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Abstract
In the field of electrochemical energy storage systems, the use of in situ detection technology helps to study the mechanism of electrochemical reaction. Our group has previously in situ detected the electrochemical reaction in vanadium flow batteries by total internal reflection (TIR) imaging. In order to further improve the detection resolution, in this study, the weak measurement (WM) method was introduced to in situ detect the electrochemical reaction during the linear sweep voltammetry or the cyclic voltammetry tests with quantitative measurement of the absolute current density, which lays a foundation for replacing the TIR for two-dimensional imaging of electrochemical reactions in vanadium flow batteries, oxygen/hydrogen evolution reaction, surface treatments, electrochemical corrosion and so on.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to energy shortages and environmental issues, the demand for renewable energy is becoming stronger. Electrochemical energy storage systems, as a method to solve the inherent intermittent problems of renewable energy including solar energy, wind energy, etc., have attracted considerable attention in recent years [1,2]. Among the electrochemical energy storage systems, vanadium redox flow batteries [3,4] show the promising prospect of industrialization with its advantages of long cycle life, environmentally friendly and independently tunable power and energy capacity.
Researchers pursue vanadium flow batteries with high electrochemical performance, however, the traditional methods for detecting electrochemical performance, such as linear sweep voltammetry (LSV) [5], cyclic voltammetry (CV) [6], electrochemical impedance spectroscopy (EIS) [7], etc., can only provide average information of the whole electrode. But the spatial distribution difference of the electrode affects the performance of energy efficiency, charge-discharge capacity, polarization and so on [8,9]. The electrochemistry distribution detection methods like potential probe [10], shunt resistor [11], printed circuit board [9] affect the battery. Therefore, it is necessary to find a method which can give spatial distribution of electrochemistry performance with little interference.
Optical refractive index detection methods—including surface plasmon resonance (SPR), total internal reflection (TIR), weak measurement (WM), and so on—has the advantages of small interference to analytes, high spatial resolution, high sensitivity and high throughput. The microscopic detection technology proposed by the Tao’s group combines the SPR method with electrochemical measurement [12,13], which achieved the same function as CV. And from each point on the result graph, a CV curve can be obtained, so the specific distribution can be obtained. Since the method was based on a microscopic light path, the light path needed to be updated to measure the macroscopic range of the electrode. Moreover, the gold film used in the SPR method not only had low durability in acid electrolyte and redox process, but also produced Faraday current interfering with the electrochemical process, thus limiting the application of the SPR method. Due to the various problems of the sensing film, our group used the TIR without sensing film to achieve a wide range of electrochemical measurements in the vanadium battery system [14,15]. But the resolution, sensitivity and signal-to-noise ratio of the TIR were worse than those of the SPR, so it is necessary to have a better performance method without sensing film.
The WM method was first proposed by Aharonov, Albert and Vaidman in 1988 [16]. This method not only provided a more in-depth explanation for quantum physics [17,18], but also was widely used in the detection of weak signals based on weak-value amplification effect, showing its potential in precise measurement [19–22]. Our group has introduced the WM to the field of biomedical testing for the first time in 2015, measured the state of biomolecules and the biological reaction process, and obtained very impressive experimental results [23–25]. We compared the performances of the WM and the SPR [26], and found that the resolution of the WM without coating has the same order of magnitude as the SPR. So it is very suitable for electrochemical measurement.
In this work, as far as we know, this is the first time that the WM method has been applied to electrochemical in situ detection, which has proved the feasibility of its application. The calibration experiment verified that its resolution was higher than that of the TIR, which laid the foundation for the subsequent replacement of the TIR in two-dimensional imaging measurement. Therefore, we believe that the WM method has great application prospects in electrochemical reaction measurement in vanadium flow batteries, oxygen/hydrogen evolution reaction [27,28], surface treatments [29], electrochemical corrosion [30] and so on.
