Efficient CO2 Utilization via a Hybrid Na-CO2 System Based on CO2 Dissolution.

Carbon capture, utilization, and sequestration technologies have been extensively studied to utilize carbon dioxide (CO2), a greenhouse gas, as a resource. So far, however, effective technologies have not been proposed owing to the low efficiency conversion rate and high energy requirements. Here, we present a hybrid Na-CO2 cell that can continuously produce electrical energy and hydrogen through efficient CO2 conversion with stable operation for over 1,000 hr from spontaneous CO2 dissolution in aqueous solution. In addition, this system has the advantage of not regenerating CO2 during charging process, unlike aprotic metal-CO2 cells. This system could serve as a novel CO2 utilization technology and high-value-added electrical energy and hydrogen production device.

Introduction

Many researchers believe that global warming and climate change are the result of carbon dioxide (CO2) generated by human activities over the centuries (Jenkinson et al., 1991, Obama, 2017). Thus, many countries and organizations have made great efforts to reduce their carbon footprint, and recently, the carbon capture, utilization, and storage/sequestration (CCUS) technology has been studied to recycle CO2 as a resource (Keith et al., 2018, Andersen, 2017, Dowell et al., 2017). In this regard, considerable research has been focused on the chemical conversion of CO2 into high-value-added carbon compounds, such as methanol, organic materials, and plastics (Liu et al., 2015, Li et al., 2016, Angamuthu et al., 2010, Darensbourg, 2007). However, owing to the low conversion efficiency, it has been pointed out that it cannot be an effective greenhouse gas abatement technology (Bourzac, 2017, Markewitz et al., 2012, Mikkelsen et al., 2010). Recently, aprotic (non-aqueous) metal-CO2 batteries have also been studied for the production of electrical energy using CO2 (Zhang et al., 2015, Qie et al., 2017, Hu et al., 2016, Al Sadat and Archer, 2016, Das et al., 2013). However, during the generation of electric energy, solid carbonate products accumulate on the surface of the electrode, which deteriorates the performance and discharge capacity. In addition, because CO2 is regenerated in the charging process, aprotic metal-CO2 batteries are not an efficient CCUS technology for utilizing and reducing CO2. Thus, we have devised a hybrid Na-CO2 battery that continuously produces electric energy and hydrogen simultaneously through efficient CO2 conversion with highly stable operation over 1,000 hr from the nature of spontaneous CO2 dissolution in an aqueous solution. We further show that unlike existing aprotic metal-CO2 batteries (Zhang et al., 2015, Qie et al., 2017, Hu et al., 2016, Al Sadat and Archer, 2016), the proposed system does not regenerate CO2 during the charging process. Therefore, this hybrid Na-CO2 cell, which adopts efficient CCUS technologies, not only utilizes CO2 as the resource for generating electrical energy but also produces the clean energy source, hydrogen.

Results and Discussion

The Proposed Hybrid Na-CO2 Cell and Its Reaction Mechanism

A schematic illustration of the proposed hybrid Na-CO2 cell is presented in Figure 1. The digital photographs of the system are also presented in Figure S1. This system could work continuously with Na metal and CO2 as fuel at the anode and feedstock gas at the cathode, respectively. Na is regarded as a promising candidate as a substitute for Li in terms of its electrochemically similar behavior along with low cost (30 times cheaper than Li) from natural abundance and environmental friendliness (Noorden, 2014, Kwak et al., 2015). The Na metal anode is kept in an organic electrolyte to prevent a direct corrosion from an aqueous electrolyte separating by Na super ionic conductor (NASICON) membrane. The overall reaction mechanisms are composed of a chemical reaction and an electrochemical reaction.

Figure 1

Schematic Illustration of Hybrid Na-CO2 System and its Reaction Mechanism

The chemical reaction of CO2 dissolution mechanism is as follows:

