Development and characterization of high-stability all-solid-state porous electrodes for marine electric field sensors

Journal Pre-proof Development and characterization of high-stability all-solid-state porous electrodes for marine electric field sensors Wang Luo, Haobin Dong, Jianmei Xu, Jian Ge, Huan Liu, Cheng Zhang

PII:

S0924-4247(19)30112-8

DOI:

https://doi.org/10.1016/j.sna.2019.111730

Reference:

SNA 111730

To appear in:

Sensors and Actuators: A. Physical

Received Date:

22 January 2019

Revised Date:

28 October 2019

Accepted Date:

7 November 2019

Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Development and characterization of high-stability all-solid-state porous electrodes for marine electric field sensors Wang Luoa,b,c , Haobin Donga,c,∗, Jianmei Xub , Jian Gea,c , Huan Liua,c , Cheng Zhanga,c a

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School of Automation, China University of Geosciences (Wuhan), Lumo Road, Wuhan 430074, China b Faculty of Materials Science And Chemistry, China University of Geosciences (Wuhan),Lumo Road, Wuhan 430074, China c Hubei Key Laboratory of Advance Control and Intelligent Automation for Complex Systems, Lumo Road, Wuhan 430074, China

Abstract

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Marine electromagnetic (EM) methods can be used for marine geophysical exploration. We proposed an all-solid-state porous Ag/AgCl electrode with high stability and a long service life as a marine electric field sensor with the ability to withstand deep sea high pressures while improving the accuracy of electric field measurements for long-period marine EM method engineering projects. Compared with a plate electrode, the all-solid-state porous electrode has excellent polarization resistance in seawater. To avoid the short service life of the sensor used in marine EM method engineering, the all-solid-state porous Ag/AgCl electrodes were manufactured by a ball-milling and sintering process, and the electrochemical channel of the electrodes was observed under a scanning electron microscope. A sensor casing based on the porous electrodes used in the deep sea environment was designed. An experimental system for evaluation and characterization was designed to measure the polarization resistance, sensitivity, noise, self-potential and drift of the proposed electrodes, which are important properties and will affect the accuracy of marine electric field measurements. The results show that the source resistance of the electrode is 2.13 Ω/cm2 , the electrode noise level is 1.57 nV/rt(Hz), the self-potential of the proposed electrode pair is less than 35 µV, and the potential drift is less than 10 µV/day. After ∗

Corresponding author Email address: [email protected] (Haobin Dong )

Preprint submitted to Sensors and Actuators B: Chemical

November 9, 2019

more than 180 days of use, the electrodes showed outstanding stable drift potentials of below 15 µV/day. The proposed all-solid-state porous electrodes can ensure the measurement accuracy of marine electric field signals and promote the exploration of marine resources. Keywords: Marine electromagnetic method, Ag/AgCl electrode, all-solid-state porous electrode, high stability 1. Introduction

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With the depletion of terrestrial resources, prospecting and exploiting marine resources has become inevitable. Marine electromagnetic (EM) methods are the primary means of surveying marine geological structure and prospecting offshore hydrocarbon[1, 2]. Marine EM methods are utilized to obtain magnetic resistivity information in the measured area, which measures the electric field by several pairs of non-polarizable electrodes and the magnetic field by high-sensitivity magnetometers[3]. Seawater will attenuate high frequency EM signal due to the conductivity; therefore the period of useful signal in marine EM methods must be sufficiently long enough to anti-attenuate (typically 1 mHz) [4]. Marine EM methods require a longer time to complete than terrestrial EM methods, and marine EM field signals are weaker. The stability and noise of the electric field sensors commonly used for terrestrial EM cannot easily meet the demand of marine EM methods [5]. Thus, improving the stability of marine electric field sensors is very important for enhancing the accuracy of deep sea electric field measurements by marine EM methods. For marine EM methods, the core of the electric field sensor is a non-polarizable electrode that is primarily composed of an inert metal and its halide ion salt (such as Ag-AgCl electrode) to avoid the corrosion problem caused by the anode reaction[6]. Ag/AgCl electrodes have excellent potential stability and low-frequency characteristics [7], which are suitable for receiving the weak electric field signals in marine EM methods[8]. The techniques used to prepare Ag/AgCl electrodes are mainly divided into two types: electrolytic plating and sintering [9]. Electrodes made through the electrolytic plating process are cost effective and have good stability and a large surface area, although the reaction layer is thin. To improve the stability of plate Ag/AgCl electrodes, Stoica. D, Melissa A et al. studied the influence of several parameters used in the preparation of thermal-electrolytic Ag/AgCl electrodes on the resulting electrode performance and proposed an optimized preparation process for Ag/AgCl electrodes used in PH measurements [10, 11]. In marine EM method applications, Filloux and Webb used electroplated Ag/AgCl 2