2. Experimental setup and theory
2.1 Materials
Vanadyl sulfate (VOSO4·3.5H2O, 99% purity) was purchased from Shenyang Haizhongtian Fine Chemical Co., Ltd. (Shenyang, China). Sulfuric acid (H2SO4) was purchased from Dongguan Dongjiang Chemical Reagent Co., Ltd. (Dongguan, China). Prism (Chinese ZF6 glass, 12 × 12 × 12 mm) was ordered from Fuzhou Alpha Optics Co., Ltd. (Fuzhou, China). All other chemical reagents were purchased from Shenzhen Tianxiang Huabo Co., Ltd. (Shenzhen, China).
2.2 Experimental setup
The schematic diagram of the electrochemical WM system in this work was shown in Fig. 1(a). The incident light from a Superluminescent diode (SLD, A in Fig. 1(a), output with a fiber, 5 mW, IPSDS0803, Inphenix, Wuhan, China) centered at 830 nm, collimated by an achromatic lens (B in Fig. 1(a), f = 10 mm, GCL-010661, Daheng Optics Inc., Beijing, China), propagated through a linear polarizer (C in Fig. 1(a), extinction ratio of 100,000:1, LPVIS050-MP2, Thorlabs Inc., Shanghai, China). The light beam then entered a self-made electrochemical detection module (D in Fig. 1(a)) and was reflected by a prism in the module. The reflected light then passed through a Soleil-Babinet compensator (SBC, E in Fig. 1(a), SBC-IR, Thorlabs Inc., Shanghai, China), which was used for continuous phase adjustment and controlling the measuring range. The light beam then passed through another linear polarizer (F in Fig. 1(a), the same as C in Fig. 1(a)), which was almost perpendicular to the first polarizer to realize weak measurement, and was focused by another achromatic lens (G in Fig. 1(a), the same as B in Fig. 1(a)), finally collected by a spectrograph (H in Fig. 1(a), input with a fiber, HR2000, Ocean Optics, Shanghai, China). Figure 1(b) is a photograph of the electrochemical WM system in this work.
Fig. 1. Electrochemical weak measurement (WM) system. (a) Schematic of the electrochemical WM system. A: Superluminescent diode (SLD); B: achromatic lens (L1); C: linear polarizer (P1); D: self-made electrochemical detection module (EDM); E: Soleil-Babinet compensator (SBC); F: linear polarizer (P2); G: achromatic lens (L2); H: spectrograph. (b) A photograph of the electrochemical WM system.
2.3 Self-made electrochemical detection module
The expanded view of the self-made electrochemical detection module in Fig. 1(a) was shown in Fig. 2(a). A prism (D3 in Fig. 2(a)) was fixed with a prism holder (D2 in Fig. 2(a)), then it was pressed against an electrochemical reaction cell (D4 in Fig. 2(a)), which was surrounded by a water bath pool (D8 in Fig. 2(a)) to reduce the interference caused by outside temperature. Finally, the prism, the electrochemical reaction cell and the water bath pool were combined by fixators (D1 and D9 in Fig. 2(a)) with screws. In the electrochemical reaction cell, a graphite plate with insulating tape (D5 in Fig. 2(a)), a graphite rod (D6 in Fig. 2(a)) and a saturated calomel electrode (SCE, D7 in Fig. 2(a)) used as working electrode, counter electrode and reference electrode respectively. The three electrodes were partially immersed in the electrolyte (0.1 M V(IV) and 2 M H2SO4). In the electrochemical reaction cell, tetravalent vanadium ions (V(IV)) and pentavalent vanadium ions(V(V)) converted into each other. The change in the valence state led to the change of the concentrations of oxidized (V(V)) and reduced (V(IV)) species, then the refractive index changed as well [31]. The change of refractive index can be detected by a high-resolution [23,26,32] weak measurement system (the sensitivity is 2795 nm∕RIU and the resolution is $8.5 \times 10^{-7}$ RIU in this work) without coating. Figure 2(c) was a schematic of them combined together. Figure 2(c) was a photograph of the self-made electrochemical detection module.
