Anodic stripping voltammetry (ASV) is a well-known powerful, sensitive, and selective analysis technique used for the detection of metal ion species in solution. While ASV was initially developed with mercury (Hg) as the plating electrode more recent applications have used bismuth (Bi) as a less toxic and less oxygen sensitive electrode that has low background currents, better reproducibility, and little to no need for electrode conditioning, as compared to Hg electrodes. ASV is most commonly performed under neutral to acidic electrolyte conditions where metals, as metal cations are readily soluble; however, the extension of an ASV method into highly alkaline environments (pH > 14) could prove useful for the in situ electrolyte monitoring of alkaline batteries and fuel cells such that potential diagnostics of their performance vs. electrolyte composition could be realized. With this in mind, we recently extended the ASV method to highly alkaline electrolytes, in order to determine Zn diffusion across separator membranes.[1] Zinc (Zn) alkaline battery chemistries such as Zn/MnO₂ and Zn/Air are currently attracting a lot of attention due to their potential as safe, low cost, high energy density rechargeable batteries. A main failure mechanism for these cells is the poisoning of the cathode material during cycling due to Zn crossover from the anode to the cathode. Advanced separators that successfully stop or limit this crossover are crucial in order to increase the cycle lifetime of these batteries. ASV, utilizing a Bi film, along with cadmium (Cd) and lead (Pb) as the plating mediators, was utilized to determine Zn concentrations in highly alkaline environments (30% NaOH or 35% KOH). To evaluate the utility of this assay, zincate diffusion through commercial off-the-shelf (COTS) Celgard 3501 and Cellophane 350P00 membranes was monitored using both ASV and inductively coupled plasma – mass spectrometry (ICP-MS) methods. The obtained zincate diffusion coefficients for both techniques are shown to compare favorably. While the traditional complexometric or elemental analysis methods can take days to weeks to determine Zn membrane diffusion metrics, this ASV method provides a real-time measurement which significantly increases the throughput for the screening of these membranes.

During our development of an ASV method for Zn, we realized that in general there was a lack of ASV methods in highly alkaline electrolyte and hence turned our attention to the development of an ASV method for the determination of Cu, as cuprate [Cu(OH)₄^2-]. Cu has been notoriously difficult to measure by ASV, even under acidic conditions.[2] In fact, most studies using Bi film electrodes for the direct determination of Cu have deemed it impossible and thus studies for the determination of the Cu concentration in a matrix are normally still performed separately with Hg electrodes. After investigation of potential metal film electrodes and mediators including Cd, Ag, Sb, and Zn, it was found that lead (Pb) serves as a good mediator for the plating of Cu in highly alkaline solutions which permits the replacement of the interfering Bi film electrode. Various aspects of this work will also be presented.

[1]. J. Duay, T. N. Lambert and R. Aidun “Stripping Voltammetry for the Real Time Determination of Zinc Membrane Diffusion Coefficients in High pH: Towards Rapid Screening of Alkaline Battery Separators” Electroanalysis 2017, 29, 2261-2267.

[2]. J. Duay, J. E. Ortiz-Santiago and T. N. Lambert “Copper Sensing in Alkaline Electrolyte Using Anodic Stripping Voltammetry by means of a Lead Mediator” Electroanalysis 2017, DOI: 10.1002/elan.201700526.

This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. Dr. Imre Gyuk, Energy Storage Program Manager, Office of Electricity Delivery and Energy Reliability is also thanked for financial support of this project.