(544ej) One Dimensional (1D) Earth-Abundant Based Nanomaterials As Oxygen Evolution Reaction Electrocatalysts for Acid Mediated Proton Exchange Membrane Based Water Electrolysis

The rising concerns of rapid depletion of conventional fossil fuels combined with the alarming indications of global warming have made it critical to develop clean, environmentally friendly and sustainable energy sources that can alleviate our reliance on the rapidly diminishing conventional fossil fuels ^1-3. In order to achieve this goal, hydrogen (H₂) gas being completely non-carbonaceous in nature and having higher energy density than petroleum based energy sources has garnered immense attention over the years as the foremost energy carrier. The generation of clean and sustainable hydrogen via advantageous proton exchange membrane (PEM) based acid mediated water electrolysis (H₂O → H₂ + 0.5O₂) is universally considered as one of the most efficient and reliable technologies among all other conventional hydrogen production approaches¹. Though PEM based water electrolysis is a promising approach to large scale generation of ultra-high purity hydrogen, the commercial implementation of this technology has been largely hindered due to the need for highly expensive and scarce platinum group metal (PGM) based electro-catalysts such as Pt, RuO₂, IrO₂. All of these PGM based electro-catalysts exhibit excellent electrochemical response for the oxygen evolution reaction (OER) in the highly energy intensive PEM based water electrolysis but they are all characterized by sluggish reaction kinetics. Therefore, there is a need to identify, synthesize and develop novel reduced noble metal containing electro-catalysts displaying excellent electro-catalytic activity and robust long term electrochemical stability similar/superior to IrO₂, the state of the art OER electro-catalyst in highly acidic operating conditions of OER^1-3. Development of such a catalyst will not only help in reducing the capital cost of PEM water electrolyzers but also help in commercialization. In addition, it is also important to generate morphologies that will exhibit improved OER kinetics while exhibiting superior electro-catalytic activity towards OER.

Generating one-dimensional (1D) architecture is a promising strategy for achieving improved electrocatalytic response. Over the past few years, electro-catalysts with 1D nanostructured morphologies such as nanowires (NWs), nanorods (NRs) as well as nanotubes (NTs) have garnered significant attention as a potentially effective materials for water splitting due to their inherent benefits such as high electro-catalytic surface area, high aspect ratios (length-to-width ratio) and facile electron transport though 1D nanotubular arrays ^4-9. Therefore, in the present study, based on the theoretical first principles calculations of the total energies and electronic structures conducted in our group, we have explored 1D structured-morphology for the F substituted and earth abundant transition metal oxide (SnO₂) based electrocatalyst system. The as-synthesized electrocatalyst system exhibits remarkably higher electrocatalytic activity and excellent stability for acidic OER.

Electrochemical characterization of these electro-catalysts has been carried out in three-electrode configuration system, using 1N H₂SO₄ electrolyte solution. Pt wire and Hg/Hg₂SO₄ are used as a counter electrode and reference electrode (+0.65 V with respect to normal hydrogen electrode, NHE) respectively. The electrochemical characterization has been performed with a scan rate of 10 mV/sec and at temperature of 40^oC. The as-synthesized 1D electrocatalyst exhibited significantly lower charge transfer resistance (R_ct) than the benchmark noble metal based OER catalysts and many other precious/non-precious electrocatalysts systems_. In addition, the as-synthesized 1D electrocatalyst displayed remarkable activity yielding a current density of ~ 10 mA/cm² at an overpotential of ~ 285 mV (1.51V). Further, the chronoamperometry test conducted in 1N H₂SO₄ solution shows the minimal loss in current density, indicating a good electrochemical stability of the as-prepared electro-catalysts.

In conclusion, we have developed highly active 1D OER electrocatalyst system for the PEM water splitting. The enhanced electrocatalytic activity of this electro-catalyst is majorly attributed to modification of the electronic structure (as evidenced from theoretical study) and lower charge transfer resistance (i.e. lower activation polarization owing to 1D architecture). Thus, we believe that the present electrocatalyst system is promising and reliable for cost-effective and sustainable hydrogen production. The results of this work will be presented and discussed.

Acknowledgements:

Financial support of NSF-CBET grant# 1511390, Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM) is acknowledged.

References:

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