Fig. 2. The self-made electrochemical detection module (EDM, D in Fig. 1(a)). (a) Expanded schematic view of the EDM, D1: fixator; D2: prism holder; D3: prism; D4: electrochemical reaction cell; D5: graphite plate with insulating tape (working electrode, WE); D6: graphite rod (counter electrode, CE); D7: saturated calomel electrode (reference electrode, RE); D8: water bath pool; D9: fixator. (b) Combined schematic view of the EDM. (c) Photograph o of the EDM. (d) Photograph of graphite plate with insulating tape (working electrode, WE).
2.4 Theory
The WM systems were used to explore about quantum physics [17,18] or detected some physical quantities [19–22]. Our group introduced the WM systems to measure the state of biomolecules and the biological reaction process [23–25]. In this work, we used the WM system to detect electrochemical reaction. The theoretical basis is as follows:
In the electrochemical reaction cell of the electrochemical detection module (EDM), V(IV) and V(V) converted into each other. The detection signal collected by the WM system was contributed by the concentration variation of the oxidized (V(V)) and reduced (V(IV)) species on the surface between the graphite plate and the prism in the EDM. Considering this redox reaction V(IV) ↔ V(V), the scanning potential [33] is:
If the charge transfer rate of the reaction is fast enough, the electrode reaction can be described by the Nernst equation:
The diffusion equation can be used to describe the real-time variation of electrolyte concentration:
Similar to the previous work [12,14], considering the detection depth, the response of the WM system is:
where $\lambda (t)$ is the change value of the centroid wavelength shift. In the WM system, the two-peak spectrum are obtained by the spectrograph (H in Fig. 1(a)). When the refractive index of the electrolyte changes, the phase of the incident light will change, and the centroid wavelength of the two-peak spectrum will shift, as shown in Fig. 3(a). The relationship between the centroid wavelength shift of the two-peak spectrum and the phase is [26] (see Supplement 1, Section 1 for details):Fig. 3. (a) The centroid wavelength shift of two-peak spectrum obtained by spectrograph. (b) Time-varying centroid wavelength shift based on the data in (a) and${\lambda _{pa}}$ is the point with the most drastic centroid wavelength shift change. (c) Denoising result by smoothing the centroid wavelength shift variations in (b) and$\Delta {\lambda _{\textrm{pa}}}$corresponds to${\lambda _{pa}}$in (b). (d) Curves by deconvolution calculation of the centroid wavelength shift variations in (c) and the peak current ipa is the result obtained by the deconvolution of$\Delta {\lambda _{\textrm{pa}}}$in (c).
${\alpha _O}$ and ${\alpha _R}$ are the refractive index changes per unit concentration of the oxidized (V(V)) and reduced (V(IV)) species respectively. K is the change of the centroid wavelength shift caused by the per unit refractive index.
Then the centroid wavelength shift deduced from the above can be obtained as follow:
Then the centroid wavelength shift variation is:
Figure 3(b) displays the time-varying centroid wavelength shift $\lambda (t)$ and ${\lambda _{pa}}$ is the point with the most drastic centroid wavelength shift change. By subtracting the initial centroid wavelength shift at 0 V from all the subsequent centroid wavelength shift ($\Delta \lambda (t){\rm = }\lambda (t)-\lambda (t_0)$), the time-varying centroid wavelength shift variations $\Delta \lambda (t)$ is denoised by the “smooth” function in MATLAB R2017a. The denoised result is plotted in Fig. 3(c) and $\Delta {\lambda _{\textrm{pa}}}$ corresponds to ${\lambda _{pa}}$ in Fig. 3(b).
Rewrite Eq. (9) as:
Therefore, the current density can be derived as:
The quantitative relationship between the current density of electrochemical reaction and the centroid wavelength shift variation is established by Eq. (11). The derived curve in Fig. 3(d) shows the relative current densities along with the potential and the peak current ipa is the result obtained by the deconvolution of $\Delta {\lambda _{\textrm{pa}}}$ in Fig. 3(b).
