Final Report - Novel Approach to Non-Precious Metal Catalysts

摘要

This project was directed at reducing the dependence of PEM fuel cells catalysts on precious metals. The primary motivation was to reduce the cost of the fuel cell stack as well as the overall system cost without loss of performance or durability. Platinum is currently the catalyst of choice for both the anode & the cathode. However, the oxygen reduction reaction (ORR) which takes place on the cathode is an inherently slower reaction compared to the hydrogen oxidation reaction (HOR) which takes place on the anode. Therefore, more platinum is needed on the cathode than on the anode to achieve suitable fuel cell performance. As a result, developing a replacement for platinum on the cathode side will have a larger impact on overall stack cost. Thus, the specific objectives of the project, as stated in the solicitation, were to produce non-precious metal (NPM) cathode catalysts which reduce dependence on precious metals (especially Pt), perform as well as conventional precious metal catalysts currently in use in MEAs, cost 50% less compared to a target of 0.2 g Pt/peak kW, & demonstrate durability of greater than 2000 hours with less than 10% power degradation. During the term of the project, DOE refined its targets for NPM catalyst activity to encompass volumetric current density. The DOE Multi-Year RD&D Plan (2005) volumetric current density targets for 2010 & 2015 are greater than 130 A/cm3 & 300 A/cm3 at 800 mV (IR-free) respectively. The initial approach to achieve these targets was to use vacuum deposition techniques to deposit transition metal, carbon and nitrogen moieties onto 3M’s nanostructured thin film (NSTF) catalyst support. While this approach yielded compounds with similar physicochemical characteristics as catalysts reported by others as active for ORR, the activity of these vacuum deposited catalysts was not satisfactory. In order to enhance catalytic activity additional process steps were introduced, the most successful of which was a thermal treatment. To withstand the high temperatures (~900 oC), alternative supports to NSTF were introduced. A variety of carbon fabrics were tested for this purpose. Vacuum deposited materials were used as precursors & physicochemically transformed via thermal treatment to produce substantially better catalytic activity. This activity was further amplified by increasing the surface area of the carbon fabrics which lead to significant gains in fuel cell performance. The second synthetic approach is based on 3M nanotechnology & involves depositing precursor catalytic materials on high surface area supports, initially carbon. These materials were subsequently thermally treated in a nitrogen-containing gas atmosphere. While this approach is similar to others reported in the literature, we exploited 3M’s nanotechnology platform & our expertise in the areas of synthesis & application of the precursor on the substrate. ORR activity proved higher for the materials produced via this approach. In fact, to our knowledge, the performance achieved on this effort exceeded the best previously reported for any NPM catalyst. With 4-nitroaniline as a precursor, the volumetric current density of our material achieved 19 A/cm3 at 800 mV, exceeding the value reported by DOE as the 2005 status (8 A/cm3) by a factor of more than two. We emphasize a unique feature of this project is that all measurements were done in real PEM fuel cells using 50-cm2 MEAs, therefore rendering credibility to the data for practical projection to a fuel cell stack application. In addition, with the price of the precursor nitroaniline only $1.5 kg on the commodity market enabling the DOE requirement of reducing the cost of the catalyst by a factor of two. A drawback of high-performing catalysts on carbon supports is their poor durability. Therefore, in the last stage of this project the focus of shifted toward improving the stability of the NPM catalyst. For that purpose alternative supports to carbon were introduced, The best catalyst synthesis methods remained practically the same for the new supports. Consequently, catalysts were made that were stable up to 1.4 V & one such material ran for over 1000 hours in a 50-cm2 fuel cell with no significant performance loss. In conclusion, by using precursor materials that are commodity items this project achieved the best performing & the most durable NPM catalyst reported thus far in PEM fuel cells. The knowledge base in the area of NPMC has been substantially increased & a solid platform for reaching the 2010 and 2015 targets of the DOE Multi-Year RD&D Plan has been established.