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1. Breaking electrolyte symmetry in induced-charge electro-osmosis

   https://doi.org/10.1017/jfm.2020.754  

   Khair, Aditya S. ; Balu, Bhavya ( December 2020 , Journal of Fluid Mechanics)

   null (Ed.)

   Induced-charge electro-osmotic (ICEO) flow caused by an alternating electric field applied around an infinitely long, ideally polarizable, uncharged circular cylinder in a binary electrolyte with unequal cation and anion diffusion coefficients is analysed. The thin-Debye-layer and weak-field approximations are invoked to compute the time-averaged, or rectified, quadrupolar ICEO flow around the cylinder. The inequality of ionic diffusion coefficients leads to transient ion concentration gradients, or concentration polarization, in the electroneutral bulk electrolyte outside the Debye layer. Consequently, the electric potential in the bulk is non-harmonic. Further, the concentration polarization alters the electro-osmotic slip at the surface of the cylinder and generates body forces in the bulk, both of which affect the rectified ICEO flow. Predictions for the strength of the rectified flow for varying ratio of ionic diffusion coefficients are in reasonable agreement with available experimental data. Our work highlights that an inequality in ionic diffusion coefficients – which all electrolytes possess to some extent – is an important factor in modelling ICEO flows. 

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2. Suppressing Co-Ion Generation via Cationic Proton Donors to Amplify Driving Forces for Electrochemical CO 2 Reduction

   https://doi.org/10.1021/acs.jpcc.3c04004  

   Guo, Wenxiao ; Liu, Beichen ; Gebbie, Matthew A. ( July 2023 , The Journal of Physical Chemistry C)

   Interfacial microenvironments critically define reaction pathways for electrocatalytic processes through a combination of electric field gradients and proton activity. Non-aqueous ionic liquid electrolytes have been shown to sustain enhanced interfacial electric field gradients at intermediate ion concentration regimes of around 1 M, creating local environments that promote CO2 electroreduction. Notably, water at low concentrations absorbed by non-aqueous electrolytes is usually assumed to be the proton donor for CO2 reduction. Consumption of protons causes proton donors to become more negative by one unit charge, which significantly modifies the local concentration of charged species and hence should strongly impact local electric fields. Yet, how the coupling between proton donation and changing interfacial electric fields influences electrocatalytic processes in non-aqueous electrolytes remains largely unexplored. In this work, we show that the high activity of 1,3-dialkylimidazolium ionic liquids for CO2 reduction in acetonitrilebased electrolytes stems from the ability to act as cationic proton donors that release neutral conjugate bases. Using in situ electrochemical surface-enhanced Raman spectroscopy, we find that the formation of neutral conjugate bases from imidazolium cations preserves local electric field strengths at electrode-electrolyte interfaces, providing a powerful strategy to maintain an active local microenvironment for CO2 reduction. In contrast, conditions where water behaves as the primary proton donor generates [OH]- anions as negative “co-ions” in the electric double layer, which weakens the interfacial electric field and significantly compromises the steady-state CO2 reduction activity. Our study highlights that electrochemical driving forces are highly sensitive to the charge state of both reactant and product species and highlights the fact that the generation of interfacial co-ions plays a key role in determining electrochemical driving forces. 

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3. Linking Electric Double Layer Formation to Electrocatalytic Activity

   https://doi.org/10.1021/acscatal.3c04255  

   Gebbie, Matthew A. ; Liu, Beichen ; Guo, Wenxiao ; Anderson, Seth R. ; Johnstone, Samuel G. ( December 2023 , ACS Catalysis)

