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Visualizing the zero-potential line of bipolar electrodes with arbitrary geometry Meng Li, Shasha Liu, Yingyan Jiang, and Wei Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04881 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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Analytical Chemistry

Visualizing the zero-potential line of bipolar electrodes with arbitrary geometry Meng Li, Shasha Liu, Yingyan Jiang, and Wei Wang* State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ABSTRACT: In a typical bipolar electrochemistry (BPE) configuration, voltage applied between the two driving electrodes induced a potential drop through solution filled in the microchannel, resulting in interfacial potential difference between solution and BPE varied along the BPE. In the present work, we employed a recently-developed plasmonic imaging technique to map the distribution of surface potential of bipolar electrodes with various geometries including round, triangle, hexagon, star and rhombus shapes under non-faradaic charging process, from which the line of zero potential (LZP) was visualized and determined. We further investigated the dependence of LZP on electrode geometry and the distribution of external electric field, and explained the experimental results with a charge balance mechanism. The triangular and star-shaped BPEs show quite different LZP features from the other ones with symmetrical geometry. These experimentally obtained potential distributions are all in good agreement with electromagnetic simulations. Finally line of zero over-potential (LZO) of the triangular-shaped BPE under faradaic reactions were investigated. The results confirm the shift of LZO when faradaic reactions occurred at the corresponding ends of BPE. The present work demonstrates the first experimental capability to map the potential distribution of BPE with arbitrary geometry under arbitrary driving field. It is anticipated to help the design and optimization on the geometry of electrodes and microchannels with implications for boosting their applications in chemical sensing and materials synthesis.

Bipolar electrodes (BPEs) have received a lot of attention during the last years due to advantages of wireless, simple configuration and easy miniaturization.1 They show promising applications in a variety of fields, such as electro-synthesis,2,3 biosensor4,5 and electro-catalysis.6,7 Most BPE studies focus on faradaic reactions occurring at BPEs, which depend on optical probes for signal reporting.8-10 Although optical probes such as fluorescent and electrochemiluminescent reagents have proven powerful to study faradaic reactions, it remains difficult for them to study non-faradaic process. The latter is an important process in many electroanalytical techniques and deserves attention.11-15 Since non-faradaic process does not require external electroactive species, it will provide opportunity to expand applications of BPEs. Surface plasmon resonance imaging (SPRi) technique has proven powerful to visualize nonfaradaic char-ging process due to the well established and quantitative optical-to-electrochemical conversion model.14,15 Also SPRi readout of non-faradaic charging paves a way towards impeda-nce-based BPE assay by introducing high frequency potential modulations.14 By SPRi technique the distribution of interfac-ial potential on rectangular BPEs can be visualized with a sen-sitivity of 10 mV and a spatial resolution as low as sub-micr-on.16 Previous reports have shown that electrode geometry exhibited significant influences on the performance of BPE-based biosensing and asymmetrical synthesis.17,18 However sy-stematic investigations on the geometry-dependent potential distribution has not been experimentally demonstrated yet.

Interfacial potential difference between solution and BPE is the driving force for electrochemical reactions.19-21 Its magnitude is a function of location across BPE in parallel to the driving electric field and it determines the reaction kinetics taking place at the location.20 To measure the potential difference at a specific point on BPE, it is essential to define a position where the electrode and the solution potentials are the same,17,22 namely line of zero potential (LZP). Note that, in absence of redox species, this line refers to zero polarization line where solution and substrate have equal potential. In this case, plasmonic image reveals the distribution of charge density change along the BPE, which maps the polarization potential via surface capacitance.16,23 In the presence of redox species, this line refers to line of zero over-potential (LZO) where electrochemical current equals zero. Under faradaic reactions the dependence of plasmonic image on the distribution of local refractive index change allows for interrogating the interfacial electron transfer rate.24 Moreover, a unidirectional driving field has been applied along a single microchannel in most studies up to date.22,25-27 In order to take full advantage of the partial polarization in BPE, a two-dimensional (2D) BPE has also been proposed by combining BPE principle and more sophisticated microfluidic channels. The 2D-BPE allowed electrochemical reactions to be focused at a specific location on the perimeter of the bipolar surface.27,28 For the 2D-BPE, driving voltages were independently applied across two orthogonal microfluidic channels. The field induced at the intersection of the two channels was 1