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electrodes as EM electric field sensors [12, 13]; however, to reduce noise, the electrodes were extremely large. Another method of eliminating the noise distribution is signal processing based on chopper amplifiers. Wang et al. developed a chopper-stabilized amplifier for electroplating Ag/AgCl electrodes to improve the signal-to-noise ratio of the electric field signal [14]. A porous structure can increase the electrochemical reaction area of the electrode, and it may enhance the electrochemical energy storage and conversion device performance. Zai et al. electroplated Ag/AgCl on porous graphene, and this technique reduced the cost and improved the stability of electrode [15]. However, maintaining this electrode was difficult because of the larger exposed surfaces. Everson et al. used a spraying method to electroplate a film material to produce a non-polarizable electrode, and this method reduced the electrode volume [16, 17, 18]. Other improved approaches for fabricating Ag/AgCl reference electrodes have been studied [19, 20]. The reaction surface of the electrode generated by the electrolytic plating process is mainly the surface layer of the electrode. As the Ag in the surface layer is oxidized or polarized to a large extent, the ratio of Ag to Ag+ decreases, and the stability of the electrode is lowered; thus, the service life of these electrodes is short. The sintered porous solid-state Ag/AgCl electrode has a reaction surface not only on the surface of the electrode but also inside the electrode core, which is inconsistent with porous plate electrodes prepared with the plating process. At the same volume, sintered porous electrodes have a much larger electrochemical reaction area and smaller polarization current than other electrodes. The void reaction surface inside the porous electrode is less likely to be oxidized, and when fully immersed in seawater, the liquid junction potential is smaller and the potential is more stable. Graphite-fibre electrodes have a smaller liquid junction potential. Corna et al. proposed the use of graphite fibre electrodes as marine EM method electric field sensors, although long-term (greater than 1000 s) test data have not been reported [21, 22]. Shen compared the performance of sintered porous Ag/AgCl with graphitefibre electrodes in the application of marine EM methods, although graphite-fibre electrodes require an external electrical device to stabilize the potential [23]. When used in long-term measurement applications, such as maritime station observations, porous solid Ag/AgCl electrodes are more stable and reliable. Sintered porous solid electrodes, which exhibit pressure resistance and anti-polarization characteristics, are more suitable for deep sea exploration engineering than the electroplated electrodes and plate electrodes. Therefore, Wei and Zhang et al. studied the effects of fabrication processes on the performance of sintered solid Ag-AgCl electrodes when used in ship corrosion detection [24, 25]. However, sintered all-solid porous electrodes are rarely used as electric field sensors for marine EM method 3

2. Ag/AgCl electrode polarization mechanism

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engineering applications. Thus, studying high-stability marine electric field sensors based on all-solid-porous Ag-AgCl electrodes is of great significance for meeting deep sea measurement engineering requirements. Based on previous works, this paper proposes a method for improving the performance of the Ag/AgCl electrodes used in marine electric field measurements, including their source resistance, noise level, sensitivity, and stability. These electrodes are manufactured by ball-milling and sintering processes. AgCl was synthesized by ball milling, and fine powders were obtained to increase the interface area between Ag and AgCl within the electrode. Polyvinyl alcohol is used as the pore-making agent to increase the specific surface area and make the electrode porous. Based on the above electrodes, sensors with high stability and long service lives were designed. This work will pave the way to improving the accuracy of marine electric field measurements in long-term marine EM method engineering.

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2.1. Electrochemical reaction process of Ag/AgCl electrode When the surface of the porous electrode contacts seawater, the surface will undergo a redox reaction, resulting in uneven polarization over the entire electrode. Therefore, the Ag/AgCl electrodes will have a contact potential. The active material of the Ag/AgCl electrode is Ag, and its ion is Ag+ . The redox reaction of the electrodes in seawater is as follows (1): AgCl Ag + + Cl− Ag + + e− Ag

(1)

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In this reaction, AgCl is first dissolved and releases Ag+ , and then Ag+ combines with free electrons to form Ag, which covers the surface of the electrode. Similarly, the electrode undergoes a reverse reaction to oxidize Ag to Ag+ , and Ag+ combines with Cl− in seawater to form AgCl. The redox reaction cause contact voltage ηAg/Ag+ , which can be calculated by the Nernst formula (2) [26]:

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ηAg/Ag+ = η 0 Ag/Ag+ +

RT K 0 sp (AgCl) ln nF CCl−

(2)

where η 0 Ag/Ag+ is the base potential of the electrode, n is the number of reactive particles, F is the Faraday constant, R is the molar gas constant, T is the temperature in Kelvin, K 0 sp (AgCl) is the concentration product constant of AgCl, and CCl− is the Cl− concentration in the seawater near the electrode. In the ocean, the temperature 4

T and ion concentration CCl− over a short vertical height are relatively stable, and the influence of these parameters on the electrode potential is small. The ηAg/Ag+ is mainly determined by η 0 Ag/Ag+ . In marine EM method engineering, because the degree of polarization of the electrode pairs changes with time and the environment, small potential drift occurs between the electrode pair and is measured by the acquisition circuit. Moreover, a smaller bias potential and drift correspond to higher accuracy of the electric field signal measurement.

