Pattarachai Srimuk

MXene as a novel intercalation-type pseudocapacitive cathode and anode for capacitive deionization

In this proof-of-concept study, we introduce and demonstrate MXene as a novel type of intercalation electrode for desalination via capacitive deionization (CDI). Traditional CDI cells employ nanoporous carbon electrodes with significant pore volume to achieve a large desalination capacity via ion electrosorption. By contrast, MXene stores charge by ion intercalation between the sheets of its two-dimensional nanolamellar structure. By this virtue, it behaves as an ideal pseudocapacitor, that is, showing capacitive electric response while intercalating both anions and cations. We synthesized Ti3C2-MXene by the conventional process of etching ternary titanium aluminum carbide i.e., the MAX phase (Ti3AlC2) with hydrofluoric acid. The MXene material was cast directly onto the porous separator of the CDI cell without added binder, and exhibited very stable performance over 30 CDI cycles with an average salt adsorption capacity of 13 ± 2 mg g−1.

Faradaic deionization of brackish and sea water via pseudocapacitive cation and anion intercalation into few-layered molybdenum disulfide

This work establishes molybdenum disulfide/carbon nanotube electrodes for the desalination of high molar saline water. Capitalizing on the two-dimensional layered structure of MoS2, both cations and anions can be effectively removed from a feed water stream by faradaic ion intercalation. The approach is based on the setup of capacitive deionization (CDI), where an effluent water stream is desalinated via the formation of an electrical double-layer at two oppositely polarized carbon electrodes. Yet, CDI can only be effectively applied to low concentrated solutions due to the intrinsic limitation of the electrosorption mechanism. By replacing the conventional porous carbon with MoS2/CNT binder-free electrodes, deionization of sodium and chloride ions was achieved by ion intercalation instead of ion electrosorption. This enabled stable desalination performance over 25 cycles in various molar concentrations, with salt adsorption capacities of 10, 13, 18, and 25 mg g−1 in 5, 25, 100, and 500 mM NaCl aqueous solutions, respectively. This novel approach of faradaic deionization (FDI) paves the way towards a more energy-efficient desalination of brackish water and even sea water.

Concentration-gradient multi-channel flow stream membrane capacitive deionization cell for ultra-high desalination capacity of carbon electrodes

We present a novel multichannel membrane flow-stream capacitive deionization (MC-MCDI) concept with two flow streams to control the environment around the electrodes and a middle channel for water desalination. The introduction of side channels to our new cell design allows operation in a highly saline environment, while the feed water stream in the middle channel (conventional CDI channel) is separated from the electrodes with anion- and cation-exchange membranes. At a high salinity gradient between side (1000 mm) and middle (5 mm) channels, MC-MCDI exhibited an unprecedented salt-adsorption capacity (SAC) of 56 mg g-1 in the middle channel with charge efficiency close to unity and low energy consumption. This excellent performance corresponds to a fourfold increase in desalination performance compared to the state-of-the-art in a conventional CDI cell. The enhancement originates from the enhanced specific capacitance in high-molar saline media in agreement with the Gouy-Chapman-Stern theory and from a double-ion desorption/adsorption process of MC-MCDI through voltage operation from -1.2 to +1.2 V.

High performance stability of titania decorated carbon for desalination with capacitive deionization in oxygenated water

Performance stability in capacitive deionization (CDI) is particularly challenging in systems with a high amount of dissolved oxygen due to rapid oxidation of the carbon anode and peroxide formation. For example, carbon electrodes show a fast performance decay, leading to just 15% of the initial performance after 50 CDI cycles in oxygenated saline solution (5 mM NaCl). We present a novel strategy to overcome this severe limitation by employing nanocarbon particles hybridized with sol–gel-derived titania. In our proof-of-concept study, we demonstrate very stable performance in low molar saline electrolyte (5 mM NaCl) with saturated oxygen for the carbon/metal oxide hybrid (90% of the initial salt adsorption capacity after 100 cycles). The electrochemical analysis using a rotating disk electrode (RDE) confirms the oxygen reduction reaction (ORR) catalytic effect of FW200/TiO2, preventing local peroxide formation by locally modifying the oxygen reduction reaction.

