The United States plans to use significant amounts of biomass as an energy source. Effective conversion of this resource to traditional fuels requires discovery and development of efficient processing methods for biorefineries. One of the key processes needed is reforming to produce hydrogen, which is then available to upgrade conversion products to liquid fuels. Reforming is especially important in biorefining, as it is estimated one-third of the biomass feedstream must be reformed to hydrogen in order to convert the remaining two-thirds to liquid fuels, largely by oxygen removal.
Aqueous phase reforming offers substantial benefits for biorefining as a hydrogen source. The question posed by the PI Eric Stuve from the University of Washington as to whether analogous electrochemical routes exist, and would there be any advantages to their use. In electrocatalytic reforming, an imposed electrical current drives electrooxidation of a biomass derived compound to form carbon dioxide or partial oxidation products at the anode and hydrogen at the cathode. Electrocatalytic reforming has not been demonstrated, however, so the goal of this high risk EAGER project is to discover whether it exists. The liquid phase reactions require extreme conditions of temperatures up to 225 °C and pressures up to 30 atm and unfortunately little is known about electrochemistry at these conditions.
The research proposed here has the potential to establish an exciting new dimension of high temperature/high pressure electrochemistry about which little is known. There is substantial potential for broader impacts in that electrocatalytic reforming could become a new and transformative technology capable of reducing biorefining costs and consequently speeding the adoption of renewable, biomass-derived energy in our society.
This project is itself a broader impact of the UW's NSF-IGERT in Bio-resource Based Energy for Sustainable Societies, a collaborative effort involving the Native American Yakama Nation and North Seattle Community College. Studies of renewable energy are of strong interest among students at all levels. Opportunities for study and research in electrocatalytic reforming will be provided by a new course in Sustainable Energy and the Environment and undergraduate research programs.
Hydrogen production is one of the main processes in refining. In both traditional and bio-refineries, hydrogen is necessary for fuel upgrading. The problem is more acute in biorefining because of the relatively low ratio of hydrogen to carbon in biomass. The low hydrogen to carbon ratio means that about 35% of the biomass feedstream must be converted to hydrogen in order to convert the remaining 65% to liquid fuels. Any further demand for hydrogen, such as for fuel cells, adds to the amount of hydrogen needed. This project investigated the feasibility of liquid phase electrochemical reforming (ECR) as a means of producing hydrogen from biomass. The ECR method offers the advantage of a relatively low temperature process—much lower than that required for gasification—and delivers a pure hydrogen stream without need of further separation. EAGER projects are, by definition, exploratory, so the goal of this project was to establish feasibility of the ECR process in its most basic form. In this case, the simplest example of a carbohydrate-like molecule is ethylene glycol, which has a 1:1 ratio of hydroxyl groups (OH) to carbon atoms. (A true carbohydrate has a 1:1 ratio of hydroxyl groups to carbon atoms, but also a 1:1 ratio of water to carbon, which ethylene glycol does not have.) To obtain the necessary reaction kinetics, this work was done at temperatures up to 137 C and pressures of 7 atmospheres. These reaction conditions are extreme for electrochemistry in water based solutions and are virtually unexplored. Experiments were conducted in a proton exchange membrane fuel cell (PEMFC) apparatus especially designed for high temperature, high pressure aqueous electrochemistry, shown in Image 1. The overall system schematic is shown in Image 2. A solution of ethylene glycol was oxidized at an anode consisting of platinum supported on carbon. Hydrogen ions produced by the reaction traveled through the electrolyte membrane (Nafion) and recombined as H2 at a platinum/carbon cathode. The reactions for complete oxidation are: Anode: C2H6O2 + 2 H2O --> 2 CO2 + 10 H+ + 10 e- Cathode: 2 H+ + 2 e- --> H2 Hydrogen gas was supplied to the cathode to maintain a stable potential for this experiment. Such an external supply would not be necessary for the ECR method in practice. Liquid and gaseous reaction products were collected and analyzed by gas chromatography and high performance liquid chromatography. The experimental results showed that ethylene glycol reaction becomes facile at temperatures approaching 137 C. Typically, this reaction is poisoned at low temperature by carbon monoxide formed as a partial oxidation intermediate. Poisoning is virtually eliminated at the high temperatures of this work, as the temperature is high enough to promote desorption of carbon monoxide from the anode. Electrode potential is a measure of reactivity (lower is better), and the electrode potential needed for reaction decreased from 0.68 V to 0.42 V as temperature increased from 40 to 137 C. (Potentials are measured with respect to a reversible hydrogen electrode.) The lower value of electrode potential is almost that required (0.3 V) for technically feasible electrochemical reforming. The overall reaction is quite complex. While carbon dioxide made up a large portion of the reaction products, appreciable amounts of glycolaldehyde and glycolic acid were detected, as were trace amounts of oxalic acid. Image 3 shows the dual-pathways reaction scheme depicting these products as well as other possible products. In the dual-pathways scheme, carbon dioxide is formed by one of two paths. In path A, reaction occurs through a carbon monoxide intermediate. In path B, reaction occurs through a number of other partial oxidation products---avoiding carbon monoxide formation---to produce carbon dioxide. Partial oxidation products decrease reforming efficiency (since fewer electrons flow), and, as can be seen, a large number of products is possible. Again, higher temperatures are beneficial, as they reduce the amoung of partial oxidation products. Lower potentials, however, increase the amount of partial oxidation products. In general, these results show that reaction occurs through both path A (since high temperatures eliminated the carbon monoxide poisoning) and path B (since glycolic acid was formed). Further studies are necessary to determine the extent of partial oxidation products as a function of reaction conditions. In summary, these results demonstrate that electrochemical reforming of ethylene glycol is possible and point the way towards further studies of more complex molecules such as glycerol. Reactions conditions will need to be extended to temperatures as high as 200 C, but this requires identifying a proton exchange membrane that can operate with liquids at high temperatures. That, in itself, is a matter of research.