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2.2. Polarization mechanism of the all-solid-state porous electrode Due to the chemical composition and the structural differences of electrodes, the partial dissolution and crystallization process of the electrode are unbalanced, which deteriorates the stability, self-noise, and potential of the electrodes and affects the measurement accuracy of the electric field signal. For the all-solid-state porous Ag/AgCl electrode, the equivalent circuit in the case of full immersion is as shown in Fig. 1. This circuit is based on the assumption that the electrode surface area is 1 cm2 and the electrode thickness is L, which consists of abundant thin layers with a thickness of dx . Among them, Rs dx is the solid-phase equivalent resistance, Rl dx is the liquid-phase equivalent resistance, and Rct /dx is the solid-liquid equivalent transfer resistance.

Figure 1: Equivalent model of the all-solid-state porous Ag/AgCl electrode in seawater

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The apparent polarization current Itotal of the electrode is: Itotal = Il(x=0)

(3)

Il(x=0) is the surface current, and the electrode potential caused by the cathode reaction is as follows: η=ϕs − ϕl +C (4)

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ϕs and ϕl are solid and liquid potentials, respectively, and C is a constant. Because the solid-phase resistance ρs is much smaller than the liquid-phase resistance ρl , we can assume the following: dη= dϕl = −Il ρl dx (5) Il = − ρ1l η 0

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If the surface polarization curve is I 0 =F (η), then the current density of the electrode surface can be calculated according to the equivalent circuit of Fig. 1.

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di di di = s = − l = S ∗ I 0 = S ∗ F (η) dx dx dx

(6)

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S ∗ is the volume-specific surface area of the electrode, and F (η) is the actual surface polarization curve. When the degree of polarization of the electrode is not negligible, the multi-layer polarization of the porous electrode is similar to that of the plate electrode. We can obtain the differential equation for the polarization process of porous electrode as follows:   nF ρl di 0 ∗ 00 η=ρl = 2i ρl S sinh η (7) η = Rct dx 2RT In this formula, i0 is the exchange current density. If we solve the differential equation, we can obtain the following: s i nF nF 4i0 S ∗ RT h ( 4RT η0 ) Itotal = e − e(− 4RT η0 ) (8) ρl nF

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This formula can characterize the dynamic polarization process of a electrode. When η 0  4RT /nF , the degree of polarization cannot be ignored, and Eq 8 can be transformed as follows: s ! 4RT 4i0 S ∗ RT 4RT 0 η =− lg + lg Itotal (9) nF ρl nF nF 6

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The polarization of plate electrode degree can be expressed by the Tafel formula [27]: RT RT η0 = − lg i0 + lg Itotal (10) αnF αnF Therefore, by comparing Eq 9 and Eq 10, the polarization degrees of plate electrode and porous electrode can be obtained as shown in Fig. 2.

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Figure 2: Comparisons of polarization curves between porous electrodes and plate electrodes

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As shown in Fig.2, the degree polarization of porous electrode is much smaller than that of plate electrode. The polarization potential η 0 of plate electrode is proportional to the current lg Itotal , which is shown as curve D in Fig. 2. When the polarization current lg Itotal is small, the effective reaction zone participating in the reaction in the pores of porous electrode is thick, and the specific surface area S ∗ and the exchange current density are both larger than those of plate electrode; thus, the polarization potential η 0 is small. As the current density increases, the thinning of the effective reaction zone of porous electrode is equivalent to the decrease in S ∗ , and curve B exhibits a tendency to approach curve D quickly. As the high polarization reaction proceeds, the polarization process of porous electrode can be equivalent to that of a plate electrode; thus, curve C is similar to curve D. 7

In general, porous solid-state electrode has a larger specific surface area and a thicker reaction area, which results in a lower degree of polarization than plate electrode under the same current density. Similarly, the sintered solid-state porous electrode in the same volume has a thicker reaction layer than the electroplated porous electrode and higher stability when fully immersed in seawater. The inside of the sintered electrode is less susceptible to polarization and has a longer service life. Therefore, the all-solid-state porous electrode can be used as an electric field signal receiver for marine EM methods in long-term measurement engineering.

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3. Electric field sensor system in marine EM method applications

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Marine EM methods consist of transmitting low-frequency high-power EM waves to the seabed and then measuring the reflected EM wave signals. The absorption of EM waves varies with different geological structures. Therefore, the echo information is accompanied by geological structure information. We can deduce the seabed structure by analysing the echo information. The existing magnetic field sensor has a very high sensitivity (0.01nT). However, the accuracy of the electric field measurement is greatly affected by various factors, such as the potential drift and noise of electric field sensors, which limit the accuracy of marine EM method inversion. A schematic diagram of the basic marine EM method instrument signal collector is shown in Fig. 3. Three sets of orthogonal electric field sensor pairs, Ex , Ey , and Ez , are fixed on the acquisition arms of the anchor base station. The acquisition system is sealed in a glass pressure chamber for signal processing of the sensor’s output. Specifically, the electric field signal is initially amplified by a chopper-stabilized amplifier, and the high-frequency background noise is removed by an analogue circuit. After processing, the signal is converted into a digital signal by a high-precision ADC and transmitted to the MCU for post-processing. The potential drift and noise of the electric field sensor are determined by the Ag/AgCl electrode which is the core of the sensor. Research on the manufacturing process of Ag/AgCl electrodes with high stability, deep sea pressure resistance and a sensor casing suitable for deep sea environments is of great significance for improving actual performance of marine EM method-based engineering.

