3.1. CO2 Capture and Release
The capture and the release of CO
2 is performed by an electrochemical cell composed of four chambers, depicted in
Scheme 1. In this system, the CO
2 recovery is performed by acidification of the sorbent solution, namely NaHCO
3. Cationic and bipolar membranes are arranged as shown in
Scheme 1, since it is necessary to provide protons to the sorbent solution and to withdraw alkaline metal ions from it. Electrodialysis is the process to perform this exchange, allowing the release of gaseous CO
2 (outlet gas in
Scheme 1), according to the following reactions:
Therefore, protons are provided to the feed compartment (compartment 3 in
Scheme 1) by the bipolar membrane placed on the anode side of the cell (membrane A), while Na
+ ions cross the cationic membrane B reaching the alkali regeneration compartment (compartment 4) where NaCl is used as electrolyte. Here, hydroxyl ions are delivered by the bipolar membrane on cathode side (membrane C), consequently forming NaOH. The latter is then turned back into NaHCO
3 by the CO
2 present in the inlet flue gas, according to the Reaction 4, and driven again to the feed compartment; hence the process can restart.
It is worth noticing that, in addition to the release of gaseous CO2, there is also production of O2 in the anodic (compartment 1) chamber and H2 in the cathodic (compartment 2) chamber, due to water electrolysis. The hydrogen can be brought at the final outlet of the electrochemical reactor responsible for the reduction of carbon dioxide and added to syngas as final product.
Different tests were carried out to optimize the electrochemical capture and release cell, by changing the pump flow rate, the inlet gas flow rate, and the applied current density.
Figure 1 reports the voltage and the gas outlet flows (
Qoutlet) trend for a typical experiment carried out at 100 mA/cm
2 with an inlet flow composed by 100% CO
2 at 20 mL/min. The voltage was stable between 12 V and 13 V for the entire 1.5 h-experiment. In the CO
2 flow, released from compartment 3, some traces of hydrogen and oxygen have been detected, probably due to the not perfect isolation among the compartments.
The effect of the variation of the current applied at the electrodes of the electrochemical cell on the release of CO
2 has been explored first. This is pointed out in
Figure 2a,b. During the course of the experiments, stable voltages were measured, independently of the applied current density. As expected, a lower current brings a smaller voltage (12.5 V for 100 mA/cm
2 and 7 V for 25 mA/cm
2). Accordingly, the number of electrons involved in the reactions decreases, so that the production of carbon dioxide drops; likewise, the outlet. In particular, the average outlet flow for
J = 100 mA/cm
2 is almost doubled with respect to the value obtained at 75 mA/cm
2. This is coherent with the CO
2 recovery rate from carbonate calculated by Iizuka [
18], which is proportional to the current applied at the electrodes. However, it is important to highlight that diminishing the voltage means to reduce the energy consumption of the capture system. This implies that, according to the needs of the conversion module that will get the carbon dioxide flow as an inlet, a proper trade-off between the current density and outlet flow is sometimes needed.
The second parameter that has been optimized is the electrolyte flow rate, controlled by the peristaltic pump. The pump must make the sorbent solution and the electrolyte recirculate through the cell and the tanks, ensuring the regeneration of the bicarbonate, and therefore the perpetuation of the reactions. The optimization of this parameter is important since it controls the turbulence of the motion of the reactants, therefore changing the reaction rate inside the reactor.
Figure 3 shows how the flow rate of reactants affects the average
Qoutlet for CO
2.
Qpump = 1 mL/min is the best solution since it provides the highest average CO
2 outlet. Probably, the optimal pump flow rate, thus the recirculation of the reactants, is linked to the number of electrons provided at the electrodes, and therefore the current, as already observed in [
19,
20].
Since at this point a capture device was being implemented, one of the most important aspects is the percentage of CO
2 released with respect the amount of CO
2 bubbled in the solution.
