Activated Carbon Paper Research Articles

High Electrocatalytic Activities of the Electrochemically Exfoliated Graphene Deposited Carbon Paper Electrodes for Vanadium Redox Flow Batteries

The cost-effective and process-efficient preparation of highly electrocatalytic carbon electrodes on a large scale is crucial for enhancing the competitiveness of vanadium redox flow batteries (VRFBs). In this work, electrochemically exfoliated graphene oxides (Exf-GOs) are prepared form graphite foil at large scale with a multi-electrodes equipped continuous Exf-GOs production system. The Exf-GOs are deposited on the activated carbon paper (A-CP) substrate by a direct infiltration-deposition approach. Collective electro-kinetics analysis applying cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) revealed that the resulting Exf-GO/A-CP electrodes exhibit two orders magnitude greater intrinsic electrocatalytic activities than the A-CP electrode for the VO2+/VO2 + redox reactions. A trade-off between the Exf-GO deposition amount and the open pore volume through the Exf-GO/A-CP electrodes was found that have critical impacts on the overall electro-kinetic performance of the electrodes. Compared to the A-CP electrode, the Exf-GO/A-CP electrode provided the VRFB with more than two-fold enhancements in the energy storage capacity and at least 10 – 15 % higher voltage and energy efficiencies. The results collectively demonstrate the high effectiveness of the Exf-GO/A-CP electrodes for VRFBs that can be prepared cost and process efficient approach at large scales.

CuO@NiCo-LDH core-shell structure for flexible fiber-shaped supercapacitor electrode material

Metal fiber electrodes are of great interest for their excellent electrical conductivity and flexibility, however the limited surface area is unfavorable for the electrochemical performance. Herein, CuO nanowires in-situ growth on one dimensional (1D) Cu wire were obtained by wet chemistry and annealing method, and NiCo-LDH nanoplates were further growth on CuO nanowires by hydrothermal method to fabricate CuO@NiCo-LDH. The combination of CuO nanowires and NiCo-LDH nanoplates creates a three-dimensional (3D) core-shell structure, which improves the specific surface area and the electrochemical properties. In addition, there is a synergistic effect between the highly reactive of NiCo-LDH and the well-conducting of CuO/Cu. The composite presents a high specific capacitance of 1220.4 mF cm−2 at 1 mA cm−2. Moreover, the CuO@NiCo-LDH//activated carbon paper (ACP) asymmetric supercapacitor exhibits an excellent energy density of 20.1 μWh cm−2 at the power density of 556.3 μW cm−2, outstanding cycling stability with specific capacitance retention of 87.1 % after 6000 cycles and good bending performance. This paper provides a novel idea for the reasonable construction of electrode material structure.

Enhanced capacitive performance of activated carbon paper electrode material

Abstract

Performance of Vanadium Redox Flow Battery By Using the Electrode with Porous Carbon Nanofiber

Introduction In recent years, mismatch between renewable energy supply and power demand has become a social problem. For this purpose, redox flow batteries (RFBs) are attracted attention because of their good characteristics, i.e., the quick response, easily measurement of state of charge, and no design limitations between power density and capacity. Vanadium RFBs (VRFBs) [1] have been widely studied because of no contamination problems of the electrolyte. The performance of VRFB was significantly improved by using carbon paper [2]. For commercialization of VRFBs, the developments of thinner electrode and improvement of voltage efficiency are essential to reduce the cost. In this work, the activated carbon nanofiber (ACNF) is prepared as a material with higher specific surface area and active sites. The nanofiber should be appropriate additive because of less obstructions compared to nanoparticles. The composite electrode with ACNF in ACP was prepared, and current-voltage curve by using ACNF/ACP composite electrode at positive electrode was measured. Experimental ACNF was prepared by electrospinning technique, heat treatments, and activation by using similar procedure of our previous work [3]. The activation was performed by flowing 2–10 vol% O2 (N2 balance) at the reactor temperature of 530 °C. The obtained fiber was milled 6 h. The redox activity of VO2+/VO2 + reaction was measured by conventional three-electrode cell (glassy carbon, Ag/AgCl (KCl sat.), and Pt mesh as the working, reference, and counter electrode, respectively). The sample was mounted on the working electrode at 0.26 mg cm−2. The measurement was conducted in a water solution of 1.0 M VOSO4 with 3.0 M H2SO4 at scan speed of 100 mV s−1. The performance of VRFB was measured by using single cell with interdigitated channel. The carbon paper (SGL 10AA) was heat treated under air, 400 °C, 24 h (activated carbon paper (ACP)). The ACNF ink was dropped on ACP and dried at 80 °C, 1h. In this procedure, 7.1 wt% ACNF/ACP composite electrode was obtained, and its thickness was not changed compared to ACP. The cell was consisted of positive electrode (ACP or ACNF/ACP), negative electrode (ACP), separator (NRE-212), gaskets, and current collectors with interdigitated channel. 1.5 M V(V) in 3.0 M H2SO4 aqueous solution was used for positive electrolyte, and 1.5 M V(II) in 3.0 M H2SO4 aqueous solution was used for negative electrolyte. The flow rate of electrolyte solution was 40 mL min−1. The current-voltage curve was obtained with scan speed of 2.0 mV s−1. Results and Discussion The mean diameter of ACNF was around 350 nm. Figure 1 shows the cyclic voltammograms of the samples with different O2 fractions at the activation. The current density of oxidation at around 1.15 V vs. RHE was increased at higher O2 fraction. This result suggests CNF activated at higher O2 fraction showed higher activity because of its larger surface area and more active sites. 7.1 wt% ACNF/ACP composite electrode was prepared by using ACNF which was activated at 530 °C and 10 vol% O2. Figure 2 shows the current-voltage curves of the VRFB single cells with ACNF/ACP or ACP at positive electrode. The voltage drop was reduced 0.12 V at 0.1 A cm−2 by using the ACNF/ACP electrode. This result is attributed to less activation overpotential by high active surface area of ACNF. Conclusion ACNF is prepared as the additive with higher specific surface area by electrospinning and heat treatments. The optimized activation condition for CNF was 530 °C, 10 vol% O2, and 1 h. The voltage drop was reduced 0.12 V at 0.1 A cm−2 by using the ACNF/ACP electrode. Acknowledgement This research was supported by The Iwatani Naoji Foundation’s Research Grant and Iketani Science and Technology Foundation. We significantly thank this foundation. H. I. appreciate to Prof. Shoji Tsushima and Dr. Takahiro Suzuki (Osaka University) for their supports and comments. Figure 1

Binder-free activated carbon papers for high-performance electric double-layer capacitors

A binder-free activated carbon paper (ACP) was simply prepared for electric double-layer capacitors by the carbonization of filter paper, followed by heat-air activation at a lower temperature. The electrochemical cells assembled using the as-prepared ACP-470 provides a high specific capacitance of 296.4 F g−1 at current density of 0.5 A g−1 and a high rate performance at a current density of 150 A g−1 with a capacitance of 191.2 F g−1 and a high cycle ability at 10,000 recycles with 100 % capacitance retention. In addition, the ACP has a lower electrical resistivity and provides an effective energy storage performance with a maximum energy density of 41.2 Wh kg−1 and a maximum power density of 138.0 kW kg−1 in a voltage range of 1 V.