Elimination of acid-base generation (‘water-splitting’) in electrodialysis (original) (raw)
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Membranes and membrane technologies, 2019
It has been found that after 300 h of operation of AMX and AMX-Sb anion-exchange membranes (Astom, Japan) in overlimiting current regimes in the process of electrodialysis desalination of 0.02 M NaCl, NH 4 Cl, NaH 2 PO 4 , and KC 4 H 5 O 6 (KHT) solutions, the limiting currents, , determined by graphic processing of current-voltage curves increase in the order NaCl < NaH 2 PO 4 < KHT. Their increments relative to those for the "fresh" membrane are 33, 90, and 128%, respectively. The growth in is accompanied by an increase in the thickness of the samples occurring in the NaH 2 PO 4 and KHT solutions. In the case of NH 4 Cl, the values of decrease. It has been shown that a small decrease in counterion transport numbers during membrane operation has almost no effect on the values of limiting currents. The main contribution to the increase in is apparently made by electroconvection, which develops according to the mechanism of electroosmosis of the first kind. Its development is facilitated by the growth in the number and size of freestanding micrometer-sized cavities on the surface of anion-exchange membranes, the area and linear dimensions of which increase in the order NaCl < NaH 2 PO 4 < KHT. These cavities are formed as a result of enhancement of electrochemical degradation of the ion-exchange material and the inert filler polyvinyl chloride at the membrane/solution interface in ampholyte solutions.
Journal of Membrane Science, 2018
Competition between homogeneous and heterogeneous ion-exchange membranes (IEMs) lasts for decades. Low fraction of conductive surface area, Θ, of IEMs causes lower limiting current density, higher voltage and water splitting rate at a same average current density. On the other hand, heterogeneous IEMs are less costly. Additionally, as it was found recently, electrically heterogeneous surface enhances electroconvection. In this paper, we consider a heterogeneous anion-exchange MA-41 membrane (Shchekinoazot) and two its modifications. The first one (MA-41P) is prepared in the same way as the MA-41 membrane and contains the same resin particles, but of a larger size; these larger particles are rearranged on the surface to form agglomerates separated by non-conductive regions. The value of Θ for the MA-41P membrane is 1.5 times greater than that for the MA-41 and the height of "hills" formed by the resin particles on the surface is 3 times higher. The second membrane (MA-41PM) is obtained from the MA-41P by treatment of its surface with a bifunctional polymer solution allowing transforming the functional tertiary and secondary amino groups into the quaternary ones, Θ remains the same. We compare the main physico-chemical (ion-exchange capacity, water content), surface (SEM-EDS analysis, optical microscopy, contact angle) and electrochemical (pH-metry, voltammetry, chronopotentiometry, impedancemetry, water splitting and mass transfer rate) properties of the three mentioned above membranes with those of a homogeneous Neosepta AMX membrane (Astom), in a 0.02 M NaCl solution. The experiments show that the water splitting rate decreases in the sequence MA-41>MA-41P>AMX≥MA-41PM. For the membranes in this sequence above the experimental limiting current densities normalized at the theoretical limiting current density are 0.6, 0.8, 1.3 and 1.25, respectively. However, the voltage at a same overlimiting current density is still greater across the MA-41PM than across the AMX membrane.
Membranes and Membrane Technologies, 2019
Long-term (over 20 h) operation of the AMX-Sb membrane in the electrodialysis desalination of 0.02 M NaCl solution in overlimiting current regimes can lead to an increase in experimentally determined limiting current by more than 30% compared to the pristine membrane. This growth is caused by electrochemical degradation of the ion-exchange material at the AMX-Sb/solution interface, which leads to (1) a decrease in hydrophilicity of the membrane surface and (2) the formation of membrane cavities with linear dimensions of 2-3 μm. Both of these effects stimulate electroconvection, which develops in underlimiting current regimes via the mechanism of electroosmosis of the first kind. The resulting microvortex structures deliver a more concentrated solution to the AMX-Sb surface, shifting the limiting state and the onset of the intense generation of H + and OH − ions to higher currents. The study has been carried out using the techniques of voltammetry and impedance spectroscopy, as well as contact angle measurements and optical visualization of the membrane surface.