Pseudocapacitive Desalination of Brackish Water and Seawater with Vanadium-Pentoxide-Decorated Multiwalled Carbon Nanotubes

A hybrid membrane pseudocapacitive deionization (MPDI) system consisting of a hydrated vanadium pentoxide (hV2 O5 )-decorated multi-walled carbon nanotube (MWCNT) electrode and one activated carbon electrode enables sodium ions to be removed by pseudocapacitive intercalation with the MWCNT-hV2 O5 electrode and chloride ion to be removed by non-faradaic electrosorption of the porous carbon electrode. The MWCNT-hV2 O5 electrode was synthesized by electrochemical deposition of hydrated vanadium pentoxide on the MWCNT paper. The stable electrochemical operating window for the MWCNT-hV2 O5 electrode was between -0.5 V and +0.4 V versus Ag/AgCl, which provided a specific capacity of 44 mAh g-1 (corresponding with 244 F g-1 ) in aqueous 1 m NaCl. The desalination performance of the MPDI system was investigated in aqueous 200 mm NaCl (brackish water) and 600 mm NaCl (seawater) solutions. With the aid of an anion and a cation exchange membrane, the MPDI hybrid cell was operated from -0.4 to +0.8 V cell voltage without crossing the reduction and oxidation potential limit of both electrodes. For the 600 mm NaCl solution, the NaCl salt adsorption capacity of the cell was 23.6±2.2 mg g-1 , which is equivalent to 35.7±3.3 mg g-1 normalized to the mass of the MWCNT-hV2 O5 electrode. Additionally, we propose a normalization method for the electrode material with faradaic reactions based on sodium uptake capacities.

Nanoconfinement of redox reactions enables rapid zinc iodide energy storage with high efficiency

A key challenge for present-day electric energy storage systems, such as supercapacitors and batteries, is to meet the worlds growing demand for high performances, low cost, and environmental-friendliness. Here, we introduce a hybrid energy storage system combining zinc iodide (ZnI2) as redox electrolyte with a nanoporous activated carbon fiber (ACF) cathode and a zinc disk anode. We found that the nanopores (<1 nm) of ACF lead to a strong adsorption behavior of iodide and triiodide. Hence, this system exhibits low self-discharge rates without applying an ion exchange membrane. The high power performance (20.0 kW kg−1) originates from the enhanced redox kinetics of the iodide system as evidenced by electrochemical analysis. Considering the high specific energy (226 W h kg−1), the ACF/Zn ZnI2 battery represents an alternative for lead acid, Ni–Zn, and Ni–Cd batteries, while providing a supercapacitor-like power performance in the range of seconds to minutes charging times.

Carbon onion/sulfur hybrid cathodes via inverse vulcanization for lithium–sulfur batteries

A sulfur–1,3-diisopropenylbenzene copolymer was synthesized by ring-opening radical polymerization and hybridized with carbon onions at different loading levels. The carbon onion mixing was assisted by shear in a two-roll mill to capitalize on the softened state of the copolymer. The sulfur copolymer and the hybrids were thoroughly characterized in structure and chemical composition, and finally tested by electrochemical benchmarking. An enhancement of specific capacity was observed over 140 cycles at higher content of carbon onions in the hybrid electrodes. The copolymer hybrids demonstrate a maximum initial specific capacity of 1150 mA h gsulfur−1 (850 mA h gelectrode−1) and a low decay of capacity to reach 790 mA h gsulfur−1 (585 mA h gelectrode−1) after 140 charge/discharge cycles. All carbon onion/sulfur copolymer hybrid electrodes yielded high chemical stability, stable electrochemical performance superior to conventional melt-infiltrated reference samples having similar sulfur and carbon onion content. The amount of carbon onions embedded in the sulfur copolymer has a strong influence on the specific capacity, as they effectively stabilize the sulfur copolymer and sterically hinder the recombination of sulfur species to the S8 configuration.

