Membrane Properties and Characterization for Zero-Gap CO2 Electrolyzers

Zero-gap electrolyzers are similar to fuel cells in design because the heart of the electrolyzer consists of two electrodes pressed against a membrane. These electrolyzers are called “zero-gap” because there is no gap between the cathodes, anodes, and the electrolyte. This design decreases the distance for ion transport because the layers are pressed or bonded together. The zero-gap CO₂ electrolyzers can achieve high current densities (≥100 mA/cm²) by delivering gaseous CO₂ to the cathode. The efficiency of these electrolyzers depends upon the catalysts used, the operating conditions, and of course, the membrane chosen.

The membrane in zero-gap CO₂ electrolyzers facilitates ion transport between the cathode and the anode while preventing reactant or product crossover. The membrane would ideally meet the following requirements: High ionic conductivity, Present an adequate barrier to the reactants, Be chemically and mechanically stable, Low electronic conductivity, Ease of manufacturability/availability, Preferably low-cost.

Currently, CO₂ electrolyzers use anion exchange membranes (AEMs), cation exchange membranes (CEMs), or bipolar membranes (BPMs) (see Figure 1). AEMs transport anions such as hydroxide (OH^-) from the cathode to the anode, CEMs transport cations such as hydrogen protons (H⁺) from the anode to the cathode, and BPMs may perform more than one type of ion transport, depending upon the chemistry. Electrolyzers that contain AEMs have demonstrated higher conversion efficiencies than CEM and BPM systems. This may be due to the AEM microstructure, which contains hydrophilic positively charged groups attached to a hydrophobic polymer backbone. The hydrophilic portion of the microstructure relies on the water content in the AEM for anion transport. Research on the water transport mechanisms is still ongoing, but the literature describes ion transport as a combination of Grotthuss, diffusion, and surface site hopping mechanisms.

Figure 1. Ion Transport for CEM and AEM in CO₂ Electrolyzers.

The Grotthuss mechanism is one of the dominant modes of anion transport through the AEM (see Figure 2). The hydroxide groups (OH^-) are transported through the hydrogen bond network of water molecules through the formation and cleavage of covalent bonds. Transport via diffusion occurs when there is a concentration or electrical potential difference between the anode and cathode side of the membrane. The movement of OH^- ions occurs through surface site hopping from one side of the AEM to the other via cationic functional groups. The dominant transport mechanism depends upon the membrane chemistry and the water content of the AEM.

Figure 2. Grotthuss Transport Mechanism in Anion Exchange Membrane (AEM).

When the membrane is not adequately hydrated, the ionic conductivity is limited because the hydrophilic areas are small for the diffusion and surface hopping water transport processes to occur (see Figure 3). The ionic conductivity is higher in a highly hydrated membrane because the water-filled channels expand, and anion transport can occur through diffusion mechanisms.

Figure 3. Diffusion and Surface Site Hopping Transport Mechanism in Anion Exchange Membrane (AEM)

CO₂ electrolyzer efficiency increases at higher temperatures, but issues with the membrane, such as membrane dehydration, reduction of ionic conductivity, decreased affinity for water, loss of mechanical strength via softening of the polymer backbone, and increased parasitic losses through high fuel permeation, become worse. Ionic membranes must be kept hydrated to retain ion conductivity, but the operating temperatures often must be kept below the boiling point of water.

The amount of water in the ion exchange membrane, such as an AEM, is critical to good performance of the CO₂ electrolyzer. Too little water in the AEM reduces ion transport, and too much water in the AEM can hinder the CO₂ diffusion to the catalyst layer. The methods for delivering water to the membrane are humidifying the CO₂ feedstock or through the anolyte that permeates the AEM. 

The net flux of water is due to a combination of diffusion, electroosmotic drag, and back convection (Figure 4). Water diffuses from one side of the membrane to the other due to the liquid concentration gradient between the aqueous anode and the gas-fed cathode. Water is also carried from the cathode to the anode through solvated anions known as electroosmotic drag. The back convection of water from the cathode to the anode is driven by the liquid pressure gradient across the membrane.

Figure 4. Water Transport Processes in Anion Exchange Membrane (AEM).

The rate of water flux depends upon the AEM properties. Water diffusivity and electroosmotic drag increase in membranes with higher water content. Also, thinner membranes have increased back convection of water. Therefore, not only is the membrane microstructure important, but also the operating conditions.

Ideally, an AEM should facilitate the anion transport while eliminating the flux of co-ions. The migration of cations is undesirable because salt can limit the mass transport of the desired ions. The transport of co-ions can occur through the AEM via diffusion due to a difference in concentration across the membrane or through electromigration due to an applied electric field. The transport of undesirable cations can be minimized through material design or optimizing the applied current or analyte concentration.

In the CO₂ electrolyzer, the electrolyte conductivity is a function of temperature and water content. Conductivity can be increased through advanced conductive materials, thinner electrolytes, or an optimal temperature/water balance. One of the most effective methods of increasing conductivity is to use a better ionic conductor for the electrolyte layer or a thinner electrolyte layer since the electrolyte component of an electrolyzer often dominates the conductivity (ohmic) losses. Depending upon the electrolyzer design, thinner membranes may also be advantageous because they keep the anode electrode saturated through “back” diffusion of water from the cathode. One must also consider that at high current densities (fast fluid flows), mass transport can cause a rapid drop-off in the voltage because the reactants cannot diffuse through the electrode and ionize quickly enough, and products cannot be moved out at the necessary speed.