2.5 Calibration experiment
In order to obtain the current density, these parameters (n, F, b, π) and variables ($\Delta \lambda (t)$, t) should be known according to Eq. (11). Among them, n, F and π were known, $\Delta \lambda (t)$ and t were obtained by the WM. The parameter b was the only unknown quantity, which was about the diffusion coefficients of the oxidized (V(V)) and reduced (V(IV)) species (DO, DR) and the refractive index changes per unit concentration of the oxidized (V(V)) and reduced (V(IV)) species (${\alpha _O}$, ${\alpha _R}$). As b was unknown, we conducted the calibration experiment to obtain the current density with a physical unit.
A working electrode with known reaction area was needed to obtain the current density because the electrochemical workstation can only provide total current. A graphite plate with smooth surface was used as the working electrode for calibrating. Only one interface of the graphite plate (area: 1×1 cm2) contacted with the prism and the others were wrapped by insulating tapes to avoid contact with the electrolyte, as shown in Fig. 2(d). Since the electrochemical reaction only occurred on the exposed interface, the current density can be obtained by dividing the current by the area assuming that the reaction on the interface was uniform [14]. Because the two contact surfaces between the graphite plate and the prism were not ideally smooth, the gap between them will be immersed by the electrolyte. But the electrolyte in the gap cannot be refreshed effectively, so we adopted the linear sweep voltammetry (LSV) to reduce the electrolyte renewal difference by shortening time instead of the CV [14].
A series of scan rate was set in the experiment in order to tune the current density. The current densities obtained by dividing the electrochemical workstation (EW) currents by the area (1×1 cm2) should be equal to the relative current densities obtained by the WM. Thus, the different peak oxidation current densities of the LSV at different scan rates were obtained to achieve calibration, that is, the relative current density obtained by the WM can be converted to the current density with a physical unit (mA cm-2).
3. Results and discussion
3.1 Linear detection range
According to Eq. (8), the phase change of the light can be measured by detecting the centroid wavelength shift of the emission spectrum [26]. The phase change and the centroid wavelength shift was taken as the abscissa and the ordinate respectively. The relationship between the two was shown in Fig. 4. It can be seen from Fig. 4 that the centroid wavelength shift of the emission spectrum was not a uniform change process with the continuous increase of the phase change. When the phase increased, the centroid wavelength shift changed slowly until it reached a minimum, increases suddenly and rapidly, and then slowly decreased after reaching a maximum value. It means that the response sensitivity of the centroid wavelength shift of the emission spectrum to the phase was not the same in different phase change regions. For the rapidly changing region in the middle, the response was sharp and relatively linear, which was very suitable for high-sensitivity phase detection. Therefore, the phase difference caused by the concentration of the oxidized (V(V)) and reduced (V(IV)) species in this experiment can be detected with this linear range.
Fig. 4. The centroid wavelength shifts with respect to the phase. The solid line is the theoretical expectation. The dots are the experimental data
3.2 Performance comparison of the TIR method and the WM method
In order to compare the resolution of the TIR and the WM, the TIR and the WM systems were built by the same experiment components (see Supplement 1, Section 1 for details) for reducing the impact of equipment as much as possible and worked within their linear range. For convenience, both systems were tested by LSV at scan rate of 1 mVs-1 and the potential window of 0.5 V-1.2 V. The results were shown in the Fig. 5. Figure 5(a) showed the light intensity variation measured by the TIR system, and Fig. 5(b) showed the centroid wavelength shift measured by the WM system. In the initial stage, the vanadium ions in the vanadium electrolyte were in the form of V(IV) and almost unchanged, so the curve was nearly flat. As the potential increased, V(IV) were oxidized to V(V), so the curve dropped sharply caused by the change of refractive index in the different concentration of the oxidized (V(V)) and reduced (V(IV)) species.
Fig. 5. Performance comparison of total internal reflection (TIR) method and weak measurement (WM) method. (a) Light intensity variation measured by the TIR. (b) Centroid wavelength shift measured by the WM.
Generally, the resolution of a system can be evaluated by the ratio of the noise level $\sigma$ to the response of the reaction $\Delta $. In this experiment, the standard deviation of the first 200 s without chemical reaction was used to calculate noise level and the relative resolution $\delta n = {{({{{\sigma _{TIR}}} / {{\Delta _{TIR}}}})} / {({{{\sigma _{WM}}} / {{\Delta _{WM}}}})}}\textrm{ = }4.21$ was used to judge the method performance. It can be seen from the result that the resolution of the WM was four times better than that of the TIR.
