Improving the Mg Sacrificial Anode in Tetrahydrofuran for Synthetic Electrochemistry by Tailoring Electrolyte Composition

Abstract

Mg0 is commonly used as a sacrificial anode in reductive electrosynthesis. While numerous methodologies using a Mg sacrificial anode have been successfully developed, the optimization of the electrochemistry at the anode, i.e., Mg stripping, remains empirical. In practice, electrolytes and organic substrates often passivate the Mg electrode surface, which leads to high overall cell potential causing poor energy efficiency and limiting reaction scale-up. In this study, we seek to understand and manipulate the Mg metal interfaces for a more effective counter electrode in tetrahydrofuran. Our results suggest that the ionic interactions between the cation and the anion of a supporting electrolyte can influence the electrical double layer, which impacts the Mg stripping efficiency. We find halide salt additives can prevent passivation on the Mg electrode by influencing the composition of the solid electrolyte interphase. This study demonstrates that, by tailoring the electrolyte composition, we can modify the Mg stripping process and enable a streamlined optimization process for the development of new electrosynthetic methodologies.

Enabling Al sacrificial anodes in tetrahydrofuran electrolytes for reductive electrosynthesis

“…Following the rest, we repeat the LSV-OCV protocol 4 times to observe how Al stripping behavior evolves with each consecutive scan. The cell configuration and LSV protocol are adapted from our work on Mg. 9 The 2nd–5th LSV scans are shown in Fig. 2b–d.…”

“…t BuBr serves as the sacrificial oxidant of the galvanostatic oxidation experiment. 9 The experiment begins with galvanostatic stripping of Al at j ≈ 1 mA cm −2 while the voltage profiles at the Al WE and graphite CE are recorded. After every 2 hours of galvanostatic oxidation, we rest the cell at OCV and measure the series resistance at the Al WE using electrochemical impedance spectroscopy (EIS).…”

“…Our previous study on a system using a Mg electrode has demonstrated a similar phenomenon where the accumulation of Mg salts on the surface on the Mg electrode leads to increased cell voltage. 9,15 Indeed, when the electrode is removed from solution following a stripping reaction, salt is observed on the surface as shown in the photograph in Fig. 6a.…”

“…The Pt|Fc/Fc + RE was prepared following literature procedure with a 0.5 mm diameter Pt wire (Sigma-Aldrich), ferrocene (Fc, 98%, Sigma-Aldrich), ferrocenium hexafluorophosphate (FcPF 6 , 95%, Combi-Blocks), TBAPF 6 , and THF. 9,64–66 Fc and TBAPF 6 were recrystallized prior to use. The Pt wire was first cleaned in concentrated HNO 3 and heated in a H 2 flame before sealed within a ceramic-fritted glass tube (inner diameter 3.5 mm, Pine Research Instrumentation).…”

“…Previously, we have demonstrated that simple electrolyte tailoring strategies can change the composition of solid surface layers, often called solid electrolyte interphases (SEI), that form on Mg and improve the performance of a Mg sacrificial anode in THF-based electrolyte. 9 In this work, we explore the influence of electrolyte composition on Al SEIs and stripping behavior. Linear sweep voltammetry (LSV) demonstrates that supporting electrolytes such as TBABF 4 , TBAClO 4 , and TBA bis(trifluoromethanesulfonyl)imide (TFSI) do not support efficient Al stripping in THF.…”