FUEL CELLS AND ELECTROLYZERS
Electrochemical energy conversion devices are not limited by traditional heat cycles, i.e. Carnot or Rankine, making them perhaps the most promising power plants to transform energy in a highly efficient and environmentally friendly manner. Energy conversion devices with these attributes are critical in both the stationary power and transportation sectors as we move into the 21st century. Unfortunately, there are several areas where improvements must be made to realize the widespread commercialization of fuel cells and electrolyzers. A few areas where we are actively working to solve these problems are briefly discussed below.
ANION EXCHANGE MEMBRANE FUEL CELLS
Over the past decade, interest in Anion Exchange Membrane Fuel Cells has grown nearly exponentially, with the number of literature citations in the field increasing by more than 6000% during this time. The primary motivating factor for this attention is cost as it is widely accepted that alkaline pH conditions have the potential to drive down materials-level, stack-level and systems-level costs below their Proton Exchange Membrane Fuel Cell. The intense effort around AEMFCs has led to the development of several very highly conducting, stable anion-exchange membranes (AEMs) and anionomers, high activity catalysts – still typically PGM-containing at the cathode and anode as well as a significant increase in state-of-the-art performance – with our group approaching nearly 2 W/cm2 peak power. Our work in this area has been focused in several areas: the creation of Pt-free and PGM-free catalysts for both the oxygen reduction reaction and hydrogen evolution reaction, the control and design of the catalyst layer composition, structure and morphology, balancing the electrode/membrane/GDL water, and understanding carbonation processes and dynamics during AEMFC startup and operation. Through these advances, our group has been able to achieve among the highest peak power for AEMFCs in the world, 1.9 W/cm2.
CATALYSTS AND CATALYST SUPPORTS
One of the main focuses of our group over the past ten years has been the development of novel chemistries and structures for electrochemical catalysts. We have a large body of work in producing noble metals, noble metal alloys and transition metal oxide catalysts for several reactions, including oxygen reduction in fuel cells - both Proton Exchange Membrane Fuel Cells and Anion Exchange Membrane Fuel Cells. We also seriously think about catalyst integration - both in the context of the MEA as well as the specific interaction between the catalyst and catalyst support. Though carbon blacks are the most widely used catalyst supports, they are thermodynamically unstable at fuel cell relevant potentials both during operation in the cathode and startup/shutdown conditions. Additionally, carbon’s graphitic π-stabilized sp2 bonding, which leads to completely saturated valences and nearly zero unpaired surface electrons, facilitates very weak bonding with Pt. The result: catalysts that agglomerate quickly and do not meet government or commercial targets for performance stability. We study non-carbon support materials - designing the support and catalyst structure and chemistry in order to control catalysts activity and stability. Several mechanisms are active including: direct electron transfer between the catalyst and support, support-initiated shape control, and corrosion suppression.
Electrolyzers have long been sold as a potentially zero emission producer of hydrogen, particularly when combined with solar energy. Electrolyzers have received interest for H2 production for fuel cells, but perhaps the largest and most promising market for this technology is the production of low cost, high purity industrial hydrogen. To be cost-competitive with delivered H2, a considerable cost reduction is still needed. A critical component of reducing cost is to reduce the noble metal loading at both the cathode (hydrogen electrode) and anode (oxygen electrode). Our work in this area has focused on improving the dispersion and processing of catalyst materials to reduce electrode loading as well as MEA processing. We have also focused on fundamental studies of new catalyst-support systems to determine what role the support plays in the Pt HER activity and stability, and developed new tools to determine the electrochemically active surface area of the IrO2 catalyst in-stu.
ELECTROCHEMICAL CO2 CAPTURE
The threat of global climate change has brought considerable attention to the need for a reduction in anthropogenic CO2 emissions. The most significant sources for carbon dioxide emissions are electric power plants, accounting for around 40% of U.S emissions. Unlike transportation emissions, which is a collection of millions of small sources, 250 power plants comprise > 50% of the total CO2 emissions in the U.S., making them prime targets for immediate reduction in the CO2 emissions. Chemical sorption is widely viewed as the state-of-the-art technology for scrubbing CO2 from flue gas; however, it has been estimated that adding an monoethanolamine sorption system to a new pulverized coal power plant would increase the cost of electricity by 80% and derate the plant’s net generating capacity by approximately 30% (and some studies show that the energy penalty may be as high as 45%). Therefore, there is an urgent need for new technologies that approach CO2 capture from a fresh perspective. Our work focuses on electrochemical CO2 capture by leveraging carbonate chemistry in anion exchange membranes as well as our experience with designing electrolysis systems. The thermodynamic energy requirement for our system is 80% less than traditional physical solvent capture. However, improvements in both catalyst and anion-exchange membrane materials are still needed to drive the performance up and the costs down, and this is exactly what our focus is.
Professor William E. Mustain
Department of Chemical Engineering
Swearingen Engineering Center
University of South Carolina
301 Main St.
Columbia, SC 29208