(Invited) Self-Supported Nanoporous Carbon Scaffold As a Novel Material for Use in Fuel Cells and Electrolysis Cells

Abstract

Several years ago, our group pioneered the use of mesoporous colloid imprinted carbon (CIC) powders (1-3) for a wide variety of electrochemical applications, including as catalyst supports in PEM fuel cells, as electrodes in supercapacitors and redox-flow batteries, sensors, and for water deionization. The CICs have an ordered, close-packed internal porous (spherical) structure and are typically prepared by imprinting a mesophase carbon pitch powder with a colloidal silica template, followed by carbonizing the imprinted pitch and then removal of the silica template. One of the biggest advantages associated with the CICs is the tunability of their pores, controlled by the size of the silica colloid (from ca. 10 to 100’s of nanometers) used in the templating step, with our CICs having pore sizes ranging from ca. 8 – 100 nm. This pore size is ideal for the deposition of catalytic nanoparticles inside the pores, as well as the infiltration of an ionomer phase, such as Nafion, for use in PEM fuel cell catalyst layers. However, one disadvantage of the CIC powders, especially in PEMFC applications, is that there are variable pore sizes between the CIC particles and thus varying access to Nafion and water. Also, the use of powders makes it challenging to image the same particle or region before and after catalyst testing to determine any changes that have occurred, including carbon corrosion.Partly for these reasons, as well as the desire for improved reproducibility of the catalyst layer microstructure, our team recently developed a novel, self-supported, nanoporous carbon scaffold (NCS) material (4,5) for use in PEMFCs, but also in electrolysis systems. The preparation method used to produce the NCS membranes is similar to what was used by our group to make the CIC powders, employing mesophase pitch as the carbon precursor and imprinting with silica colloids of controlled diameter. The pore size can be similarly tailored (from 5 nm to > 100 nm) and the thickness can be controlled to 1-100 microns, with the NCS being formed using a tape casting method. We have also successfully modified the internal wettability of the NCS through surface functionalization, examples being a hydrophilic group (e.g., -PhSO3H) or a hydrophobic group (e.g., PhF5), and have also successfully loaded the NCS with Pt nanoparticles using a variety of approaches. The electrochemistry of the NCS materials, carried out in aqueous solutions using 3-electrode circuitry, shows that the internal surface of the NCS is fully percolating, with all of the pores being 3-D interconnected. Similar to the CICs, the ink bottle-shaped pores within the NCS material are connected bypore necks, which are typically 25-50% of the diameter of the corresponding spherical pores. The necks may hinder mass transport within the NCS during device operation and thus efforts have been made to tailor the pore neck size. The performance of the NCS material within a full PEMFC MEA has also been determined as a function of NCS pore and pore neck size. As well, the behavior of the NCS material in a flow through mode, including for CO2reduction in an electrolysis cell, has been demonstrated, while also having been characterized using nitrogen gas sorption, field emission microscopy (FE-SEM), transmission electron microscopy (TEM), 3-D TEM, TEM tomography, and elemental analysis.