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4. Characterization and discussion 4.1. Manufacturing the marine electric field sensor In this section, we propose a novel method for fabricating a porous solid-state Ag/AgCl electrode and design a sensor housing for the electrode to use in actual marine EM method engineering applications. 8

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Figure 3: Marine EM method electric field measuring instrument diagram

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4.1.1. Fabricating the all-solid-state porous Ag/AgCl electrode The electric field signal in marine EM method engineering has a long period with a small amplitude, the electrode pair has a strong anode reaction, and the pressure in the deep sea is high. For these reasons, in this paper, we proposed a method for preparing an all-solid-state porous Ag-AgCl non-polarized electrode with low noise and excellent stability, which is suitable for deep sea environments. First, the analytical reagents AgNO3 and NaCl powders are ball milled in absolute alcohol, and then the powders are freeze-dried to obtain fine AgCl particles. Next, polyvinyl alcohol is used as a binder and pore-making agent and added to a mixture with a certain proportion of Ag and AgCl. The mixture is moulded under high pressure and sintered at a certain temperature. Finally, after the sintering process, the electrodes are spoiled in dilute hydrochloric acid and polished by sandpaper. The finished electrodes should be immersed in NaCl solution and stored in the dark until use. The finished all-solid-state porous Ag/AgCl electrode is shown in Fig. 4(a), and the surface of the electrode is smooth after being polished. For comparison, a plated Ag/AgCl electrode (NO.CHI111) from Shanghai ChenHua is shown in Fig. 4(c). The structures of two electrodes are different because of the fabricating methods, which determine their dissimilar abilities. The internal structures of the all-solidstate porous Ag/AgCl electrode and plated Ag/AgCl electrode were scanned using an electron microscope as shown in Fig. 4(b) and Fig. 4(d). The inside of the all-solid-state porous Ag/AgCl electrode has a large number of 9

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Figure 4: (a) Proposed all-solid-state porous electrode, (b) scanning electron micrograph inside the proposed electrode, (c) CHI111 plated Ag/AgCl electrode, and (d) scanning electron micrograph over the surface of the CHI111

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micro-diameter pores. Conversely, only a layer of uniform AgCl particles cover the surface of the plate electrode. For the all-solid-state porous Ag/AgCl electrode, as the sintering temperature increases, the pore-making agent polyvinyl alcohol transforms into water and carbon dioxide and then volatilizes out of the pores. Therefore,some micropores were formed. For the plated Ag/AgCl electrode, AgCl particles were adsorbed onto the surface of the Ag substrate by electroplating, to form a unique non-polarized surface. However, when fully immersed in seawater, the micropores of the all-solid-state porous Ag/AgCl electrode could provide a channel for seawater to the inside of the electrode, which enhanced the surface area and the thickness of the reaction layer between the seawater and the electrode compared with the plate electrode. Thus, this solid-state porous electrode is more difficult to be polarized than the plate electrode.

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4.1.2. Design of electric sensor shell The proposed electrodes are composed of Ag and AgCl. Ag is easily oxidized by oxygen, and AgCl is prone to decomposition by light. Therefore, this electrode is difficult to store for a long time. To avoid this problem, we designed a suitable electrode shell to protect the electrode. Meanwhile, the electrode is easily broken by the impact of ocean currents when in service. In addition, the Cl− concentration changes due to ocean current movement, which affects the stability of the electrode. The electrode shell reduces the current flow velocity near the electrode core. The Ag wire that connects the electrode and the external circuit should also be sealed within the shell. A sensor shell for the proposed electrode is shown in Fig. 5. The electrode shell is 10

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made of high-strength polyester plastic in the shape of a drop to reduce the influence of ocean currents and silt. Many inlet holes are regularly distributed over the lower half of the electrode shell, which not only provides a way for the electrode core to contact the seawater but also slows the flow of ocean current into the shell. The Ag wire connects the watertight connector with the electrode, and it is sealed by a waterproof and pressure-resistant adhesive and finally connected to the measuring system. The electrode shell is filled with silicon dioxide to prevent damage to the shell caused by electrode shaking, thereby weakening the impact of ocean currents and improving the stability of the electrode. The high-strength electrode shell and the high-stability non-polarizable electrode provide good electric field measurement conditions for marine EM methods.

