(<i>Invited) </i>Mesoporous Carbon Powders and Self-Supported Scaffolds for Electrochemical Energy Storage and Conversion Applications

Improving the composition, nanostructure, and activity of electrodes and supported electrocatalysts used in catalyst layers is critical to both understanding and making positive step changes in the performance and durability of electrochemical devices, including fuel cells and electrolysis cells, as well as batteries and capacitors. In most cases, the catalyst layers (or active layers) must be porous and have a high surface area, while also being conducting (usually both ionically and electronically) and as stable as possible, especially considering the often aggressive oxidative/reductive conditions encountered during device operation. As part of the quest particularly for high performance electrocatalysts, we have developed a novel family of nano-templated carbon materials, with carbon being a ubiquitous material that is a key component in almost all electrochemical energy conversion/storage systems. However, most carbons are microporous in nature and thus their internal surface area cannot be easily accessed. They are also usually used in powder form, requiring binders to hold the particles together, which then can lead to surface and channel blockage and even poisoning of catalytic sites. Furthermore, packing carbon particles together with a binder leads to uncontrolled tortuosity in the pores and channels between the particles, resulting in mass transport limitations. To overcome as many of these problems as possible, we have been developing a family of mesoporous carbon powders that have organized and tunable internal pore sizes (colloid imprinted carbons, CICs) and are ~90% porous. More recently, we have been able to produce the analogous material with monodisperse mesopore diameters, but in the form of a binder-free, 100% carbon, self-supported, nanoporous carbon scaffold (NCS), or sheet. In another direction, we have prepared a parallel family of materials but with an ordered bimodal pore structure (BCS, ‘ball and stick’ nanostructure), containing 0.2 to 0.8 um spheres that are saturated with ordered, 3-D-interconnected mesoporous. These spheres are held together with carbon nanofibers, with large secondary pores between the spheres facilitating mass transport. All of these materials have been produced using a hard-templating method, using colloidal silica powder with particles of a single controlled size (5 to 100 nm) and then thermally imprinting the close-packed silica structure that forms after solvent evaporation with mesophase pitch. These composites are then carbonized, followed by silica removal, resulting in a templated morphology with pores of the same size as the silica particle size used to produce them. For PEMFC cathode applications, Pt nanoparticles (NPs) can be easily loaded into the carbon scaffold materials, including via atomic layer deposition, where the excellent line-of-sight microstructure resulting from the organized carbon pores makes the scaffolds nearly ideal for ALD purposes. The result is excellent Pt NP dispersion and controllable Pt NP size, resulting in high electrochemical surface areas, as determined both in aqueous solutions and MEA testing. These cathodes exhibit highly competitive oxygen reduction kinetics, shown especially by the BCS under kinetically-controlled conditions at high cell voltages. The durability of Pt within these cathodes has also been found to be exemplary, as the proximity of the Pt catalyst and the Nafion ionomer can be well-controlled, while the highly defective internal surfaces of these carbons serve to stabilize the Pt NPs against dissolution and agglomeration. Our mesoporous carbon powders and scaffolds are also being investigated for electrochemical CO 2 reduction after heteroatom surface-doping with nitrogen, giving an estimated internal surface coverage of roughly 20 at% N. Because of the ordered and highly reproducible carbon backbone in the CIC powders and also in the NCS and BCS sheets, this has allowed the development of a robust property-structure-performance relationship between the morphology, surface properties, and defect chemistry of N-doped carbons and the kinetics and selectivity of CO2 reduction to form CO. Also, the subsequent attachment of single Fe atoms to the N-doped mesoporous carbons has resulted in the best durability during CO 2 reduction yet reported. Notably, the mesoporous carbon scaffolds are expected to be ideal for flow-through and MEA testing of the CO2RR process at larger scale. The NCS and BCS materials have also been used as flow-through electrodes in redox-flow batteries and as a model materials for the study of imbibition of fluids into nanoporous structures. Because of the many applications of both the mesoporous carbon powders and self-supported carbon scaffolds, they are being developed further by Momentum Materials in Calgary.

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