Effect of Bi Doping on the Kinetics of Nickel Oxides-Based Electrocatalyst in Alkaline Oxygen Evolution Reaction
Authors: Jo Seunghyun , Eom Kwang Sup 
Published: December 2024

As a generation process of hydrogen fuel, an alkaline water electrolysis is considered as one of the most promising pathway. The water electrolysis is able to divide into two half-cell reaction: hydrogen evolution reaction (HER) in cathode and oxygen evolution reaction (OER) in anode. However, high activation energy of OER is regarded as a major bottleneck for utilization of water electrolysis, therefore it is necessary to use an electrocatalyst to improve the reaction rate. Currently, commercially used electrocatalysts of OER is precious metal such as platinum (Pt), ruthenium (Ru), and iridium (Ir) based compound which have critical drawback for high cost and low stability in alkaline electrolyte.
As a substitute of platinum group materials for electrocatalyst of oxygen evolution reaction (OER), the development of cost-effective and high-active nickel oxide catalyst has attracted intense research interest. Alkaline OER is composed by five states: M, M-OH, M-O, M-OOH, and M + O2, and the gibbs free energy of final state is 4.92 eV higher than the first state. Thus, the free energy difference between each states equally becomes 1.23 eV to minimize the activation energy of rate determining step (RDS). In this point, nickel oxide shows a remarkable catalytic activity because a rapid and reversible electrochemical oxidation on the surface of nickel oxide triggers reconstructions to form hydroxide which is suitable structures for intermediate of OER. However, the nickel oxide still has high activation energy of reaction step from nickel(II) oxide to nickel(III) oxyhydroxide (3rd step of OER, Ni-O à Ni-OOH), which becomes RDS of OER. Because the catalytic activity seriously reduces without the balanced rate between each step of reaction, it is necessary to control the electric structure of nickel oxide to optimize adsorption energy of intermediates.
In this context, bismuth doped nickel oxide is synthesized for electrocatalyst of OER. Because bismuth has higher electronegativity than nickel, the adsorption energy of positively charged nickel with proton decreases and the adsorption energy of positively charged nickel oxide with hydroxide ion increases. Therefore, the activation energy of RDS of OER is expected to reduce than the pristine nickel oxide.
In practice, the bismuth doped nickel oxide is synthesized by hydrothermal process and the composition of bismuth is controlled by the concentration of precursors. The crystal structure of nickel oxide is maintained after the addition of bismuth, but the inter-planar spacing of overall plane of nickel oxide structure decreases according to selected area electron diffraction (SAED) pattern of transmission electron microscopy (TEM). In this context, the bismuth additive does not form the bi-metal oxide with nickel, but it substitutes the nickel site of nickel oxide structure. According to the nickel 2p3/2 spectra of x-ray photoelectron spectroscopy (XPS), a binding energy of nickel oxide peak positively shifts when the composition of bismuth additive increased due to the shifted electron from nickel to bismuth atom. As electrochemical measurement for evaluation of catalytic activity, the bismuth doped nickel oxide shows 59 mV lower overpotential at 20 mA cm-2 in alkaline OER, respectively, than the nickel oxide. In particular, the addition of bismuth is especially influenced to intrinsic activity, the bismuth doped nickel oxide catalyst shows higher turnover frequency than the pristine nickel oxide although it has similar active surface area.

Figure 1