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the vector sum of the individual electric field within the microfluidic channels. Accordingly the distributions of interfacial potential on BPEs located at the junction part will be very different from that under one electric field. A capability to visualize the complete distribution of potential as well as the location of LZP under 2D driving electric field is thus highly desired. In the present work, we employ plasmonic imaging technique to map the complete distribution of interfacial potential on BPEs with different geometries under 1D and 2D driving electric fields during non-faradaic charging process. We further investigate how the localization of LZPs are affected by the electrode geometry and the driving field. The experimental results nicely matched with numerical simultaneous under a static electromagnetic field, which can be explained with a charge balance mechanism. Finally LZO of the triangular-shaped BPE under faradaic reactions are studied. The SPR amplitude maps reveal the shift of LZO when comparing with these obtained under non-faradaic process. These interesting results provide a new route for the design of BPE and are anticipated to expand their applications in chemical and biosensing and material synthesis.

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a laser engraver machine. Note that the long axis of the bipolar electrode was in parallel with that of the microchannel. PDMS reservoirs were placed on top of the holes at the end of the microchannel. The whole piece was subsequently placed on top of a prism (gold side up) in the presence of immersion oil. The microchannel was then filled with the electrolyte (1×PBS) and Pt driving electrodes were situated in the reservoirs, respectively. COMSOL simulation. Distributions of interfacial potential on the BPEs with different geometries were numerically simulated by COMSOL multiphysics software (5.1). In the simulations, the electrostatics modules was used to calculate the interfacial potential difference between a gold film (relative permittivity=8) and solution (relative permittivity =20).

RESULTS AND DISCUSSION Potential distributions on BPEs with different geometries. We investigated potential distribution on BPEs by a prism-based SPR imaging (Figure 1A). An array of BPEs with different shapes was fabricated (Figure 1B). A microchannel with two PDMS reservoirs at both ends was placed on top of the BPE array while the long axis was in parallel with that of the microchannel (Figure 2A). Voltages applied to the driving electrodes were controlled with a potentiostat and a function generator was used to apply sine-wave modulation to the potentiostat (Figure 2B).

EXPERIMENTAL SECTION Chemicals and reagents. The phosphate buffer saline (1×PBS, Thermo Scientific) was used as the electrolyte. Potassium nitrate was purchased from Shantou Xilong Chemical Reagent Co., Ltd (China). Hexaammineruthenium(III) chloride (Alfa Aesar) and hexaammineruthenium(II) chloride (Alfa Aesar) were purchased and used without further purification. The deionized water (18.2 MΩ) produced by Smart2Pure 3 UF (Thermo Fisher) was used to rinse the electrodes, electrochemical cells and microchannels. Apparatus. Prism-based SPR imaging (SPRi) setups were used in the present work. The home-built SPRi apparatus was consisted of a 670 nm light-emitting diode (L7868-01, Hamamatsu, Japan), a triangle SF-11 prism (Edmund Optics, USA), and a gold-coated coverslip placed on the prism (with immersion oil). Detailed description on the optical configuration of SPRi apparatus can be found in our previous report.29 Glass coverslip (No. 1 BK7 glass from Fisher) was coated with gold film with a thickness of 50 nm using a magnetron sputter. Gold film serves as both the substrate for SPR imaging, and the conducting electrode in BPE. Five patterns of gold film with different geometries are fabricated to act as five BPE electrodes. Five patterns are physically close to each other but electrically isolated (with a minimal gap of 100 microns). A potentiostat (Pine Instruments, Model AFCBP1) was used to apply the driving potential along the microchannel filled with 1×PBS (see below for details). The applied potential is modulated by using a function generator (Agilent 33210A) to generate square and sine waves for faradaic and non-faradaic process respectively. A CCD camera (Pike F032B, Allied Vision Technologies) was used to capture SPRi images. An image acquisition rate of 20 frames per second (fps) was routinely used in the work to collect the data shown in all figures. Fabrication of the microchannel. A glass slide was drilled with two holes separated by a distance of 10 mm (four holes for the 2D-BPE). A gold-coated coverslip (gold side up) was attached to the bottom of the drilled glass slide, separated by a 100 µm-thick double-side tape. A 10 mm length × 2.5 mm width channel had been previously carved (two orthogonal channels for the 2D-BPE) in the double-side tape by using

