Advanced Carbon Nanomaterials as Non-Precious Metal Catalysts for Fuel Cells

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) are electrochemical devices that efficiently convert hydrogen and oxygen into electricity and water. Their clean point of operation emissions and continuous operation have resulted in PEMFCs being highly touted as integral components of sustainable energy infrastructures, most notably in the transportation sector as a green alternative to the internal combustion engine. The issues associated with hydrogen production and distribution aside, the commercial viability of PEMFCs into the auto- motive sector is hindered by their high cost and inadequate long-term operational stability. The main factor behind both of these problems is the platinum-based electrocatalysts used at the cathode to facilitate the inherently sluggish oxygen reduction reaction (ORR). These expensive precious metal catalysts comprise almost half of the overall PEMFC stack cost and tend to degrade in the cathode environment that is very corrosive due to the acidic and potentiodynamic conditions. The current cost targets for PEMFCs are unattainable unless the extensive reliance on this precious metal is alleviated. The cost reduction can ultimately be accomplished by developing alternative cathode catalysts for the ORR. Research on new platinum catalyst supports or nanostructured platinum alloys to increase ORR activity on a precious metal mass basis have been largely successful. This approach is not ideal, however, due to the volatile pricing and geopolitical instabilities that can likely affect the supply of platinum. For these reasons, the development of entirely non-precious metal catalysts (NPMCs) for the ORR is highly desirable. This is the objective of this thesis, as will be presented in the following sections. ‎Chapter 4 describe the operation of one-dimensional nanofibers are prepared by electrospinning an iron–polyaniline/polyacrylonitrile (Fe–PANI-PAN) metal-polymer blend, followed by subsequent heat treatment. PANI was selected as it has previously been shown to be an ideal nitrogen precursor to produce some of the most active NPMCs to date, owing to its aromatic ring structure with a high content and uniform distribution of nitrogen species that can readily form nitrogen-doped graphitic carbon structures during heat treatments. PAN was also helpful as a low-cost polymer carrier to overcome the poor solubility of PANI in solution and as a secondary source of nitrogen. The addition of 10 wt. % PANI to the electrospinning mixture provides 100 and 70 mV improvements to the ORR onset potential and half-wave potential, respectively, rendering the most active NPMCs prepared by electrospinning to date. The high activity is attributed to the porous structure of the nanofibers, combined with the increased nitrogen content provided by the PANI incorporation. This unique synthetic approach, therefore, provides practical progress towards the development of one-dimensional NPMCs for PEMFC applications. Nitrogen-functionalized graphene substances have proved to be promising electrocatalysts for the ORR due to their high activity and exceptional stability in the alkaline environment. However, they exhibit much lower catalytic activity in acidic electrolytes. Hence, in ‎Chapter 5, a hierarchically porous Co-N functionalized graphene aerogel is provided as an active catalyst for the ORR in an acid medium. In the synthesis procedure, PANI is introduced as a pore-forming substrate to promote the self-assembly of graphene structures into the porous aerogel networks and as a nitrogen precursor to induce in-situ nitrogen-doping. Accordingly, a Co-N decorated graphene aerogel framework with a large surface area (485 m2 g−1) and a plenty of meso/macropores are formed after pyrolysis. Such complex structures provide an excess amount of exposed active sites for the ORR and also ensure secure mass transfer. These advantages render significant catalytic activity with the improved onset and half-wave potentials, low peroxide yield and remarkable stability in acid medium. In the next project (‎Chapter 6), we apply an ammonia treatment to tune the structure and activity of electrocatalysts derived from iron, polyaniline and carbon nanotubes (CNTs). By controlling the NH3 reaction conditions, we were able to tune the chemistry of nitrogen incorporation, including concentration and dopant type. The final catalyst had a robust morphology consisting of highly porous 2-D in-situ formed graphene-like structures that, along with the intermixed 1-D CNTs, were decorated with an abundance of nitrogen and iron species. The catalyst derived under the optimized condition (F-P-C_Ar-NH900) exhibited high catalyst activity, including an E1/2 of 0.80 V vs RHE through RDE testing. Under H2-air conditions that are application-friendly, current densities of 77 mA cm-2 at 0.8 V and 537 mA cm-2 at 0.6 V were achieved. Furthermore, a maximum power density of 335 mW cm-2 at 0.6 V was observed. The number of electrons transferred per reduced oxygen molecule was determined to be 3.90 by RRDE indicating that the catalyst exhibited very good selectivity toward the 4-electorn transfer reaction. These electrochemical evaluations indicate that the chemical modification of Fe-PANI-CNT catalyst by NH3 results in a highly promising Pt-free PEMFC ORR electrocatalyst. In ‎Chapter 7, we report the design of 3-dimensional graphitic meso-porous carbon spheres wrapped with 2-dimensional graphenized sheets. This heterostructure has a large electroactive surface area, abundant pore channels and tuned chemical structures that leads to improved electrocatalytic performance. The nano-channels, acting as nanoscale reactors, provide easily accessible active sites, effective mass transfer and smooth charge transfer across the highly conductive carbon matrix. The obtained catalyst delivers a high maximum power density of 0.82 W cm−2 in a single H2−O2 fuel cell measurement, ranking it as one of the most promising NPMCs in PEMFCs. Moreover, fairly good fuel cell stability was also observed through accelerated degradation testing. This work provides a new avenue for NPMC design that can be a step towards practical commercial PEMFCs. Following the previous studies, an efficient strategy of utilizing dual nitrogen sources for preparing highly active Fe-N-C electrocatalyst with in-situ formed graphene-like structures and tuned micro/meso/macro-porous morphology is reported in ‎Chapter 8. This approach is achieved by simultaneously using PANI as a graphene precursor and introducing phenanthroline (Phen) as a pore-forming agent, followed by several post–treatments. This research was accomplished via introducing Phen into the pores of carbon support by ball-milling, which was then covered with a PANI shell through polymerization of aniline, followed by several subsequent pyrolysis and acid leaching steps leading to the formation of in-situ 3D porous graphene-like morphologies with multiple types of pores. Here, Phen acts as a pore-forming agent that is capable of expanding the external PANI shell during the decomposition. Simultaneously, PANI shell converted to graphene-like structures through graphenization in the presence of iron species during pyrolysis processes. Extensive physical characterization indicates the final catalyst provides rich, porous graphene frameworks decorated with uniformly dispersed active sites. The catalyst exhibits high maximum power densities of 1.06 W cm−2 and 0.38 W cm−2 in H2−O2 and H2−air fuel cell tests, respectively, representing one of the highest reported values to date for NPMCs in PEMFCs. Moreover, good fuel cell durability is also observed through accelerated degradation testing. The unprecedented performance of this electrocatalyst in fuel cell is linked to the highly porous graphene frameworks with a vast distribution of pore sizes that maximizes the number of active sites with enhanced accessibility, facilitates the mass-transport properties, and improves the carbon corrosion resistance. ‎Chapter 9 provides a summary of the conclusions of this body of work, along with strategies that can be engaged to capitalize on the scientific advancements made in this thesis. In summary, this research extends from catalyst synthesis to their actual use in a PEMFC, in order to develop commercially viable NPMCs. Various suggestions for prospect works are recommended in the last part of this chapter to further relate the knowledge to design highly active, durable, and low-cost NPMCs.