The rapid depletion of fossil fuels and increased environmental pollution due to vast fossil-fuel consumption is a thrust for efficient use of energy and exploration of clean and non-carbonaceous fuel to meet the global energy demand.^1-3 As the most lightweight fuel, hydrogen has received special attention due to its ability to provide clean, reliable and affordable energy supply without any greenhouse gas emission and thus, its capability to meet global energy demand.⁴However, economic production of hydrogen, along with cost effective storage and distribution are the bottlenecks for the commercialization of hydrogen as a fuel, which need to be addressed, in order, to solve the global energy crisis.

           Electricity driven hydrogen production from water splitting reaction is considered indeed a promising approach for the economic and efficient production of hydrogen, as it does not involve greenhouse gas emissions and toxic byproducts.⁵ However, the high capital cost, mainly due to use of expensive noble metal electro-catalysts (e.g. Pt, IrO₂, RuO₂) is a major constraint for commercialization of PEM water electrolysis system.⁵ The engineering of non-noble metal electro-catalyst or reduced noble metal containing electro-catalysts with high electrochemical activity for hydrogen evolution reaction (HER), which constitutes half of water splitting process, will offer significant reduction in the overall capital cost of PEM based water electrolysis system. It is hence important to identify electro-catalysts for HER (at cathode of PEM water electrolyzers) with lower overpotential, high cathodic current density (i.e., electrochemical activity) and superior long term stability than the state of the art, Pt/C. These identical electro-catalyst can also be used for driving HER at cathode in photoelectrochemical (PEC) water splitting, where a semiconductor is used as photoanode to absorb the solar energy.⁵

           In the present study, nanostructured Co-Ir based solid solution electro-catalysts, denoted as Co_1-x(Ir_x) (x=0.2, 0.3, 0.4) have been studied for HER using first principles electronic structure calculations. The results of the theoretical studies are experimentally verified by synthesizing Co_1-x(Ir_x) solid solution. The TEM image of synthesized Co_1-x(Ir_x) (x=0.4) is shown in Fig. 1. The electrochemical characterization of Co_1-x(Ir_x) (x=0.2, 0.3, 0.4) as cathode electro-catalyst for PEM water electrolysis system has been carried out using 0.5 M sulfuric acid (H₂SO₄) electrolyte, Pt wire counter electrode and Hg/Hg₂SO₄ reference electrode (+0.65V vs NHE) at a scan rate of 10 mV/sec and temperature of 26⁰C. Co_1-x(Ir_x) exhibit onset potential for HER similar to Pt/C (~10 mV vs RHE). The overpotential required for Co_1-x(Ir_x) (x=0.3, 0.4) to reach 100 mA/cm² is lower than that of Pt/C in acidic, neutral and basic media, indicating the excellent electrochemical activity of the alloys for HER. Moreover, Co_1-x(Ir_x) system exhibits electrochemical stability in acidic media similar to that of Pt/C reflecting its potential as an alternative electro-catalyst system. Additionally, these novel electro-catalysts with unique composition and electronic structure have also been studied as cathode electro-catalysts in PEC water splitting cell. The results of the synthesis, structural, microstructural, and electrochemical activity of these novel electro-catalysts will thus be presented and discussed.

 References:

1.             P. P. Patel, M. K. Datta, O. I. Velikokhatnyi, P. Jampani, D. Hong, J. A. Poston, A. Manivannan and P. N. Kumta, Journal of Materials Chemistry A, 2015, 3, 14015-14032.

2.             P. P. Patel, M. K. Datta, P. H. Jampani, D. Hong, J. A. Poston, A. Manivannan and P. N. Kumta, Journal of Power Sources, 2015, 293, 437-446.

3.             J. R. Miller, Science, 2012, 335, 1312-1313.

4.             P. P. Patel, P. H. Jampani, M. K. Datta, O. I. Velikokhatnyi, D. Hong, J. A. Poston, A. Manivannan and P. N. Kumta, Journal of Materials Chemistry A, 2015, 3, 18296-18309.

5.             P. P. Patel, P. J. Hanumantha, O. I. Velikokhatnyi, M. K. Datta, D. Hong, B. Gattu, J. A. Poston, A. Manivannan and P. N. Kumta, Journal of Power Sources, 2015, 299, 11-24.

Acknowledgement:

The authors gratefully acknowledge the financial support of NSF-CBET grant# 1511390. The authors also acknowledge the Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials for partial support of this research.