Application of the Maxwell–Stefan theory to the membrane electrolysis process
Computers & Chemical Engineering, 2001
A model is developed which describes the mass transfer in ion-selective membranes as used in the chloralkali electrolysis process. The mass transfer model is based on the Maxwell-Stefan theory, in which the membrane charged groups are considered as one of the components in the aqueous mixture. The Maxwell-Stefan equations are re-written in such a way that the current density can be used as an input parameter in the model, which circumvents an extensive numerical iterative process in the numerical solution of the equations. Because the Maxwell-Stefan theory is in fact a force balance, and the clamping force needed to keep the membrane charged groups in its place is not taken into account, the model is basically over-dimensioned: the mole fraction of the membrane can be calculated by using the equivalent weight (EW) of the membrane or by using the equations of continuity. In this work, the latter method has been chosen. The results of the computer model were verified in several ways, which show that the computer model gives reliable results. Several exploratory simulations have been carried out for a sulfonic layer membrane and the conditions as encountered in the chloralkali electrolysis process. As there are no (reliable) Maxwell-Stefan diffusivities available for a Nafion membrane, in this trend study the diffusivities were all chosen equal at a more or less arbitrary value of 1.10 − 10 m 2 s − 1 . Due to this, the absolute values of several performance parameters are incorrect as compared with industrial chloralkali operation (e.g. an unrealistically high current efficiency of 95.7% was found), but the model can still be used to obtain trends. For example, it is shown that the thickness of the membrane hardly increases the current efficiency (CE), however, the required potential drop proportionally increases with thickness. The pH rapidly increases to values greater than 12 just inside the membrane at the anolyte side. Moreover, for different values of the pH in the anolyte, the pH profiles inside the membrane nearly coincide with each other. A change in the anolyte strength does not have a significant effect on the performance of the membrane. At low values of the current density, a high value of the current efficiency is found. However, this is not due to a low OH − counter flux, but to the simultaneous transport of OH − and Cl − towards the catholyte.
Journal of Membrane Science, 2017
The mechanism of electroconvection at a permselective surface presents a high interest for electrodialysis separation processes as well as for microfluidics and other applications. We have studied a commercial Neosepta AMX-Sb anion-exchange membrane and its three modifications differing in the surface charge and, as a consequence, in the degree of hydrophobicity. The zeta-potential and the contact angle were measured; the membranes were characterized by chronopotentiometry and voltammetry. It is shown that at the current densities slightly lower or equal to the limiting current density, the mass transfer rate is mainly affected by the membrane surface charge. However, at the higher current densities, the main factor is the degree of hydrophobicity: the samples with a weakly charged highly hydrophobic surface show lower voltage under the same current density. This peculiarity is explained by the fact that the mechanism of electroconvection (EC) depends on the current density. At underlimiting currents and low voltages, EC occurs as electroosmosis of the first kind; the surface charge determines the parameters of the (quasi)equilibrium electric double layer (EDL), playing the main role in the phenomenon. At overlimiting currents and high voltages, it is the extended space charge region (much thicker than the EDL), which controls EC occurring apparently as electroosmosis of the second kind (nonequilibium EC). Then the contribution of the EDL is less important, while the impact of hydrophobicity increases. It is shown that the equilibrium EC may be quite strong at the AMX-Sb membrane having a highly developed surface roughness of different scales. In the 2 range of 0.03-0.06 V there is an "anomaly": with increasing current density the potential drop over the AMX-Sb is decreasing instead of increasing.