Figure 4 reports the average outlet CO
2 flow for different inlet gas flow rates. Considering
Qinlet = 10 mL/min and 5 mL/min, the percentage of carbon dioxide captured and released is larger (45% and 91% respectively) with respect to the case of
Qinlet = 20 mL/min (32%). It is important to take into account that part of the CO
2 released from the capture device in 1.5 h arises from the initial NaHCO
3 present in the electrochemical reactor, as shown in
Figure 4 for
Qinlet = 0 mL/min. Once the initial NaHCO
3 is completely consumed, the outlet flow will be produced only by the acidification of carbonate regenerated by the bubbling carbon dioxide [
18]. However, it has to be highlighted that this device has been designed expressly for the coupling with the conversion module. Since, overall, the outlet flow of CO
2 captured with
Qinlet = 20 mL/min is the largest one, this will make the performance of the whole capture/conversion system higher. For this reason, the optimized
Qinlet will be selected to maximize the performance of the whole coupled system, i.e., 20 mL/min.
So far, the parameters have been changed to optimize the outlet flow, but the real capture of carbon dioxide from flue gas has never been simulated. In this regard, the outlet was studied dependently on the percentage of the composition of the inlet gas. In order to simulate a flue gas, a mixture of CO
2 and N
2 was being bubbled through.
Figure 5 points out that the presence of nitrogen in the inlet gas does not influence the outlet, which is almost the same for the case in which no bubbling gas is present. Indeed, the presence of an additional bubbling flow, such as nitrogen, could even help in the regeneration of the bicarbonate, increasing the turbulent motion in the tank. In addition,
Figure 5 provides the result with a flue gas as the inlet, in which the CO
2 component is kept at 20 mL/min. It can be observed that the final average CO
2 outlet flow, with 50% and 84% of nitrogen as the inlet, does not depart too much from the case of pure CO
2.Figure 6 considers the findings mentioned so far and highlights the dependence of the CO
2 outlet flow rate on both CO
2 inlet flow rate and pump flow rate (the current density is not taken into account here since the output flow rate is directly proportional to
J). The surface has been constructed employing Matlab, implementing a fit through a piecewise cubic interpolation of the data obtained during the tests. It can be noticed that for large pump flow rates the CO
2 outlet flow decreases. Moreover, having a higher CO
2 inlet helps to have a larger outlet. In order to maximize the gas outlet and perform the coupling with the conversion module, looking at
Figure 6, the best condition seems to be described by the area where the parameters
Qinlet and
Qpump are about 20 mL/min and 1 mL/min, respectively. For this reason, the following set of parameters have been employed for the subsequent part of the work:
Qinlet = 20 mL/min,
Qpump = 1 mL/min and
J = 100 mA/cm
2.
A stability test of the capture/release system of 8 h has been performed with the optimized parameters employing pure CO
2 flow as inlet. Three parameters have been monitored during the whole experiment: the outlet gas flow, the voltage, and the outlet gas composition.
Figure 7 reports the trend of these three parameters and shows how after around 2 h the voltage and the outlet gas reach a stable value, around 12 V and 5 mL/min respectively. Probably, this is the time necessary for the hydraulic system to completely mix the sorbent and the electrolyte solutions between the tanks that feed compartments 3 and 4 (see
Scheme 1). The composition of the outlet gas has been monitored during the entire test. After 1 h, the output gas phase was composed by 98% of CO
2 and it remained stable for the rest of the experiment. Therefore, this stability test shows how the capture module is able to provide a constant pure amount of carbon dioxide for a long time interval. Similar purities of the outlet stream have been obtained in analogous experiments performed with a simulated flue gas (CO
2/N
2 mixtures of 50%/50% and 16%/84%) as the inlet.
Considering the results obtained in the stability test, the energy spent by the system per unit of carbon dioxide recovered from the bicarbonate is 42.5 MJ/kgCO
2. This result makes clear that this particular capture system has to be further optimized. In fact, the comparison with the similar system introduced by Nagasawa [
13], characterized by a minimum recovery power requirement of 2.1 MJ/kgCO
2, and with a typical amine-based process (3–4 MJ/kgCO
2), shows how this precursory system has the possibility to be proficient, avoiding the use of toxic agents like amines. One possibility would be a stack composed by two of this system in order to decrease the ohmic losses. In addition, two different nanostructured electrocatalysts could replace the platinum foil at the anode and the cathode, increasing the energy efficiency.
