(LKE) Using Microfabricated Square Microelectrodes - ACS Publications

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Enhanced electroanalysis in lithium potassium eutectic (LKE) using microfabricated square microelectrodes. Damion Kevin Corrigan, Ewen Blair, Jonathan G. Terry, Anthony J. Walton, and Andrew R. Mount Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac5030842 • Publication Date (Web): 04 Oct 2014 Downloaded from http://pubs.acs.org on October 12, 2014

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Analytical Chemistry

Enhanced electroanalysis in lithium potassium eutectic (LKE) using microfabricated square microelectrodes. Damion K Corrigan1, Ewen Blair2, Jonathan G. Terry2, Anthony J. Walton2 and Andrew R. Mount1. 1

School of Chemistry, EaStCHEM, The University of Edinburgh, Joseph Black Building, The King’s Buildings, West Mains Road, Edinburgh, EH9 3JJ, Scotland (UK) 2

Institute for Integrated Micro and Nano Systems, School of Engineering, The University of Edinburgh, SMC, The King’s Buildings, West Mains Road, Edinburgh EH9 3JF, Scotland (UK) Corresponding author – Prof A R Mount. Email - [email protected]

Tel – 0131 650 4747

Abstract Molten salts (MSs) are an attractive medium for chemical and electrochemical processing and as a result there is demand for MS-compatible analysis technologies. However, MSs containing redox species present a challenging environment in which to perform analytical measurements because of their corrosive nature, significant thermal convection and the high temperatures involved. This paper outlines the fabrication and characterisation of microfabricated square microelectrodes (MSMs) designed for electrochemical analysis in MS systems. Their design enables precise control over electrode dimension, the minimization of stress due to differential thermal expansion through design for high temperature operation, and the minimization of corrosive attack through effective insulation. The exemplar MS system used for characterisation was lithium chloride/potassium chloride eutectic (LKE), which has potential applications in pyrochemical nuclear fuel reprocessing, metal refining, molten salt batteries and electric power cells. The observed responses for a range of redox ions between 400 and 500oC (673 and 773 K) were quantitative and typical of microelectrodes. MSMs also showed the reduced iR drop, steady-state diffusionlimited response and reduced sensitivity to convection seen for microelectrodes under ambient conditions and expected for these electrodes in comparison to macroelectrodes. Diffusion coefficients were obtained in close agreement with literature values, more readily and at greater precision and accuracy than both macroelectrode and previous microelectrode measurements. The feasibility of extracting individual physical parameters from mixtures of redox species (as required in reprocessing) and of the prolonged measurement required for online monitoring was also demonstrated. Together, this demonstrates that MSMs provide enhanced electrode devices widely applicable to the characterisation of redox species in a range of MS systems. Introduction Molten salts (MSs) allow the solubilisation, stabilisation, controlled formation and reaction of highly reactive redox species, particularly those for which their oxides are particularly stable. They are therefore a key element of a number of industrial processes, including metal refining1, pyrochemical reprocessing of spent nuclear fuel2;3, molten salt batteries4 and catalysis5. Chloride melts are the MS of choice for a wide range of applications, due to their relatively low melting points and viscosities, their ligating ability, and their reduced corrosion compared to fluoride melts6. A particularly intensively studied chloride system is the lithium chloride-potassium chloride eutectic (LKE) which has a relatively low melting point of 625 K at the eutectic composition (59mol % LiCl: 41 mol % KCl), a relatively high ionic conductivity of 1.57 S cm-1 at 723 K7 and a wide electrochemical window of 3.62 V, within which a wide range of reactions can be studied. As a result of its industrial importance, there has been a desire to develop analysis techniques suitable for such a MS system, which address the challenges of withstanding the high temperatures and the corrosive nature of the melt, particularly when it contains reactive species. It would also be desirable to develop such a technique into an online monitoring system. The literature on MS online monitoring can be divided into two major groups: spectrophotometric methods8-11 and electrochemical methods12-17. Spectrophotometric methods suffer from the significant disadvantages of requiring the engineering of sampling access points and being limited to those