Since the ohmic overpotential (conductivity losses) for the electrolyzer is mainly due to ionic resistance in the electrolyte, this can be expressed as:

where A_electrolyzer is the active area of the electrolyzer, δ is the thickness of the electrolyte layer, σ is the conductivity, i is the current density, and j is the current density/area. As seen from the equation, the ohmic potential can be reduced by using a thinner electrolyte layer and using a higher ionic conductivity electrolyte. Using highly conductive materials for the electrodes and contacts will also help to reduce ohmic polarization. 

Three essential techniques for characterizing ion exchange membranes (i.e., AEMs and CEMs) are ionic conductivity, ion exchange capacity, and water uptake.  

Ionic conductivity can be measured along the plane (in-plane) or through the thickness of the membrane (through-plane). As we just saw in equation (1), conductivity values change with membrane thickness, but they will also change across the membrane area (in-plane) due to anisotropic resistances that arise from the polymer casting method or use of support materials in the membrane. The through plan conductivity is produces practical results because anions in a CO₂ electrolyzer migrate between the cathode to the anode through the thickness of the AEM. However, the through-plane testing can be more challenging to implement because it requires components that can introduce other resistances. 

The through-plane membrane resistances are usually determined using electrochemical impedance spectroscopy (EIS). EIS is a noninvasive technique that uses a small sinusoidal perturbation potential at one or several frequencies to the electrolyzer stack, and the response is an alternating current (AC) signal of the same frequency with a possible shift in phase and change in amplitude. The recorded response is used to calculate the impedance using a mathematical technique. By repeating this at several frequencies, an Electrochemical Impedance Spectroscopy (EIS) is obtained. EIS helps characterize fast and slow transport phenomena because it tests both single and wide range of frequencies.

There are two main electrochemical impedance spectroscopy (EIS) approaches: ex-situ or in-situ two-probe. In the ex-situ method, a membrane is sandwiched between two electrodes, and this conductivity cell is submerged in a water bath or placed in a controlled humidity chamber to ensure homogeneity of the water content in the membrane. In-situ conductivity measurements are performed using a similar setup and operating conditions as an operating zero-gap cell. Most current ex-situ conductivity measurements assume that OH– is the sole charge carrier; however, AEMs contain a mixture of OH^– and less mobile HCO^3– and CO₃^2– ions in CO₂ electrolysis. In-situ EIS measurements yield more representative anion conductivities of the conditions in a CO₂ electrolyzer but are more time-consuming and may require extensive setup to gather the data reproducibly.

The Ion Exchange Capacity (IEC) helps to compare the suitability of AEM membranes for electrochemical CO₂ reduction. IEC (meq/g) is the number of exchangeable functional groups per dry weight of the polymer and is typically reported for AEMs in Cl^– form because Cl^– is the mobile counter-ion in the membrane. To determine the IEC, the membrane is pretreated with an aqueous electrolyte (1.0 M KOH) to exchange the Cl^– ions with the ions in the solution (OH^-). The concentration of the Cl^– ions can then be determined through titration, ion-selective electrode, FTIR, or NMR spectroscopy. Titration-based techniques are the most common method of determining IECs; however, there is variability in this technique because a color change is used to indicate the equivalence point. FTIR and NMR spectroscopy can determine the total number of functional groups in the membrane, but this may differ from the number of functional groups participating in ion exchange. Table 1 illustrates the properties of some commercial ion-exchange membranes, including their IEC values.

Table 1. Properties of Commercial Ion-Exchange Membranes.

An important membrane property is how the membrane structure changes as a function of water content (where l is the moles of water per mole of reactive sites). This property is well documented in the literature and can be measured by examining the weight gain of an equilibrated membrane. The initial water content is associated strongly with the reactive sites, and the addition of water causes the water to become less bound to the polymer and causes the water droplets to aggregate. The water clusters eventually grow and form “water channels” that are transitory. A transport pathway forms when water clusters are close together and become linked. As the amount of water in the membrane increases, a complete cluster-channel network will form. As the membrane becomes saturated, the channels will fill with water, and the uptake of the membrane will increase without a change in the chemical potential of water. This phenomenon is known as Schroeder’s paradox.  

Since water facilitates anion transport through the AEM, it is essential to characterize the amount of water in the membrane. Water content is needed for ionic transport, but high-water content can compromise the mechanical integrity of the AEM due to excessive swelling. The water content of the AEM can be described by the water uptake property, which is the % increase in the mass of the AEM when it is fully hydrated in liquid or water vapor (m_H) relative to when the membrane is completely dry (m_d). Water uptake of AEMs is typically reported in the Cl^– form and when the membrane is hydrated with liquid water. 

Water uptake can be measured by immersing the AEM in Cl^- form in deionized water for 24 hours and replacing the water at least three times to ensure excess ions are removed. The membrane is then placed in a desiccator for a length of time (i.e., five days), and then the mass of the dried membrane is weighed. The water uptake value is then calculated:

The membrane for CO₂ electrolyzers must be a good ion conductor, chemically stable, and able to withstand the temperatures and compression forces of the electrolyzer stack. The membrane requirements include high ionic conductivity, an adequate barrier to the reactants, chemically and mechanically stable, and low electronic conductivity. In addition, the operating conditions must ensure adequate membrane water hydration and uptake to maintain a predetermined level of electrolyzer efficiency. There are many choices for the membrane for CO₂ electrolyzers, and the decision regarding the type chosen must depend upon the factors described in this blog post, along with the electrolyzer design, cost and mass manufacturing capabilities.