- Electrochemical capacitors and batteries are two major electrochemical energy storage technologies, which have been investigated extensively to meet the rapidly-growing demand for higher energy, higher power, lower cost and enhanced safety in the past few decades. With the charge storage mechanism of electrostatic charge adsorption desorption via electrical double layers, electrochemical capacitors deliver higher power, but store less energy, compared to batteries, where redox reactions usually take place inside bulk electrode materials. Depending on the electrolytes, electrochemical capacitors can be divided into aqueous and non-aqueous capacitors. Aqueous electrolytes are more electrically conductive, non-flammable, and more sustainable, compared to non-aqueous electrolytes. However, non-aqueous electrolytes are overwhelmingly dominating the electrochemical capacitor markets, because they provide a larger electrochemical window, and consequently enable capacitors to store more energy. To take advantages of aqueous electrolytes and facilitate safer and more sustainable electrochemical capacitors, tremendous research has been conducted toincrease energy density of aqueous capacitors. Traditional approach is to utilize redox-active electrodes, e.g. metal oxide and conducting polymers in pseudocapacitors, most of which increase energy density at the expense of largely sacrificing power and cycle life. It is important to make progress in one performance aspect of electrochemical capacitors, while retaining other desirable properties as much as possible.There are two effective ways to store more energy in electrochemical capacitors. One is by increasing capacitance, and the other one is by increasing operating voltages.Higher capacitance can be obtained when introducing redox reactions in electrochemical capacitors. Instead of employing redox-active electrodes, which may experience ion diffusion in solid, aqueous redox-active electrolyte was studied to retain high power while storing more energy. The redox pair of IOx- I- in 4 M KI and 1 M KOH is reported for the first time, which enables aqueous capacitors to store a maximum energy of 7.1 Wh kg, on a par with state-of-the-art non-aqueous capacitors, while delivering a maximum power of 6222 W kg, and retaining 93% capacitance after 14,000 cycles.Higher operating voltages are realized in aqueous electrochemical capacitors by maintaining pH 1 and pH 10 at the positive and negative electrode, respectively, with a bipolar assembly of ion-exchange membranes. The theoretical electrochemical window of aqueous electrolytes is expanded from 1.23 to 1.76 V. A practical operating voltage of 1.8 V is proved to be safe for aqueous capacitors with the bipolar assembly,which allows to store a specific energy of 12.7 Wh kg, as well as retain 97% capacitance after 10,000 cycles.Although batteries, especially lithium-ion batteries, have been successful in different fields, e.g. portable electronics, electric vehicles, etc., an intrinsic drawback still exists: low abundance of lithium and therefore high cost of lithium-ion batteries. To address this issue, different types of batteries have been studied, which utilize Earth-abundant elements, such as Na, K, Al, etc. In this dissertation, a novel battery is reported, where hydronium ions perform as charge carriers. For the first time, hydronium ions are found to be reversibly stored in 3,4,9,10-perylenetetracarboxylic dianhydride crystals, contributing 85 mAh g at 1 A g after an initial conditioning process. As an aqueous battery storing hydronium ions instead of metal cations, it may deliver higher power, significantly lower the battery cost, and increase the margin of safety. Although this technology is not as mature as lithium-ion batteries, it provides new opportunities and possible solutions for future energy storage.A new deposition technology, namely ambient hydrolysis deposition, is also studied in this dissertation, which enables nanoparticles grown in porous substrate in a simple and cost-effective way. As a proof-of-concept, by controlling the amount of pre-adsorbed water vapor in the porous carbon, various amount of TiO2 nanoparticles are grown in porous carbon. The TiO2 nanoparticles can be converted into TiN nanoparticles by nitridation, which improve the electrical conductivity of porous carbon. Electrodesprepared from porous carbon with TiN nanoparticles coating exhibit enhanced rate capability in electrochemical capacitors.