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Researches on electrochemical energy storage devices with a high energy density have attracted the tremendous attention in recent years due to the ever-growing energy demands and rapid development of electric vehicles. Carbon-based materials are widely employed as the electrode materials in supercapacitors (SCs) which are promising devices for highly efficient energy and power managements. However, such devices suffer from the moderate energy densities in common organic liquid electrolytes compared to secondary batteries, significantly limiting their future applications. To circumvent this issue, enlarging the working cell voltage of SCs is a more effective strategy for increasing the energy density in comparison with enhancing the specific capacitance. This critical review points out the potential strategies for enlarging the working cell voltage and surveying recent achievements of high cell voltage SCs obtained through the modification and development of hybrid systems. Various factors influencing the operating cell voltage of carbon-based materials are discussed from the intrinsic property viewpoints of carbon. Consequently, we make an in-depth summary on various promising approaches of significant breakthroughs in recent years and provide the strategies to choose suitable carbon materials or modification methods for approaching the high-voltage SCs. The AC-based EDLCs employing common organic liquid electrolytes suffer from the moderate energy density because of the relatively low cell voltages of 2.5–2.7 V. Enlarging the working cell voltage of SCs, achieved by different combinations among electrode materials and electrolytes, is a more effective strategy for increasing the energy density in comparison with enhancing the specific capacitance of electrode materials. In addition, there are four general strategies: change in electrolytes, control of surface functional groups on carbons, modification of carbon surface, and asymmetric/hybrid designs, to achieve the high-voltge SC purpose. Image 10517932 [ABSTRACT FROM AUTHOR]

Abstract In capacitive deionization (CDI) systems, more Faradaic reactions generally occur at a higher cell voltage, resulting in the poor ion-removal efficiency and high energy consumption, but a low cell voltage may lead to a low deionization capacity. In this study, an integrated strategy, consisting of (1) selecting the acceptable charge/discharge time ratio, (2) finding the potential windows of both positive and negative electrodes, and (3) using the charge-balanced method for determining the optimal mass ratio between the positive and negative electrodes (denoted as (m+)/(m−)), is proposed to find the optimal cell voltage of a CDI system with a high desalination efficiency and low energy consumption. According to this strategy, two newly designed cells using two types of activated carbon (AC) show excellent CDI performances; cell 1 with AC = ACS679, (m+)/(m−) = 1:1.4, and cell voltage = 1.4 V: salt adsorption capacity of 12 mg g−1, charge efficiency of 62%, and energy consumption of 109 kJ mol−1 and cell 2 with AC = ACS25, (m+)/(m−) = 1:1, and cell voltage = 1.2 V: salt adsorption capacity of 13 mg g−1, charge efficiency of 81%, and energy consumption of 72 kJ mol−1 although the potential window of water decomposition for such AC-coated electrodes in 8 mM NaCl is about 2 V. This universal strategy is applicable to all CDI systems utilizing various electrode materials. Graphical abstract Image 1 Highlights • We demonstrate a universal strategy for constructing the AC//AC cell with high CDI performances. • The same charge/discharge time ratio with high reversibility on both electrodes is the key. • Influences of two-electrode mass ratio on the desalination performances are studied. • The pH change is affected by the Faradaic reactions from the potential windows of electrodes. [ABSTRACT FROM AUTHOR]

Abstract A dual-anode consists of stainless steel and TiO 2 /Ti electrodes is used to study the kinetics of the degradation of hazardous chemicals exemplified by azo dye orange G (OG) using a coupling photoelectrochemical catalytic and photoelectro-Fenton (PEC/PEF) system. Concurrent generation of hydroxyl radicals on the TiO 2 /Ti photocatalyst and in-situ generation of Fenton reagents on the stainless steel electrode greatly enhances the performance of the PEC/PEF electrodes over that of the PEC and the PEF alone process. The efficiency of the PEC/PEF process is a function of Fe2+ and H 2 O 2 concentration OH⋅ in the solution bulk, which promotes the oxidative degradation of OG and its byproducts. The mean carbon oxidation state (COS) is estimated to reflect the degree of mineralization. Based on the pseudo first-order kinetics with respect to OH , OG, Fe2+, the corresponding reaction rates is established. UV–Vis spectrometry reveals the presence of four major intermediates, which helps establish the OG degradation pathways. Graphical abstract Unlabelled Image Highlights • Coupling photoelectrochemical and photo-electro-Fenton process effectively degrades OG. • UV-Vis spectrometry reveals the presence of four major intermediates • Pseudo first-order kinetics describes well OG degradation. • Carbon oxidation state (COS) is a convenient indicator for the extent of mineralization. • Constant and sustaining OH⋅ attack of OG arrives at high degree of mineralization. [ABSTRACT FROM AUTHOR]

