A variety of functionalized mesoporous carbon materials have been developed to potentially overcome limitations in current materials employed for a number of electrochemical applications. The heteroatomic sites of these materials introduce local structural and electronic defects that result in useful macroscopic physical and chemical properties. However, characterization of such materials on the atomic scale has proven difficult due to generally low heteroatom contents which can be present in a variety of bonding environments, as well as their general disorder, which has hindered their rational development.
The principle focus of this presentation will be on the use of heteroatom-containing mesoporous carbon materials is as replacements for expensive Pt-based oxygen reduction reaction (ORR) catalysts in the cathode of fuel cells. For this application, nitrogen- and transition-metal-functionalized carbon materials which exhibit high electrical conductivities, large surface areas, and uniform, adjustable, mesopores, have been synthesized. Following the application of novel methods to selectively alter their compositions, surface areas, and extents of graphitization, the Fe,N-containing mesoporous carbon catalysts exhibit ORR activities that are comparable to commercial electrocatalysts that rely on platinum supported on activated carbon (Pt/C). Through close feedback of synthesis, characterization, and electrocatalytic activity testing, we have acquired detailed atomic-level understanding of the origins that underlie the optimal catalyst properties. In particular, X-ray photoelectron spectroscopy (XPS) and solid-state nuclear magnetic resonance (NMR) spectroscopy were used to show correlations between heteroatom-containing moieties and demonstrate that the surface N moieties are strongly influenced by inclusion of transition-metal codopants, as well as local interactions with the surfaces of structure-directing template materials. Variable-temperature electron paramagnetic resonance (EPR) spectroscopy and NMR relaxometry are used to provide insights into the high electrical conductivity of the materials, while combined analyses by XPS, Mössbauer spectroscopy, and catalytic measurements reveal the proximities of Fe and N species and the importance of surface Fe sites to the catalytic activity of the materials.
The synthetic and characterization methods demonstrated here are expected to be applicable to similar functionalized carbon materials and permit their rational development for use in other applications, including for water treatment, battery electrodes, and supercapacitors.