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species containing optically distinct chromophores. Electrochemical methods are suited to redox characterisation and do not suffer from these disadvantages, but to date have typically focused on macroelectrode measurements which suffer from low signal-to-noise, a time-dependent signal, iR distortion of the response and sensitivity to convection. These effects at best complicate and often preclude quantitative analysis. Microelectrodes can be fabricated using a number of methods18 and have the well-established advantages under ambient conditions of high signal-to-noise, and steady-state (time-invariant) and quantitative signals which are relatively insensitive to convection when compared to macroelectrodes19-22. They have therefore been developed for use in the monitoring of ambient industrial processes23. However, to date there has been a significant technological barrier to the development of microelectrode sensors for reproducible and quantitative MS analysis and monitoring. For example, microelectrodes fabricated by metal wire drawing, glass encapsulation and cleavage to expose the metal wire end with surrounding glass insulation as a disk microelectrode have been employed with limited success, as this has been shown to result in both initial and time-dependent variation of response24-28. The initial variability can be attributed to differences in the wire size and cross section due to variation in the wire drawing process, which as a result requires optical and/or electrochemical characterization of electrode size and shape. The time-dependent variation can be attributed to one or more of: the tendency of the MS to attack the glass and compromise the electrode insulation; the differential thermal expansion of the microelectrode materials, which causes materials stress and cracking; and the chemical corrosion of the electrode, which can cause it to progressively recess with time. All of these effects cause a progressive change in effective electrode area. In this study, we have developed and characterized a microelectrode system (the microfabricated square microelectrode, MSM) designed to overcome the above issues. We have built on our previous work, which established photolithographic fabrication techniques as a suitable methodology for the reproducible fabrication of square microelectrodes, and established the resulting characteristic microelectrode response under ambient aqueous conditions22. In this work, we again employ photolithography to produce similar square microelectrode systems with precisely controlled dimensions, and in addition utilize, through design, the layering of selected materials, first to relieve the stress from thermal expansion when operating at high temperature and secondly, to resist chemical attack through effective electrode insulation. The subsequent characterization demonstrates the success of this approach in producing a MS compatible microelectrode capable of reproducible and quantitative high temperature measurement. Materials and Methods Design and microfabrication of the electrode devices The devices used in this study were all fabricated on 100 mm (3.92 inch) diameter (100) p-type silicon wafers using standard photolithographic deposition and patterning methods. Figure 1A shows a cross-section through this microelectrode structure, Figure 1B a top down plan view and Figure 1C shows an image of a finished device after testing. Further details of the systematic integrated materials of microfabrication development programme that resulted in this device are reported elsewhere29. From the bottom of the diagram in Figure 1A, the device consisted of the silicon wafer substrate (1), a 500 nm thick silicon dioxide layer designed to minimise the development of stress when operating at high temperature through the thermal mismatch of materials30, which also electrically insulates the substrate from the deposited electrode (2), a 20 nm thick layer of titanium nitride that acts as an adhesion layer31 between the tungsten and silicon oxide layers (3), a 200 nm thick tungsten layer (4) that acts as the electrode and a 500 nm layer of silicon nitride (5) that provides insulation of the tungsten layer and defines the electrode area. High temperature measurements in LKE Experiments using LKE were conducted at 673-773 K in a sealed quartz cell (See Figure S1A in the supporting information) within a vertical tube furnace (Carbolite, Derbyshire, UK). Tungsten wire (Goodfellow, Cambridge, UK) was employed as the auxillary/counter electrode with a 1% Ag+/Ag electrode used as the