The electrochemical reduction of CO 2 is strongly influenced by both the applied potential and the surface adsorption status of the catalyst. In this work a gas diffusion electrode (GDE) coated with Pd nanoparticles/carbon black (Pd/XC72) was used to study the electrochemical reduction of CO 2 . Cyclic voltammetric (CV) analysis of Pd/XC72 between 1.5 V and − 0.6 V (vs. RHE) shows the formation of intermediates and the blocking of hydrogen absorption on the Pd nanoparticles (NPs) under a CO 2 atmosphere. The relationships between the Faradaic efficiency/current density and the applied potential reveal that the onset potential of CO formation is around − 0.4 V. Moreover, the presence of adsorbed CO was confirmed through CV analysis of Pd/XC72 under CO 2 and CO/He atmospheres. This demonstrates that H atoms and CO intermediates co-adsorb on the surface of the Pd NPs at an applied potential of around − 0.4 V. When the applied potential is more negative than − 0.6 V, adsorption of CO intermediates on the surface of the Pd NPs becomes dominant. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate that vertically grown ZnO nanowire (NW) arrays of the wurzite phase were successfully fabricated on fluorine doped tin oxide (FTO) substrates via a hydrothermal method. The coating of a seed layer onto the FTO substrates was found to favor the growth of a uniform ZnO NWs array which shows saturation in the photocurrent density with a relatively low potential bias. Furthermore, prolonging the calcination time of the seed layer makes the ZnO NWs behave the better charge separation and improve the photo-electrochemical performance. Under the irradiation at a 75 mW cm −2 from a simulated sunlight source, the ZnO NWs array electrode prepared from the seed layer with calcination at 350 °C for 5 h shows a saturated photocurrent density of 514 μA cm −2 and a maximum half-cell solar-to-hydrogen (HC-STH) efficiency of 0.26% was obtained at 0.6 V versus reversible hydrogen electrode (RHE) in neutral electrolyte. [ABSTRACT FROM AUTHOR]

The degradation of azo dye Orange G (OG) on TiO 2 /Ti electrode was studied using a photoelectrocatalytic system. TiO 2 /Ti electrode was prepared by electrophoretic deposition (EPD) process. Results of surface analyses, namely, X-ray diffraction (XRD), SEM, X-ray photoelectron spectroscopy (XPS), and linear sweep voltammetry (LSV), revealed that the optimal EPD condition was a deposition potential of 180 V for 1 min and annealed at 350 °C. The TiO 2 /Ti electrode was used as photoanode and a graphite cathode for the degradation of OG. The coupling effect of photocatalytic and electrochemical oxidation processes was evaluated in terms of the decolorization of OG. The degradation of OG increased in the following increasing order: photocatalysis < electrochemical oxidation < photoelectrochemical degradation. A decolorization of 54.3% was achieved at an applied current density of 75 μA cm −2 . The degradation of Orange G follows the first-order kinetics in those three treatment processes, which suggests the participation of OH radicals in the decolorization reaction. [ABSTRACT FROM AUTHOR]

An inverted-capacitive deionization (i-CDI) cell of MnO 2 //polypyrrole-coated activated carbon (MnO 2 //PPy@AC) with pseudocapacitive characteristics enables unique salt-removal/salt-concentration capability. The salt adsorption capacity (SAC) of MnO 2 //PPy@AC highly depends on the discharging time and a high SAC of 27.6 mg·g−1 is obtained at the 60-min discharging time. The SAC retention of this unique cell is 100% after 10 charge/discharge cycles at 1.2/0 V. The "memory effect" of MnO 2 //PPy@AC is confirmed by the unchanged conductivity of the cell when the applied cell voltage is interrupted, which has been demonstrated for salt-removal and salt-concentration applications. Unlabelled Image • The pseudocapacitive MnO 2 //PPy system is demonstrated to be an ideal i-CDI cell. • A unique "memorized" capability of MnO 2 //PPy@AC avoids to release counter ions at OCP is demonstrated. • The ion-adsorption time is recognized as the key factor determining the SAC of MnO 2 //PPy@AC. • The memorized MnO 2 //PPy@AC cell shows the salt-removal/-concentration applications. [ABSTRACT FROM AUTHOR]

Here we develop a novel but fundamental strategy for optimizing the ion-removal capacity of electrode materials in capacitive deionization (CDI) and inverted-capacitive deionization (i-CDI) systems without sacrificing their coulombic efficiency. The most efficient working cell voltages of different systems can be precisely determined by our newly designed cell voltage-programming method. We employ a palladium hydride reference electrode to monitor the electrode potentials of positive and negative electrodes during the double-layer charging-discharging processes. Accordingly, charge balance and maximum working cell voltages of CDI and i-CDI systems in brackish water can be achieved simultaneously to optimize the desalination efficiencies. Hence, the ion removal capability of an activated carbon//activated carbon (AC//AC) cell in 0.3–1.9 V reaches 12.2 mg g −1 with minor pH change in deionized solutions. The asymmetric CDI cell consisting of a positive graphene oxide-CNT composite and a negative AC electrode (GO@CNT//AC) is the most efficient CDI system with ion removal capacity of 21.3 mg g −1 in 0.2–1.45 V. The asymmetric AC//reduced GO-CNT composite (AC//rGO@CNT) cell is an exceptional i-CDI system which exhibits ion removal ability of 25.2 mg g −1 in 1.0–1.7 V. The above concepts in optimizing CDI/i-CDI performances are applicable to all types of electrode materials in various cell designs. [ABSTRACT FROM AUTHOR]