Enhancement of counter-ion transport through ion-exchange membranes in electrodialytic processes
Desalination and Water Treatment, 2014
Ion-exchange membranes (IEMs) are ionic conducting materials. They have various applications such as: fuel cell (PEMFC), electrochemical synthesis (Cl 2 /NaOH), desalination, purification, separation, and environment. Despite these applications, several aspects are still unknown, such as: the membrane structure, the conduction mechanisms, and concentration polarization. The main obstacle in electro-membrane processes such as electrodialysis is the concentration polarization phenomenon, which remains one of the incomprehensible phenomena in IEM transport. This phenomenon is common to all systems operating a selective ionic transfer through an interface; it arises from the difference in ions mobility in the solution and in the membrane. A better understanding of concentration polarization can help to improve the membrane performance, the process efficiency, and in the reduction the process operation cost. In this research, we studied the effect of the ammonia buffer (NH 3 /NH þ 4 ) on the counter-ion transfer through the anion-and the cationexchange membranes AMX and CMX, respectively. The results show that the ammonia addition facilitates the counter-ion transfer in both cases and gives a total elimination of the system polarization, but with different behaviors of CMX and AMX membranes. The classical concentration polarization theory remains insufficient to explain the obtained results.
Energy and Environmental Science, 2018
The lamination of a cation exchange layer (CEL) and an anion exchange layer (AEL) to form a hybrid bipolar membrane (BPM) can have several unique advantages over conventional monopolar ion exchange membranes in (photo-)electrolysis. Upon application of a reverse bias, the ordinarily slow water dissociation reaction at the AEL/CEL junction of the BPM is dramatically accelerated by the large electric field at the interface and by the presence of catalyst in the junction. Using electrochemical impedance spectroscopy (EIS), we have found a counterbalanced role of the electric field and the junction catalyst in accelerating water dissociation in a BPM. Experimental BPMs were prepared from a crosslinked AEL and a Nafion CEL, with a graphite oxide (GO) catalyst deposited at the junction using layer-by-layer (LBL) assembly. BPMs with an interfacial catalyst layer were found to have smaller electric fields at the interface compared to samples with no added catalyst. A comprehensive numerical simulation model showed that the damping of the electric field in BPMs with a catalyst layer is a result of a higher water dissociation product (H + /OH À) flux, which neutralizes the net charge density of the CEL and AEL. This conclusion is further substantiated by EIS studies of a high-performance 3D junction BPM that shows a low electric field due to the facile catalytic generation and transport of H + and OH À. Numerical modeling of these effects in the BPM provides a prescription for designing membranes that function at lower overpotential. The potential drop across the synthetic BPM was lower than that of a commercial BPM by more than 200 mV at 4100 mA cm À2 reverse bias current density, with the two membranes having similar long-term stability. Broader context Bipolar membranes (BPMs) have recently been shown to offer important advantages over conventional monopolar membranes in avoiding polarization losses in (photo-)electrochemical water splitting and carbon dioxide reduction. The rate of water dissociation at the junction between the cation and anion exchange layers (CEL and AEL) limits the energy efficiency of BPM-based electrolysis devices. This rate is dramatically increased by the high electric field and the presence of catalysts in the junction region. Realizing high performance BPMs requires knowledge of the relative importance of and correlation between the electric field and the catalyst in promoting water dissociation. Here a combined electrochemical impedance and simulation study reveals that the electric field across the AEL/CEL interface is weakened by the H + /OH À flux from catalyzed water dissociation, which partially neutralizes the unbalanced fixed charges on the AEL and CEL. The understanding gained from this study highlights the need to optimize the amount of catalyst in the AEL/CEL junction, as found empirically in previous studies, and offers some basic principles for designing higher performance BPMs.
Innovations in electromembrane processes
Copernican Letters, 2015
Electromembrane processes are increasingly important group of separation methods, widely used for removal of charged components from solutions. It is a growing field of research with a plethora of both existing and still developed applications. The separation is based on ion migration across the charged membranes (a polymeric matrix with fixed charged groups, counterbalanced with mobile counter-ions), placed in the electric field. This paper presents the main electromembrane processes: electrodialysis (ED), electrodialysis reversal (EDR), electrodialysis with bipolar membrane (EDBM), electrodeionization (EDI), membrane capacitive deionization (MCDI), reverse electrodialysis (RED). We present the common applications of electromembrane processes and discuss the physical basis of the electromembrane processes. The most important parameters of the ion-exchange membranes are discussed, as well as the novel approaches towards mitigation of scaling, enhancement of mass transfer, decreasing the concentration polarization, and new hybrid electromembrane processes. Critical analysis of the possibility of energy production by reverse electrodialysis is presented.