Development of an External Pressure Balance Reference Electrode for Use in Supercritical Water Systems

超临界水氧化（SCWO）是一种高效、快速地将有害有机废弃物降解为无害无机物的方法。然而，超临界水氧化反应器遭受严重的腐蚀，卤化氢，硝酸盐，硫酸盐和磷酸盐，这是在销毁废物。因此，有必要弄清超临界水氧化环境中的腐蚀机理，并制定相应的防腐蚀对策。为此，电化学测量的发展是必要的。研制了一种用于超临界水中电化学测量的新型外压平衡参比电极（EPBRE），并对其性能进行了评价，将其应用于超临界水中阳极极化曲线的测量。该外部参比电极的压力保持在相同的值 ...更多信息 通过用高压泵泵送内部溶液（0.1M KCl），在该参比电极系统中，在室温侧设置液体接界，使得在EPBRE和高温侧的测试电极之间产生热液体接界电势（TLJP）。为了评估TLJP的价值，首先在高温高压釜中安装Pt-氢电极（PHE），并测量PHE和EPBRE电极之间的电位差。将0.1M KCl、0.05M Na_2SO_4、1 mM Na_2SO_4、纯水、1 mM HCl和1 mM NaOH用于测试溶液，并在298 - 773 K的温度范围内测量溶液的TLJP。因此，评估的TLJP可用于将测量的电位转换为涉及SHE的电位。在563- 673 K温度范围内测量了SUS 304不锈钢的阳极极化曲线，发现1mMNa_2SO_4和0.05MNa_2SO_4的TLJP值相差很小或没有差别，表明在本实验所用的范围内TLJP与Na_2SO_4浓度无关。在0.1M KCl中的TLJP与在Na_2SO_4中的TLJP大致相同。在本实验使用的中性溶液中，TLJP随溶液组成的变化较小。SUS 304钢在0.05MNa_2SO_4溶液中的阳极极化曲线呈现出四个特征区：溶解区、钝化区、过钝化区和析氧区。随着温度的升高，过钝性和析氧的起始电位向负方向移动。SUS 304钢在673 K的0.1MKCl超临界溶液中发生点蚀。在673 K下的阳极极化曲线中，随着pH值的降低，SUS 304钢的过钝化溶解起始电位正移，同时过钝化溶解电流密度增大。少

Supercritical water oxidation (SCWO) is a promising method for decomposing hazardous organic wastes into harmless inorganic compounds in a high efficiency and a short time. However, SCWO reactors suffer from severe corrosion by hydrogen halide, nitrate, sulfate, and phosphate, which are formed during destruction of the wastes. Consequently, it is necessary to make clear corrosion mechanism in the SCWO environment and to establish the countermeasure to prevent corrosion. For this purpose, the development of electrochemical measurement is necessary. The objective of this study is to develop a new type of external pressure balanced reference electrode (EPBRE) for electrochemical measurement in supercritical water, to evaluate its performance, and to apply it to the measurement of anode polarization curves in supercritical water.A Ag/AgCl electrode was installed in a small pressure vessel at room temperature. The pressure of this external reference electrode was maintained at the same valu … More es as that of an autoclave at high temperature by pumping an internal solution (0.1M KCl) with a high-pressure pump. In this reference electrode system, liquid junction is set in the room temperature side, so that thermal liquid junction potential (TLJP) generates between the EPBRE and a test electrode in the high temperature side. In order to evaluate the value of TLJP, a Pt-hydrogen electrode (PHE) was first installed in the high temperature autoclave, and the potential difference between PHE and EPBRE electrodes was measured. 0.1M KCl, 0.05M Na_2SO_4, 1mM Na_2SO_4, pure water, 1mM HCl and 1mM NaOH were used for the test solution and TLJPs for the solutions were measured in the temperature range of 298〜773K. Thus evaluated TLJPs can be used to convert the measured potential into the potential referred to SHE. Anode polarization curves of SUS304 stainless steel were measured in the temperature range of 563-673K.It was found that there is little or no difference in the value of TLJP for 1mM Na_2SO_4 and 0.05M Na_2SO_4, indicating that the TLJP is independent of the concentration Na_2SO_4 over the range used in this experiment. The TLJP in 0.1 M KCl was approximately the same as that in Na_2SO_4. The change of TLJP with solution composition is small in neutral solutions used in this experiment. The anode polarization curves of SUS304 steel in 0.05M Na_2SO_4 solution showed four characteristic regions : active dissolution, passivity, transpassivity, and oxygen evolution. Starting potentials of transpassivity and oxygen evolution shifted to negative direction with increasing temperature. In supercritical 0.1M KCl solution at 673K, SUS304 steel was suffered from pitting. In the anode polarization curves of SUS304 steel at 673K, the starting potential of transpassive dissolution shifted to the positive direction with decreasing pH and at the same time the transpassive dissolution current density increased. Less