The silver chloride electrode is covered withSilver chlorideOf porous metal silver immersed in Cl-An electrode formed in a solution of.Silver chloride electrode can be expressed as Ag/AgCl/Cl-，electrode reaction Is AgCl+e=Ag+Cl-。
The standard electrode potential of silver chloride electrode in 1M KCL is+0.2224 V (25 ℃), and the standard electrode potential in saturated KCl is+0.199 V (25 ℃).
Because Ag/AgCl electrode has very small solubility and extremely highstabilityandreversibilityAnd even in the presence of hydrogenelectrodeThe surface will also be well protected, and these characteristics are unmatched by other electrodes.Therefore, the research on Ag/AgCl electrode is more in-depth and detailed.
Electrochemical performance
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Research background
The exploration and development of marine resources is of great significance to the development of national economy.However, some conventional geophysical methods have lost good detection effect after extending the detection object from land to sea.With the in-depth study of marine geophysics, it has been found thatElectrical prospectingBased on the principle of using electrical parameters to reflect the distribution law of medium in seawaterElectromagnetic method, can provide valuable information, and the measurement of this information requires high-performance electric field sensors.The characteristics of submarine electric field signal are: (1) due to the attenuation of the seawater layer, the electric field signal at the seabed is much weaker than that on land, and the amplitude can reach mV level;(2) The frequency components of the seabed electric field signal are mainly medium and low frequency, so the signal acquisition time is relatively long. Generally, the instrument needs to be placed on the seabed for a week, or even longer.Therefore, the detection of underwater electric field signal requires that the electric field sensor has the characteristics of low self noise, high measurement sensitivity, taking into account the withstand voltage and long-term stability.To this end, the search and development of highly stable electric fields in seawatersensorEspecially important.
Based on the comprehensive analysis of commonly used electrodes and the specific requirements of the deep-sea high-pressure service environment, an Ag/AgCl all solid electrode was developed, which can be directly inserted into the measured sea water, thus avoidingLiquid connection potentialGeneration of.The preparation conditions, factors affecting the potential stability and the self noise performance of the electrode were studied, and the stability and self noise performance of the electrode were analyzed.
Preparation of Ag AgCl electrode
To prepare in a certain proportionAgClAnd granularity is 200orderThe Ag powder with purity of 99.97% is evenly mixed and granulated, and is pressed into a cylindrical mold under uniaxial pressure to make an electrode blank. The initial electrode is sintered at 400~600 ℃, and is threaded and sealed with a silver rod after polishing and hydrochloric acid activation to make an ocean electric field measuring electrode.
Electrode stability test
Fig. 1 SEM Photo of Silver Powder
It can be seen from the comprehensive consideration of the electrode microscopic morphology that the range potential is mainly caused by the difference of the reaction active areas of the two electrodes, and the large difference of the reaction active areas causes that the range potential cannot be controlled within the specified range.According to the working principle of the electrode, there are two phase interfaces of Ag/AgCl electrode in seawater, namely Ag AgCl Cl-, Ag and AgCl on the electrode surface are used asanodeandcathodeParticipate in the reaction process and tend to balance, that is, AgCl+e=Ag+Cl.
Since AgCl is a kind of insoluble salt, there is another equilibrium relationship in the reaction process: AgCl → Ag++Cl-。
Fig. 2 SEM Photo of Electrode Surface Morphology after Sintering
When an electric field signal is transmitted, a small amount of current passes through the electrode interface, and the above balance deviatesChloride ionThe double electric layer of concentration control is destroyed, and the double electric layer re establishes balance, resulting in instability of the range potential.Therefore, the difference potential and its stability are determined by the electric double layer on the electrode surface, and the composition, morphology and structure of the two electrode surfaces determine the distribution of the electric double layer.The particle size of silver powder is distributed between 2-5 μ m. As shown in Figure 1, the number of AgCl particles is far more than that of silver powder. The AgCl particles are evenly distributed around the silver powder, and the AgCl of small particles is filled between the silver powder of large particles, thus effectively avoiding the generation of large pores and greatly improving the density and uniformity of the electrode.As shown in Figure 2, there are few defects on the electrode surface, and the interface between particles can be seen when AgCl melts slightly. For metals, due to the existence of grain boundariesbarrier，resistivityWill increase with the increase of the number of grain boundaries, which is attributed to thescatteringeffect;Since there is more silver in the sample electrode, electrons will be transmitted through the scattering of AgCl interface and silver interaction, so that the signal can be effectively transmitted.[1]
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Table 1 Study of Ag/AgCl reference electrode in high-temperature and high-pressure aqueous phase system
During the electrochemical research at high temperature and pressure,Reference electrodeThe stability of is very important for all electrochemistry and related research.Before the 1990s, people had carried out extensive research on reference electrodes in high-temperature and high-pressure environment, such as hydrogen electrode, palladium hydride electrode, silver/silver halide electrode (Ag/AgCl, Ag/AgBr, Ag/AgI, etc.), metal/metal oxide electrode (Hg/HgOAg/Ag2O、Pt/PtO、Ir/IrO、Rh/RhtwoOthree, Zr/ZrO2, etc.), metal/metal sulfate electrode (Ag/AgtwoSOfour、Hg/HgtwoSOfour、Pt/PtSOfourThe calomel electrode and the redox electrode of inactive metals (Pt, Ni) have been tried and studied, and have been widely used in related fields.However, because Ag/AgCl electrode has very small solubility, extremely high stability and reversibility in high-temperature and high-pressure aqueous solution system, and the electrode surface will be well protected even in the presence of hydrogen, these characteristics are incomparable to other electrodes.Therefore, the research on Ag/AgCl electrode is more in-depth and detailed[2]It can be seen from the study that whether the Ag/AgCl electrodethermodynamicsordynamicsIt has been widely used in the research.This paper reviews and summarizes the research on Ag/AgCl reference electrode for high-temperature and high-pressure aqueous solution system in recent decades, analyzes its development trend, and puts forward its own suggestions.
