Soo Min Hwang

Na-ion storage performance of amorphous Sb2S3 nanoparticles: anode for Na-ion batteries and seawater flow batteries

Amorphous (a-) Sb2S3 nanoparticle aggregates were synthesized using a facile polyol-mediated process at room temperature and their Na-ion storage capability was evaluated. Owing to the spherical nano-aggregate morphology and amorphous structure, the a-Sb2S3 anode exhibited superior rate capability and cycling stability compared to that of crystalline, microscale Sb2S3 particles. The a-Sb2S3 electrode was also examined as an anode for eco-friendly, Na metal-free seawater flow batteries.

Seawater battery performance enhancement enabled by a defect/edge-rich, oxygen self-doped porous carbon electrocatalyst

Low-cost oxygen evolution/reduction reaction (OER/ORR) catalysts are critically important for energy conversion/storage systems. Here, a porous carbon (PC) catalyst is prepared as a low-cost catalyst from grapefruit peel biowaste using a facile hydrothermal carbonization combined with a chemical activation process. When examined as an OER/ORR catalyst in seawater, the PC exhibits an unexpected, highly efficient catalytic activity. From density functional theory (DFT) calculations, we found that sp3-bonded carbon atoms, zigzag edges with or without –COOH and –OH, and armchair edges with –O– are responsible for its superior OER/ORR activity. The suitability of the PC as a catalyst is tested in the half-cell of a Na/seawater battery, which exhibits a decreased voltage gap of ∼0.47 V between charge and discharge voltage curves. This value is even lower than those of Pt/C (∼0.68 V), IrO2 (∼0.66 V) and MnO2 (∼0.73). Also, a full cell of the metal-free seawater battery is assembled using hard carbon and PC as an anode and a catalyst, respectively. The full cell shows a lower voltage gap (∼0.65 V) with the voltage efficiency of ∼83–84% and excellent cycle life over 100 cycles. Our results confirm PC derived from grapefruit peels as an alternative to expensive Pt/C and IrO2 catalysts for the OER/ORR activities in seawater batteries.

Upcycling of nonporous coordination polymers: Controllable-conversion toward porosity-tuned N-doped carbons and their electrocatalytic activity in seawater batteries

Herein, we report the preparation of highly porous N-doped carbons (PNCs) via thermolysis of nonporous Zn-based coordination polymers (CPs) constructed with nitrogen-containing ligands. So far, the thermal conversion of Zn-CPs, including metal–organic frameworks (MOFs), has mainly yielded microporous carbon materials, and to change the textural properties of end carbons, new CPs/MOFs with different properties were introduced. However, present studies show that just varying the conversion conditions of a parent CP results in porosity-tuned PNCs, in which especially mesoporosity is developed, and this is applicable for even nonporous CPs. This is conducted based on the understanding of conversion phenomena which is that during thermal conversion of Zn-based CPs, the in situ generated Zn metal species act as porogens and their agglomeration can be controlled by the reaction conditions. Different reaction temperatures, ramping rates and retention times allow control over the ratio of micro- to meso-pore volume, while a slower ramping rate and longer retention time at lower heating temperature induced the agglomeration of the porogens, yielding greater mesoporosity, and holding the Zn-CPs at high temperature for a short period afforded the micropore-dominant PNCs due to rapid porogen elimination. The superiority of the mesopore-developed PNCs as electrocatalysts, attributed to greater mass-transport-accessible surfaces, was examined for the electrodes in a rechargeable seawater battery system as an example of a practical application. Therefore, our synthetic approach provides a facile method for the preparation of PNCs with suitable hierarchical pore distributions for use as energy-related materials without exerting significant effort in the design of coordination compounds.

Hybrid Na–air flow batteries using an acidic catholyte: effect of the catholyte pH on the cell performance

Metal–air batteries show great promise because of their high theoretical energy density resulting from the use of an unlimited, low-mass O2 gaseous reactant. Nevertheless, several issues remain to be tackled for their practical implementation in ambient air. In this work, we constructed a hybrid-type Na–air battery with a flow-through configuration for direct use of ambient oxygen as the cathode, and studied the effect of the flow of the aqueous catholyte on its electrochemical properties. In addition, the effect of the catholyte pH on the open-circuit and discharge–charge voltage behavior of the hybrid Na–air battery was systematically investigated. An enhanced operation voltage was found for the flow cell using an acidic catholyte composed of 1 M NaNO3 and 0.1 M citric acid (pH = ∼1.8). Further improvement in the cell performance was observed in a cell employing both Pt/C and IrO2 electrocatalysts, which showed an average voltage gap of ∼0.4 V between the charge (∼3.7 V) and discharge (∼3.3 V) voltages vs. Na+/Na at a current rate of 0.1 mA cm−2 over 20 cycles (200 h total). These findings suggest that the hybrid Na–air battery system using a flow-through mode and an acidic catholyte could be a promising way to achieve practical Na–air cells with desirable performance.

Saltwater as the energy source for low-cost, safe rechargeable batteries

The effective use of electricity from renewable sources requires large-scale stationary electrical energy storage (EES) systems with rechargeable high-energy-density, low-cost batteries. We report a rechargeable saltwater battery using NaCl (aq.) as the energy source (catholyte). The battery is operated by evolution/reduction reactions of gases (mostly O2, with possible Cl2) in saltwater at the cathode, along with reduction/oxidation reactions of Na/Na+ at the anode. The use of saltwater and the Na-metal-free anode enables high safety and low cost, as well as control of cell voltage and energy density by changing the salt concentration. The battery with a hard carbon anode and 5 M saltwater demonstrated excellent cycling stability with a high discharge capacity of 296 mA h ghard carbon−1 and a coulombic efficiency of 98% over 50 cycles. Compared with other battery types, it offers greatly reduced energy cost and relatively low power cost when used in EES systems.

High energy density rechargeable metal-free seawater batteries: a phosphorus/carbon composite as a promising anode material

A new energy conversion and storage system, named the ‘seawater battery’, has recently been a subject of research in the field of electrochemistry. Using natural seawater as the catholyte in an open-structured cathode, the battery stores sodium (Na) ions from the seawater on the anode side during charging, and then delivers electricity by discharging on demand. Herein, we report an amorphous red phosphorus/carbon composite anode material which was successfully employed as the anode of a seawater battery. It exhibited a stable cycling performance with a high reversible capacity exceeding 920 mA h gcomposite−1 with a coulombic efficiency of more than 92% over 80 cycles, as well as good rate capabilities. In terms of the full-cell performance of lithium ion and sodium ion batteries, the seawater batteries with the amorphous red phosphorus/carbon composite anode exhibited the highest reversible capacity and specific energy. These results indicate that the use of an open-structured cathode system, which provides the anode with an unlimited supply of Na ions, would allow a seawater battery to overcome the limitations associated with the high-capacity alloying reaction-based anodes used in conventional batteries with a closed system.