When CO2 is purged into an aqueous solution (e.g., distilled water, seawater, NaOH solution), CO2 dissolution proceeds and carbonic acid (H2CO3(aq)) is formed through the hydration of CO2 (Equation 1). For a standard state condition in pure water, this spontaneous chemical equilibrium of CO2 hydration is determined by the hydration equilibrium constant (Kh = 1.70 × 10-3) (Housecroft and Sharpe, 2005). Then, the carbonic acid dissociates into HCO3- and H+ determined by the first acid dissociation constant (Ka1 = 4.46 × 10-7), shown in Equation 2 (Harris, 2010). Because carbonic acid is a polyprotic acid dissociating multiple steps, an in-depth understanding of CO2 dissolution requires that the second acid dissociation step, i.e., HCO3-(aq) ⇌ CO32-(aq) + H+(aq) (Ka2 = 4.69 × 10-11), be considered (Harris, 2010). However, the second acid dissociation constant is significantly smaller than the first (Ka1 ≫ Ka2), making it negligible in calculating the proton concentration. Thus, when CO2 dissolved in water, it acidifies the aqueous solution and HCO3-(aq) is predominant over CO32-(aq). The concentration of carbonate ions when CO2 dissolves in water at normal atmospheric pressure is provided at Table S1. The mole fractions of carbonate ions depending on the pH of solution is shown in Figure S2.

The electrochemical reactions are composed of anodic reaction of sodium metal oxidation (Equation 3) and cathodic reaction of hydrogen evolution (Equation 4):

Then, the electrochemical net equation is simply given as the oxidation of Na metal and the spontaneous evolution of hydrogen (Equation 5). Because the potential of cathodic reaction is closely influenced by the pH of aqueous solution, the dissolution of CO2 renders a favorable electrochemical reaction environment by acidifying the aqueous solution.

Half-Cell Configured Electrochemical Analysis

The cathodic electrochemical profiles were closely examined using a cyclic voltammetry (CV) technique on the Pt electrode (Figure 2A). An apparent cathodic peak in O2-saturated NaOH was observed near of −0.1 V versus Ag/AgCl, which could be ascribed to an oxygen reduction reaction (ORR) on the Pt electrode (Park et al., 1986, Kim et al., 2016). When ORR occurred, a diffusion-controlled region was found near of −0.2 V and a limiting current was observed due to the typical O2 mass transfer limitation in ORR profiles (Kim et al., 2016, Bu et al., 2017). At the lower potential, a cathodic peak corresponding to hydrogen evolution reaction (HER) was observed around −0.95 V in O2- and N2-saturated conditions (Mahmood et al., 2017, Xu et al., 2016, Ahn et al., 2018). Meanwhile, in the case of CO2-saturated condition, hydrogen evolution occurs more positively by 0.35 V due to the higher concentration of H+. In addition, HER profiles, contrary to ORR profiles, presented sharply increasing cathodic curves without a mass transfer limitation. For depth analysis, the kinetics of these electrochemical reactions were interpreted by an analysis of the Tafel slope (Figure 2B). Because ORR is one of the most complex electrochemical reactions, involving 4 electrons with 2 reactants (O2 and H2O), the reaction kinetics is sluggish, even on a state-of-the-art Pt electrode, with a value of 78 mV dec.−1. However, HER only involves 2 electrons with 1 reactant (H+ or H2O depending on the pH) and thus presented a low Tafel slope of 48 mV dec.−1 near the onset potential. Furthermore, the Tafel slope is more decreased to 28 mV dec.−1 at an activation-controlled Tafel region, indicating a highly efficient cathodic reaction. Furthermore, the cathodic CVs and the corresponding Tafel plots were investigated in seawater (Figures 2C and 2D). Likewise, it has been confirmed that CO2 dissolution in seawater provides the electrochemically favorable environment toward HER. The hydrogen evolution potential based on pH is described in Figures 2E–2I. These electrochemical profiles have significant implications; the less corrosive environment of the quasi-neutral condition (pH ∼ 7) could potentially allow the adoption of abundant and non-noble-metal-based electrocatalysts. Thus, notably, this combined cathodic reaction not only utilizes CO2 to generate H2 but also possesses highly efficient reaction kinetics, possibly overcoming the key issue of sluggish discharge rates for common metal-air batteries (Wang et al., 2014).

Figure 2

Half-Cell Configured Electrochemical Analysis

(A) Cathodic CV profiles measured in O2-, N2-, and CO2-saturated 0.1 M NaOH at 10 mV s−1, using Pt as the working and counter electrode and Ag/AgCl electrode as the reference electrode. A reference potential is described with Ag/AgCl instead of reversible hydrogen electrode (RHE) for the clarification of potential difference with respect to purging gases and pH.

(B) Tafel analysis of the cathodic profiles.

(C) Cathodic CV profiles measured in O2- and CO2-saturated seawater.

(D) Corresponding Tafel plots.