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reference electrode. The latter was prepared by encapsulating a silver wire in the eutectic containing 1% by mass silver (I) chloride (Sigma Aldrich, Poole, Dorset, UK) in a mullite tube (Multilab, Newcastle upon Tyne, UK). All potentials quoted in this paper are with respect to this reference electrode. For each experiment, anhydrous analytical grade salts of LiCl and KCl (Sigma Aldrich, Poole, Dorset, UK) were mixed in a molar ratio of 7:5 in a glassy carbon crucible (see Figure S1B) and then melted in vacuo to form LKE. The microelectrode was connected to a tungsten wire via a crocodile clip; this was sealed in ceramic putty and deployed on a tungsten rod for electrical connection through a port in the cell lid (see Figure S1C). Experiments were performed using a PC-controlled Autolab PGSTAT12 potentiostat (EcoChemie, Utrecht, Netherlands) and results were analysed with GPES 4.9 software (EcoChemie). Results and Discussion Electrochemical measurements were performed in LKE and in LKE containing various redox agents. Silver (I) chloride and samarium (III) chloride were chosen to characterise microelectrode behaviour in the melt. Silver (I/0) was used as an exemplar plating/stripping reaction, as many processes of industrial interest involve full reduction (metal plating) of dissolved ionic metal species. Samarium (III/II) was chosen as an exemplar solution-based electron transfer reaction, as this couple is known to be electrochemically reversible over the temperature range tested and samarium (III) is used in nuclear pyroprocessing research as a surrogate compound for americium12. Device Development through Assessment of Device Performance The chosen electrode was designed to be produced by the insulation of a large area of metal electrode, leaving only a square hole of micron dimension exposed to act as the microelectrode32;33. This ensured that either delamination or corrosive attack of the insulation would be readily apparent through relatively large changes in the metal area exposed to solution, and hence the resulting electrochemical response. Examples are shown in Figure 2A&B, which show the presence or growth of large redox currents due to (electro)chemical attack of the insulator device and the resulting exposure of additional electrode area. Electrochemical signatures when plating and stripping were: firstly, the magnitudes of the currents were larger than expected for a microelectrode, meaning that additional electrode area was present in the melt; secondly, there was a large disparity between the charge passed during the plating and stripping of silver, along with spiking of the current, consistent with mechanical drop off of the plated silver; and finally a steady-state diffusion limited current characteristic of a microelectrode was not observed. Where this loss of insulator was substantial, this could be and was subsequently confirmed by visual observation (Fig 2C&D), but this was not required, as cyclic voltammetric electrochemical characterisation allowed for more precise and quantitative characterisation of MSM longevity before, during and after electrochemical experimentation. Initial failure rates of the device described in the materials and methods section were in the region of 90-95% (where this is strictly defined as an MSM that cannot maintain quantitative microelectrode response under cyclic voltammetric cycling over several hours in LKE); however optimisation of the individual processing steps involved in fabrication has now resulted in a markedly improved failure rate of below 10% of devices. It is the successful devices that were then fully characterised as to their response and benchmarked to the macroelectrode response.



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Figure 1. (A) Schematic cross-section of the MSM device; (B) Top down view of the MSM device. The relatively large exposed contact pad for the electrode can be seen at the top of the device, and the relatively small electrode is near the bottom (C) Image of the microfabricated device

Figure 2. Cyclic voltammetric characteristics of failed L = 50 µm tungsten MSM in LKE containing 40 mM AgCl at 723 K; (A) initial cycle showing large currents indicative of insulator compromise (B ) Successive cyclic voltammograms (CVs) of a device indicating progressive insulator compromise (as evidenced by the increasing currents observed for each cycle). (C&D) Images of failed MSM devices after cycling in LKE due to removal of the top insulating film. In (D), insulator failure has occurred away from the L = 50 µm MSM (circled on right of image).

Electrochemical characterisation of MSMs Silver (I)


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Figure 3A shows the electrochemical behaviour of silver (I) in LKE using a tungsten wire macroelectrode. A reduction peak was observed for the plating of silver metal below -0.3 V with a concomitant silver oxidation peak above -0.2 V. Silver plating was found to be predominantly under diffusion control with a linear relationship between the square root of the scan rate and the cathodic peak current (see inset). This also results in an apparent square root dependence of the sweep rate on the stripping peak current, as the total amount of silver deposited in plating (which determines the magnitude of the peak stripping current) also has this square root of sweep rate dependence. When an L = 50 µm Tungsten MSM was used to perform similar measurements (see Figure 3B) it can be seen that a markedly different response was observed. Figure 3B shows a response typical of a microelectrode, i.e. a reductive wave leading to a steady state limiting current, a sharp stripping peak and a response independent of scan rate. As expected, the charges obtained by integrating the plating and stripping currents were equal (at 0.7 µC). From simulation and measurement under ambient aqueous conditions, it has been established that the limiting current to a microsquare electrode is given by22 2.341

