## 3 Results and Discussion
### Electrochemical Behaviour  
Figure 5 shows charge-discharge profiles of the three prepared samples at 30 mA g\({}^{-1}\), between 1.8 and 0.8 V. The observed behaviour of the different charge-discharge profiles of the three samples, KMn4, KMn8, and HKMn, are similar to that reported previously in the literature for Zn-manganese oxide batteries [18]. Figure 5b gathers the efficiency and specific capacity values for all samples. It can be observed that at current density values of 30 mA g\({}^{-1}\), all electrodes achieve capacity values lower than the theoretical one (308 mA h g\({}^{-1}\)). In the initial cycles, the following order in the capacity values is observed: KMn8 < KMn4 < HKMn. This result agrees with the average oxidation state obtained by XPS as well as the amount of K in the interlayer space commented above. Moreover, after 50 cycles, the specific capacity reached are 90, 105 and 110 mA h g\({}^{-1}\), which mean capacity losses of 12, 14, and 32% for KMn8, KMn4, and HKMn, respectively. Figure 6 shows the CV curves for assembled ZIBs using different currents for KMn8, KMn4 and HKMn. Their general aspect is similar to that previously found in other Mn-oxide ZIBs [19; 20; 21]. From the second cycle, two reduction peaks appear at 1.30 and 1.20 V. An oxidation peak at 1.55 V and a small shoulder at 1.65 V are also observed at lower speeds, i.e., 0.1-0.3 mV s\({}^{-1}\). The voltage values of each peak are dependent on the scan rate and the sample morphology. Thus, at higher rates (0.7 and 1.1 mV s\({}^{-1}\)) the CV curves exhibit one strong cathodic peak located at 1.28 V and the other peak is now seen as a  
Figure 5: (a) Charge–discharge profiles at 30 mA g\({}^{-1}\) and (b) specific capacity and efficiency for KMn8, KMn4 and HKMn during 50 cycles. shoulder at 1.18 V and even disappear for the sample HKMn. In the same way the anodic peak at 1.55 V disappears and the shoulder at 1.65 hexagonal (2H) increases in intensity. This gradual voltage change can be attributed to the ion diffusion [22]. At the discharge processes, as the CV rate increases the high-voltage peak remains present and even more intense (due to H\({}^{+}\) insertion), while the low-voltage peak gradually decays (related to Zn\({}^{2+}\) reaction). In order to study the performance at high density current, the three samples were cycled at a rate of 300 mA g\({}^{-1}\). Corresponding galvanostatic curves as well as capacity and efficiency versus cycle number are shown in Figure 7. In agreement with above statement, in the discharge stage the second plateau at this high speed is hardly seen for the three samples, which is related to Zn\({}^{2+}\) insertion. It is only weakly appreciated in the HKMn sample, in which Zn\({}^{2+}\) can be accumulated in the hollows between the leaves of the flower-like morphology. In addition, the capacity values obtained are in the same order than at low current density, i.e., KMn8 \(<\) KMn4 \(<\) HKMn. However, a different tendency is observed for capacity losses, which are practically negligible after 50 cycles. The slight increase of capacity observed in Figures 5 and 7 can be due to the Mn\({}^{2+}\) present in the electrolyte, that can be related to deposition of MnO\({}_{2}\) during the cycle as has been evidenced in the literature [23; 24]. As has been suggested [25], the high voltage plateau in the discharging process (i.e., at 1.4 V) is due to a conversion reaction between MnO\({}_{2}\) and H\({}^{+}\) leading to MnOOH as intermediate, which is dissolved in acid medium through a disproportion. Therefore, the reductive reaction can be described as follows:  
\[\mathrm{MnO_{2}(s)+H^{+}(aq)+1e^{-}\to MnOOH(s)} \tag{2}\]  
\[\mathrm{MnOOH(s)+H^{+}(aq)\to\frac{1}{2}Mn^{2+}(aq)+\frac{1}{2}MnO_{2}(s)+H_ {2}O(l)}. \tag{3}\]  
In consequence, during the discharge process both dissolved Mn\({}^{2+}\) and solid MnO\({}_{2}\) will form. Moreover, the H\({}^{+}\) are consumed in this reaction.