|Schematic of the HP-BPMED device used in the renewable fuels research. (a) Atmospheric CO2 separation using a continuous-flow electrodialysis system. (b) Detailed view of the stack. Source: PARC, portions courtesy of Ameridia Corp. Click to enlarge.|
PARC (Palo Alto Research Center), the home of a long line of technology innovations, including laser printing, object-oriented programming, and personal workstations with graphical user interfaces (e.g., the Alto), is building a portfolio of research in the cleantech area. PARC was founded in 1970 at Xerox; in 2002, it was incorporated as a wholly owned independent research and development subsidiary of Xerox Corporation, providing services to external customers as well as to Xerox.
Dr. Scott Elrod, VP and Director of PARC’s Hardware Systems Laboratory (HSL) research organization also directs the Cleantech Innovation Program at PARC, which develops solutions for delivering affordable solar energy, increasing solar cell efficiency, purifying water, managing energy utilization, and producing renewable fuels. Two of the projects Elrod and PARC were discussing at last week’s ARPA-E Energy Innovation Summit in Washington DC were a technology for the co-extrusion printing of novel battery electrodes, enabling higher energy and/or power densities; and an approach to producing carbon-neutral renewable hydrocarbon fuels using air, water and CO2 captured from the atmosphere.
Electrodes. PARC has developed a technology for co-extrusion printing—i.e., for depositing thick films of pastes of densely interdigitated functional materials. For batteries, these functional material pastes would be the electrode active materials. A post deposition processing step dries and sinters the deposit into the final electrode structure. The stripes can be as narrow as several microns wide and as tall as several hundred microns.
The first application of this technique has been for silver gridlines on the front surface of solar cells. Compared to screen-printed gridlines, the narrower and taller front gridlines created by the technique cover less solar cell surface area, resulting in increased absolute cell efficiency. A production prototype machine is under test at a customer site, with an additional gain of 1% absolute cell efficiency and process speeds up to 200 mm/sec having been demonstrated.
While the solar cell application has a near-term sales opportunity, commercial application of the technology to battery electrodes is probably 2-3 years out, Elrod noted. There is further opportunity for the method in air cathodes. The current density in an air-breathing electrode is proportional to the amount of electro-catalytic surface area that is exposed to air. The PARC technology provides a directed-assembly printing method for producing a greater proportion of this “three-phase boundary” than conventional electrode manufacturing methods—up to 10x the air-breathing surface area of conventional electrodes.
Carbon-neutral liquid fuel. PARC is developing a non-biological approach for producing liquid fuels from renewable energy, air, CO2 and water. PARC’s proposed approach captures CO2 from the air and reacts it with hydrogen produced via the electrolysis of water to produce liquid hydrocarbon fuels. As long as the energy for the process is renewably generated, PARC notes, the overall process is carbon-neutral.
Current PARC research (funded by DARPA) focuses on the critical step of capture CO2 directly from the atmosphere. PARC is focusing on recovery using a novel electrochemical approach developed at PARC: high-pressure bipolar membrane electrodialysis (BPMED), and has designed, constructed and begun testing of a prototype for CO2 separation.
The technique uses a solvent such as sodium or potassium hydroxides, converted by reacting with CO2 to aqueous carbonates or bicarbonates. (One possible technique for this might be the use of spray towers, Elrod noted.) The spray solution is then pressurized to 5-10 atmospheres, Elrod said, and introduced to the BPMED unit. BPMED then regenerates the CO2 gas.
The bicarbonate is transferred across the anion exchange membrane to the CO2-rich acid stream that is held at a constant pH of 3 to 4 by a combination of acidic buffers and flow-rate control. The capture solution is buffered against excessive pH increases and held at a constant pH of 8 to 10 by the presence of significant concentrations of HCO3- and CO32- ions. The capture solution is regenerated by the OH--ion flux from the bipolar membrane and by partially depleting it of HCO3- and CO32- ions via electrodialysis.
The high-pressure acid stream is transferred to a gas evolution/separation tank where the pressure is reduced with concomitant release of pure CO2. The now CO2- depleted acid stream is returned to the electrodialysis unit via a repressurization pump while the regenerated capture solution is returned to the spray tower.—Eisaman et al. (2009)
PARC is focusing on three key innovations to enable this approach: (1) active pH control for energy efficient CO2 concentration; (2) very high-pressure operation for suppression of in-stack gas evolution; and (3) very high current densities in a gas-evolving system.
In a paper published in the RSC journal Energy & Environmental Science, the PARC researchers present results indicating that the energy consumption required to regenerate CO2 gas from aqueous bicarbonate (carbonate) solutions using this method can be as low as 100 kJ (200 kJ) per mol of CO2 in the small-current-density limit.
Matthew D. Eisaman, Luis Alvarado, Daniel Larner, Peng Wang, Bhaskar Garg and Karl A. Littau (2011) CO2 separation using bipolar membrane electrodialysis. Energy Environ. Sci., doi: 10.1039/C0EE00303D
M. D. Eisaman, D. E. Schwartz, S. Amic, D. Larner, J. Zesch, F. Torres, and K. Littau (2009) Energy-efficient electrochemical CO2 capture from the atmosphere