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Figure 5: Electric field sensor

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4.2. Performance test of Ag/AgCl electrode The non-polarizable performance of the electric field sensor is macroscopically reflected in the polarization current of the Ag/AgCl electrode caused by the multiphase reaction. The stability of the Ag/AgCl electrode is determined by parameters such as electrode impedance, potential drift, and noise. The ability of the electric field sensor to acquire weak signals is also determined by the sensitivity of the electrode pair. Therefore, to test the sensor performance index, we designed experimental characterization devices and corresponding electrochemical impedance spectroscopy, noise, sensitivity, self-potential and potential drift tests for the proposed electrodes. The experimental results are analysed and discussed. 4.2.1. Electrochemical impedance spectroscopy of the Ag/AgCl electrode The multiphase reaction of the porous Ag/AgCl electrode with seawater is complicated; thus, its reactance is not pure resistance and can be represented by the equivalent circuit shown in Fig. 6, where Rs is the equivalent resistance, Rp and Qp are the charge transfer resistance and capacitance, respectively, and Rd and Qd are 11

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the dynamic polarization resistance and double-layer capacitance, respectively. The porous electrode has a dispersion effect in seawater, resulting in phase angle elements Qp and Qd in the equivalent circuit; thus, the electrode can also be equivalent to a high-pass filter.

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Figure 6: Equivalent circuit of proposed electrode impedance

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To test the electrode impedance, the electrode system is excited by a sinusoidal signal with a small amplitude and a variable frequency, and the output signal is measured. The ratio between the output and input signal is the electrochemical impedance of the electrode. In this experiment, the electrochemical impedance of all solid porous electrodes and plate electrodes is tested and compared.

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Figure 7: Electrochemical impedance spectroscopy (EIS) characterization of the proposed all-solid-state electrode and plate electrode: (a)EIS of proposed electrode, (b) impedance mode versus frequency spectrum of proposed electrode, (c) EIS of plate electrode, and (d) impedance mode versus frequency spectrum of plate electrode The characterization experiments use three electrodes to form a three-electrode measurement system[28] that includes a standard Ag/AgCl electrode used as the ref12

Table 1: Equivalent circuit parameter of the proposed and plate electrodes EIS Proposed Plate

Rs (Ω/cm2 ) 1.52 21.7

Rp (Ω/cm2 ) Qp (F/cm2 ) 0.17 0.89 920.60 3.83 × 10−7

Rd (Ω/cm2 ) Qd (F/cm2 ) 0.72 0.61 -

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erence electrode (Japanese ALS NO.RE-1CP,a plate electrode and AgCl is thin solid film formed on surface of the Ag wire), a Pt auxiliary electrode, and the proposed Ag/AgCl electrode or the plate electrode, which is used as a working electrode. The reference electrode of three-electrode measurement system can isolate other component impedances to the measurement. The AC impedance of the electrodes is measured using a dual potentiostat (Shanghai Chenhua NO.CHI700E). The excitation frequency of the CHI700E step change and the excitation amplitude is 5 mV, and the impedance of the electrode is measured at different frequencies and used to obtain the electrode electrochemical impedance spectrum. As shown in Fig. 7, the electrochemical impedance spectra of the proposed allsolid-state Ag/AgCl electrode and plate Ag/AgCl electrode are tested. The electrochemical impedance of the proposed electrode is shown in Fig. 7(a). The data show two semicircles that correspond to the dynamic polarization equivalent circuit (DPEC) and the charge transfer equivalent circuit (ETEC) shown in Fig. 6. The fitted electrical parameters can be seen in Table 1, and they reflect the electrochemical reaction mechanism of the electrode in the low-frequency band and the high-frequency band. Similarly, the impedance spectra of the plate electrode CHI111 is tested using the same method as shown in Fig. 7(c). The EIS data of the plate electrode show one semicircle, which represents only one circuit with a time constant, and the fitted electrical parameters can also be seen in Table 1. After taking the imaginary part of Fig. 7(a) and (c) obtaining the impedance model and frequency relationship spectrum of Fig. 7(b) and (d), the relationship of the electrode source resistance with the frequencies of the received signal can be observed. The source resistances of two types electrode show a downward trend with increasing frequency, meaning that the electrode exhibits a high-pass filter effect. For porous electrodes, the solid-liquid phase exchange mechanism of the porous electrode is mainly based on the charge exchange mechanism in the low-frequency region, while the surface of the electrode exhibits a double capacitance in the high-frequency region due to the dispersion, and the equivalent capacitance of the circuit increases. The containment ability of high-frequency signals is weakened. However, the plate electrode only has a surface unpolarized reaction and causes a liquid junction 13

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capacitor, which has a low-pass effect at both high- and low-frequency domains. The electric field sensor for marine EM method engineering has higher requirements on its low-frequency impedance, and the proposed electrode can guarantee a source resistance of 2.13 Ω/cm2 @ 1 Hz. On the contrary, the source resistance of the plate electrode is very large at approximately 942.27 Ω/cm2 @ 1 Hz. The smaller source resistance has larger output voltage amplitude and a smaller thermal noise under the condition when the excitation source power is constant. Thus, the all-solid-state porous electrode is more suitable for use in EM applications than the plate electrode. In fact, pure Ag is an electrical conductor, and the source resistance of the all-solid Ag/AgCl electrode should be much less than 2.13 Ω/cm2 @ 1 Hz. However, AgO is insulated, and it is certain that an amount of Ag is oxidized during production or use, resulting in a different colour on the surface of the electrode. Therefore, during production and use, protection from light and constant temperature conditions require special attention.

