Ever increasing energy demands and environmental concerns necessitate development of technologies which are clean, efficient and environmentally friendly such as hydrogen based proton exchange membrane (PEM) fuel cells. One of the technological challenges identified here is the slow kinetics of the oxygen reduction reaction (ORR) at the cathode. Platinum based catalysts currently used to catalyze ORR suffer from their high costs, limited availability and susceptibility to presence of poisons in the fuel. Our research effort has focused on development of alternative electrocatalyst materials for PEM fuel cells.
Carbon materials doped with hetero-atoms, with or without an active metal center offer promise to substitute Pt. Early phase of our work was focused on development of nitrogen containing carbon nanostructures (CNx) synthesized by pyrolyzing C and N containing precursors on a variety of metal-doped supports. By controlling the carbon growth process, we were able to synthesize different nano-geometries with different levels of basal and edge planes.
We have also compared the CNx materials with so called iron-nitrogen coordinated catalysts supported on carbon (FeNC) with an aim to resolve existing debate on the nature of ORR active sites in NNMC(expand) catalysts. Recently, we showed FeNC to be a promising cathode material for phosphoric acid fuel cells owing to its phosphate poisoning resistance arising from the inaccessibility of phosphate to ORR active Fe centers present deep inside the pores of its carbon support.
We used several poisons such as CO, cyanide and H2S as probes to investigate the nature of ORR active sites in CNx. It was found that these catalysts do not show any decrease in their activity even after exposure to these poisons whereas commercial platinum sample supported on Vulcan carbon showed marked decrease in its activity under identical conditions.
The nature of the active sites was further explored using post‐reaction X‐ray photoelectron spectroscopy and DFT studies to reveal changes in its nitrogen functionalities after potential application in an oxygen saturated acidic electrolyte.
More recently, these versatile materials are being tested as precious metal-free carbon-based materials to be used in proton exchange membrane (PEM) fuel cells and electrolyzers for other applications such as oxygen evolution, electrochemical synthesis of halogens and removal of halide ions from waste water.
Carbon Dioxide poisoning of CNx for ORR
We have also compared the CNx materials with so called iron-nitrogen coordinated catalysts supported on carbon (FeNC) with an aim to resolve existing debate on the nature of ORR active sites in NNMC(expand) catalysts. Extensive and systematic experimentation from our group has revealed that FeNC and CNx are infact fundamentally different materials and very different ORR active sites with iron playing a key role to catalyze ORR in the former whereas remaining encased within carbon in case of CNx catalysts.
Our most recent research has focused on understanding the formation of ORR active sites in FeNC electrocatalysts as they evolve from their crude (inactive) form to the most active form and the effect of acid-washing on their ORR performance.
Electrochemical Bromine Evolution using CNx
We perform electrocatalytic measurements to investigate the potential of using heteroatom-doped carbon-based materials as both anode and cathode catalysts for electrochemical bromine production. Significant energy savings can be attained by an alternative oxygen depolarized cathode (ODC)-based process where oxygen is reduced at the cathode instead of protons.
This project focuses on tailoring the composition of CNx for the bromine evolution reaction (BER) on the anode side, by systematically modifying the synthesis procedure and parameters. This work also involves developing a fundamental understanding of the effect of halide exposure on the ORR activity of CNx to be used on the cathode side as well.
We investigate the electrocatalytic activity of these materials using a standard three – electrode rotating disk electrode (RDE) set up in an acidic electrolyte at room temperature. In addition, characterization techniques such as Raman, TPD/TPRxn with mass spectroscopy, XPS, NEXAFS, and DRIFTS are performed to get an understanding of the nature of active sites for BER. This work proposes the use of a noble metal free carbon based catalyst, CNx, which is immune to Br- poisoning as an inexpensive alternative to the state of the art Pt/C catalyst. Moreover, CNx is also proposed as an active and durable anode for BER compared to less active commercial carbon catalysts. The findings from this study so far demonstrate the promise of these materials as cathode catalysts for an oxygen depolarized cathode-based bromine evolution process to manufacture bromine in a more energy efficient scheme. Tuning the nitrogen configuration and characterization of active sites will aid in preparing model catalysts to improve BER kinetics further.