3.2. Capture–Conversion Coupling
At this point, the capture system has been optimized and its outlet has been connected to the CO
2 reduction reaction (CO
2RR) module, as shown in the
Scheme 2.
The CO
2 released by the capture module enters the chamber III, where it can diffuse through the GDL and reach the ZnO catalyst/electrolyte interface in chamber II. For a cathodic applied potential of −1.2 V (whole cell potential of 3.2 V), the CO
2 can be reduced to CO (Equation (5)) and HCOOH or HCOO
− (Equation (6)). Hydrogen can also be produced (Equation (7)). The ZnO catalyst is more selective towards CO, and thus the formic acid production is scarce [
15]. The gaseous products are CO and H
2, which are the components of syngas.
In order to test the catalyst performance under optimal conditions, a preliminary experiment has been conducted employing a CO2 flux of 25 mL/min from an external cylinder as the CO2 source for the cathodic side. A current density of 40.9 mA/cm2 and faradaic efficiencies of 69% for CO, 27% for H2, and 4% for HCOOH have been obtained. Then the CO2 gas flow has been decreased to 3 mL/min, to mimic the release rate of CO2 from the capture module. The current density maintains a similar value of 43.6 mA/cm2, while the selectivities exhibit a notable difference, namely 56% for CO, 40% for H2 and 4% for HCOOH. This outcome indicates that the CO2RR performance is significantly influenced by the CO2 flow rate; smaller values can decelerate the mass diffusion process and induce an increase of hydrogen evolution.
Then, two different tests with CO
2/N
2 gas mixtures equal to 50%/50% and 16%/84% were carried out. The capture module worked at the optimized conditions described in the previous section. The 50%/50% gas mixture releases an outlet gas flux of 3 mL/min. It is lower than that obtained in the previous tests (see
Figure 5), probably due to the coupling with the conversion module, which introduces an obstacle to the gas flux, raising the pressure and therefore reducing the flow rate. During the test, the conversion module showed stable selectivity and current density, as displayed in
Figure 8. An average current density of 35.9 mA/cm
2 and faradaic efficiencies of 56% for CO, 41% for H
2, and 3% for HCOOH have been achieved. These results are in line with those obtained with a CO
2 flux of 3 mL/min from an external cylinder. It is clear that not all the provided CO
2 can react, and around 14.8% of the total amount is converted into CO and HCOOH.
For the second test, the CO
2/N
2 16%/84% gas stream has been used. The outlet flow rate from the capture module was 1.5 mL/min, smaller if compared to the one obtained without the conversion module, due to the same reason described above. As shown in
Figure 9, the current density is 39.6 mA/cm
2 and the faradaic efficiency is 37% for CO, 59% for H
2, and 4% for HCOOH. The selectivity towards CO
2RR products is further decreased, due to the lower CO
2 flow rate at the cathodic side of the flow cell, while the percentage of reacted CO
2 rises up to 22.6% during this test.
By comparing the results of the two tests, it can be observed that with decreased CO
2 percentage in the initial gas stream, the absorption module has reduced its released gas flux. This induces a CO
2 mass diffusion limitation at the conversion module as described before. Despite this issue, which reserves additional investigations aimed at optimizing the coupling between the modules, the electrochemical platform demonstrated its ability to separate CO
2 from simulated flue gas mixtures and convert it into value added products. Syngas with CO:H
2 ratios from 1.4 to 0.6 has been produced at good current densities of 35–40 mA/cm
2. The composition of the obtained syngas is similar to the one commonly used for the synthesis of ethanol [
21]. This system can be employed for the capture of harmful emissions and their transformation into useful resources in both industrial and civil scenarios. Moreover, the use of green chemicals such as sodium chloride and sodium/potassium bicarbonates makes it particularly appealing in the framework of sustainable CCU.