The oxygen evolution reaction
The oxygen evolution reaction (OER) plays an eminent role in electrochemical science and technology as it is participate in many necessary applications, e.g., energy storage devices and energy conversion 1.This half cell reaction (i.e., the OER) act as a main source of over potential in the industrial water electrolysis processes.
The water electrolysis for hydrogen production requires a large amount of energy to drive the reaction. The most important key to increasing the efficiency of the water electrolysis system is to develop highly effective electrocatalysts for the OER 2. An electrocatalyst is a catalyst that is used to accelerate electrochemical reactions (reactions involving charge transfer, as the following equation :-
O + ne- R (1)
It can be accelerated by structural or chemical modification of the electrode surface, increasing the operating temperature and developing stable and cheap electrocatalysts that give higher rates of charge transfer 3,4,5.
In the past 20-30 years, huge evolution have been made in the development of efficient electro- catalysts including metal oxides (RuO2 and IrO2
based electrode) 6,7 , base metal (Co, Fe, Ni, Mn) 8,9 or hydroxides layers 10,11 and metal oxide carbon (CNTs) hybrid for the OER. The metal oxide based catalysts are relatively poor electric conductivity, so the carbon nanotubes (CNTs) used to support the metal oxides for the development of efficient OER catalysts because CNTs have high surface area, Corrosion resistant and high conductivity 1.
Principles of water electrolysis:-
Water electrolysis is the process when a water molecule split into oxygen and hydrogen electrically. The basic equation of water electrolysis can be described by the following equation:
2H2O 2H2 + O2 (2)
The overall process is consists of oxygen evolution reaction (OER) on the anode and hydrogen evolution reaction (HER) on the cathode of the electrolyzer. where OER could be difficult than HER because OER is participate in the transportation of four-electrons coupled with the generation of the O?O bond and split of the O?H bond (OER in alkaline medium described by Eqs.4), which needs high overpotential to overcome the high kinetic energy barriers whereas HER is participating in the transportation of only two-electrons for the formation of an H2 molecule. (HER in the alkaline medium described by Eqs.4) 12.
Fig. 1 Polarization curves for HER (left) and OER (right). The ?_c and ?_a are the overpotentials for cathode and anode at the same current ( j), respectively 13.
When the process is run in an alkaline solution (PH=14) , the Processes that occurs at the cathode and anode are described by Eqs.3 and 4:
4H2O + 4e 2H2 + 4OH- E0c = -0.826 V (3)
4OH- O2 + 2H2O + 4e E0a = 0.404 V (4)
In case of acidic solution , the Processes that occurs at the cathode and anode are described by Eqs.5 and 6:
2H+ + 2e H2 E0c =0.0 V (5)
2H2O O2 + 4H+ + 4e E0a = 1.23 V (6)
In neutral conditions (pH 7), the Processes that occurs at the cathode and anode are described by Eqs.7 and 8:
4H2O + 4e 2H2 + 4OH- E0c = -0.413 V (7)
2H2O O2 + 4H+ + 4e E0a = 0.817 V (8)
Where E0c and E0a are the equilibrium half-cell potentials at standard conditions of 1 atm and 25 0C. For water electrolysis, the equilibrium potential is 1.23 V at standard condition.
it is relatively Preferable in acid and alkaline conditions due to the existence of deprotonated water molecules available for HER in acid solutions or OER in alkaline solutions14.
2.1. Materials, Electrodes, pretreatments, and measurements
All chemicals used in this study were of analytical grade purchased from (Merch or sigma Aldrich).All solutions were used without further purification and prepared by double distilled water. Two compartments three electrode electrochemical cell was used for electrochemical measurements, These measurements were conducted in 0.5M KOH (pellets of potassium hydroxide) using a Bio-logic SAS potentiostat (model Sp-150) operated with EC-lab software. A pt spiral wire and Ag/AgCl/KCl (sat.) were used as a counter and reference electrodes. A Convential pretreatment methods were applied to clean the glassy carbon GC ( 5 mm in diameter ) electrode which is served as working electrode before the deposition of the metal oxide nanostructure15. Typically, the GC electrode was polished with fine emery paper, then with aqueous slurries of fine alumina powder using a polishing microcloth then washed thoroughly with double distilled water to remove any of alumina particles on the electrode.
2.2 Electrode’s modification
Cobalt oxide nanostructures (nano-CoOx) are electrodeposited onto the GC electrode by the cyclic voltammetry technique. Typically, the potential was biased using different cycles (10, 20, 30, 40, 50) with a scan rate of 100 mVS-1 in the potential range between 1.2 V and -1.1 V Vs Ag/AgCl/KCl (sat.) in 0.5M phosphate buffer solution ( 0.1M disodium hydrogen phosphate + 0.1M HCl) (PBS , PH=7) containing 2mM CoCl2 (Cobalt chloride).linear sweep voltammetry (LSV) were used to evaluate the electrocatalytic activity and stability of the nano-CoOx /GC electrodes towards the OER at different preparation and measuring temperature ( respectively Tp and Tm).
2.3 Materials Characterization
Field emission scanning electron microscope (FE-SEM, QUANTA FEG250) was employed to know the electrode’s morphology of the various modification GC electrodes, the GC surface had the following specifications (geometric area= 0.07 cm2 , d=3mm ,h= 3mm ) then modified with nano-CoOx at different cycles and different temperatures as described above, and placed in the FE-SEM chamber for surface analysis.