(1)

Where n is the number of electrons transferred, F is Faraday’s constant, c is the concentration of redox agent and L the edge length of the microsquare. In the case of silver (I) chloride, a diffusion coefficient of D = (2.6 ± 0.1) x 10-5 cm2 s-1 was calculated at 723 K using the limiting current obtained from three microelectrode measurements. This showed good agreement with the literature value of D = 2.42 x10-5 cm2 s-1 at 730 K34, indicating for the first time the applicability of eqn. (1) to high temperature MS systems and the successful production of MSM devices of high fidelity.

Figure 3. Typical first scan CVs at a variety of sweep rates from (A) a tungsten macroelectrode and (B) an L = 50 µm tungsten MSM in LKE containing 1.75 mM AgCl at 723 K.

Samarium (III) Figure 4A shows Samarium III/II voltammetry recorded using a tungsten macroelectrode at 773 K. The measured response is consistent with a reversible, predominantly diffusion limited, solution-based one-electron transfer reaction. In this case, the experimental value of peak separation was 156 mV at 400 mVs-1, compared to the theoretical value of 2.3RT/F = 153 mV. The small but significant difference between these values is indicative of the growing importance of iR drop at higher currents (faster sweep rates and higher concentrations). Figure 4B shows the result of convolving the current, i, to give I, to account for the effects of time-dependent planar diffusion, according to35:

/

/

(2)

As expected, when diffusion is the predominant form of mass transport35, at faster sweep rates these curves show a reasonable correspondence to mass transport independent steady-state curves (as shown in Figure 4C).



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However, there are significant and increasing contributions attributable to the convolution of an additional current arising from non-Faradaic charging at the faster sweep rates and of an additional current arising from mass transfer of the Sm (III) species in the MS due to thermal convection at lower sweep rates. These at best complicate and at worst precludes quantitative analysis (Figure 4D). (Note that, the hysteresis in I seen at slower sweep rates is not due to the presence of a background current, which it has been shown can be responsible for the distortion of convolved voltammograms36, as background currents are not substantial and when present, have been subtracted prior to convolution.) In contrast, Figure 4C shows typical cyclic voltammograms for an L = 100 µm tungsten MSM in a 23 times more concentrated solution of SmCl3 (340 mM; in pyroprocessing, typical concentrations of redox agents can be several grams per kilogram of LKE, which corresponds to similarly high concentrations16. This produces significant distortion of the macroelectrode response through the contribution of significant iR drops of tens of millivolts.). As with silver (I), it is again important to note the sweep rate independence of the response and the “wave” like form of the voltammogram characteristic of a microelectrode. There is also the characteristic absence of product electrochemistry expected for a soluble redox couple. The inset in Figure 4C shows a modified Tafel plot22 for this MSM response. The experimentally obtained gradient of 16.55 ± 0.03 V-1 is in good agreement with the expected gradient, F/RT = 16.63 V-1 at 698 K and indicates reversible one electron transfer with an insignificant contribution from iR drop even at these high concentrations. From equation (1) and these data, a value of D = (9.2 ± 0.9) x10-6 cm2 s-1 was calculated from three independent measurements. This is consistent with the three previously reported but significantly different values of D = 7.6, 9.5 and 5.1 x10-6 cm2 s-1 obtained from tungsten macroelectrodes using cyclic voltammetry, convolution voltammetry and chronopotentiometry12 respectively. MSM measurement and analysis therefore provide a more accurate and precise method for obtaining D than equivalent macroelectrode measurements. The actual MSM measurement accuracy is expected to be nearer that observed for silver (I), as the main origin of the larger error in D compared to silver (I) is experimental, through the volatility of samarium (III), which causes uncertainty in the final concentration.

