Innovative Materials for Electrocatalysis

There are growing concerns about global warming and the depletion of petroleum resources. Thus, developing renewable energy production and storage technolgies is one of the magjor scientific challenges in our world. The oxygen reduction reaction (ORR) and its reverse reaction, oxygen evolution reaction (OER), are amongst the most studied electrochemical reactions for fundamental reasons as prototypes of four electron

transfer reactions, and for technological relevance in renewable energy production. Specifically, both ORR and OER are at the center of many applications in electrochemical energy conversion processes including polymer electrolyte membrane fuel cells (PEMFCs), direct-solar and electrolytic water-splitting devices, and metal-air batteries. However, a key element for the commercialization of these reactions is the need of an efficient and cost-effective catalyst that solves their slow kinetics at the oxygen electrodes, namely the cathode in fuel cells and the anode in electrolyzers. For example, Pt-based alloys are among the best catalysts in PEMFCs despite the fact that the fuel cells have a significant power loss due to a low operating voltage of 0.7 V as measured with respect to standard hydrogen electrode (SHE); this is only 57% of the available free energy. Additionally, and more importantly, large amounts of the precious metal Pt are needed to boost the cathode kinetics that are significantly slower than the hydrogen evolution at the anode.

In our recent study, we investigated using first-principles density functional theory the electrocatalytic activity towards the four-electron oxygen reduction-reaction in N-doped graphene quantum dots.

Relevant Publications

The use of water electrocatalysis for hydrogen production is a promising, sustainable and greenhouse-gas-free process to develop disruptive renewable energy technologies. Transition metal carbides, in particular β-phase Mo2C, are garnering increased attention as hydrogen evolution reaction (HER) catalysts due to their favourable synthesis conditions, stability and high catalytic efficiency. We use a thermodynamic approach in conjunction with density functional theory and a kinetic model of exchange current density to systematically study the HER activity of β-Mo2C under different experimental conditions. We show that the (011) surface has the highest HER activity, which is rationalized by its lack of strong Mo-based hydrogen adsorption sites. Thus, the HER efficiency of β-Mo2C can be tuned using nanoparticles (NPs) that expose larger fractions of this termination. We give definite maps between NP morphologies and experimental synthesis conditions, and show that the control of carbon chemical potential during synthesis can expose up to 90% of (011) surface, while as H2 ambient has little effect on NPs morphology. The “volcano” plot shows that under these optimum conditions, the NP exchange current density is ~10-5 A/cm2, that is only slightly smaller than that of Pt (111).
First-principles investigations of the electrocatalytic activity toward the four-electron oxygen reduction-reaction in N-doped graphene quantum dots reveal that pyridinic and graphitic nitrogen are the most active sites with overpotentials of 0.55 and 0.79–0.90 V, respectively. This agrees with experimental findings. Our calculations account for van der Waals interactions, solvent effects, and describe the electrochemistry using standard hydrogen electrode model. The results show correlations between OH*, OOH*, and O* binding energies that impose a lower limit on the oxygen reduction overpotential.

The Effect of Metal Catalyst on the Electrocatalytic Activity of Nitrogen-Doped Carbon Nanotubes

Authors Yifan Tang, Seth C. Burkert, Yong Zhao, Wissam A. Saidi, and Alexander Star
Nitrogen-doped and undoped carbon nanotubes (CNTs) were synthesized from ferrocene, nickelocene, and cobaltocene metal catalysts. Electrochemical testing for an oxygen reduction reaction (ORR) showed that nitrogen-doped CNTs synthesized from ferrocene had improved catalytic activity while nanotubes synthesized from nickelocene and cobaltocene, doped with a comparable amount of nitrogen and having similar stacked-cups structure as nitrogen doped CNTs from ferrocene, had a performance only slightly better than that of undoped CNTs. Ferrocene-based nitrogen-doped CNTs also demonstrated similar long-term stability and higher CO tolerance compared to Pt/C catalyst. Detailed ORR mechanisms were also studied and carbon nanomaterials showed different ORR processes as a result of the metal catalyst utilized in the chemical synthesis. Nitrogen-doped and undoped CNTs synthesized from nickelocene show a preferential 4-electron process as compared to materials synthesized from ferrocene and cobaltocene. We believe that the metal used in the growth process regulates the mechanism of oxygen reduction and can be used to develop improved nitrogen-doped carbon nanomaterials as nonprecious-metal catalysts for fuel cells.