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4.2.2. Sensitivity of the electrode pair The sensitivity of the electric field sensor can be preliminarily characterized by a pair of electrodes that test the response under the excitation signal. Using the electrode signal response measurement system shown in Fig. 8 below, we can obtain the potential response of the electrode under a fixed AC excitation. The two proposed electrodes were placed in a 3.5wt% NaCl aqueous solution at a distance of 10 cm, and two Pt plates were placed outside the electrode at a distance of 20 cm. The Pt plates were connected to the signal generator to obtain the solution excitation frequency and sine wave with variable amplitude. The oscilloscope simultaneously observes the excitation signal of the Pt plates and the response signal at both ends of the proposed electrode. The electrical signal response curve of the proposed electrode pair is shown in Fig. 9. When the excitation amplitude is 1 mV, the frequencies are 1 mHz, 10 mHz, 100 mHz, 1 Hz, and 10 Hz, where the received signal changes synchronously with the excitation signal, and the curve is smooth and has no breaks. As the frequency increases, the induced signal is attenuated due to the induced current caused by the excitation signal in the conductive solution. In the low-frequency range, the electrode exhibits excellent frequency response characteristics and is suitable as a marine EM method electric field signal receiver. 4.2.3. Noise of the proposed electrode In marine EM methods, the electric field signal is seriously attenuated when it is transmitted in seawater; thus, compared with terrestrial applications, marine applications require electric field sensors with better sensitivity and noise. 14

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Figure 8: Signal response test system for the electrode pair

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Figure 9: Electrode signal response curve: (a) Excitation signal waveform, where the frequency is 0.001 Hz, 0.01 Hz, 0.1 Hz, 1 Hz and 10 Hz, and (b) response signal waveform of the proposed electrode pair to the excitation signal in (a)

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Because natural or industrial noise is much larger than the electrode self-noise, the electrode noise test needs to be performed in an EM shielding environment. Therefore, we build an electrode noise measurement system, as shown in Fig. 10. Only considering engineering applications for noise analysis, two identical proposed electrodes can be used as the measuring electrodes to measure the sensor noise level. The electrode noise is first amplified using an ultra-low noise preamplifier and then acquired by an ultra-precise 32-bit ADC. The frequency range of the marine electric field signal is 0.001 Hz to 1000 Hz; therefore, the sampling frequency of the ADC is set to 2.4 kHz. To reduce the influence of external EM interference, the preamplifier is pre-designed as a 6th-order low-pass filter with a cutoff frequency of 1 kHz to filter out high-frequency interference.

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Figure 10: Electrode noise measurement system

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The noise level of the measuring electrode system after the electrode pair was stabilized for 24 hours is shown in Fig. 11. Under the condition of 1 Hz, the voltage noise density was 1.69 nV/Hz. According to the electrochemical impedance data shown in Fig(b), the resistance of the electrode pair at 1 Hz is approximately 2 * 2 = 4 Ω. Therefore, a 4 Ω equivalent noise resistor is connected to the front end of the test system to measure the measured system and ambient noise, and the voltage noise density is 0.62 nV/Hz. The noise density of the proposed electrode can be calculated as follows: p 1.692 −0.622 nV (Hz)−1/2 =1.57nV (Hz)−1/2 (11) The noise level is low enough to detect marine electric field signals [29]. Theoretically, the self-noise of the all-solid-state porous Ag/AgCl electrode consists of two 16

components, the shot noise and the 1/f noise, and the electrode voltage noise can be expressed by the following formula: V 2 = (K1 V0 2 R + f −n ) • ∆f

(12)

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K is a constant proportional to the material; V0 is the electrode terminal voltage, including the polarization potential; f is the frequency; and n is the square times of flicker noises.

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Figure 11: Electrode and system noise power spectrum

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Below 1 Hz, the electrode noise is mainly controlled by 1/f noise, and its slope can be used to characterize the internal polarization process of the electrode to some extent. In the high-frequency range, the electrode voltage noise density level is controlled by shot noise and thermal noise. The noise of the proposed electrode is also determined by the polarization of the electrode caused by the uniformity of Ag and AgCl after actual sintering. Therefore, the ratio of Ag to AgCl within the proposed electrode is uniformly distributed. Of course, the larger the source resistance of the electrode is, the larger the noise floor level. In fact, the measurement of noise in the actual use of the electrode is controlled not only by the electrochemical process inside the electrode but also by the external environment. 4.2.4. Stability of the proposed electrode Even for non-polarized electrodes, there is still a self-generated differential potential between the electrode pairs. The differential potential is disturbed by the 17

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electrochemical polarization process, which can be divided into two kinds of changes: no-time-varying bias potential and time-varying potential drift. When using as a marine EM method electric field sensor, the non-polarized electrode pair needs a lower bias potential and drift. These abilities indicate that the paired electrodes maintain a constant difference, and even when the external conditions change, the same change can be produced without causing irregular changes in the electrode potential. Typically, the electrode pair used in marine EM method measurements has a potential drift of several microvolts (µV) in one day; thus, the accuracy of the potential measurement system must be better than 0.1 µV. Traditional methods of measuring the electrode pair potential mostly use electrochemical workstations, although the accuracy and efficiency are low. Therefore, a set of precise multi-group electrode potential acquisition systems for measuring multi-pair electrode bias potential and potential drift is designed as shown in Fig12. LabVIEW software for PCs controls the Fluke8845 voltmeter and four (or more) sets of relays that connect different pairs of electrodes to the voltmeter to measure the potential. LabVIEW controls the relay to open one set of electrode pairs every minute, and the meter measures the potential at a frequency of 0.5 Hz. After the data are processed, the electrode pair potential curve is displayed and stored in LabVIEW.