   Electric double layers form at electrode-electrolyte interfaces and often play defining roles in governing electrochemical reaction rates and selectivity. While double layer formation has remained an active area of research for more than a century, most frameworks used to predict electric double layer properties, such as local ion concentrations, potential gradients, and reactant chemical potentials, remain rooted in classical Gouy-Chapman-Stern theory, which neglects ion-ion interactions and assumes non-reactive interfaces. Yet, recent findings from the surface forces and electrocatalysis communities have highlighted how the emergence of ion-ion interactions fundamentally alters electric double layer formation mechanisms and interface properties. Notably, recent studies with ionic liquids show that ionic correlations and clustering can substantially alter reaction rates and selectivity, especially in concentrated electrolytes. Further, emerging studies suggest that electric double layer structures and dynamics significantly change at potentials where electrocatalytic reactions occur. Here, we provide our perspective on how ion-ion interactions can impact electric double layer properties and contribute to modulating electrocatalytic systems, especially under conditions where high ion concentrations and large applied potentials cause deviations from classical electrolyte theory. We also summarize growing questions and opportunities to further explore how electrochemical reactions can drastically alter electric double layer properties. We conclude with a perspective on how these findings open the door to using electrocatalytic reactions to study electric double layer formation and achieve electrochemical conversion by engineering electrode-electrolyte interfaces. 

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4. Controlling the direction of steady electric fields in liquid using nonantiperiodic potentials

   https://doi.org/10.1103/PhysRevE.107.054608  

   Hashemi, Aref ; Tahernia, Mehrdad ; Ristenpart, William D. ; Miller, Gregory H. ( May 2023 , Physical Review E)

   When applying an oscillatory electric potential to an electrolyte solution, it is commonly assumed that the choice of which electrode is grounded or powered does not matter because the time average of the electric potential is zero. Recent theoretical, numerical, and experimental work, however, has established that certain types of multimodal oscillatory potentials that are “nonantiperiodic” can induce a net steady field toward either the grounded or powered electrode [A. Hashemi et al., Phys. Rev. E 105, 065001 (2022)]. Here, we elaborate on the nature of these steady fields through numerical and theoretical analyses of the asymmetric rectified electric field (AREF). We demonstrate that AREFs induced by a nonantiperiodic electric potential, e.g., by a two-mode waveform with modes at 2 and 3Hz, invariably yields a steady field that is spatially dissymmetric between two parallel electrodes, such that swapping which electrode is powered changes the direction of the field. Furthermore, we show that, while the single-mode AREF occurs in asymmetric electrolytes, nonantiperiodic electric potentials create a steady field in electrolytes even if the cations and anions have the same mobilities. Additionally, using a perturbation expansion, we demonstrate that the dissymmetric AREF occurs due to odd nonlinear orders of the applied potential. We further generalize the theory by demonstrating that the dissymmetric field occurs for all classes of zero-time-average (no dc bias) periodic potentials, including triangular and rectangular pulses, and we discuss how these steady fields can tremendously change the interpretation, design, and applications of electrochemical and electrokinetic systems. 

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5. A perturbation solution to the full Poisson–Nernst–Planck equations yields an asymmetric rectified electric field

   https://doi.org/10.1039/D0SM00417K  

   Hashemi Amrei, S. M. ; Miller, Gregory H. ; Bishop, Kyle J. ; Ristenpart, William D. ( August 2020 , Soft Matter)

   null (Ed.)

   We derive a perturbation solution to the one-dimensional Poisson–Nernst–Planck (PNP) equations between parallel electrodes under oscillatory polarization for arbitrary ionic mobilities and valences. Treating the applied potential as the perturbation parameter, we show that the second-order solution yields a nonzero time-average electric field at large distances from the electrodes, corroborating the recent discovery of Asymmetric Rectified Electric Fields (AREFs) via numerical solution to the full nonlinear PNP equations [Hashemi Amrei et al. , Phys. Rev. Lett. , 2018, 121 , 185504]. Importantly, the first-order solution is analytic, while the second-order AREF is semi-analytic and obtained by numerically solving a single linear ordinary differential equation, obviating the need for full numerical solutions to the PNP equations. We demonstrate that at sufficiently high frequencies and electrode spacings the semi-analytical AREF accurately captures both the complicated shape and the magnitude of the AREF, even at large applied potentials. 

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