Gas diffusion electrodes, membrane-electrode assemblies and methods for the production thereof

INV- 15070

Background

Proton exchange membrane fuel cells (PEMFC) are considered to be one of the most promising sources of clean electrical energy for the near future. PEMFC are electrochemical generators which produce direct electrical current from a gaseous fuel (typically hydrogen, pure or in a mixture) and a gaseous oxidant, normally consisting of oxygen or air.

The importance of mixed metal coatings is commonly related to the properties of binary and ternary metal alloys in the field of electrocatalysis, for instance in imparting tolerance to carbon monoxide and other organic species in the oxidation of impure hydrogen feeds, or in enhancing the catalytic activity of platinum metal in the oxygen reduction reaction.

The core component of the cell is the membrane-electrode assembly, consisting of anion-exchange membrane, which is the solid electrolyte supporting the whole process and the physical separation of the anode and cathode cell compartments, bonded or otherwise coupled to gas diffusion electrodes.

The gas diffusion medium usually comprises an electrically conductive web and one or more gas diffusion layers and the conductive web can be metallic or carbon-based. Gas diffusion electrodes of this kind coupled to proton-exchange membranes that give rise to membrane-electrode assemblies, are characterized by excellent performances. Nevertheless, the noble metal component is exploited to such a low extent in structures of this kind, that very high specific loadings are required.

The high amount of noble metal required for obtaining suitable performances in fuel cells is perhaps the single most important factor preventing PEMFC from having a commercial success. Direct metallization of ion-exchange membranes with a catalyst layer has been proposed as a means to achieve a better catalyst-membrane interface, allowing a better catalyst exploitation and therefore the use of lower noble metal loadings. However, no means for direct metallization of membranes has proven effective and practical up to now. High temperatures required by Sputtering or ultra high vacuum deposition (UHV) are destined to impart consistent damages to the delicate ion-exchange membranes, and even the common physical and chemical vapor deposition techniques (PVD or CVD) have proven too difficult to control and cumbersome to scale up. Instead, the dual IBAD (Ion Beam Assisted Deposition) process, has the advantage of being a low temperature process and very easy to scale up.

Technology Overview

This invention provides a gas diffusion electrode comprising of a patterned mixed‑metal coating obtained by the dual IBAD deposition process while using at least two metals on a gas diffusion medium overcoming the limitations of the prior art.

The inventors have found that the best method for obtaining mixed metal coatings of high performances, especially in fuel cell applications, while retaining the benefits of the dual IBAD single metal coatings of the prior art, comprises depositing the different metals in subsequent overlaid layers. The method of the invention subjects electrically conductive web to a first ion beam having an energy not higher than 500 eV, preferably comprised between 100 and 500 eV, then to a second beam having an energy of at least 500 eV, preferably between 500 and 2000 eV and containing the ions of a first metal, then to at least a third beam having an energy of at least 500 eV, preferably between 500 and 2000 eV and containing the ions of one noble metal. Subsequent high energy beams may be used for the deposition of other metal layers. In a preferred embodiment, the first metal is a transition metal, preferably cobalt or chromium, and the noble metal in the third beam is platinum.

Benefits

A method for forming a preferably patterned mixed metal coating on a gas diffusion medium by direct metallization
Low loading of one noble metal and of at least one second metal used in the process
High electrical performance
A membrane-electrode assembly substantially free of ionomeric fluorocarbon components