Figure 1. (A) Schematic illustration of the plasmonic imaging setup. A triangular prism is used with a LED as the light source and a CCD camera as the detector. The gold coated cover slip is contacted with the prism by immersion oil. The light beam passes a collimating system and a polarizer, then propagates to the prism and onto the sensing surface. When the incident angle is larger than the critical angle, the evanescent wave appears in about 200 nm depth from the surface. The reflected light carrying the surface information is finally imaged by the CCD camera. (B) Photograph of a BPE array including different gold patterns, and (C) the corresponding SPRi image of the star-shaped gold pattern in figure (B). Reflectivity from gold region is relatively low because of the surface plasmon resonance effect, while surrounding glass region exhibits higher reflectivity due to total internal reflection.

Figure 2. Schematic illustration of the microchannel fabrication (A) and experimental configuration (B). The microchannel consists of three parts, that are a glass slide with two holes, a doublefaced tape with a 10 mm length × 2.5 mm width gap and a gold coated coverslip. After microchannel fabrication, two PDMS reservoirs were put on top of the two holes. The whole piece was subsequently placed on top of the prism showed in Figure 1A. Pt electrodes were situated in the reservoirs filled with PBS and connected to a potentiostat modulated by a function generator. 2


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As we have mentioned, a sine wave potential as driving potential can get a good signal-to-noise ratio.16 Here we applied a sine wave potential with an amplitude of 4 V and a frequency of 6 Hz to the driving electrodes. SPRi images of BPEs under this potential modulation were captured by a camera at 20 fps for 10 min. These images were subsequently processed through Fourier transform-based electrochemical impedance method to create the amplitude and the phase maps of these BPEs.30,31 The amplitude was then converted to the interfacial potential (Figure 3B). As shown in Figure 3B, the external ac driving potential created an overall potential drop of nearly 500 mV between the two poles of the BPEs along the driving electric field, which was consistent with the rough estimation by assuming a linear potential drop in the entire microchannel. Considering the size of holes where the driving electrodes were situated, potential drop on each BPE along the driving electric field fell within a potential range of 400~560 mV (∆ ,where ∆ is potential drop over a

the BPE and it was determined by the principle of charge neutrality. During a non-faradaic charging process, modulations of the driving potential cause interfacial potential at a specific location on the BPE to change. Local charge density change (∆Q) is related to local interfacial potential change ∆U by ∆Q C∆U （1） Where C is local capacitance density (capacitance per unit area).14,31 Assuming the interfacial capacitance density (C) was constant at any location of the BPE, one can estimate local charge density change using local potential change. Due to charge neutrality, charge accumulated at the anodic and cathodic regions should have an equal value which meant their ratio should be 1.00. According to formula (1), if C was constant, ratio of charge density change between the two regions was equal to ratio of potential change. Here we calculated the ratio of potential change between the two regions (Table 1) as ratio of charge change and determined LZP on these BPEs (Figure 3B). Also in Table 1, ratio of areas divided by LZP was calculated to show geometric position of LZPs on different shapes. BPEs with round, rhombus and hexagon shapes were all central symmetric. Their distributions of interfacial potential and areas were symmetric about LZP. However potential distributions on the triangle and star BPEs were quite different. On the triangular BPE, LZP was close to the horizontal line which went through the centre of its circumscribed circle (area ratio between the upper and lower parts was 0.8) while on the star BPE LZP was along the waist. The asymmetric potential distributions on the triangular and star BPEs may be caused by their sharp corners along the driving electric field. Local charge density changed more in the corners along the driving electric field. This can be reflected on the potential distribution maps, where corners right along the driving electric field usually have relatively large potential values. Potential distributions on BPEs under 2D electric fields. Inspired by the concept of 2D-BPE, we configured two types of 2D-BPEs shown in Figure 4A and 4B respectively. It consisted of planar gold electrodes situated at the intersection of two orthogonal microfluidic channels. When two driving voltages were applied across the two channels, the resulting field at the intersection was the vector sum of the individual field in the micro-fluidic channels. Accordingly, the distributions of interfacial potential on the BPEs will be different from that under unidirectional field. Figure 4 shows potential distributions on the circular and hexagonal BPEs after image processing. Potential distributions of other BPEs are provided in the Supporting Information (Figure S1). In type I 2D-BPEs, the amplitude of the two driving voltages were the same, directions of their vector sum were at an angle between horizontal and vertical directions, as shown in Figure 4C and 4E. The maximum potential drop on the 2DBPE was about 1.3 times (close to √2) as much as that under one driving voltage, which satisfied the principle of vector sum. Potential distributions on type II 2D-BPEs were similar to those under one driving voltage. However the potential distributions of type II 2D-BPEs had a smaller angle (about 20 degree) to the vertical direction, compared with type I 2D-BPEs. That is because the intensity of electric field along the horizontal direction was weaker in type II 2D electric field.

