Sung-Hyun Yun

A review of current developments in non-aqueous redox flow batteries: characterization of their membranes for design perspective

The non-aqueous redox flow battery (RFB) is one of the emerging large-scale energy storage systems that may overcome the low energy density limited by breakdown of water at a high voltage in aqueous RFBs. Yet development of the non-aqueous RFB is at an early stage, so its components are not thoroughly understood. As a key component of non-aqueous RFBs, the role of the membrane is to suppress cross-contamination between the anolyte and catholyte confined in two separate compartments, and to transport the charge carrier ions selectively for the completion of the circuit during cell operation. In this review, recent studies on non-aqueous redox flow systems are summarized including redox couples, electrolytes, and systems including membranes. A focus is placed on comparison of battery performance in terms of the current and power density through membranes. In addition, we introduce syntheses and characterization of membranes used for non-aqueous RFBs.

Preparation of ion exchanger layered electrodes for advanced membrane capacitive deionization (MCDI).

A noble electrode for capacitive deionization (CDI) was prepared by embedding ion exchanger onto the surface of a carbon electrode to practice membrane capacitive deionization (MCDI). Bromomethylated poly (2, 6-dimethyl-1, 4-phenylene oxide) (BPPO) was sprayed on carbon cloth followed by sulfonation and amination to form cation exchange and anion exchange layers, respectively. The ion exchange layers were examined by Scanning electron microscopy (SEM) and Fourier transform infrared spectrometer (FT-IR). The SEM image showed that the woven carbon cloth was well coated and connected with BPPO. The FT-IR spectrum revealed that sulfonic and amine functional groups were attached on the cationexchange and anionexchange electrodes, respectively. The advantages of the developed carbon electrodes have been successively demonstrated in a batch and a continuous mode CDI operations without ion exchange membranes for salt removal using 100 mg/L NaCl solution.

Hydrogen bonding: a channel for protons to transfer through acid-base pairs.

Different from H(3)O(+) transport as in the vehicle mechanism, protons find another channel to transfer through the poorly hydrophilic interlayers in a hydrated multiphase membrane. This membrane was prepared from poly(phthalazinone ether sulfone kentone) (SPPESK) and H(+)-form perfluorosulfonic resin (FSP), and poorly hydrophilic electrostatically interacted acid-base pairs constitute the interlayer between two hydrophilic phases (FSP and SPPESK). By hydrogen bonds forming and breaking between acid-base pairs and water molecules, protons transport directly through these poorly hydrophilic zones. The multiphase membrane, due to this unique transfer mechanism, exhibits better electrochemical performances during fuel cell tests than those of pure FSP and Nafion-112 membranes: 0.09-0.12 S cm(-1) of proton conductivity at 25 degrees C and 990 mW cm(-2) of the maximum power density at a current density of 2600 mA cm(-2) and a cell voltage of 0.38 V.

End-group cross-linked large-size composite membranes via a lab-made continuous caster: enhanced oxidative stability and scale-up feasibility in a 50 cm2 single-cell and a 220 W class 5-cell PEFC stack

We present successful scaling-up feasibility of a large-size membrane-electrode assembly (MEA) using a newly developed thin composite polymer electrolyte membrane for fuel cell applications. The highly sulfonated end-group cross-linkable polymer electrolytes were synthesized and reinforced using a porous polytetrafluoroethylene (pPTFE) substrate of 15 μm thickness. Here, a lab-scale continuous caster was developed in order to fabricate thin composite membranes with a uniform thickness of 25 μm. The reinforced thin composite membranes exhibit an efficiently reduced water uptake and swelling ratio, and improved oxidative stability in Fentons reagent compared to normal cast membranes. The optimized composite membrane was subsequently tested in a single-cell and a five-cell stack to demonstrate their feasibility for the scaled-up polymer electrolyte fuel cells (PEFCs). Eventually, the composite membrane successfully performed in the stack with low scale-up losses of about 2–3%. As a preliminary investigation, an in situ accelerated degradation test (ADT) is performed to evaluate the relative chemical durability of the composite MEA in a 50 cm2 single cell (at 100 °C with approximate relative humidity of 10%), and the degradation kinetic of the MEA (1.47 mV h−1) is comparable to Nafion®212 (1.45 mV h−1) during 24 h ADT.

Estimation of approximate activation energy loss and mass transfer coefficient from a polarization curve of a polymer electrolyte fuel cell

A simple electrochemical approach is presented to quantitatively predict activation energy and mass transfer coefficient from a polarization curve of polymer electrolyte fuel cells to examine the membrane-electrode assembly (MEA) performance. It is assumed that the initial voltage drop at open circuit voltage is due to kinetic activation energy and that the current loss at short circuit current is due to mass transfer resistance. Accordingly, voltage drop in the activation polarization is converted into a change in the Gibbs free energy to determine the activation energy requirement. The mass transfer coefficient for current losses is derived from Fick’s law, based on the mass transfer limitation of oxygen at the oxygen reduction reaction sites. Case studies from the literature show reasonable correlations to the operating conditions, thereby providing a useful tool for prediction of the preliminary values of the activation energy and mass transfer coefficient for an MEA under various conditions.