全钒液流电池高浓度下V(IV)/V(V)的电极过程研究

Investigations on the Electrode Process of Concentrated V(IV)/V(V) Species in a Vanadium Redox Flow Battery

采用循环伏安、低速线性扫描和阻抗技术,以石墨为电极,研究了V(IV)/V(V)在较高浓度下的电极过程.结果表明,采用2.0mol·L-1的V(IV)溶液时,H2SO4浓度低于2mol·L-1,V(IV)/V(V)反应极化大,可逆性差,表现为电化学和扩散混合控制;H2SO4浓度增至2mol·L-1以上,V(IV)/V(V)反应的可逆性提高,转为扩散控制,且增加H2SO4浓度有利于阻抗的降低;但H2SO4浓度超过3mol·L-1,溶液的粘度和传质阻力大,阻抗反而增大.在3mol·L-1的H2SO4中,随着V(IV)浓度的增加,体系的可逆性和动力学改善,阻抗减小;但V(IV)浓度超过2.0mol·L-1,较高的溶液粘度导致溶液的传质阻力迅速增加,V(IV)/V(V)的电化学性能衰减,阻抗增大.因此,综合考虑电极反应动力学和电池的能量密度两因素,V(IV)溶液的最佳浓度为1.5～2.0mol·L-1,H2SO4浓度为3mol·L-1.

The electrode process of concentrated V（Ⅳ）/V（Ⅴ） species has been studied at a graphite electrode using cyclic voltammetry, linear polarization, and impedance techniques. The results have revealed that in H2SO4 solution below 2 mol·L^-1 containing 2.0 mol·L^-1 V（Ⅳ）, the electrode process of V（Ⅳ）/V（Ⅴ） is controlled by the electrochemical polarization and diffusion with poor reversibility. When the concentration of H2SO4 is above 2 mol·L^-1, the electrode process of V（Ⅳ）/V（Ⅴ） turns into the diffusion control with improvement of the reversibility. An increase in concentration of H2SO4 solution facilitates a decrease in impedance, whereas too high concentration of H2SO4 solution（ 〉 3 mol·L^-1） results in a remarkable increase in the viscosity of solution leading to a large mass transportation polarization, and thus the impedance starts to increase a bit. In 3 mol·L^-1 H2SO4 solution, the reversibility and kinetics of V（Ⅳ）/V（Ⅴ） are improved gradually as well as the characteristics of impedance with increasing concentration of V（Ⅳ） solution. But, when the concentration of V（Ⅳ） solution exceeds 2.0 mol·L^-1, the viscosity of the solution is so high that the mass transportation polarization increases considerably, resulting in the deterioration of electrochemical performance of V（Ⅳ）/V（Ⅴ） species and an increase in impedance. Therefore, considering from standpoint of increasing the energy density and electrode kinetics comprehensively, the optimal concentration of H2SO4 is 3 mol·L^-1 containing 1.5-2.0 mol·L^-1 of V（Ⅳ）.