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This project investigates the use of nitrogen-doped carbon supports to improve the catalytic activity of various low temperature electrochemical reactions relevant to fuel cells. Recent research suggests that catalytic activity for both methanol electrooxidation and oxygen reduction is dramatically improved when Pt catalysts are loaded onto nitrogen-containing carbon supports vs. pure-carbon supports [[1],[2],[3]].This effect is so significant that it could be game-changing for fuel cell catalysis. The principle goal of this research effort is to determine why nitrogen-doping improves catalytic activity. The project employs highly controlled, geometrically simple Pt/graphite test-systems to clearly measure and explain nitrogen-doping effects on catalytic activity. In particular, this approach will determine whether structure, chemistry, or a combination of effects leads to the nitrogen-doping enhancement.

Our research findings have been detailed in several recently published papers [[4],[5]]. A brief summary is provided below:

·         Geometrically well defined Pt/HOPG and PtRu/HOPG model catalyst systems have been successfully developed as a uniquely powerful platform to investigate the effects of N-doping.

·         N-doping leads to smaller average catalyst particle size for both Pt and PtRu materials (as quantified by statistical TEM/SEM analysis methods) and narrower particle size distribution. Furthermore, N-doping significantly reduces Pt and PtRu particle agglomeration, as measured by statistical nearest neighbor analysis.

·         The catalytic effects of N-doping have been quantified by IV, CV, and EIS. N-doping leads to substantial catalytic enhancements (up to 10-fold on a Pt-mass normalized basis) for both MOR and ORR activity. The onset potential for the MOR is lowered by 130 mV in the PtRu/N-HOPG system vs. the undoped control.

·         Careful analysis of structural versus catalytic effects reveals that catalytic enhancement due to N-doping can only be partially explained by improved catalyst utilization due to decreased particle size. (Decreased catalyst particle size accounts for about ½ of the observed mass-normalized enhancement effect.) The remainder of the enhancement effect appears to be due to a dopant-induced increase in intrinsic catalytic activity. This is a significant conclusion, because it suggests that N-doping can be used to intrinsically boost the performance of nanoparticle catalysts. In contrast, Ar-doped control samples do not show this intrinsic catalyst activity enhancement effect, suggesting this effect may be specific to nitrogen doping.

·         N-doping improves the degradation resistance of the Pt-catalyst system (by at least a factor of ten) during MOR cycling (as measured by final vs. initial peak CV currents obtained from N-doped and undoped HOPG/Pt systems after 10,000 cycles). In contrast, Ar-doping does not provide any degradation resistance (and may in-fact accelerate degradation).

·         The above findings are significant, because they show that only N-doping provides increased agglomeration resistance and significantly enhanced intrinsic catalyst activity. This indicates that N-doping provides a significant chemical influence that is not present with Ar-doping. N-doping also improves the degradation resistance of the PtRu-catalyst system during MOR cycling (as measured by SEM/TEM particle size statistical analysis obtained from N-doped and undoped HOPG/Pt systems before and after 3,000 CV cycles).

·         A quantitative study of nitrogen doping effects as a function of nitrogen doping dosage for the HOPG/PtRu system indicate that a nitrogen implantation level of ~ 3.0x10¹⁶ nitrogen atoms/cm² provides the best performance (resulting in about 10% nitrogen content in the carbon near-surface region). These nitrogen implantation levels are significantly higher than what can be typically accomplished using chemical doping routes, and indicate that higher-nitrogen-content materials should be investigated for fuel cell catalyst applications.   

·         We are now implementing our nitrogen implantation method to commercial high-surface-area carbon support materials (Ketjen Black, Vulcan) to see if similar doping-enhancement effects can be obtained in these systems. Much of this work will be continued in a recently awarded Presidential Early Career Award for Science and Engineering (PECASE) project. Alternative dopants to nitrogen (such as fluorine, iodine, and sulfur) will also be investigated in the PECASE project.

In summary, the major goals and tasks associated with quantifying the N-doping effect in both Pt/C and PtRu/C fuel cells have been successfully accomplished. Our efforts now focus on developing a deeper understanding of the fundamental mechanisms underlying the N-doping effect, exploring whether these impressive catalytic enhancements can be achieved with other dopants and/or other catalyst systems, and applying our doping process to commercial high-surface area carbon support materials with integration into fuel cells.

This young investigator's grant from the ACS PRF has been instrumental in providing seed funding for my laboratory's initial results in this area. This grant has yielded early results which have enabled me to seek additional research funding from government sources. In particular, partly as a result of this work, I have been awarded the Presidential Early Career Award in Science and Engineering (PECASE). The budget flexibility provided by this ACS PRF grant has been particularly important, as I've required adjustments based on staffing situations and a capital equipment acquisition opportunity which occurred during the first year. In the second year, funds were used to partially support a female, minority, masters-level graduate student on the project (April Corpuz), who will continue on with a PhD project in the same area.