Charge and Potential Balancing for Optimized Capacitive Deionization Using Lignin‐Derived, Low‐Cost Activated Carbon Electrodes

Lignin-derived carbon is introduced as a promising electrode material for water desalination by using capacitive deionization (CDI). Lignin is a low-cost precursor that is obtained from the cellulose and ethanol industries, and we used carbonization and subsequent KOH activation to obtain highly porous carbon. CDI cells with a pair of lignin-derived carbon electrodes presented an initially high salt adsorption capacity but rapidly lost their beneficial desalination performance. To capitalize on the high porosity of lignin-derived carbon and to stabilize the CDI performance, we then used asymmetric electrode configurations. By using electrodes of the same material but with different thicknesses, the desalination performance was stabilized through reduction of the potential at the positive electrode. To enhance the desalination capacity further, we used cell configurations with different materials for the positive and negative electrodes. The best performance was achieved by a cell with lignin-derived carbon as a negative electrode and commercial activated carbon as a positive electrode. Thereby, a maximum desalination capacity of 18.5 mg g-1 was obtained with charge efficiency over 80 % and excellent performance retention over 100 cycles. The improvements were related to the difference in the potential of zero charge between the electrodes. Our work shows that an asymmetric cell configuration is a powerful tool to adapt otherwise inappropriate CDI electrode materials.

Electrodeposition of hydrated vanadium pentoxide on nanoporous carbon cloth for hybrid energy storage

Electrodeposition is a simple and effective method for the synthesis of disordered hydrated vanadium pentoxide (V2O5·nH2O). For the synthesis of energy storage electrodes with high power performance, electrodeposition of hydrated V2O5 inside carbon micropores is particularly attractive to synergize electric-double layer formation and lithium ion intercalation. Here, we demonstrate that hydrated V2O5 can be effectively electrodeposited in carbon micropores of activated carbon cloth. Our study indicates that carbon pores larger than 1 nm are essential for the effective decoration with hydrated V2O5. A thermal treatment after the electrodeposition is often used to enhance the crystal structure of hydrated V2O5. However, thermal annealing of the hydrated vanadium pentoxide decorated activated carbon cloth under an oxygen-rich environment at high temperature (>330 °C) leads to a significant loss of pore volume, leading to a decreased electrochemical performance. At low annealing temperature (200 °C), the vanadium pentoxide electrodeposited activated carbon cloth electrode exhibits a maximum specific capacity of 137 mA h g−1 with stable cycle performance over 1600 cycles at a rate of 4C.

In Situ Tracking of Partial Sodium Desolvation of Materials with Capacitive, Pseudocapacitive,and Battery-Like Charge/Discharge Behavior in Aqueous Electrolytes

Aqueous electrolytes can be used for electrical double-layer capacitors, pseudocapacitors, and intercalation-type batteries. These technologies may employ different electrode materials, most importantly high-surface-area nanoporous carbon, two-dimensional materials, and metal oxides. All of these materials also find more and more applications in electrochemical desalination devices. During the electrochemical operation of such electrode materials, charge storage and ion immobilization are accomplished by non-Faradaic ion electrosorption, Faradaic ion intercalation at specific crystallographic sites, or ion insertion between layers of two-dimensional materials. These processes may or may not be associated with a (partial) loss of the aqueous solvation shell around the ions. Our work showcases the electrochemical quartz crystal microbalance as an excellent tool for quantifying the change in effective solvation. We chose sodium as an important cation for energy storage materials (sodium-based aqueous electrolytes) and electrochemical desalination (saline media). Our data show that a major amount of water uptake occurs during ion electrosorption in nanoporous carbon, while battery-like ion insertion between layers of titanium disulfide is associated with an 80% loss of the initially present solvation molecules. Sodiation of MXene is accomplished by a loss of 90% of the number of solvent molecules, but nanoconfined water in-between the MXene layers may compensate for this large degree of desolvation. In the case of sodium manganese oxide, we were able to demonstrate the full loss of the solvation shell.