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This study focuses on aqueous electrochemical capacitors (ECs) for optimising the cell voltage and understanding the charge storage mechanism of transition metal oxides and hydroxides. The first part discusses how to optimise cell voltage of symmetric and asymmetric ECs. In Chapter 4, a commercial activated carbon (ACS–679) was employed as an electroactive material to demonstrate the necessity of charge–balanced condition of aqueous symmetric ECs in order to improve the utilisation of electrochemically stable potential window of electroactive material. Besides, the effect of stable open–circuit potential on electroactive materials is also addressed here. The concept is on the basis of electrochemical charge–balance principle, therefore, it is believed to be applicable to all the symmetric ECs no matter what kind of material is used as electroactive material. The charge–balanced electric double–layer capacitors (EDLCs) was assembled to evaluate the acceptable cell voltage by means of electrochemical analyses, including cyclic voltammogram, constant current charge–discharge, electrochemical impedance spectroscopy and inductance–capacitance–resistance meter measurements. Moreover, charge–unbalanced EDLCs were used to demonstrate the necessity of charge balance in symmetric ECs since those exhibit lower cell voltage and specific energy compared with the charge–balanced case. Similarly, in Chapter 5, asymmetric ECs consisting of reduce graphene oxide and manganese oxide (RGO//MnOx) were employed as an example to establish criteria for determining the highest acceptable cell voltage of asymmetric ECs with excellent reversibility and capacitor−like behaviour. It is very useful to evaluate the practical specific energy of asymmetric ECs by understanding the highest acceptable cell voltage. Besides, RGO//RuO2 was also demonstrated to verify the validity of the proposed criteria. The second part probes the charge storage mechanism of transition metal oxides and hydroxides in aqueous ECs. In Chapter 6, a wide–ranging study on electrochemical activation from Mn3O4 to MnO2 in 0.5 M Na2SO4 medium were presented, which includes in operando monitoring of the structural evolution during the activation process via in situ Raman microscopy. Other advanced material characterisation techniques, such as powder X–ray diffraction, X–ray photoelectron spectroscopy, scanning electron and transmission electron microscopy, were also applied to analyse the as–prepared and activated MnOx. Due to the structural disordering of electrochemically activated birnessite−MnO2 and residual Mn3O4, the charge storage is attributable to the redox reaction between Mn(III) and Mn(IV) at outer surface active sites, rather than cations and/or protons intercalation into layer structures. In Chapter 7, the redox behaviours of γ−NiOOH/α−Ni(OH)2 in various electrolytes (LiOH, NaOH, KOH, CsOH and NH4OH) are discussed. The charge storage mechanism of γ−NiOOH/α−Ni(OH)2 was studied by means of EQCM and in situ Raman microscopy. Moreover, the computational simulation (DFT+U) based on EQCM results gives a better idea on the compositional changes in the first few potential cycles. The insertion/removal of Li+ does not alter the content of water inside the structure, while the insertion of Na+ and K+ leads to a significant removal of water. The removed water molecules cannot be reversibly re−inserted back into the nickel structure, leading to the loss of water molecules in Ni(OH)2 structure during charge–discharge process. The capability to retain water molecules inside the Ni(OH)2 is crucial for the stability of γ−NiOOH/α−Ni(OH)2 redox reaction.