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 significantly. 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 the incumbent 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 containing platinum group metals (PGMs). In recent years, our group has increased the achievable energy density of AEMFCs from 0.5 W/cm2 to 3.5 W/cm2 while simultaneously demonstrating 2000 hour durability. 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. This work has been published in several high impact journals including Nature Energy, Energy & Environmental Science and Advanced Energy Materials.
CATALYSTS AND CATALYST SUPPORTS
One of the main focuses of our group over the past ten-plus years has been the development of novel chemistries and structures for electrochemical catalysts. We have a large body of work in producing noble metal, noble metal alloy, transition metal oxide and single-atom 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 also 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.
ANION EXCHANGE MEMBRANE WATER ELECTROLYSIS
Electrolyzers have long been sold as a potentially zero emission producer of hydrogen, particularly when combined with renewables. Electrolyzers have received interest for H2 production as a feedstock for industrial sources as well as for energy storage - where the H2 is later utilized to produce energy either electrochemically in fuel cells or even combustion. Alkaline electrolysis has been commercialized for decades. They are a well-established technology that uses very low cost materials for both the catalysts and separator. However, their operating current is very low, meaning large system size and high capital cost. Also, they are only able to operate at low differential pressures. Lastly, throughout the plant, there must be an extensive distribution and handling of concentrated KOH, which is a safety and corrosion issue. This led to the development and commercialization of PEM-based electrolyzers. They have been able to operate at much higher current density and high differential pressures. However, PEM electrolyzers use expensive perfluorinated membranes and expensive PGM-based catalysts. In recent years, anion exchange membrane electrolyzers (AEMELs) have been proposed that combine the advantages of alkaline electrolyzers and PEM electrolyzers. Our work in this area has focused on electrode engineering. This includes both new electrode designs as well as component optimization. The result has been high performing cells - with an operating voltage as low as 1.55 V at 1.0 A/cm2 - as well as long-life. Lastly, our success with AEMFCs and AEMELs have afforded us the opportunity to investigate AEM-based unitized regenerative fuel cells (URFCs). In this regard, we have worked to achieve very high round-trip efficiency (~50% @ 0.5 A/cm2) and we have the ability to operate reversibly while spending considerable time operating the cell in both directions (shown in the picture above).
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