SRNL Nanomanufacturing Project Explores Sub-Nanometer Properties of Platinum for Fuel Cell Catalyst

 Proton Exchange Membrane Fuel Cells

Image Source: SRNL

Proton exchange membrane fuel cells use nano-scale platinum catalyst to increase efficiency and lower costs. However the problem of catalyst degradation in fuel cells remains a hurdle that researchers hope to leap.

A Savannah River National Laboratory research project to study the use of highly dispersed platinum as a fuel cell electrode catalyst is one of 20 project proposals selected by the U.S. Department of Energy (DOE) for funding under its Nanomanufacturing for Energy Efficiency 2008 Research Call. The funded projects promise to make revolutionary improvements in a broad range of energy production, storage, and consumption applications that will reduce energy and carbon intensity in industrial processes.

The SRNL project will examine catalyst structure at the sub-nanometer and even the single-atom level to determine whether dispersing the platinum will allow a significant reduction in the amount of the expensive precious metal used in a fuel cell.

Nanotechnology, the understanding and control of matter at the atomic or molecular level, has the potential for major improvements in energy applications. Over the past seven years, the U.S. government has invested $8.3 billion in nanotechnology and made great strides in gaining fundamental knowledge at the nanometer scale.

An important next step in realizing the promise of nanotechnology is to improve production and manufacturing techniques for nanomaterials and nano-enabled products, many of which are “stuck at the lab scale.” The selected projects will advance the state of nanomanufacturing by improving the reliability of nanomaterials production and scaling-up manufacturing processes that use nanomaterials.

SRNL was awarded $250,000 for its 12-month project to evaluate the use of highly dispersed platinum on electrical conductive porous supports as a fuel cell electrode catalyst. Fuel cells use platinum as a catalyst to facilitate the reaction of hydrogen and oxygen.  Mathematical modeling indicates that the amount of precious metal could potentially be reduced by a factor of 100, if the platinum catalyst were dispersed so that every platinum atom is active for catalytic reaction, rather than being stacked against each other.  SRNL’s Steve Xiao, who is leading the project, will bring industrial catalyst experience to the fuel cell research project, which will examine catalyst structure in sub-nanometer and ultimately single atom or mono layer.

DOE national laboratories responded to the research call intending that innovative technologies developed will be further developed and deployed commercially by industry. The research call was geared toward “quick-win” nanomanufacturing projects with a realistic path to commercialization in 3–5 years.

The 20 research projects total over $17 million in DOE funding. The National Energy Technology Laboratory manages the Nanomanufacturing Program and will oversee the selected projects for the DOE Office of Energy Efficiency and Renewable Energy’s Industrial Technology Program

3M Nano-Structured Thin Film Catalyst Increases Fuel Cell Cathode Acitivity by a Factor of Five

The use of 3M’s optimized nano-structured thin film (NSTF) catalyst in a proton exchange membrane fuel cell (PEMFC) cathode contributes to an increase in both performance and durability, according to U.S. Patent 7,622,217.  3M’s patented fuel cell cathode catalyst is comprised of nanostructured elements that are microstructured support whiskers bearing nanoscopic catalyst particles.  The catalyst comprises platinum and manganese (Mn) and at least one other metal selected from the group consisting of Group VIb metals, Group VIIb metals and Group VIIIb metals other than platinum and manganese. Typically, the at least one other metal is nickel (Ni) or cobalt (Co) , according to its inventors.  