3.3 Calibration of weak measurement (WM) system
As we know, the gold film used in the SPR method has low durability in acid electrolyte and produces Faraday current interfering with the electrochemical process, so our group introduced the TIR method without sensing film to detect the electrochemical process [14]. But the resolution of the TIR method is worse than that of the SPR, so it is necessary to have a better performance method without sensing film. From above experiments, we know that the resolution of the WM without sensing film was better than that of the TIR, so it was very suitable for electrochemical measurement. Now we verified that the WM method can be applied to electrochemical measurement, and the calibration experiment was conducted to obtain the current density with a physical unit (details can be seen in Section 2.5).
In order to verify the feasibility of the WM method, LSV test on the positive electrolyte (0.1 M V(IV) and 2 M H2SO4) at different scan rates (0.5 mVs-1, 1 mVs-1, 1.5 mVs-1, 2 mVs-1) and the potential window of 0.5 V-1.2 V was conducted, the results were shown in Fig. 6. The LSV curves of the graphite plate with tape recorded by the EW were obtained, as shown in Fig. 6(a) and the LSV curves obtained by deconvolution calculation of the centroid wavelength shift variations recorded by the WM were shown in Fig. 6(b).
Fig. 6. (a) Linear sweep voltammetry (LSV) curves of the graphite plate with tape in the positive electrolyte of 0.1 M V(IV) and 2 M H2SO4 at different scan rates (0.5 mVs-1, 1 mVs-1, 1.5 mVs-1, 2 mVs-1) recorded by the EW. (b) LSV curves by deconvolution calculation of the centroid wavelength shift variations recorded by the WM. (c) The relationship between the current density obtained by the EW and the relative current density obtained by the WM with the square root of the scan rate. (d) The relationship between the current density obtained by the EW and the relative current density obtained by the WM.
According to the comparison of Fig. 6(a) and Fig. 6(b), the results obtained by the WM were similar to the results obtained by the EW, the peak oxidation currents were both increase along with the increasing scan rates. The peak oxidation currents (Ipa (EW)) from the LSV curves in Fig. 6(a) were picked out and calculated their densities (ipa (EW)). The relationship between the current density obtained by the EW (ipa (EW)) with the square root of the scan rate (v1/2) and the relationship between the relative current density obtained by the WM (ipa (WM)) with the v1/2 were both plotted in Fig. 6(c). The linear fitting relations are ${i_{pa}}(EW) = 5.911 \times {v^{{1 / 2}}} - 0.2185$ and ${i_{pa}}(WM) = 0.0547 \times {v^{{1 / 2}}} + 0.0029$ respectively. Both ipa (EW) and ipa (WM) were linear with the v1/2, proving the Randles-Sevcik equation [33]. This verified that the WM method can be applied to electrochemical measurement.
The current density of the graphite plate with tape obtained by dividing the EW currents by the cross-sectional area (1×1 cm2) should be equal to the relative current densities obtained by the WM. In order to obtain the quantitative relationship between the current density obtained by the EW and the relative current density obtained by the WM, the peak oxidation values at different scan rates from the LSV curves in Fig. 6(a), b was picked out and plotted. After linear fitting of the peak oxidation values, the quantitative equation between the current density obtained by the EW and the relative current density obtained by the WM is
as shown in Fig. 6(d). Therefore, the relative current density obtained by the WM was converted to the current density with a physical unit (mA·cm-2).3.4 CV test by the weak measurement system
As mentioned above, the LSV test was conducted for calibration. But in general, CV test is a more common experiment to evaluate the electrochemical performance in vanadium flow batteries. So the CV test was conducted to further demonstrate the capability of our WM system. A weak measurement system with the electrochemical detection module containing graphite felt was rebuilt for CV testing. The only difference between the CV test and the LSV test was the working electrodes. A platinum electrode (D6, Pt017, ϕ 1 mm × 37 mm, purity: 99.95% Tianjin Aida Hengsheng Technology Development Co., Ltd, Tianjin, China) was inserted through the graphite felt (10 mm × 10 mm × 5.4 mm, Gansu Haoshi Cabon Fiber Co., Ltd, Gansu, China) as the working electrode in CV test (see Supplement 1, Section 3 for details), while the working electrode in LSV test was a graphite plate with tape. The scanning rate was set as 1 mVs-1, and the potential window was set as 0.55 V-1.15 V. Figure 7(a) showed the curve of centroid wavelength shift caused by refractive index change collected by the spectrograph during one CV cycle.