(E–I) (E) Schematic diagram of hydrogen evolution potential related to pH. RHE calibration profile corresponding to hydrogen evolution potential measured in (F) 0.1 M NaOH, (G) CO2-saturated 0.1 M NaOH, (H) seawater, and (I) CO2-saturated seawater.

Performance and Stability of Hybrid Na-CO2 Cell

The actual working performance of a hybrid Na-CO2 cell is evaluated using a composite of Pt/C and IrO2 (Pt/C + IrO2) as a catalyst. Figure 3A presents the chronopotentiometric discharge profiles at a current density of 50–200 mA g−1 under N2- or CO2-saturated 0.1 M NaOH. Discharging CV profiles measured in various gas-saturated conditions were also investigated, and three distinctive reduction peaks were found, as observed in the half-cell CV profiles (Figure S3). These findings confirmed that the dissolution of CO2 led to a favorable HER environment in both NaOH solution and seawater. The full discharge profile was investigated in a CO2-saturated NaOH solution (Figure 3B) with a mechanical recharge by replacing the Na metal anode. As shown in Figure 3B, the highly stable operation over 1,000 hr was achieved because only a gas phase H2(g) was produced during the discharge process, suggesting the similar nature of fuel cell systems (Park et al., 2000, Sengodan et al., 2015, Yang et al., 2009). Also, the full discharge profile measured under CO2-saturated seawater presented a highly stable operation over 500 hr (Figure 3C). In other words, there is no deposition of solid discharge products that possibly causes clogging or physical damage on the electrode as examined from scanning electron micrographs and X-ray diffraction (XRD) patterns (Figures 3D–3G). In contrast, conventional aprotic metal-CO2 batteries have exhibited typical clogging phenomenon by the deposition of solid M2CO3(s) (M = Li or Na), Al2(C2O4)3(s), or MgCO3(s) on the surface of the electrode (Zhang et al., 2015, Qie et al., 2017, Hu et al., 2016, Al Sadat and Archer, 2016, Das et al., 2013), which results in a performance drop with limited capacity. A comparison of the capacities of various batteries is provided in Table S2. Furthermore, the pH of the CO2-saturated NaOH solution after the 1,000-hr operation was investigated and determined to be 6.62, indicating that the pH of the solution is stably maintained over 1,000 hr (Figure S4). The produced gas during operation was analyzed by gas chromatography (GC), which confirms that this system generates only H2, as expected from Equation 4, during the discharge process (Figure S5). To identify a soluble product, the aqueous solution was freeze-dried and obtained in the form of a white powder (the inset of Figure S6). The XRD patterns of the white powder identifies it as pure NaHCO3 (Figure S6), commonly known as baking soda. It is notable that the continuous enrichment of NaHCO3(aq) in the aqueous media from the discharge does not affect the discharge performance, as shown in the 1,000-hr discharge profile (Figure 3B). Therefore, CO2 gas has been successfully captured and converted to baking soda. The additional XRD profiles of the powder obtained through different drying processes are provided in Figure S7. Furthermore, we investigated the practical CO2 conversion efficiency through quantitative GC analysis. As shown in Figure S8, the practical efficiency of CO2 conversion during the discharge reaction was determined to be 47.7%. Although this value is lower than the theoretical conversion rate, it is meaningful in that it proves the additional CO2 dissolution during the discharge process. The detailed discussion is available in Supplemental Information.

Figure 3

Performance and Stability of Hybrid Na-CO2 Cell

(A) Chronopotentiometric potential profiles on the hybrid Na-CO2 system under various current densities. Discharge processes are conducted in CO2- and N2-saturated 0.1 M NaOH to observe the effects of CO2 dissolution.

(B and C) (B) The chronopotentiometric discharge profile of Pt/C + IrO2 catalyst at 200 mA g−1 in CO2-saturated 0.1 M NaOH. (C) Discharge profile of hybrid Na-CO2 system measured in CO2-saturated seawater. Surface observation of carbon felt cathode before and after test.

(D–F) (D) Scanning electron micrograph of carbon felt before and (E) after discharge in 0.1 M NaOH and (F) after discharge in seawater.

(G) XRD profiles of carbon felt electrode before and after discharge in 0.1 M NaOH and seawater.

