(376c) Engineering High Performance and Durable PGM-Free Electro-Catalysts for Oxygen Evolution Reaction in PEM Water Electrolysis

The rising concerns related to rapid depletion of conventional fossil fuels and alarming indications of global warming has triggered the pursuit of environment friendly and sustainable alternative energy sources. In this regards, hydrogen (H₂) gas having non-carbonaceous nature and higher energy density than carbonaceous energy sources is indeed the foremost energy carrier^[1]. The generation of clean and sustainable hydrogen via acid assisted i.e. proton exchange membrane (PEM) based water electrolysis is hitherto hailed as one of the most efficient and promising technologies among all other conventional hydrogen production methods^[2]. Although PEM water electrolyzers offer a reliable way to produce high-purity hydrogen in large scales, commercial development of this technology has been largely restricted due to the need for costly and scarce platinum group metal (PGM) based electro-catalysts such as Pt, RuO₂, IrO₂; which demonstrate superior electro-catalytic activity towards energy intensive and sluggish oxygen evolution reaction (OER) in PEM water electrolysis^[2]. Therefore, to overcome this bottleneck, exploration of earth abundant, economic and PGM-free electro-catalysts, exhibiting comparable or superior electro-catalytic activity and robust durability compared to state-of-the art noble metals based electro-catalysts in harsh acidic media is extremely desired, which can lead to significant reduction in capital cost of the PEM water splitting process^[3].

In line with this objective, exploiting theoretical first principles approaches, we have engineered anionic fluorine (F) doped transition metals non-oxide pnictide (TMN) based electro-catalyst for OER in PEM water electrolysis. The dopant-F plays a vital role in enhancing the electro-catalytic activity by altering the electronic structure of the electro-catalyst. The as-synthesized electro-catalyst with F content of 10 wt. % consequently exhibits excellent electro-catalytic performance, outperforming the benchmark IrO₂ for OER in PEM water splitting. The as-synthesized electro-catalyst powder was coated on porous titanium (Ti) substrate (total catalyst loading=1 mg/cm²) and used as an anode. Electrochemical characterization of the electro-catalyst has been carried out in three-electrode configuration system, using 1N sulfuric acid (H₂SO₄) solution as the proton source as well as the electrolyte. Pt wire and Hg/Hg₂SO₄ are used as a counter electrode and reference electrode (+0.65 V with respect to the normal hydrogen electrode, NHE) respectively. Electrochemical evaluation has been conducted at a scan rate of 10 mV/sec and at temperature of 40^oC. The generated PGM-free electro-catalyst exhibits an onset potential of ~1.43 vs NHE, similar to that of IrO₂ but significantly lower charge transfer resistance (R_ct) than benchmark IrO_2. In addition, the as-synthesized electro-catalyst with F content of 10 wt. % displays a remarkable ~1.4 fold higher electro-catalytic OER activity (i.e. current density at 1.5 V) than that of IrO₂ approaching a current density of ~ 10 mA/cm² at an overpotential of ~ 245mV. Chronoamperometry tests conducted in 1N H₂SO₄ solution at ~1.5 V (vs NHE) for 24 hours also show marginal loss in current density, indicating good electrochemical stability of the as-prepared electro-catalyst. In summary, we have synthesized highly efficient PGM-free F doped TMN pnictide based electro-catalyst for OER in PEM based water electrolysis. The superior electrocatalytic activity of this electro-catalyst is attributed to the modification of the electronic structure (as evidenced by the theoretical studies) and lower charge transfer resistance (i.e. lower activation polarization). Thus, the present electro-catalyst system is indeed promising and beneficial for the economical and efficient hydrogen production from PEM water splitting. Results of these studies will be presented and discussed.

References:

[1] A. S. Aricò, V. Baglio, N. Briguglio, G. Maggio, S. Siracusano, Fuel Cells: Data, Facts and Figures 2016.

[2] S. D. Ghadge, P. P. Patel, M. K. Datta, O. I. Velikokhatnyi, R. Kuruba, P. M. Shanthi, P. N. Kumta, RSC Advances 2017, 7, 17311-17324.

[3] S. A. Grigoriev, V. I. Porembsky, V. N. Fateev, International Journal of Hydrogen Energy 2006, 31, 171-175.