Performance Evaluation of Aqueous Organic Redox Flow Battery Using Methylene Blue and Vanadium Redox Couple

메틸렌블루와 바나듐을 활물질로 활용한 수계 유기 레독스 흐름 전지의 성능 평가

Abstract

In this study, methylene blue which is one of dye materials was introduced as active material for aqueous redox flow battery. The redox potential of methylene blue was shifted to negative direction as pH increased. The full-cell performance was evaluated by using methylene blue as the negative active material and vanadium as the positive active material with acid supporting electrolytes. The cell voltage of methylene $blue/V^{4+}$ is very low (0.45 V). In addition, the maximum solubility of methylene blue in water is only 0.12 M. Therefore, the cell test was performed with very low concentration (0.0015 M methylene blue, $0.15M\;V^{4+}$) at first time. Cut-off voltage range was 0 to 0.8 V and $1mA{\cdot}cm^{-2}$ current density was adopted during cycling. As a result, current efficiency (CE) was 99.67%, voltage efficiency (VE), 88.83% and energy efficiency (EE) was 85.87% and discharge capacity was ($0.0500Ah{\cdot}L^{-1}$) at 4 cycle. In addition, the cell test was performed with increased concentration (0.1 M methylene blue, $0.15M\;V^{4+}$) with $10mA{\cdot}cm^{-2}$ current density, leading to higher discharge capacity ($3.8122Ah{\cdot}L^{-1}$) with similar efficiency (CE=99%, VE=85%, EE=85% at 4 cycle).

본 연구에서는 염료 물질 중 하나인 메틸렌 블루(methylene blue)를 수계 레독스 흐름 전지의 활물질로 처음으로 도입하였다. Methylene blue의 레독스 전위는 pH가 높아짐에 따라 음의 방향으로 이동하는 것을 확인할 수 있었다. 이 methylene blue를 음극 활물질로 활용하고, 양극 활물질로는 바나듐(vanadium) 을 활용하여 산 전해질을 기반으로 셀성능 평가를 진행하였다. Methylene $blue/V^{4+}$ 레독스 조합의 산 전해질에 대한 셀 전압은 0.45 V로 낮으며, Methylene blue의 물에 대한 용해도 또한 0.12 M로 굉장히 낮다. 이에 따라 0.0015 M의 낮은 농도로 단전지 셀 성능을 평가하였으며, Nafion 212 멤브레인을 사용하여 0~0.8 V 컷-오프 전압으로 $1mA/cm^2$ 전류밀도 하에서 4 cycle에서 충방전 효율 96.67%, 전압효율 88.83%, 에너지효율 85.87%, 방전 용량($0.0500Ah{\cdot}L^{-1}$)의 성능을 보였으며, 낮은 방전용량은 활물질의 낮은 농도에 의한 것이므로 활물질인 메틸렌 블루의 농도를 0.1 M로, 전류밀도는 $10mA/cm^2$로 더 높였을 때 4 cycle에서 CE 99%, VE 85%, EE 85%의 효율로 더 높은 방전 용량($3.8122Ah{\cdot}L^{-1}$)을 도출함을 확인할 수 있었다.

Fig. 1. Scheme of redox reaction of (a) Methylene blue and (b) V⁴⁺/V⁵⁺.

Fig. 2. Cyclic voltammetry results of 0.0015 M methylene blue in different pH supporting electrolytes at scan rate 100 mV·s^-1.

Fig. 3. Cyclic voltammetry results of 0.0015 M methylene blue and V⁴⁺/V⁵⁺ redox reactions at scan rate 100 mV·s^-1.

Fig. 4. (a) Charge-discharge graphs, (b) efficiency graphs and (c) discharge capacity and SOC versus cycle number graphs of 0.0015 M methylene blue/0.15 M vanadium RFB single cell tests in 2 M H₂SO₄ with 1 mA·cm^-2 current density at 0~0.8 V cutoff voltage.

Fig. 5. (a) Charge-discharge graphs, (b) efficiency graphs and (c) discharge capacity and SOC versus cycle number graphs of 0.0015 M methylene blue/0.15 M vanadium RFB single cell tests in 2 M HCl with 1 mA·cm^-2 current density at 0~0.8 V cut-off voltage.

Fig. 6. (a) Charge-discharge graphs, (b) efficiency graphs and (c) discharge capacity and SOC versus cycle number graphs of 0.1 M methylene blue/0.15 M vanadium RFB single cell tests in 2M H₂SO₄ with 10 mA·cm^-2 current density at 0~0.8 V cut-off voltage.