Electrodeposited Na-Birnessite on Carbon Cloth as Positive Electrode for Capacitive Deionization

Capacitive Deionization (CDI) based on traditional activated carbon (AC) electrodes faces some important intrinsic hurdles, such as the co-ion expulsion phenomenon and unwanted faradaic reactions, harming efficiency, operational stability, and electrode lifetime. The incorporation of ion-exchange membranes (IEM) in CDI, as free-standing films applied onto the electrodes, was shown to be an effective solution to improve charge efficiency and has led in fact to the commercialization of MCDI (membrane-CDI). An alternative way to improve CDI performance is the use of ion insertion materials, such as metal oxides and layered double hydroxides. In this work, we examine the performance of sodium-birnessite electrodeposited on commercial carbon cloth (CC) as the positive electrode of a flowby CDI cell, coupled to an ordinary AC / AEM stack as the negative electrode

Evaluation of Co-Ion Desorption and Faradaic Losses in Capacitive Deionization

The efficiency of Capacitive Deionization (CDI) is largely determined by the charge loss associated with two distinct parasitic processes, namely, co-ion expulsion and faradaic reactions. There is wide agreement that the first factor dominates the inefficiency of CDI; however, the evaluation of the relative incidence of co-ion repulsion on CDI inefficiency remains somewhat elusive. In this work, in the assumption of relatively small ohmic losses, we propose a simple model to quantify charge losses due to either of these processes in CDI cells, disregarding ohmic losses

Boosting the Power of Na0.44MnO2: Unlocking Its Potential for Aqueous Sodium-Ion Storage through Nanostructuring and Hybridization

We report an effective processing route, combining nanostructure formation and hybridization, to improve the rate performance of the tunnel-structure sodium manganese oxide Na0.44MnO2 (NMO) as a cathode material for aqueous sodium ion storage. We use hydrothermal synthesis to prepare an NMO/CNF (Carbon NanoFiber) hybrid, consisting of uniform oxide nanowires with an average width of 70 nm and length in the range of several tenths of µm. The highly dispersed CNFs impart high conductivity to the NMO/CNF electrode, allowing high-rate performance at a C-rate of up to 20 C, with a delivered capacity of more than half the theoretical value in a 1 M Na2SO4 electrolyte. Moreover, the NMO/CNF hybrid shows good electrochemical stability under several hundred cycles at a high C-rate. However, the NMO nanowire electrodes reveal a lower-than-expected capacity, probably as a result of the tendency of nanowires to form bundles, which prevents direct contact with conductive fibers and induce the under-utilization of active material. With this study, we demonstrate a strong improvement of the otherwise inherently low-rate performance of NMO through oxide nanostructuring and hybridization with carbon fibers, paving the way for further research on NMO-based materials for aqueous sodium ion storage

Evaluation of the operating potential window of electrochemical capacitors

Widening of the operating potential window is a straightforward route towards increasing the specific energy of electrochemical capacitors. Usually, the assessment of the viable potential range is committed to thermodynamic considerations over the electrolyte potential window and experimental probing of the electrochemical stability of both electrolyte and electrode materials through cyclic voltammetry. However, while the former approach is too conservative, the latter is prone to failure. In the present work, we consider quantitative approaches for monitoring the influence of the potential window on the dissipative behaviour of aqueous electrochemical capacitors. For proving the concept underlying this work, we analyse as a case study an asymmetric cell with a reduced graphene oxide / MnO2 positive electrode and a carbon nanotube based negative electrode, as well as nominally symmetric cells assembled with either electrode. We apply and compare different procedures to define the safe potential window of these systems: the usual potential window opening technique, applied either to single electrodes or packed cells; and not conventional methods, based on the analysis of either the energy efficiency or the cell impedance as a function of the cell potential. Precisely, we analyse the trend of the energy efficiency, derived from galvanostatic charge / discharge experiments, and that of the real component of the cell impedance vs. the potential window, to discriminate the onset of irreversible processes leading to dissipative losses. The viability of the proposed methods and the reliability of the attendant criteria are finally checked in the light of the results of cycling performance of the asymmetric cell

Boosting the Power of Na_0.44MnO₂: Unlocking Its Potential for Aqueous Sodium-Ion Storage through Nanostructuring and Hybridization

We report an effective processing route, combining nanostructure formation and hybridization, to improve the rate performance of the tunnel-structure sodium manganese oxide Na0.44MnO2 (NMO) as a cathode material for aqueous sodium ion storage. We use hydrothermal synthesis to prepare an NMO/CNF (Carbon NanoFiber) hybrid, consisting of uniform oxide nanowires with an average width of 70 nm and length in the range of several tenths of µm. The highly dispersed CNFs impart high conductivity to the NMO/CNF electrode, allowing high-rate performance at a C-rate of up to 20 C, with a delivered capacity of more than half the theoretical value in a 1 M Na2SO4 electrolyte. Moreover, the NMO/CNF hybrid shows good electrochemical stability under several hundred cycles at a high C-rate. However, the NMO nanowire electrodes reveal a lower-than-expected capacity, probably as a result of the tendency of nanowires to form bundles, which prevents direct contact with conductive fibers and induce the under-utilization of active material. With this study, we demonstrate a strong improvement of the otherwise inherently low-rate performance of NMO through oxide nanostructuring and hybridization with carbon fibers, paving the way for further research on NMO-based materials for aqueous sodium ion storage

Charging processes of Na4Mn9O18 electrode in aqueous electrolyte

Recent trends in electrochemical energy storage –the renewed interest in aqueous electrolytes, the development of nanostructured and/or hybridized materials, the advent of unconventional systems– call for detailed analyses of charging processes. We address this issue in studying a sodium manganese oxide (Na4Mn9O18, NMO) electrode in aqueous environment. Charge storage is examined by cyclic voltammetry (CV) in a wide range of sweep rate (ν) and by equivalent circuit modelling of the electrode impedance response. Voltammetry shows that, with increasing ν, the insertion process evolves from a quasi-equilibrium behavior (ν ≤ 0.1 mV s–1) towards a diffusion controlled regime overlapping with capacitive charging (ν ≥ 0.2 mV s–1), and culminates at even higher rate (ν > 2 mV s–1) in mixed mass transport ohmic control. Impedance analysis permits to discriminate the varying character of charge storage, revealing the low frequency dominance of faradaic insertion and the rising contribution of pseudocapacitive and double layer charging at higher frequency. We show that the frequency decomposition of charging mechanisms obtained by this analysis can be reconciled with the CV analysis. For further clarification of the above analysis in particular, and as a relevant aspect of the NMO behavior in general, we evaluate the chemical diffusion coefficient of Na-ion as a function of potential