Mark K. Debe,  Susan M. Hendricks, George D. Vernstrom, Alison K. Schmoeckel, Radoslav Atanasoski and  Clayton V Hamilton, Jr. state in the patent that the amount of overpotential losses due to resistance, mass transport loss (MTL), and kinetic losses are independent. The kinetic activity depends directly on the catalyst's ability to efficiently oxidize hydrogen on the anode and reduce oxygen on the cathode. The oxygen reduction reaction (ORR) is far less facile (by about 10.sup.-6) than hydrogen oxidation reactivity, and therefore the cathode catalyst ORR overpotential dominates the 70 mV/decade loss. The cathode catalyst ORR activity, defined as current generated at a specific voltage in the absence of internal resistance (IR) and MTL losses, is a product of two factors: the catalyst's area-specific activity (SA).times the catalyst's electrochemical surface area (ECSA). The area-specific activity is measured in units of amperes/(cm.sup.2 of ECSA), and the ECSA is the surface area of the catalyst actually active for ORR in the PEM fuel cell. Thus, the present invention aims to increase the cathode catalyst's specific activity and/or its ECSA in order to optimize the PEM fuel cell performance.

One mode of MEA failure is breakdown of the polymer which comprises the PEM due to the action of peroxide radicals. One method for measuring the rate or degree of proton exchange membrane degradation is the rate of fluoride ion release, nanograms/min of fluoride ion.

Peroxide radicals may be generated from H2O2 produced on the cathode from incomplete oxygen reduction during the ORR process. The overall 2H.sub.2+O.sub.2=>2H.sub.2O reaction is a four electron process but there are competitive 2e.sup.- paths leading to H.sub.2O.sub.2 from incomplete oxygen reduction. The higher the specific activity of the catalyst for ORR, the greater the current generated through the 4e.sup.- pathways compared to the 2e.sup.- pathways, meaning more H2O relative to H2O2 is produced. Thus the greater the specific activity, the lower the amount of peroxide radicals produced.

It is an aim of 3M’s optimized NSTF catalyst to increase the durability of the membrane by reducing the peroxide radicals generated at their source without loss of catalyst performance, and in fact while optimizing performance. 3M achieves these objectives through selection of the catalyst composition and structure. The 3m MEA: a) uses nanostructured thin film catalysts instead of dispersed fine particle catalysts to give 5 times higher specific activity, thus reducing the ratio of H.sub.2O.sub.2/H.sub.2O produced by the cathode catalyst; b) eliminates carbon particles as the catalyst support, by use of the NSTF catalysts, and thus eliminates a source of peroxide radical generation by the cathode catalyst; c) uses PtCoMn and PtNiMn ternary NSTF catalysts, having volumetric ratios of Pt to transition metal of greater than about 2, to maximize the specific activity for ORR, fundamentally reducing the ratio of H.sub.2O.sub.2/H.sub.2O produced; d) uses NSTF PtCoMn or PtNiMn ternary catalysts having volumetric ratios of Pt to transition metal of less than about 4 and Mn content equal to or greater than about 5 micrograms/cm.sup.2 areal density to minimize the amount of fluoride ions released during fuel cell operation.

The layered fuel cell cathode catalyst may be made by any suitable method. Typically, the layered catalyst is made by alternate steps of vacuum deposition of a layer comprising or consisting essentially of platinum and a second layer, or a second and a third layer, on a film of microstructures. Typically the vacuum deposition steps are carried out in the absence of oxygen or substantially in the absence of oxygen. Typically, sputter deposition is used. 

 FIG. 1 represents a typical polarization curve, which is a plot of the cell voltage versus current density for a fuel cell membrane electrode assembly in operation, used herein to illustrate the three major contributors to cell voltage loss.

FIG. 2 represents a typical polarization curve, plotted as a Tafel plot of the cell voltage versus current density for a fuel cell membrane electrode assembly in operation, along with an IR-corrected Tafel plot, and a plot of the high frequency resistance, used herein to illustrate measurement of the three major contributors to cell voltage loss.

FIG. 3 is a plot of specific activity of PtNiMn (Trace A) and PtCoMn (Trace B) NSTF ternary catalysts according to the present invention versus Pt/transition metal bi-layer thickness ratio (volume ratio) obtained.

FIG. 4 is a graph of average rate of fluoride ion release from the cathode effluent of an MEA in operation as a function of the Mn content in the cathode catalyst of the MEA.