Highly Efficient Fluorine (F) Doped Transition Metal Non-Oxide Pnictide (TMN) Based Electro-Catalyst for Oxygen Evolution Reaction in Alkaline Water Electrolysis
Authors: Ghadge Shrinath Dattatray , Patel Prasad Prakash , Velikokhatnyi Oleg I , Datta Moni Kanchan , Jampani Prashanth , Kumta Prashant N 
Published: February 2020

There is a critical need to develop clean, environmental friendly and sustainable energy sources to substitute the rapidly depleting conventional fossil fuels - causing serious global environmental concerns. In the pursuit of this need, hydrogen is attracting immense attention due to its clean, non- carbonaceous nature and also, high energy density as compared to petroleum based energy sources[
1
]. Over the past few decades, catalytic methane conversion, steam reforming and coal gasification etc. have been widely used for hydrogen production. However, the toxic greenhouse gases (CO/CO2) produced render these techniques environmentally less attractive. Therefore, hydrogen production from water splitting (H2O® H2+ 0.5 O2) is considered a far cleaner and more efficient technique among all other conventional hydrogen production methods. However, the efficiency of water electrolysis has been severely constrained by the energy intensive anodic oxygen evolution reaction (OER) (4OH−→ O2+ 2H2O + 4e−) which is the other important half reaction in addition to hydrogen evolution reaction (HER). To date, platinum group metal (PGM) based materials such as Pt, RuO2, IrO2 are well known for their superior electrochemical performance towards OER. Commercial implementation of these electrocatalsts for water splitting has been largely limited due to their high cost and scarcity[
2
]. Thus, identification and development of earth abundant, cheap and PGM-free electro-catalysts, demonstrating excellent electro-catalytic activity and robust stability (for OER); comparable/superior to state-of-the art PGM/noble metals based electro-catalysts is critically desired[
3
].

Therefore, in the present study, exploiting theoretical first principles approaches, we have engineered anionic fluorine (F) doped transition metal non-oxide pnictide (TMN) based electro-catalyst for OER in alkaline mediated water electrolysis. We have observed remarkable enhancement in the electro-catalytic activity by incorporation of F into the as-synthesized TMN based electro-catalyst.

The electro-catalyst powder with F content of 5 wt. % (Fig. 1) has been synthesized using solid state approaches. The as-synthesized electro-catalyst powder was coated on porous titanium (Ti) substrate (total catalyst loading=1 mg/cm2) and used as an anode. The electro-catalyst with F content of 5 wt. % exhibited excellent electro-catalytic performance, outperforming the benchmark IrO2 for OER in alkaline water splitting.

The electrochemical response of the electro-catalysts has been evaluated in a three-electrode configuration system, using 1N KOH solution. Correspondingly, 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. Electrochemical characterization of the electrocatalyst has been performed with a scan rate of 10 mV/sec at 40oC. The generated PGM-free electro-catalyst exhibited significantly lower charge transfer resistance (Rct) than benchmark IrO2. In addition, this PGM-free electro-catalyst displayed remarkable ~7.6 fold higher electro-catalytic OER activity (i.e. current density, at 1. 55V) than that of IrO2 and reached a current density of ~ 10 mA/cm2 at an overpotential of ~ 300 mV. The chronoamperometry test conducted in 1N KOH solution at ~1.55 V (vs NHE) for 24 hours shows the minimal loss in current density, signifying a robust electrochemical stability of the as-prepared electro-catalyst.

In summary, we have synthesized high performance PGM-free F doped TMN pnictide based electro-catalyst for alkaline mediated water splitting. The observed enhanced electrocatalytic activity of this electro-catalyst is mainly attributed to the modified electronic structure (as supported by the theoretical study) and lower charge transfer resistance (i.e. lower activation polarization). Based on these results we believe that this PGM-free electro-catalyst is indeed a promising and reliable system for sustainable and economic hydrogen production. Results of this work will be presented and discussed. References:

[1] M. Momirlan, T. N. Veziroglu, Renewable and Sustainable Energy Reviews
2002, 6, 141-179.

[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] M. K. Datta, K. Kadakia, O. I. Velikokhatnyi, P. H. Jampani, S. J. Chung, J. A. Poston, A. Manivannan, P. N. Kumta, Journal of Materials Chemistry A
2013, 1, 4026-4037.

Figure 1