Cyclic voltammetry

CV is one of the widely used techniques to study the electrochemical performance of the electrode and battery.³⁸ The CV test allows important data such as peak potentials, peak separation, peak current, and peak reversibility to be gathered. These information are beneficial to predetermine the electrochemical behavior of the electrode or the cell before the actual charge–discharge test. There are two different setups for CV testing, as shown in Figure 12b. A three-electrode CV consists of three different electrodes, which are the working electrode (WE), the counter electrode (CE), and the reference electrode (RE). The three-electrode setup can be used to examine the electrochemical performance of each individual electrode before being assembled as a complete cell. In this setup, the WE is connected to the sample, whereas the CE is connected to an inert electrode, preferably platinum or any other alternative such as graphite [Fig. 12b(i)]. To run the three-electrode setup for anode CV, the RE is Hg/HgO in NaOH solution, the CE is a platinum electrode, and the WE is the printed ZnO. For cathode CV, the WE is connected to the printed Ni(OH)₂, whereas the other two electrodes are unchanged.^39,40

The electrochemical behavior of the full cell at a specific potential window can be observed using the two electrode setup. In this configuration, WE is connected to the sample under test, whereas the CE and RE are connected together as a single node [Fig. 12b(ii)]. In this case, the ZnO anode is connected to WE, and CE/RE is connected to Ni(OH)₂ cathode.

CV can be performed to examine electrode stability, efficiency, and impedance to improve the cell's performance. There are also variations in the numbers of the theoretical specific energy of Ni-Zn, which is 334 Wh/kg compared with an actual specific energy of the Ni-Zn battery, which is in between 70 and 110 Wh/kg, or around 21% and 33% of the theoretical specific energy.²⁸ Another important feature for rechargeable batteries is the reversibility of the electrode. This feature can also be studied using CV. Ideally, the output of cyclic voltammograms should be in the form of a “duck shape,” as shown in Figure 13(i), whereby the anodic (i_pa) and cathodic (i_pc) peak currents are equal. Besides that, the peak-to-peak separation potential for both anodic (E_pa) and cathodic (E_pc) peak must be within 57 mV⁴¹ to signal that the electrode has a reversible redox reaction. The location of the anodic and cathodic peaks can sometimes be interchanged depending on the accepted convention (United States or International Union of Pure and Applied Chemistry) for anodic and cathodic reactions.⁴¹

(i) Duck shape cyclic voltammogram. (ii) Example of cyclic voltammogram for zinc oxide converted to IUPAC convention.³⁹ (iii) Example of cyclic voltammogram for nickel hydroxide (IUPAC convention).⁴⁰ IUPAC, International Union of Pure and Applied Chemistry. Color images are available online.

In reality, an ideal cyclic voltammogram is very difficult to achieve since it requires an entirely reversible electrochemical system that consists of electrodes, electrolytes, and other items such as separator and additives. Other factors such as cost-effectiveness, practicality, safety, environment friendliness, and availability of resources need to be considered before the optimization of the electrodes to fit the ideal CV curve can be done. The printed zinc oxide anode's cyclic voltammogram should be close to the curve of the conventional zinc oxide CV curve, as shown in Figure 13(ii)³⁹ for both zinc and zinc oxide electrodes with a few additives added to the KOH such as bismuth(III) oxide (Bi₂O₃), lithium hydroxide (LiOH), and sodium carbonate (Na₂CO₃).

The potential sweep was from −1.6 to 0 V with a sweep rate of 100 mV/s. The peak potential for the anodic reaction is located somewhere around −1 V, whereas the cathodic potential peak is located around 1.5 V. Validation of the printed zinc oxide anode can be done by comparing the measurement results to this curve.

Similar methods can be applied to the anode,⁴⁰ where the CV analysis can be compared with measurements conducted for Ni(OH)₂, as shown in Figure 13(iii). The sweep potential was set to be between 0 and 0.6 V with a scan or sweep rate of 10 mV/s. Different scan rates can affect the peak current output for the CV curve without horizontally shifting the peak potential.

Thus, it is a good practice to just stick with a single scan rate that is found to be the most convenient. CV curves at lower scan rates are time-consuming and can be challenging to be analyzed, whereas higher scan rates are deemed faster but may lose some essential details of the curve, such as the noise and minor fluctuations. The anodic peak potential of Ni(OH)₂ is located somewhere around 0.5 V and cathodic peak potential around 0.4 V. Thus, it is expected that the printed nickel hydroxide cathode should exhibit similar characteristics.

The maximum battery charging and nominal voltages can be calculated using the oxidation–reduction peak potentials after both half-cell CV results for anode and cathode have been obtained,. The oxidation peak potential of cathode, E_opc, and reduction peak potential of the anode, E_rpa, can be used in Equation (10) to calculate the estimated charging voltage. The estimated nominal voltage can be obtained by subtracting the oxidation peak potential of the anode, E_opa, from the reduction peak potential of the cathode, E_rpc, as shown in Equation (11).

Besides the use of CV, there is another test method that can be used to characterize the batteries before subjecting them to charge–discharge testing, known as EIS. This is a more sophisticated method of testing an electrochemical cell and is used to gain more data regarding the cell, such as electrode resistance, electrolyte resistance, charge-transfer resistance, polarization resistance, diffusion, and double layer capacitance. These information are determined from the measured Nyquist plot and by fitting the data to an equivalent circuit. The data obtained using EIS can be used to complement the CV data.⁴²