Electrochemical impedance spectroscopy as a performance indicator of water dissociation in bipolar membranes

ACS Appl. Energy Mater.

Chanda

et al. 2021

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Bipolar membranes (BPMs) are recently emerging as a promising material for application in advanced electrochemical energy systems such as (photo)electrochemical CO2 reduction and water splitting. BPMs exhibit a unique property to accelerate water dissociation and ionic separation that allows for maintaining a steady-state pH gradient in electrochemical devices without a significant loss in process efficiency, thereby allowing a broader catalyst material selection for the respective oxidation and reduction reactions. However, the formation of high-performance BPMs with significantly reduced overpotentials for driving water dissociation and ionic separation at conditions and rates that are relevant to energy technologies is a key challenge. Herein, we perform a detailed assessment of the requirements in base materials and optimal design routes for the BPM layer and interface formation. In particular, the interface in the BPM presents a critical component with its structure and morphology influencing the kinetics of water dissociation reaction governed by both electric field and catalyst driven mechanisms. For this purpose, we present, among others, the advantages and drawbacks in the utilization of a bulk heterojunction formed in 3D structures that recently have been reported to demonstrate a possibility of designing stable and high-performance BPMs. Also, the outer layers of a BPM play a crucial role in kinetics and mass transport, particularly related to water and ion transport at electrolyte–membrane and membrane–catalyst interfaces. This work aims at identifying the gaps in the structure–property of the current monopolar materials to provide prospective facile design routes for BPMs with excellent water dissociation and ionic separation efficiency. It extends to a discussion about material selection and design strategies of advanced BPMs for application in emerging electrochemical energy systems.

Section: Bpm Properties As Design Criteriamentioning

confidence: 99%

Bipolar Membrane and Interface Materials for Electrochemical Energy Systems

Tufa

ACS Appl. Energy Mater.

Chanda

et al. 2021

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“…When a reverse bias is applied ( i.e. , when the CEL faces the cathode), the protons and hydroxide ions are removed from the IL through the CEL and AEL, respectively, which keeps the WDR to proceed. − …”

Section: Introductionmentioning

confidence: 99%

Reduced Ion Crossover in Bipolar Membrane Electrolysis via Increased Current Density, Molecular Size, and Valence

Verdonk

ACS Appl. Energy Mater.

et al. 2020

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A bipolar membrane (BPM) can be used to accelerate water dissociation to maintain a pH gradient in electrochemical cells, providing freedom to independently optimize the environments and catalysts used for paired reduction and oxidation reactions. The two physical layers in a BPM, respectively, selective for the exchange of cations and anions, should ideally reject ion crossover and facilitate ionic current via water dissociation in an interfacial layer. However, ions from the electrolyte do cross over in actual BPMs, competing with the water dissociation reaction and negatively affecting the stability of the electrolytes. Here, we explore the mechanisms of ion crossover as a function of pH and current density across a commercial BPM. Our unique series of experiments quantifies the ion crossover for more than 10 electrolyte combinations that cover 10 orders of magnitude in acid dissociation constant (K a ) and current densities spanning over more than 2 orders of magnitude. It was found that the ion crossover is dominated by diffusion for current densities up to a maximum of 10−40 mA cm −2 depending on the electrolyte, while migration is of higher importance at high current densities. The influence of the electrolyte pK a or pH on the ion crossover is not straightforward. However, ions with a higher valence or ion size show significantly lower crossover. Moreover, high current densities are the most favorable for high water dissociation efficiencies for all electrolyte combinations. This operational mode aligns well with practical applications of BPMs in electrolysis at industrial relevant current densities.

J Mater Sci: Mater Electron

“…Each point on the Nyquist plot represents a distinct frequency that decreases from left to right [41,47]. Explicitly, the Nyquist plot does not tell us about the frequency corresponding to an impedance.…”

Section: Electrochemical Impedance Spectroscopy Of Pdnps-azomentioning

confidence: 99%

Linker-free chemical preparation of palladium nanoparticles on aluminum-doped zinc oxide electrodes for electrochemical water oxidation

Buliyaminu

Aziz

Shah

et al. 2021

Despite the growing popularity and promising properties of aluminum-doped zinc oxide (AZO) among other transparent conducting oxides (TCOs), modification of AZO with a suitable nano-catalyst can help to enhance its electrocatalytic properties. In this study, the surface of AZO is decorated with palladium nanoparticles (PdNPs) by simply Pd ion capturing on bare AZO from an aqueous solution of K 2 PdCl 4 and successive reduction with NaBH 4 to enhance the electrocatalytic properties toward water oxidation. The effect of K 2 PdCl 4 concentration is optimized for optimum PdNPs-modified AZO (PdNPs-AZO) electrodes for electrochemical water oxidation. The surface morphology, elemental composition, and electrical properties of the prepared PdNPs-AZO were examined by field emission scanning electron microscopy, energy dispersive spectroscopy, and four-in-line probe, respectively. The PdNPs-AZO electrodes, prepared with various concentrations of Pd precursors, exhibited a significant change in terms of electrode sheet resistance and resistivity from each other. The electrochemical impedance spectroscopy and cyclic voltammetry were conducted in a 0.1 M NaOH (aq.) solution to achieve quantitative information about the electrodes and electrochemical reactions toward water oxidation. The PdNPs-AZO prepared with 5.0 mM K 2 PdCl 4 exhibited optimum behavior toward water oxidation with the starting oxidation potential of 625.7 mV vs. Ag/AgCl and current density of 13.8 mA cm -2 at 1.5 V vs. Ag/AgCl.