Figure 12: Multi-pair electrode potential and drift measurement system

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(a).Bias potential of the electrode pair. The bias potential between the electrode pair is determined by factors such as temperature and salt ion concentration. Theoretically, this kind of potential between the two Ag/AgCl electrodes can be calculated from the Nernst equation in the case where the above factors are determined. 18

For any single proposed Ag/AgCl porous electrode, the standard potential ϕAg/AgCl can be given by the Nernst equation[26], ϕAg/AgCl = ϕ0 Ag+ /Ag +

RT K 0 sp(Ag/AgCl) ln nF aCl−

(13)

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where ϕ0 Ag+ /Ag is 0.7996V, which is the standard electrode (half-cell) potential of the silver (Ag) electrode between the hydrogen (H) electrode under the standard state [30]. K 0 sp(Ag/AgCl) is 1.77×10−10 , which is the Ag/AgCl concentration product constant, and aCl− is the concentration of Cl− in the solution. The bias potential of the proposed porous Ag/AgCl electrode (PE) is measured in a two-electrode system, and a ChenHua NO.CHI111 plate Ag/AgCl electrode mentioned above is used as the reference electrode (RE). If the Ag/AgCl electrode pair is immersed in a saturated NaCl solution at 25 ◦ C, the proposed Ag/AgCl electrode standard potential is 234.5 mV according to Eq 13. According to Bates’s research [31], the standard potential of Ag/AgCl reference electrode is 222.4 mV ±0.13 mV; thus, the bias potential between the RE and PE is 12.1 mV ± 0.13 mV. For further analysis, the actual bias potential of the proposed electrode is measured by the system shown in Fig 12. Because the Cl− concentration in the saturated NaCl solution is highly sensitive to temperature, the test needs to be performed at a constant temperature of approximately 25 ◦ C and in a dark environment. Prior to the test, the new electrode should be soaked in a hydrochloric acid solution to activate the surface and then washed and immersed in a 3.5wt% NaCl solution for 24 hours to stabilize the electrode pair potential. Ten PEs and ten standard REs were randomly selected to measure the bias potential between PE and RE. The experimental results are shown in the shaded column Fig.13 (PE with RE). All the bias potential data are distributed at approximately 12.1±0.3mV, which is similar to the theoretical results, although the standard deviation of potential is slightly larger than that of the theoretical results. These results proved to some extent that the polarization properties of the PEs are stable. However, the standard deviation of the potential may be caused by the RE, and two similar performance PEs must be paired as the marine EM method electric field sensor pair. The potentials between two PEs with the same performance are zero in theory, although due to problems with the process, material ratio, volume, etc., diverse performances between the electrode pairs will cause differential potentials. Therefore, it is necessary to test the bias potential between PEs to provide data support for actual applications, and 9 PEs replaced the REs to measure the bias potential between PE and PE. The substantial column shown in Fig. 13 indicates the bias potential of the 19

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Figure 13: Data on the bias potential of ten pairs of electrodes

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proposed electrode pair (PE with PE). The potential between PEs is mainly concentrated at 0.025±0.01mV, and the distribution area is as narrow as 0.035mV. These extremely small and stable bias potentials of the PEs show that the proposed electrodes have high stability and durable consistency.

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(b).Potential drift and activation method of the proposed electrode pair. Changes in the environment with time cause an imbalance in the polarisation degree of the electrode pairs, leading to changes in the bias potential between the electrode pairs with time, which is called potential drift. The potential drift of the electrode tends to be the most intuitive method of reflecting the polarization characteristics and long-term stability of the electrode. In marine EM method engineering, the sampling periods may exceed one week or even one month, and the drift potential of the electrode pair will be collected by the amplifier; it is difficult to compensate for and eliminate the drift potential, and an excessive drift potential may damage the integrity of the signal. The potential drift data of the electrode pairs are measured by the system as shown in Fig.12, which can provide a reference for pairing the electrodes at the engineering site. Hence, the difference in potential drift between the plate electrodes and the proposed all-solid-state porous electrodes is tested. The experiment data are shown in Fig. 14. Fig. 14(a) and (b) show the measured potential drift data of 5 pairs of REs and 10 pairs of PEs within 15 days (360 hours). However, the bias potential of the RE pairs is sufficiently larger than that of the PE pairs, and the potential drift is obviously greater as well. Curve (a) shows that the potential drift of all RE pairs is greater than 1 mV within 15 days, representative 20

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Figure 14: Drift data of PE and RE pairs: (a) Potential drift data of five RE pairs for 15 days and (b) potential drift data of ten PE pairs for 15 days

