Schematic Structure of a Proton Exchange Membrane Fuel cell

Fuel cells are devices that convert chemical energy of a fuel such as hydrogen into electrical energy. This technology is very attractive for replacing fossil fuel-based cars and electrifying the transport sector, therefore reducing the huge amounts of carbon emissions from vehicles every year. Unfortunately, the efficiency of today’s fuel cells is largely limited by the oxygen reduction reaction (ORR) at the cathode side. Using atomistic methods such as density functional theory, our group is in forefront of getting fundamental insight into the nature of active sites and designing/developing efficient materials for ORR. To design future generation of ORR catalysts we follow two different directions:

 

Hybrid Materials, Novel Chemistry

  • Single Atom Catalysts

  • Support Effect

  • Confinement Effect

 

Cost-effective Materials

  • Carbon-based Materials

  • Nitrides, Sulfides and Oxides.

  • Bimetallic Alloys

 

Hybrid Materials, Novel Chemistry

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  • Single Atom Catalysts

Single metal atoms doped in two‐dimensional materials have attracted particular attention for various catalytic reactions, due to their unique properties beyond metal catalysts. Herein we present density functional theory (DFT) calculations to study a wide range of such systems for oxygen reduction reaction (ORR). We find that the scaling relation of adsorption free energies of relevant ORR intermediates changes in the direction of improved activity. By considering more than 50 combinations, various ORR candidates are identified with improved catalytic activities compared to the state‐of‐the‐art Pt (111).

 

Back, Kulkarni, Siahrostami*, ChemCatChem, 10, 3034 (2018). [DOI]

  • Support Effect:

a) Metal Supported Boron Nitride:

Herein, we use density functional theory calculations to systematically study metal supported hexagonal boron nitride as alternative ORR catalysts. Our results indicate that metal support increase the stability against CO poisoning as well as to activate inert h-BN toward the ORR.

 

Back, Siahrostami*, Nanoscale Advances, 1, 132 (2019). [DOI]

b) Metal Supported Ultrathin Oxides:

Economical earth-abundant catalysts based on first-row transition-metal oxides suffer from low electrochemical stability, which is difficult to improve without compromising their activity. Here, using density functional theory calculations, we demonstrate that noble-metal supports lead to bifunctional enhancement of both the stability and the oxygen reduction reaction (ORR) activity of metal (oxy-hydro) oxide nanoislands. We discover that interfacial active sites (located between the nanoisland and the support) reinforce the binding strength of reaction intermediates, hence boosting ORR activity. 

 

Back, Hansen, Torres, Zhao, Nørskov, Siahrostami*, Badjich*, ACS Appl. Mater. Interfaces, 11, 2006 (2019). [DOI]

c) Metal Supported Molecular Catalysts:

Molecular complexes capable of efficiently catalyzing ORR, when supported on a conducting solid surface could provide interesting hybrid materials combining the best of homogeneous and heterogeneous catalysis. We use density functional theory to study the ORR activity of molecular (unsupported) and gold supported 3d metal (II)‐porphycenes (MPc). This work highlights the important role of the support in preserving/increasing the activity of molecular catalysts.

 

Anand, Siahrostami*, Nørskov*, ChemCatChem, 10, 5505 (2018). [DOI]

  • Carbon-based Materials

 

We have identified the critical role of carbon defects as active sites for ORR activity through density functional theory calculations. This work highlights the potential of carbon-based materials to replace commercial precious metals in regenerative fuel cells and possibly metal-air batteries for cost-effective storage of intermittent renewable energy.

 

To, Ng, Siahrostami, Koh, Lee, Chen, Fong, Chen, He, Bae, et al, Nano Research, 10, 1163 (2017). [DOI]

  • Transition Metal Nitrides

 

Owing to the scarcity of the current state-of-the-art Pt-based catalysts, cost-effective Pt-free materials such as transition metal nitrides and their derivatives have gained overwhelming interest as alternatives. In particular, cobalt nitride (CoN) has demonstrated a reasonably high ORR activity. However, the nature of its active phase still remains elusive. Here, we employ density functional theory calculations to demonstrate that the cobalt nitride surface is highly susceptible to oxidation under ORR conditions. The as-formed oxide overlayer presents a significant promotional effect in reducing the ORR overpotential, thereby increasing the activity in comparison with those of the pure CoNs. The results of this work rationalize a number of experimental reports in the literature and disclose the nature of the active phase of cobalt nitrides for the ORR. Moreover, they offer guidelines for understanding the activity of other transition metal nitrides and designing efficient catalysts for future generation of PEMFCs.

 

Abroshan, Bothra, Back, Kulkarni, Nørskov, Siahrostami*, J. Phys. Chem. C 122, 4783 (2018). [DOI]

  • ​​​​​​​​​Bimetallic Alloys

 

Increasing the activity of Ag-based catalysts for the oxygen reduction reaction (ORR) is important for improving the performance and economic outlook of alkaline-based fuel cell and metal–air battery technologies. In this work, we show that Cu atoms in the Ag-rich phase, favorably tune the surface electronic structure and binding energies of reaction species and enhance the ORR activity.

 

Higgins, Wette, Gibbons, Siahrostami, et al. ACS Appl. Energy Mater., 1, 1990 (2018). [DOI]

Cost-effective Materials