Figure 4. (A) CVs from a tungsten macroelectrode in LKE + 15 mM SmCl3 at 773 K. (B) Convolution analysis of the CVs presented in (A) according to equation (2), (C) CVs from an L = 100 µm tungsten microelectrode in LKE + 340 mM SmCl3 at 698 K (inset: modified Tafel plot obtained from the CV at a sweep rate 100 mVs-1). (D) Comparison of the convolved CV recorded at a sweep rate of 50 mVs-1 using a tungsten macroelectrode and the CV from the L = 100 µm tungsten microsquare also at 50 mVs-1. Each is normalised to the magnitude of the maximum current, |iMAX|, or convolved current, |IMAX|, as appropriate.


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Accuracy and reproducibility of MSM measurements Figure 5A shows typical chronoamperometric responses of L = 20 and 100 µm MSMs when reducing Sm (III). As predicted by equation (1) and ambient square microelectrode studies22, the mass transfer limited currents, obtained as t → ∞, scale with edge length, with a percentage difference in current of 20.3%. When measuring diffusion limited silver (I) plating currents, the relative standard deviation was 1.25% when using the same MSM in the same melt (with the number of measurements, n = 3) and 4.4% when using different electrodes of L = 20 µm in the same melt (with n = 4). These values could be expected to decrease further with increasing n; even so, they correlate favourably with reported literature ambient temperature measurements of 1.5% for limiting current measurements on the same electrode in standard solutions and 11% for sensing measurements in the chosen sensing environment (sea water) 37. This relatively high reproducibility between different electrodes is a demonstration of an advantage of using photolithographic fabrication techniques, as other methods of microelectrode fabrication have been shown to produce significant deviation from the expected linear dependence of limiting current on electrode dimension attributable to deviation of the fabricated dimension from that predicted37. Figure 5B shows values of D at four different temperatures, obtained from equation (1) and the diffusion limited currents obtained using a single MSM when measuring the two-electron reduction of zinc(II) in LKE. As expected from the Stokes Einstein equation38, an excellent straight line was obtained for ln(D/T) vs 1/T (R2 = 0.9977) from which the activation energy, Ea, attributable to changes in the MS viscosity with temperature was calculated as 40.7 ± 1.8 kJ mol-1. It is comforting that previous attempts at deriving the activation energy through the diffusion of electroactive species in LKE have yielded similar values of 40.27 kJ mol-1 for the temperature variation of the mean value of D for Sm(III) measured by three different methods, and of 38.1839 kJ mol-1 ± 1.95 for the temperature variation of D for Sc(III) from chronopotentiometric studies, both at macroelectrodes, which demonstrates that MSMs can be used for the relatively facile determination of such physical parameters.

Figure 5. (A) Chronoamperometry in LKE + 200 mM SmCl3 from L = 20 and 100 µm MSMs at 723 K. The potential, E of the electrode was stepped from 0 V to -1.35 V (at which diffusion limited reduction of Sm(III) takes place) (B) Plot of ln(D/T) vs 1/T for the values of D obtained when carrying out diffusionlimited two electron Zn(II) reduction at E = -1.0 V at the temperatures, T = 663, 688, 733 and 773 K.

Application of microelectrodes for online monitoring of pyroprocessing In order to demonstrate MSM applicability to continued online monitoring in LKE, repetitive electrochemical cycling was carried out, with thorough inspection of the electrodes before and after immersion in the melt. Figures 6 A&B show images of an L = 100 µm MSM device before and after its potential was cycled between 0 and -2.3 V for 2.5 hours in the presence of the redox ions cerium(III) and samarium(III); the electrochemical response remained unchanged throughout and, consistent with this, the active area of the electrode after cycling



