure allows the direct calculation of experimental observables which are not readily accessible otherwise, and gives some new insights into the steps associated with the formation of higher hydrocarbons (C2 and above) on copper. It also serves as a useful validation of the methodology, which sets the stage for the third part, which is a progress report on efforts to rationally design new catalyst systems capable of yielding improved selectivity and energy efficiency. Our particular focus is breaking the well-accepted scaling relations between proton reduction and CO2 reduction by altering the catalyst chemistry.
Alnald Javier, "Reduction of CO2 on Cu and Au/W Electrode Surfaces: A Study by Differential Electrochemical Mass Spectrometry" (Poster Presentation)
Abstract: This work describes results from an attempt to employ differential electrochemical mass spectrometry (DEMS) of selectively pre-adsorbed reactants and (postulated) intermediates as a supplementary experimental approach in the study of the reaction mechanism of the Cu-catalyzed electrochemical reduction of CO2. The results prompt the following empirical inferences: (i) CO is the first product of CO2 reduction, as well as the first intermediate in more advanced reactions that include formation of pure and oxygenated hydrocarbons; this is in conformity with the (almost) unanimously held view. (ii) HCHO is not a precursor for C=C double-bond formation. (iii) HCHO is an intermediate for the production of methane and ethanol. (iv) The generation of CH4 and CH3CH2OH from adsorbed CO occurs via two pathways: one requires a theoretically postulated surface species, CO protonated on the C atom, and the other involves adsorbed HCHO, constituted after the rate-limiting protonation step. (v) The generation of CH4 and CH3CH2OH from CO has a much higher activation barrier than conversions from HCHO; not unexpected since the reactions transpire after the slow Cu–OCH+ formation and, consequently, are not highly activated. This work also presents results from an experimental study based on DEMS that tested the theoretical prediction that suggested the viability of a bimetallic near-surface alloy (NSA) electrode made up of Au and W as a CO2-reduction electrocatalyst selective towards the formation of CH3OH as a product, away from methane, ethylene or ethanol. At an overlayer NSA that consisted of n monolayers (ML) of Au on a polycrystalline W electrode, W(pc)-n[(1×1)-Au], no methane, ethylene or ethanol were detected, when the coverage of Au was at submonolayer (n = 0.5) or multilayer (n ≥ 2) coverage. However, when the overlayer contained only 1 ML of Au, methanol was generated exclusively. The anticipated CH3OH-product-selectivity of the W(pc)-(1×1)-Au NSA has thus been (qualitatively) confirmed. The CH3OH-selective activity was 52 µA cm-2 for a Faradaic efficiency of 0.50%; the bulk of the current was expended towards H2 evolution and, since the topmost layer was Au, most likely in the production of CO and formates that are undetectable by DEMS.
Clifford Kubiak, "Molecular Electrocatalysts for the Reduction of CO2 and the Effects of Bioinspired Secondary-Sphere Interactions on Mechanism (ENFL)"
Abstract: The efficient electrocatalytic functionalization of carbon dioxide (CO2) for use in fuels and commodity chemicals represents a continuing challenge for the storing of electrical energy from renewable sources in chemical bonds. Some of the most effective CO2 reduction catalysts are based on the Group VII bipyridine fac-tricarbonyl complexes ReI(2,2'-bipyridine)(CO)3Cl [2,2'-bipyridine = bpy] and MnI(2,2'-bipyridine)(CO)3X [X = Br and OTf], which form carbon monoxide (CO) and water (H2O) with near perfect Faradaic efficiency via a unimolecular [M(bpy)]–/2e– pathway. Interestingly, Re(bpy) compounds can also reduce CO2 via a bimolecular 2[Re(bpy)]/[1e– + 1e–] pathway, producing 0.5 equiv CO and 0.5 equiv carbonate (CO32–). We recently reported that the modification of the bipyridine ligand with methyl acetamidomethyl groups at the 4 and 4' positions enhanced the rate for this mechanism in acetonitrile (MeCN) for Re((4,4'-bis(methyl acetamidomethyl)-2,2'-bipyridine)(CO)3Cl, 1 [(4,4'-bis(methyl acetamidomethyl)-2,2'-bipyridine = dac]. Computational models, electrochemical measurements, and infrared spectroelectrochemistry (IR-SEC), revealed that the bimolecular catalytic response resulted from the supramolecular assembly of a hydrogen-bonded dimer. These results led us to the following question: could the monomer and dimer catalyst systems based on these compounds be improved in terms of current efficiency and turnover frequency (TOF) through the incorporation of additional amino acid residues? These efforts are inspired by nature, specifically metalloproteins, which place multi-metallic active sites and/or co-catalytic residues in close proximity. In these systems the inherent flexibility of the non-covalent interactions, which direct the formation of the higher-order structure, accommodate a variety of conformational changes during the course of a reaction. We reasoned that by adding amino acid residues to the previously reported monomer and dimer catalyst systems we could improve the activity by increasing the stability of the bimetallic active state and incorporating pendant proton sources
Charles McCrory, "Immobilization of Molecular Electrocatalysts in a Coordinating Membrane to Enhance Their Activity and Selectivity for CO2 Reduction"
Abstract: Cobalt phthalocyanine (CoPc) is a modestly-active electrocatalyt for the CO2 reduction reaction (CO2RR) in aqueous solution. When adsorbed onto edge-plane graphite surfaces, CoPc reduces CO2 to CO along with significant co-generation of H2 due to concurrent water reduction. However, upon immobilization within a poly-4-vinylpridine (P4VP) film, the resulting CoPc-P4VP system shows dramatically enhanced activity and selectivity for CO generation compared to the parent system, operating with a turnover frequency of ~4.8 s-1 at just -0.75 V vs RHE with ~90% Faradaic efficiency for CO production. We propose that two properties of the P4VP polymer contribute to this catalytic enhancement: 1) axial coordination of individual pyridine residues to the square planar cobalt center of CoPc increases the activity of the catalyst, and 2) the uncoordinated pyridine residues enable secondary and outer-coordination sphere effects that increase the selectivity of the system for CO production from CO2. These effects were independently investigated by studying the electrocatalytic performance of the CoPc catalyst immobilized in non-coordinating poly-2-vinylpyridine films and as polymer-free, five coordinate CoPc-pyridine films. We also report initial investigations into other polymer-immobilized catalyst systems for electrocatalytic CO2 reduction.
Ian Sharp, "Plasma-enhanced Atomic Layer Deposition of Transition Metal Oxides for Photoelectrochemical Energy Conversion"
Abstract: To achieve efficient and durable devices for solar water splitting, it is necessary to overcome thermodynamic limitations on material stability, integrate catalysts with light absorbers, and engineer interfaces to reduce recombination loss. Atomic layer deposition (ALD) has recently emerged as a powerful tool for integrating conformal thin film corrosion protection layers onto the surfaces of intrinsically unstable light absorbers, thereby enabling stable operation under the harsh aqueous conditions required for solar water splitting. For example, we show that plasma-enhanced ALD (PE-ALD) can be used to directly deposit a highly active cobalt oxide catalyst for the oxygen evolution reaction (OER) onto silicon. However, the composition and structure of PE-ALD materials can differ significantly from those synthesized by other methods. Using a combination of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), together with functional (photo)electrochemical testing, we show that multi-functional bilayer cobalt oxide coatings significantly enhance the performance of integrated semiconductor/catalyst assemblies, thereby enabling efficient and sustained photoelectrochemical water splitting under alkaline conditions. These bilayers consist of nanocrystalline spinel Co3O4, which provides a stable interface, and a Co(OH)2 surface layer, which can chemically transform to the catalytic phase with a high concentration of active sites. The energetics of the interface between p-type Co3O4 and the underlying semiconductor substrate promote efficient interfacial charge transfer. Chemical transformations of the coating are directly probed by operando electrochemical XPS. Beyond solar water splitting, the prospects for using these catalysts for OER under conditions relevant to (photo)electrochemical CO2 reduction will be discussed. This work shows that engineering the catalyst and the semiconductor/catalyst interface at the nanoscale is essential for simultaneously achieving efficient charge extraction, catalytic activity, and chemical stability.