Fig. 7. (a) Time-varying centroid wavelength shift of a graphite felt in the positive electrolyte of 0.1 M V(IV) and 2 M H2SO4 at a scan rate of 1 mVs-1 during the CV. (b) CV curves obtained by deconvolution calculation based on the data in (a). (c) CV curve recorded by the EW.
According to the changes in the electrochemical reaction, the curve of centroid wavelength shift can be divided into four stages. The stage I (t1→t2) was the initial stage of the reaction, when the vanadium ions in the vanadium electrolyte were at V(IV). In the stage II (t2→t4), as the applied potential increased, vanadium ions gradually lost electrons and were oxidized to V(V). The different concentration of the oxidized (V(V)) and reduced (V(IV)) species caused a change in refractive index, which led to a sharp drop in the centroid wavelength shift. In the stage III (t4→t5), the reverse potential was provided by the EW, the vanadium ions from the previous oxidation were still in the form of V(V). At the stage IV(t5→t7), the vanadium ions V(V) were gradually reduced to V(IV), so the centroid wavelength shift rose again.
Figure 7(b) showed the relationship between the potential and the relative current density obtained by the deconvolution of the centroid wavelength shift variations in Fig. 7(a). Figure 7(c) showed the relationship between the potential and the absolute current recorded by the EW. According to the comparison of Fig. 7(b) and Fig. 7(c), the results obtained by the WM were similar to the results obtained by the EW, so the WM method can be used to conduct the CV test.
We noticed that the oxidation peak (ipa) obtained by the WM was basically consistent with that obtained by the EW, but the reduction peak (ipc) appeared later than that obtained by the EW. The WM system detected a point on the interface where the graphite felt closed to the prism. However, a graphite felt was a cube with a thickness of 5.4 mm and a length and a width of 10 mm. When an electrochemical reaction occurred, it took a certain time to diffuse from the reaction center to the detection point. But the EW detected the graphite felt electrode as a whole, so there was no diffusion problem. In addition, the internal distribution of the graphite felt was not strictly uniform, which caused the point we detected to be different from the average value. Therefore, the result of the WM lag behind that of the EW. This is the reason why we used only one exposed surface graphite plate to do the calibration experiment and why we needed to study two-dimensional imaging.
4. Conclusion
In this paper, we reported that electrochemical reaction can be in situ detected by the weak measurement (WM) method. The feasibility of the WM method was verified by conducting the LSV and the CV tests. Compared with the traditional optical methods such as SPR [12], the WM method without coating can be used in the acid electrolyte of vanadium flow battery, and the higher resolution than TIR in our previous work [14] helped to more accurately detect tiny electrochemical reactions. This extended the methods of electrochemical in situ detection and laid the foundation for two-dimensional (2D) imaging. Based on the existing SPR imaging work [34], a swept frequency light source and a high-speed area array CCD camera are needed for 2D imaging in the next work. In the 2D imaging, a centroid wavelength shift curve can be obtained from each pixel on the area array CCD camera, from which the current density of the point can be obtained, so as to realize the detection of the current density distribution. Thus, we believe that the WM method will be helpful to study the mechanism of electrochemical reaction and to evaluate the performance of electrochemical devices.
Funding
National Natural Science Foundation of China (61875102, 61975089); Oversea cooperation foundation, Graduate School at Shenzhen, Tsinghua University (HW2018007); science and technology research program of Shenzhen City (JCYJ20180508152528735, JCYJ20200109110606054); Tsinghua University Spring Breeze Fund (2020Z99CFZ023).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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