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was seen to remain unchanged throughout this extended time period (Figure 6B). It is interesting that in Figure 6B some circular discolouration of the insulator can be seen around the electrode; this is consistent with the precipitation of a small amount of solid product as a result of the cerium redox reaction, and it confirms the expected hemispherical diffusion regime for the MSM, which is the origin of the steady-state diffusion limited currents21. A further example of the longevity of the MSM devices can be seen in Figure 6C, which shows stable plating and stripping of silver and zinc over 30 mins (the time course of the experiment). Figure 6C also demonstrates the potential for multiple detection of redox species in the same MS; it is clear that the response obtained is essentially the combination of the plating/stripping CVs of the individual redox species (Figure 6D). Again, it was possible to obtain diffusion coefficients from the limiting currents, iL,Zn and iL,Ag, displayed in Figure 6C and equation(1). These were (2.42 ± 0.2) x 10-5 cm2 s-1 for silver (I) and (1.02 ± 0.1) x 10-5 cm2 s-1 for Zn (II), which were in the expected range for MSs and showed close agreement with the literature values of 2.44 and 1.15 x 10-5 cm2 s-1 respectively34. It is also highly satisfying that these values are in good agreement with those obtained from Figure 6D of (2.64 ± 0.3) x 10-5 cm2 s-1 and (1.23 ± 0.1) x 10-5 cm2 s-1 respectively, which demonstrates the ready extraction of diffusion coefficients for individual ions in mixtures (as would be required for many applications, including reprocessing) when using MSMs. This is in contrast to the extraction of data from macroelectrode CVs, with their time and sweep rate dependent electrochemical response. The standard potentials for the plating of silver and zinc individually and in a mixture were obtained and found to be in agreement within experimental error. The plating peaks however, were found to differ in position and shape as would be expected in a mixture where alloys can form during the plating and stripping processes. During the study a number of CVs were collected for a range of redox agents (see Supporting Information Figure S2) and their diffusion coefficients calculated suing eqn. (1) (see Table 1). Table 1 includes measurements of cerium (III) and samarium (III), which as they have similar reduction potentials to plutonium and americium, are surrogates for use in pyrochemical reprocessing research3. Due to its high volatility it was only possible to carry out qualitative measurements with zirconium (IV) chloride, which has a similar reduction potential to uranium (IV) chloride. Together, in principle, these experiments represent a clear demonstration of the utility of these electrode systems in measuring redox species of relevance to nuclear pyroprocessing.


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Figure 5. Images of a 100 µm tungsten square electrode prior to immersion in LKE (A) and following electrochemical cycling in LKE for 2.5 hours (B). (C) Cyclic voltammogram of silver (I) chloride (2.5 mM) and zinc (II) chloride (7.5 mM) at a sweep rate of 25 mVs-1 in LKE. Note that this experiment consisted of 20 cycles over a period of 32 mins. (D) CVs normalised to the plating current, |iMAX| (at a sweep rate of 100 mVs-1) for silver (I) and zinc (II) respectively. Table 1. Diffusion coefficients for various redox agents in LKE recorded at 723 K. Species

D / 10-5 cm2 s-1

Europium (III) Samarium (III) Silver (I) Nickel (II) Bismuth (III) Cerium (III) Iron (III) Zinc (II)

0.69 ± 0.03 1.02 ± 0.04 2.58 ± 0.1 2.22 ± 0.09 0.25 ± 0.01 1.01 ±0.04 1.47 ± 0.06 1.23 ± 0.07

Conclusions This paper reports the construction and electrochemical characterisation of MSM devices. They are shown to be highly suited to the performing of electrochemical measurements in the MS LKE and to the quantitative



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characterisation of redox species in MS systems. The use of photolithographic methods for device fabrication was shown to give precise and reproducible control of electrode size and shape. The MSM microelectrodes were also shown to have a quantitative response consistent with established theory and aqueous ambient measurements, which allowed the accurate and reproducible determination of the physicochemical parameters of redox species in LKE. The devices were shown to be robust and appropriate for continual monitoring, with the electrochemical response being maintained for at least 2.5 hours of continual operation without failure. Their potential for online analysis and monitoring has been demonstrated through the quantitative measurement of the response of a range of redox species at enhanced reproducibility and precision compared to previous measurements. Several of these species are of direct relevance to pyrochemical nuclear processing. The steadystate response also more readily enables deconvolution of the response from multiple redox species. In combination, this performance demonstrates the potential for MSM devices in the development, characterisation and monitoring of industrial processes in molten salts. Acknowledgements The authors acknowledge financial support for and development of this work through the UK EPSRC REFINE project (EP/J000779/1) and the EC ACSEPT and SMART Microsystems (FS/01/02/10 IeMRC Flagship) programmes.

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179x80mm (96 x 96 DPI)


(LKE) Using Microfabricated Square Microelectrodes - ACS Publications

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