BPE, is the driving potential, is length of the BPE along the driving electric field, and is the distance between the two driving electrodes). To demonstrate the theoretical distributions of interfacial potential, COMSOL simulations were applied and the results are shown in Figure 3C. The experimental and simulation results were consistent with each other in terms of overall potential drop and potential distribution.

Figure 3. (A) SPRi images of BPEs with different shapes including circle, triangle, rhombus, star and hexagon. Diameter of the circle and circumcircle of the star, diagonal of the rhombus, altitude of the triangle and the hexagon along vertical direction were all 700 µm long. Driving electric field was also applied along the vertical direction, as shown in the figure. (B) Experimentally obtained distributions of interfacial potential (Black dashed lines show LZP) and corresponding COMSOL simulation results (C).

Line of zero potential. Voltage applied between the driving electrodes induced an electric field through solution filled in the microchannel. BPE situated in the solution became an equipotential body and floated to an equilibrium potential which depended on potential gradient in the solution. The potential of the solution along the BPE changed, resulting in interfacial potential difference between the BPE and the solution varied at different locations on the BPE. The two poles of the BPE along the direction of the driving electric field presented opposite overpotential, one was anodic while the other was cathodic. This generated a location where potentials of the electrode and the solution were the same. The location had a zero potential and was named as LZP. LZP defined a boundary on 3



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Table 1. Ratio of charge between anodic and cathodic regions and area divided by LZP on BPEs with different geometries. Geometry

Circle

Triangle

Rhombus

Pentagram

Hexagon

Ratio of Charge

1.00

1.00

1.00

1.03

1.06

Ratio of Area

1.09

0.71

1.02

1.21

1.01

The experimental results in Figure 4 can be compared to COMSOL simulations of potential distributions formed when the same potentials were applied across the two channels (Figure 5 and Figure S2). The simulations showed the same tendency with the experimental results in terms of potential distribution.

Figure 5. COMSOL simulations of interfacial potential distributions on type I and II 2D-BPEs. (A) and (B) are the circular BPE under type I and II 2D electric fields respectively. (C) and (D) are the hexagonal BPE under type I and II 2D electric fields respectively.

where no reaction occurs. In this case, this line refers to the LZO. Previous studies have shown that the LZO would shift upon faradaic reactions occurring at the end of BPE.18 Here the triangular-shaped BPE was chose to investigate influence of faradaic reactions on the location of LZO. In two different experiments, 10 mM Ru(NH3)6Cl2 or 10 mM Ru(NH3)6Cl3 were employed in 0.1 M KNO3 to enable oxidation and reduction reaction at the corresponding end, respectively. Simultaneously Ru(NH3)63+ reduction or Ru(NH3)62+ oxidation occurred at the opposite pole to enable current flow in the BPE for Ru(NH3)62+ oxidation or Ru(NH3)63+ reduction, respectively (Figure S4). From SPRi images, the amplitude and phase maps of the BPE were obtained through fourier transformbased electrochemical impedance method (Figure 6). As shown in Figure 6, when there was no faradaic reactions occurring at the BPE, LZP was close to the horizontal line which went through the centre of its circumscribed circle (Figure 6A). When there are redox species in the solution, the line of zero amplitude of optical fluctuation reflects LZO (Figure 6B-C). As shown in Figure 6B-C, faradaic reactions led to the shift of LZO on the BPE. It shifted up by ~80 µm along the direction of the driving electric field due to Ru(NH3)62+ oxidation (Figure 6D). While in Ru(NH3)63+ solution LZO shifted down by ~40 µm. Based on the electrode length (700 µm), the channel length (10 mm) and the driving voltage (6 V), potential difference between the cathodic and anodic pole of the BPEs was estimated to be around 420 mV, which enabled the coupling between Ru(NH3)62+ oxidation and Ru(NH3)63+ reduction (Figure S4). In order to examine the influence of electrode geometry on the shift of LZO, the same experiments were conducted on a symmetrical hexagon-shaped BPE. Resu-lts revealed a shift of ~45 µm and ~30 µm in Ru(NH3)62+ and Ru(NH3)63+ solution respectively (Figure S3). These results demonstrated that both electrode geometry and onset oxidation or reduction potential of faradaic reactions influenced the posi-tions of LZP and LZO.