Reference electrode for high-temperature and high-pressure aqueous solution system
(1) Requirements for reference electrode at high temperature and pressure
1) The electrode potential has good reproducibility, and the potential is stable during the experiment.
2) It shall not react with the test system solution.
3) If the reference electrode is used for thermodynamic research, it must have correct thermodynamic behavior characteristics and meetNernst equationFormula.
4) ElectrodesolubilityNot too large;The electrode material will not corrode, decompose and distort and deform the electrode performance under high temperature and pressure;The material at the hydraulic connection part shall maintain good electrical conductivity and thermal stability under high temperature and pressure.
Table 2 Advantages and Disadvantages of Reference Electrode in High Temperature and High Pressure System
Of course, when designing and using the reference electrode, a series of problems such as liquid connection potential, interaction between solutions and pollution should also be considered in detail according to the specific requirements of the experiment.In this regard, Ives and Janz have given some suggestions on the design of reference electrode at high temperature and high pressure.
(2) Form of reference electrode
Reference electrodes for high temperature and high pressure can be divided into built-in and external types according to the electrode placement mode.The former is in the same temperature, pressure and solution environment as the working electrode;The latter is to place the reference electrode outside the test system and connect it with the solution in the high-pressure container through a salt bridge.The two electrodes show great differences in characteristics due to different placement environments. Table 2 lists the advantages and disadvantages of the two reference electrodes.In general, the built-in reference electrode is mostly used for the study of electrode thermodynamic behavior, while the external reference electrode is mostly used for the study of electrode dynamics.
Ag AgCl Built in Reference Electrode Structure for High Temperature and High Pressure
The biggest problem of the built-in reference electrode for high-temperature and high-pressure water in use is the aging of the electrode in a short time.Between the electrode body component and the component and the internal solutionCoefficient of thermal expansionThe poor sealing and damage caused by the difference are related to the interruption of the solution circuit.In addition, the hydrolysis of Ag/AgCl at high temperature and the blocking of pores in the connection of the liquid by AgCl hydrolysis products are also important reasons for electrode aging.Many workers solve this problem from various aspects, such as adding circulating water for cooling and increasing the surface area of Ag exposed to electrode solution.Iwamoto et al. developed a new composite absorbent, which is composed of alkali metal ions and fourPhenylboronic acidIt is composed of salt ions (or nitrogen-containing organic substitutes such as triamine salts), which are filled in porous framework structures such as porous activated carbon or silica gel. At high temperatures, it can prevent the hydrolysis reaction of Ag/AgCl, and the generated complex is larger than the original ion diameter, which prevents the possibility of the hydrolysis products blocking the micropores.
Structure of Ag AgCl External Reference Electrode for High Temperature and High Pressure
According to different liquid connection methods, external reference electrodes can be divided into the following two types: the first type is to take a small amount of test liquid from the high-temperature and high-pressure system as the connection liquid. In order to achieve this goal, it must be kept flowing out. This is a disadvantage, but the electrode structure is relatively simple;The second is the pressure balanced external reference electrode. The potential of this electrode can be easily converted into SHE reference potential, so it is often used as the reference electrode.
External type in liquid outflow typeReference electrodeIn order to ensure the electrical continuity of the hydraulic connection, pressure sealing (or called pressure connector) becomes a crucial issue.On the one hand, the pressure sealing part should limit the speed of liquid flowing out of the high-pressure vessel, on the other hand, it should maintain the electrical conductivity of the hot solution.In general, the pressure balance electrode is mainly used to evaluate the potential of the hydrothermal connection, measure and calibrate the temperature coefficient of the built-in reference electrode in terms of its characteristics. However, this type of electrode is simpler to handle than the liquid outflow electrode, more stable than the built-in reference electrode, and has a long service life, and can correct the liquid potential,Therefore, it will be more widely used in the future corrosion research.[3]
Electrode maintenance
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1. Reference electrodeCopper sulfate solutionPreparation of chemical pure copper sulfate crystal: pour the chemical pure copper sulfate crystal into a clean glass beaker, then pour an appropriate amount of distilled water (the temperature of the water used for preparation is 25 ℃), stir and dissolve it with a clean glass rod (metal rod is not allowed), and deposit part of it, then prepare it with saturated copper sulfate solution.
2. Open the upper cover of the reference electrode, pour out the liquid in the reference electrode, and remove all components.
3. Check whether the contact is well connected, and reconnect the poor contact.Remove the surface attachment of each part, especially the copper bar shall be polished with sandpaper.Check whether the semi permeable membrane is in good condition, and replace it in time if it is damaged. If it is blocked, soak it in hot water and clean it carefully.
4. Pour the prepared saturated copper sulfate solution to submerge more than two-thirds of the copper rod, tighten the upper cover, and check that the bottom semi permeable membrane should have solution leakage, but there should be no solution leakage, otherwise the semi permeable membrane should be replaced.[4]
