Saline water electrolysis system with double-layered cation exchange membrane for low-energy consumption and its application for CO2 mineralization

https://doi.org/10.1016/j.jcou.2020.101269Get rights and content

Highlights

  • Double-layered cation exchange membrane shows ion conductivity superior to a commercially available membrane.

  • The membrane exhibits stable electrochemical performances with a small standard deviation.

  • CO2 emission can be reduced by using the double-layered cation exchange membrane.

Abstract

Saline water electrolysis (SWE) plays a key role in mineralizing CO2 to carbonate products (e.g., NaHCO3) as it determines the conversion and energy efficiency of the whole CO2 mineralization process. SWE is highly energy intensive and cation exchange membranes (CEMs) represents the highest resistance contributing component of the overall system. Hence, the use of highly Na+ ion-conductive membranes can effectively reduce overpotentials and lower the energy consumption in a SWE system. Motivated by this, a double-layered cation exchange membrane composed of perfluorinated sulfonic acid (PFSA) and perfluorinated carboxylic acid (PFCA) ionomer layers is investigated as an alternative to the commercially available Aciplex-F® membrane, and its performance is evaluated in terms of current-voltage polarization, energy consumption, and caustic current efficiency in single cell mode, followed by technical reliability, electrochemical durability, and the purity of produced NaOH in stack mode. Finally, CO2 reduction achieved by replacing the conventional Aciplex-F based SWE system with the proposed CEM based SWE system is estimated for various electric power sources.

Introduction

Carbon capture and utilization (CCU) is about capturing carbon dioxide (CO2) to recycle it for further usage [1]. The recent interest in CCU has been motivated by the high risk and cost associated with subsurface storage of CO2 emissions and opportunity to generate extra revenues by producing fuels (e.g., CH4 [2,3], syngas [4,5], methanol [6], and formic acid [7]), and other useful chemicals including polyolefin polymers [8] (e.g., polyethylene and polypropylene) to defray the cost. Among various CCU options, CO2 mineralization reacts acidic CO2 with basic alkali metal or alkali earth metal cations to generate carbonate products such as NaHCO3, Na2CO3, and CaCO3 [9]. Some of the suggested CO2 mineralization technologies have sufficiently matured that pilot- to demonstration-scale verifications (e.g., Skyonic skymine® process [10] and Calera process based on aqueous precipitation [11]) have appeared.

CO2 mineralization system is composed of the saline water electrolysis (SWE) step to provide alkali metal or alkali earth metal cations, and the CO2 carbonation step to convert the cations and the CO2 into carbonate products. For example, SWE in the Skyonic skymine® process takes a sodium chloride (NaCl) aqueous solution and converts into caustic soda (NaOH) in addition to hydrogen (H2) and chlorine (Cl2) gases at high-purity levels [12]. Then, the caustic soda is chemically reacted with CO2 to produce sodium bicarbonate (NaHCO3) or sodium carbonate (Na2CO3) by controlling the pH value. Ca(OH)2 and Mg(OH)2 coproduced from the saline water sources in the SWE process become calcium carbonate (CaCO3) and magnesium carbonate (MgCO3) after the carbonation, which can be used as raw materials for cements.

A key component of the CO2 mineralization system is SWE, which affects the overall conversion of CO2 and also the energy consumption of the whole process [13]. Here, saline water is defined as water that contains a certain concentration level of dissolved salts (e.g., NaCl), and includes natural and artificial sea water which have the NaCl concentration of 3.5 wt.% and above 25 wt.%, respectively [14]. In SWE, Na+ ions dissolved in the anode reservoir are transported through the cation exchange membranes (CEMs) to the cathode reservoir, when a certain amount of electric power is applied. The electrochemical reaction results in Cl2 at the anode and H2 and NaOH at the cathode [15]. SWE is known to be energy-intensive technology. For instance, the Skymine SWE plants in San Antonio, TX, USA consumed around 18.4 MW, which is 87 % of the total energy used to operate the Skyonic skymine® process [10]. The reported energy consumption was 2.63 MW h/ ton NaOH for the chlor-alkali (CA) system, which is known for its high energy efficiency. It is noteworthy that the CA system was operated at 90 °C to reduce their energy consumption as much as possible. To build an economically feasible CO2 mineralization process, it is necessary to develop a SWE system with energy consumption lower than the reported value.

To date, there have been diverse approaches to reduce SWE energy consumption. These efforts include a SWE cell using a fuel cell configuration [16], the zero-gap method to minimize the interfacial resistances between SWE cell components [17], and the use of porous electrodes activating the generation and emission of the product gases (i.e., H2 and Cl2) [18]. Another suggested approach is to employ oxygen depolarized cathode (ODC), which can lower the energy consumption but with some sacrifice in the H2 evolution [19].

In this study, the use of novel double-layered cation exchange membrane composed of perfluorinated sulfonic acid (PFSA) and perfluorinated carboxylic acid (PFCA) ionomer layers is investigated as a way to reduce the ohmic resistance of CEM in order to lower the energy consumption. The efficacy of the idea is tested via electrochemical performance comparison between the experimental double layered CEM and a commercially available SWE membrane in both single cell and stack modes. All the electrochemical evaluations were conducted in the SWE cells employing the above-mentioned approaches except the ODC. The study was also intended to test whether the experimental CEM can maintain the electrochemical durability under an accelerated SWE operation condition. The overall CO2 reduction depends on the energy generation source and such reduction is estimated for various electricity types when the CO2 mineralization is carried out with the CEM-based SWE stack module.

Section snippets

Materials

Sodium chloride (NaCl, 99.5 %) and sodium hydroxide (NaOH, 97.0 %) were purchased from Samchun Chemicals (Republic of Korea) and Daejung Chemical (Republic of Korea), respectively, and used as received without any further purification in preparing the anode and cathode feed solutions. For the SWE electrodes, titanium foam and Pt-coated titanium foam (CNL energy, Republic of Korea) were used as the anode and the cathode for easy emissions of Cl2 and H2, respectively [18]. Aciplex-F® (Asahi

Results and discussion

The theoretical voltage of saline water electrolysis is about 2.20 V, which is the sum of standard reduction potentials at the anode and cathode as shown below:

Reduction half-reactionStandard reduction potentialReaction site
2Cl → Cl2 + 2e1.360 VAnode
2H2O +2e → H2 + 2OH−0.828 VCathode
2NaCl +2H2O → Cl2 + H2 + 2NaOH2.188 VWhole

The realistic cell voltage is, however, much higher than the theoretical one, owing to the overpotentials resulting from the ohmic resistance of each cell component and

Conclusions

A perfluorinated cation exchange membrane in the form of double-layered structure composed of PFSA and PFCA ionomer layers was employed as a SWE membrane. The membrane showed Na+ ion conductivity superior to a commercially available Aciplex-F® SWE membrane owing to a synergistic effect of a reduced membrane thickness and the use of lower EW ionomer. The highly Na+ ion-conductive membrane was shown beneficial in reducing the ohmic resistance of the corresponding SWE cell, leading to lower energy

CRediT authorship contribution statement

Ji Hyun Lee: Conceptualization, Investigation, Writing - original draft. In Kee Park: Investigation, Data curation. Denis Duchesne: Resources. Lisa Chen: Resources. Chang Hyun Lee: Supervision. Jay H. Lee: Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Technology Development Program to Innovative Hydrogen Energy of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2019M3E6A1064093), and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20183010032380).

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