LZO on BPE under faradaic reactions. SPRi is not only sensitive to electron density within the gold film but also to th-e refractive index change of solution. Therefore, it was able to map the faradaic current based on the different refractive index between oxidation and reduction states of electroactive species.23,24 The relationships converting SPR optical signal to electrochemical parameters are different between non-faradaic charging and faradaic reaction scenarios. In non-faradaic charging, SPR signal is proportional to the change in local electron density (∆Q). Therefore one can directly access the interfacial potential through surface capacitance (∆U ∆Q/C). However, the relationship is more complicated under faradaic conditions because the SPR signal is determined by the concentration distribution of redox species, which is affected by the nature of redox species and the diffusion. Consequently it is difficult to directly obtain the potential distribution from the optical signal. The bottom line is that, despite of this limitation, the line of zero amplitude of optical fluctuation still reflects the location

Figure 6. Experimentally obtained distributions of SPR amplitude on the triangular-shaped BPE under a potential modulation from 0 V to 6 V with a frequency of 1 Hz in (A) 0.1M KNO3, (B) 0.1M KNO3 containing 10 mM Ru(NH3)6Cl2 and (C) 0.1M KNO3 containing 10 mM Ru(NH3)6Cl3 respectively (Black dashed lines show LZP and LZO). (D) SPR amplitude curves along the triangularshaped BPE (Length is from top to down).

Figure 4. Schematic diagram of type I (A) and II (B) 2D-BPEs. The yellow shapes in A and B represent BPEs with different geometries. The marks “+” and “-” represent the driving electrodes. There are two orthogonal microchannels filled with PBS between the electrodes. (C), (D), (E) and (F) are obtained distributions of interfacial potential on the circular and hexagonal BPEs under the two types of 2D electric fields respectively.

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CONCLUSIONS To summarize, we have obtained spatial distributions of interfacial potential on BPEs with various conventional geometries under non-faradaic charging process by the developed plasmonic imaging technique. Geometric positions of zero potential on these BPEs were analyzed. Locations of LZP on the round, diamond and hexagon BPEs were diagonals perpendicular to the driving electric field. Also the diagonals divided areas of these geometries equally. While locations of LZP on the triangular and star BPEs were quite different from those on geometrically symmetric BPEs, it is attributed to their asymmetric and sharp corners along the driving electric field. Furthermore distributions of interfacial potential on the BPEs under two orthogonal electric fields were obtained. The potential distributions were all in good agreement with numerical simulateons. Finally LZO on the triangular-shaped and the hexagonshaped BPEs under faradaic reactions were investigated. The results show different features with these obtained under nonfaradaic process. The present study may provide a new route for the design of BPEs to boost their applications in chemical sensing and materials synthesis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Experimental and COMSOL simulations of interfacial potential on BPEs with star, triangle and rhombus shapes under 2D electric fields.

AUTHOR INFORMATION Corresponding Author * [email protected] (W.W.)

ORCID Wei Wang: 0000-0002-4628-1755 Meng Li: 0000-0001-8399-8483

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank financial support from the National Natural Science Foundation of China (NSFC, Grant No. 21705077, 21605078, 21522503), the Natural Science Foundation of Jiangsu Province (BK20150570, BK20150013), the Fundamental Research Funds for the Central Universities (020514380090) and China Postdoctoral Science Foundation.