Reversibility of Hybrid Na-CO2 Cell

To confirm the reversibility of hybrid Na-CO2 cell, the anodic charge profile (electrolysis profile) was observed. Because Na is one of the most abundant elements on earth, Na metal anode could be easily recycled through a charging process in Na-ion-containing aqueous solution, such as seawater. Figure 4A shows an oxidation rotating disk electrode profile for examining whether CO2 was reproduced during the charging process. Generally, the charging process is regarded as the opposite reaction of the discharging reaction. In this work, however, the generated H2 gas from the discharging process is naturally removed on the surface of electrode, and thus the oxidation reaction proceeds as the oxygen evolution reaction (OER) from the water oxidation (Equation 6).

Figure 4

Reversibility of Hybrid Na-CO2 Cell

(A) Anodic rotating disk electrode profile of Pt/C + IrO2 catalyst measured in CO2-saturated 0.1 M NaOH and seawater at 10 mV s−1, using Pt as a counter electrode and Ag/AgCl electrode as a reference electrode.

(B) Discharge-charge profiles measured in three-electrode configuration using Ag/AgCl reference electrode at 100 mA g−1.

(C) Charge-discharge profiles at various current densities under CO2-saturated 0.1 M NaOH and seawater.

(D) Cyclic charge-discharge performance measured in CO2-saturated 0.1 M NaOH and seawater at a current density of 200 mA g−1 for 700 hr.

The oxidation curve corresponding to OER (Kim et al., 2016, Bu et al., 2017) was observed in a CO2-saturated NaOH solution near 1.0 V versus Ag/AgCl (from the Nernst equation, the OER potential can be calibrated by 0.0592 V × pH). In addition, the qualitative GC profiles indicate that O2 was generated during the oxidation process (Figures S9 and S10). We further investigated the oxidation profiles in seawater, which presents the typical chlorine evolution reaction (Kim et al., 2015) instead of OER (Figure 4A). It is noteworthy that the charging process does not generate CO2, which had already been consumed during discharge, as opposed to the conventional metal-CO2 battery system, which emits CO2 during the charging process (Zhang et al., 2015, Qie et al., 2017, Hu et al., 2016, Al Sadat and Archer, 2016). The discharge-charge performance of this system was evaluated in the three-electrode configuration using Ag/AgCl reference electrode to closely distinguish the potential applied on the cathode and anode (Figure 4B). Since a cell potential (Ecell) is defined as a potential difference of cathode and anode (Ecathode − Eanode), the potential gap decreases during discharging and increases during the charging process. On repeating the discharge-charge process, the cathode potential profile (Ecathode) presents discharging and charging plateau, clearly proving that this system is rechargeable. Furthermore, the charge-discharge profiles at various current densities under CO2-saturated NaOH solution and seawater are examined as shown in Figure 4C. Cyclic charge-discharge performance was evaluated to verify its reversibility and reproducibility (Figure 4D). Both cyclic performances were highly reproducible and obtained without variations over a period of 700 hr, indicating that H2 is stably produced utilizing CO2 and that the cathode was kept fresh, without clogging or damage, during a repeating discharge and charge process.

In summary, we have devised hybrid Na-CO2 cell utilizing CO2 as a useful resource. This new system has three distinctive advantages. First, it uses a kinetically fast HER as a discharge reaction thanks to a spontaneous CO2 dissolution, enabling the provision of high current compared with the present aprotic system. Second, unlike conventional aprotic CO2 batteries, wherein solid products are clogged on the electrodes, this system can continuously produce gas-phase hydrogen during discharge without damaging the electrode. This ability enabled highly stable performance to be achieved over 1,000 hr. Third, the proposed system has the unprecedented great advantage of not regenerating CO2 while recycling Na metal through charging process. Therefore, this hybrid Na-CO2 cell truly fulfills the purpose of a real CCUS technology, as it consumes CO2 efficiently throughout the process. This novel system could potentially serve as a new CO2 utilization technology and a stepping stone for the future utilization of renewable energy technologies.

Limitations of Study

We have devised hybrid Na-CO2 cell utilizing carbon dioxide as a useful resource. Although we have utilized HER as the facile cathodic reaction rather than ORR in aqueous electrolyte, we could not exclude the fact that the discharge reaction of hybrid Na-CO2 cell is relatively slow because of the low conductivity of the ceramic NASICON electrolyte, which can allow only Na+ ions to pass through. The present work indicates the novel hydrogen generation technology from CO2 utilization and is meaningful in that it proves the additional CO2 dissolution during the discharge process, but further work is required to improve the CO2 conversion efficiency and power densities of the hybrid Na-CO2 cell.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.