Stability Testing and X-ray Characterization of Novel Cathode Electrocatalysts

Deborah Myers demonstrates the fuel cell fixture created to allow in situ X-ray study of transition metals.

Researchers at Argonne are engaged in a multi-lab project, led by Los Alamos National Laboratory, on improved, durable, low-cost cathode electrocatalysts for polymer electrolyte fuel cells (PEFCs). This multi-lab project focuses on developing non-platinum (or ultra-low Pt loading) PEFC cathode electrocatalysts using three approaches and classes of materials:

1. Nonprecious metal-heteroatomic polymer nanocomposites (e.g., Co-polypyrrole-carbon, at Los Alamos National Laboratory),
2. Core-shell base metal-precious metal nanoparticles (e.g., Pt monolayers on palladium-iron alloy (Pd3Fe) nanoparticles, at Brookhaven National Laboratory), and
3. Nanoparticle chalcogenides (e.g., ruthenium-selenium skin on iron-group metal or zirconia nanoparticles, at the University of Illinois at Urbana-Champaign).

One of Argonne's roles in this collaborative project is to study--at the atomic level--the properties of these three classes of catalysts by using ex situ and in-situ X-ray absorption spectroscopies at Argonne's Advanced Photon Source. The properties studied include particle size, oxidation state, identity of the reactive site, and local environment of the reactive site.

The oxidation state of the catalyst is important in determining oxygen reduction activity; it determines the availability of catalyst sites for adsorption and dissociation of the oxygen molecule. Characterization of these properties, especially while the catalyst is in the reactive environment, allows a correlation of these properties with activity, identification of the active catalyst site, and an understanding of degradation mechanisms. Identification of the active site, for example, would enable study of synthetic routes that maximize site density, leading to higher catalyst specific activities. Ultimately, the characterization findings will lead to ways to improve catalytic activity and the durability of the cathode electrocatalyst.

A cell fixture was designed and fabricated to allow the in situ X-ray study of low-atomic-number transition metals, such as cobalt and iron, at low concentrations in the fuel cell environment. Thus far, the project has focused on in situ characterization of the cobalt and cobalt-iron catalysts developed by Los Alamos National Laboratory. Argonne researchers have found that the catalyst's atomic structure changes with cathode conditions, such as the water content of the oxygen stream and the fuel cell voltage. The atomic structure is altered from a cobalt oxide-like structure at high voltages or high water content to a structure containing hydroxy and water linkages between cobalt atoms at low voltages and low water content.

This work is funded by the Hydrogen, Fuel Cells and Infrastructure Technologies Program of the DOE Office of Energy Efficiency and Renewable Energy.

Non-Platinum Bimetallic Cathode Electrocatalysts for Polymer Electrolyte Fuel Cells

Xiaoping Wang measures the stability of a platinum cathode electrocatalyst.

One of the major barriers to the commercialization of polymer electrolyte fuel cell (PEFC) power systems, especially for the automotive application, is cost. The high cost is largely due to the use of platinum as the fuel cell's electrocatalysts. In the past decade, there have been significant advances in reducing the platinum (Pt) loading of both the anode and the cathode through the development of highly dispersed Pt nanoparticles on a carbon support, thin-film Pt electrodes, and Pt alloy nanoparticles supported on carbon. However, further significant cost reductions will require replacing platinum, especially in the cathode layer, with less expensive but active and stable electrocatalytic materials.

Argonne investigators are at work on an alternative to Pt-containing electrocatalysts: bimetallic base metal-noble metal nanoparticles. In these nanoparticles, the bulk of each particle is an inexpensive base metal or alloy, the surface of which is coated by a noble metal. The base metals are chosen for their ability to modify the electronic properties of the noble metal and thus the bonding characteristics of the noble metal with the cathode reactant, oxygen. The noble metal protects the base metals against dissolution caused by the acidic environment of the fuel cell.

Thus far, Argonne investigators have studied copper, iron, cobalt, and nickel base metals alloyed with the noble metal palladium. Oxygen reduction activities that are 75% of those of commercial platinum catalysts (per gram of platinum-group metal) have been achieved; these results could lead to a 60% reduction in the cost of the cathode electrocatalyst. Argonne scientists are currently determining the durability of these materials in an operating PEFC.

This work is funded by the Hydrogen, Fuel Cells and Infrastructure Technologies Program of the DOE Office of Energy Efficiency and Renewable Energy.