REFERENCES (1) Xing, H. H.; Zhang, X. W.; Zhai, Q. F.; Li, J.; Wang, E. K. Anal. Chem. 2017, 89, 3867-3872. (2) Mayorga-Martinez, C. C.; Khezri, B.; Eng, A.Y. S.; Sofer, Z.;

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Figure 1. (A) Schematic illustration of the plasmonic imaging setup. A triangular prism is used with a LED as the light source and a CCD camera as the detector. The gold coated cover slip is contacted with the prism by immersion oil. The light beam passes a collimating system and a polarizer, then propagates to the prism and onto the sensing surface. When the incident angle is larger than the critical angle, the evanescent wave appears in about 200 nm depth from the surface. The reflected light carrying the surface information is finally imaged by the CCD camera. (B) Photograph of a BPE array including different gold patterns, and (C) the corresponding SPRi image of the star-shaped gold pattern in figure (B). Reflectivity from gold region is relatively low because of the surface plasmon resonance effect, while surrounding glass region exhibits higher reflectivity due to total internal reflection. 190x142mm (300 x 300 DPI)


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Figure 2. Schematic illustration of the microchannel fabrication (A) and experimental configuration (B). The microchannel consists of three parts, that are a glass slide with two holes, a double-faced tape with a 10 mm length × 2.5 mm width gap and a gold coated coverslip. After microchannel fabrication, two PDMS reservoirs were put on top of the two holes. The whole piece was subsequently placed on top of the prism showed in Figure 1A. Pt electrodes were situated in the reservoirs filled with PBS and connected to a potentiostat modulated by a function generator. 190x142mm (300 x 300 DPI)



Figure 3. (A) SPRi images of BPEs with different shapes including circle, triangle, rhombus, star and hexagon. Diameter of the circle and circumcircle of the star, diagonal of the rhombus, altitude of the triangle and the hexagon along vertical direction were all 700 µm long. Driving electric field was also applied along the vertical direction, as shown in the figure. (B) Experimentally obtained distributions of interfacial potential (Black dashed lines show LZO) and corresponding COMSOL simulation results (C). 190x142mm (300 x 300 DPI)


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Figure 4. Schematic diagram of type I (A) and II (B) 2D-BPEs. The yellow shapes in a and b represent BPEs with different geometries. The marks “+” and “-” represent the driving electrodes. There are two orthogonal microchannels filled with PBS between the electrodes. (C), (D), (E) and (F) are obtained distributions of interfacial potential on the circular and hexagonal BPEs under the two types of 2D electric fields respectively. 190x142mm (300 x 300 DPI)



Figure 5. COMSOL simulations of interfacial potential distributions on type I and II 2D-BPEs. (A) and (B) are the circular BPE under type I and II 2D electric fields respectively. (C) and (D) are the hexagonal BPE under type I and II 2D electric fields respectively. 190x142mm (300 x 300 DPI)


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Figure 6. Experimentally obtained distributions of SPR amplitude on the triangular-shaped BPE under a potential modulation from 0 V to 6 V with a frequency of 1 Hz in (A) 0.1M KNO3, (B) 0.1M KNO3 containing 10 mM Ru(NH3)6Cl2 and (C) 0.1M KNO3 containing 10 mM Ru(NH3)6Cl3 respectively (Black dashed lines show LZOs). (D) SPR amplitude curves along the triangular-shaped BPE (Length is from top to down). 190x142mm (300 x 300 DPI)



Figure S1. Distributions of interfacial potential on BPEs with star, triangle and rhombus shapes under type I (A, B, C) and II (D, E, F) 2D electric fields. 190x142mm (300 x 300 DPI)


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Figure S2. COMSOL simulations of interfacial potential on BPEs with star, triangle and rhombus shapes under type I (A, B, C) and II (D, E, F) 2D electric fields. 190x142mm (300 x 300 DPI)



Figure S3. SPR amplitude curves along the the hexagonal-shaped BPE (Length is from top to down). 190x142mm (300 x 300 DPI)


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Figure S4. CVs of a gold chip in 0.1M KNO3 (the red line) and 0.1M KNO3 containing both 10 mM Ru(NH3)6Cl3 and 10 mM Ru(NH3)6Cl2 (the black line) respectively. The scan range is -230~230 mV (vs. SHE) and scan rate is 50mV/s for all curves. 190x142mm (300 x 300 DPI)


Visualizing the zero-potential line of bipolar ... - ACS Publications

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