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value is 31.8µV/24 h. However, the potential drift of all PE pairs is smaller than 0.02 mV within 15 days, representative value is 6.5µV/24 h. Thus, the 24-hour potential drift data can be used to characterize the stability of sensors in engineering applications. All 24-hour potential drift data of every single pair electrode over 15 days are first taken out and then averaged to obtain the 24-hour typical potential drift data of every electrode pair as shown in Table 2. As shown in Table 2, within 24 hours, the potential drift of part of the RE pairs is greater than 150 µV/24 h, with most greater than 33 µV/24 h, which will seriously impact EM method engineering measurement data. The potential drift of PE pairs with excellent performance does not exceed 10 µV/24 h, which is considerably lower than that of RE pairs, whereas the performance is also lower than the standard potential drift range of 0.1 mV/24 h, which is suitable for use as a marine EM electric field sensor. However, the potential drift of PE pairs is unstable, and most electrode pairs exhibit a more pronounced change. This result is due to the influence of the ion concentration of the electrolyte solution caused by the change in the temperature difference between day and night during the long-term operation of the electrode and the presence of external EM interference. After storing the PE for approximately half a year, the surface of the electrode is clearly oxidized as shown in Fig. 15. The oxidized proposed all-solid-state porous electrodes can be subjected to an activation treatment to restore electrode performance. The electrode is wetted with alcohol and then sanded, and finally, hydrochloric acid is used to remove the oxide layer on the surface of the electrode. After the electrode was immersed in saline for 24 hours, the drift of the electrode pairs was 21

Table 2: Typical potential drift data collected over 24-hour periods within 15 days for ten proposed electrode pairs and five reference electrode pairs 1

2

3

4

5

6

7

8

9

10

electrode

5.52

4.64

6.26

3.01

4.23

5.61

6.81

3.51

2.78

3.17

Reference pairs (µV)

electrode

33.55

150.39

47.18

57.51

42.91

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-

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-

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Proposed pairs (µV)

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measured, and the data are shown in Fig. 16(a). As shown, the electrode drift can still be stabilized at 15 µV within 24 hours as shown in Fig. 16(b), and some special electrode pairs can maintain a potential drift below 10 µV within 24 hours. The regular jitter in the drift data is external periodic interference.

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Figure 15: Electrode activation process after oxidation

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In the flat plate or electroplated multi-layer electrode, the reaction area is mostly on the surface, and when the reaction layer is oxidized, the electrode stability is drastically lowered. When the stability affects the marine EM method engineering application electric field accuracy, the electrode is abandoned. In fact, even if the oxidized layer is removed and subjected to a secondary activation treatment, the electrode stability is still reduced because the process creates an inconsistent ratio of Ag to AgCl inside the electrode compared with the surface. The greatest advantage of the proposed all-solid-state porous electrodes is that it not only has a high-pressure pressing process for the electrode core during production, which allows it to have good deep sea pressure resistance, but also has a consistent reaction between the internal and external electrochemical processes; moreover, surface oxidation does not greatly affect the actual performance of the electrode.

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Figure 16: Proposed electrode pair potential drift data after reactivation: (a) Drift data for nine pairs of proposed electrodes over 120 hours and (b) from 40 to 64 hours in (a)

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5. Conclusions

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Considering the high stability, low noise and long life of marine EM method electric field sensors, this paper proposes an all-solid-state porous Ag/AgCl electrode based on the sintering method. Compared with the flat electrode, the proposed electrode fabricated by the new preparation method has smaller source resistance, drift, and noise, and the weak electric field signal has a stronger sensing capability. The pores within the electrode provide a large surface area and channels for electrochemical reactions in seawater, and the internal reaction layer is thicker and less susceptible to oxidation; thus, the electrode life is significantly improved. For deep sea engineering applications, a high-strength shell adapted to the proposed electrode was designed, and corresponding characterization experiments of the proposed electrode were performed to identify the key properties like impedance, sensitivity, noise, bias potential, and drift. The 24-hour potential drift of the proposed electrode pair is better than 10 µV, and the self-noise level is 1.57 nV. The experiments show that the sensor pair can sense a 1 mV low-frequency electric field signal without distortion. The sensors are suitable for marine electric field measurements and can guarantee the accuracy of electric field measurements in long-term marine EM method engineering applications. ACKNOWLEDGEMENTS This work is partly supported by the National Natural Science Foundation of China (Grant No. 41904164, 41874212, 41474158), the Foundation of National Key R&D 23

Program of China(Grant No. 2018YFC1503702), the Foundation of Qingdao National Laboratory for Marine Science and Technology (Grant No. QNLM2016ORP0201), the National Key Scientific Instrument and Equipment Development Project of China (Grant No. 2014YQ100817), the Foundation of Science and Technology on Near-Surface Detection Laboratory (Grant No. 6142414180913, TCGZ2016A005, 614241409040218, 614241409041217), the Foundation of Wuhan Science and Technology Bureau(Grant No. 2017010201010142, 2019010701011411), and the Chinese Postdoctoral Science Foundation (Grant No. 2016M592410)

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