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、短时间将有害有机废物分解为无害无机化合物的有前景的方法。然而，SCWO反应器受到在废物销毁过程中形成的卤化氢、硝酸盐、硫酸盐和磷酸盐的严重腐蚀。因此，有必要明确SCWO环境中的腐蚀机理，并制定防止腐蚀的对策。为此，发展电化学测量是必要的。本研究的目的是研制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），对其性能进行评价，并将其应用于超临界水中阳极极化曲线的测量。在室温条件下，将Ag/AgCl电极置于小型压力容器中。用高压泵泵送内部溶液（0.1M KCl），使该外部参考电极的压力保持在与高压灭菌器在高温下的压力相同的值。在该参考电极系统中，在室温侧设置液结，使得EPBRE与高温侧测试电极之间产生热液结电位（TLJP）。为了评估TLJP的价值，首先在高温高压灭菌器中安装了一个pt -氢电极（PHE），并测量了PHE和EPBRE电极之间的电位差。用0.1M KCl、0.05M Na_2SO_4、1mM Na_2SO_4、纯水、1mM HCl和1mM NaOH作为测试溶液，在298 ~ 773K的温度范围内测量了溶液的TLJPs。因此，评价的TLJPs可用于将测量电位转换为SHE参考电位。测量了SUS304不锈钢在563 ~ 673k温度范围内的阳极极化曲线。结果表明，在1mM Na_2SO_4和0.05M Na_2SO_4溶液中，TLJP与Na_2SO_4浓度无关。0.1 M KCl溶液中的TLJP与Na_2SO_4溶液中的TLJP基本相同。在本实验使用的中性溶液中，TLJP随溶液组成的变化很小。SUS304钢在0.05M Na_2SO_4溶液中的阳极极化曲线呈现出活性溶解、钝化、透化和析氧四个特征区。透过率和析氧起始势随温度升高而向负方向移动。在673K的0.1M KCl超临界溶液中，SUS304钢发生点蚀。SUS304钢在673K时的阳极极化曲线上，随着pH值的降低，透溶起始电位向正方向移动，同时透溶电流密度增大。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