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.
Non-Carbon Supports for Proton Exchange Membrane Fuel Cells
Carbon blacks are the most widely used catalyst supports. It is no wonder; they have several very impressive credentials: chemical tolerance to strongly acidic media, high electronic conductivity, low cost, high specific surface area, and a complex microporous structure. Unfortunately, carbon is thermodynamically unstable at high potentials, relevant to the oxygen reduction reaction 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 DOE or commercial targets for performance stability.
Our focus in this area lies in two areas: 1) the advancement of non-carbon supports; and 2) chemical and microstructural design of carbon to improve interaction with Pt and stability. We have worked on several classes of materials, including Sn-doped indium ixide (ITO), tungsten carbide (WC) and nitrogen-functionalized ordered mesoporous carbon. We have shown that the activity and stability of Pt nanoparticles is strongly affected by the support. Several mechanisms are active including: direct electron transfer between the catalyst and support, support-initiated shape control, and both corrosion suppression and enhancement.
Anion Exchange Membrane Fuel Cells
An additional limitation of proton exchange membrane fuel cells is cost. Both the anode and cathode catalysts are Pt-based; the Nafion(R) membrane is expensive; balance of plant is complex. pH plays such a significant role in electrochemical behavior, and it has been suggested by many that raising the effective pH of fuel cell systems should reduce cost considerably since we could use less complex chemistries for the membrane, transition from noble-metal to metal-free catalysts and reduce the balance of plant. Unfortunately, AEMFC development is decades behind the PEMFC. Hence, there are materials stability limitations, including the membrane, that remain unsolved and non-noble metal catalysts with sufficient activity at T < 100oC in the presence of stationary cation groups have yet to be identified.
Our focus in this area lies in understanding the nature of the electrode/electrolyte interface, and controlling the reacting environment to improve materials durability.
Catalysts for O2 and H2 Evolution in PEM Electrolyzers
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. We have investigated aqueous and flame-based catalyst and MEA processing (in collaboration with Prof. Radenka Maric). We have also focused on fundamental studies of new Pt-support systems to determine what role the support plays in the Pt HER activity and stability.
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