Novel Nonnatural Catalysts

Hydrogen production and fuel cells: Current density is the main problem with fuel cell function. For a particular sized cell, the amount of current derived from it is simply too low for many desir-able applications. A key limitation is the actual maximum rate of the reaction at the surface of the fuel cell electrodes. Thus developing better catalysts for both the splitting of hydrogen into H+ and for the combining of H+ with molecular oxygen to form water are critical needs in this area. Similarly, the opposite reaction, forming hydrogen using electricity from renewable sources is also a key issue for the storage and transportation of renewable energy. Nature, of course, has developed catalysts for each of these reactions. Complex IV in the mitochondrion catalyzes the combination of protons and oxygen to form water. The reverse reaction is per-formed by the Mn-containing oxygen evolving complex of photosystem II. Various hydro-genases catalyze either the formation or oxidation of hydrogen gas. A particularly important re-action that comes into play in essentially all these applications is the combination of protons with oxygen to form water (or the reverse – splitting of water). This four-electron reaction has a substantial activation energy. We currently have a DOE-funded project to develop a catalyst for the water-splitting reaction. We also plan to use a very similar approach to develop biomimetic catalysts for oxidation of H2 to protons (as well as reduction of protons to H2 during production via electrolysis).

Carbon dioxide reduction: Another specific objective of the novel catalyst phase of this project is to develop an effective electrocatalyst for CO2 reduction. This will be performed starting with porphyrin-based molecules, already known to effectively catalyze CO2 reduction on electrodes, and adding to this a catalytic pocket created from four covalently attached heteropolymers. By varying the composition of the heteropolymer, we will stabilize the reactive intermediates in the CO2 reduction process and in so doing reduce the overpotential, improve reaction specificity, and increase the longevity of the catalyst itself. Through our novel method of in situ heteropolymer synthesis and screening, tens to hundreds of thousands of potential catalysts will iteratively be created and screened for activity. Such catalysts could be used directly for the conversion of solar electrical power into a chemical fuel. The long-term goal is to directly couple solar power to CO2 reduction and water-splitting using potential created via direct excitation of optically active components and a water-splitting electrocatalyst we are developing using similar methods under a DOE contract.

Catalyst development: We have developed a novel approach to catalyst generation that can be applied to the generation of each of the electrocatalysts described above, resulting in inexpen-sive, robust synthetic systems. Using chemistry based on that developed by the DNA chip in-dustry, we are developing systems to create and screen hundreds of thousands of potential catalysts at a time on a surface. These catalysts, like enzymes, are heteropolymers, but they can be made from a large array of natural and non-natural building blocks and they are much smaller than the original enzymes they mimic.

Electrocatalysts are particularly well suited for large scale synthesis and parallel assays for the simple reason that current as a function of voltage is an easy and reliable parameter to measure and a direct indication of catalytic function in electrochemical catalysis. Our light-directed, high-throughput synthesis and screening methods can be used directly on electrode arrays. Better yet, we have teamed up with Combimatrix in Washington State to develop an entirely electro-chemical means of making these heteropolymers. They already use arrays of 12,500 elec-trodes as the elements upon which they make DNA arrays. Instead of using light to release the blocking group at each step, they use electrochemistry. Such an approach is perfectly suited for development of electrocatalysts based on heteropolymers and we have installed their equip-ment in our lab and have generalized their chemical approach to make heteropolymers other than DNA.

Solar powered catalysts: While the catalysts for conversion between electrical and chemical energy are themselves the major goal of our work, it should also be possible once we create such a catalyst to add to that a mechanism for directly providing the potential from light by inte-grating photoelectron transfer components into the system (e.g., direct CO2 reduction powered by light). Here the ability to both build the components on electrodes and to individually address the electrodes with light will be important.