Recent Development of CO2 Electrochemistry from LiâCO2 Batteries

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Recent Development of CO2 Electrochemistry from Li−CO2 Batteries to Zn−CO2 Batteries Jiafang Xie and Yaobing Wang*

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CAS Key Laboratory of Design and Assembly of Functional Nanostructures and Fujian Key Laboratory of Nanomaterials, State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China CONSPECTUS: Metal−CO2 batteries with CO2 as cathode active species give rise to opportunities to deal with energy and environmental issues simultaneously. This technology is more appealing when CO2 is flexibly reduced to chemicals and fuels driven by surplus electricity because it represents a low-cost and controllable approach to maximized electricity utilization and value-added CO2 utilization. Nonaqueous metal−CO2 batteries exhibited high discharge voltage and capacity with carbon and oxalate as reduction products from CO2 electrochemistry that lacks proton. In contrast, aqueous Zn−CO2 batteries implemented flexible CO2 electrochemistry for more value-added products accompanied by energy storage based on a proton-coupled electron transfer mechanism. In this Account, we have exemplified our recent results in the development of CO2 electrochemistry from nonaqueous Li−CO2 batteries to aqueous Zn−CO2 batteries toward practical value-added CO2 conversion. Aimed at the challengingly limited CO2 electrochemistry and high cost of nonaqueous Li−CO2 batteries, we proposed aqueous Zn−CO2 batteries. Our previous works on nonaqueous Li−CO2 batteries, aqueous Zn−air batteries, and aqueous CO2 reduction electrocatalysts further shed light on battery mechanism, device construction, and electrocatalyst design. For example, bipolar membranes maintain the stability of the basic anolyte and neutral catholyte, as well as the kinetics of ion transport at the same time, forming the device base for aqueous Zn−CO2 batteries. Moreover, in terms of the electrocatalyst catalyzing both discharge and charge reactions on the cathode, the design of multifunctional electrocatalysts is of great importance for not only CO2 electrochemistry but also spontaneous discharge and energy efficiency of aqueous Zn− CO2 batteries. We have explored a series of multifunctional electrocatalyst cathodes, including noble metal, transition metal, and metal-free materials, all of which facilitated CO2 electrochemistry in aqueous Zn−CO2 batteries with value-added carbon-based products. Meanwhile, several operating models for practical complicated situations are presented, such as rechargeable, reversible, dual-model, and solid-state batteries. Zn−CO2 batteries with different models require different design mechanisms for electrocatalyst cathodes. Reversible aqueous Zn−CO2 batteries with HCOOH generation were enabled by electrocatalysts capable of catalyzing the interconversion of CO2 and HCOOH at low overpotentials, rechargeable aqueous Zn−CO2 batteries were allowed by electrocatalysts capable of catalyzing efficient CO2 reduction and O2 evolution, and dual-model aqueous Zn− CO2 batteries were realized by electrocatalysts capable of catalyzing CO2 reduction, water oxidation, and oxygen reduction. Concluding remarks include a summary of recent CO2 electrochemistry in metal−CO2 batteries and a brief discussion of future challenges and opportunities for practical aqueous Zn−CO2 batteries, such as highly reduced products and high production rate.

1. INTRODUCTION The increasing demand for energy supplies and limited reserves of fossil fuels cause a challenging conflict for sustainable human society. New energy technologies, especially renewable ones, are being profusely developed to partially replace fossil fuels.1 Electrochemical energy conversion and storage devices are thus of importance to maximize the utilization efficiency of generated electricity derived from not only surplus fossil fuel and nuclear energy but also renewable energy sources. On the other hand, continued dependence of the chemical industry on fossil fuels and increasing anthropogenic CO2 emissions also cause a challenge to sustainable human society.2,3 New CO2 utilization technolo© XXXX American Chemical Society

gies, especially ones with value-added products and driven by renewable energy, are being developed. Electrochemical CO2 splitting cells can implement value-added CO2 conversion with surplus electricity but require a timely electricity supply. Metal−CO2 batteries are thus important and advantageous to address both issues via providing surplus electricity storage and value-added CO2 conversion in one device, as shown in Figure 1. Metal−CO2 batteries derived from the study of metal−O2 batteries toward practical application in air. Metal−O2 batteries Received: April 11, 2019

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DOI: 10.1021/acs.accounts.9b00179 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Schematic of possible energy conversion and storage systems with Zn−CO2 batteries. Green lines are involved with matter transformation in outer space and discharge process in inner space. Blue lines are involved with energy transfer in outer space and charge process in inner space. Zn−CO2 batteries can store surplus electricity derived from different kinds of energies and also produce kinds of chemicals and fuels derived from flexible CO2 electrochemistry in aqueous electrolyte.

have been appealing in recent decades for their high energy densities and inexhaustible cathodic active species O2 in air.4 However, when supplied with real air, metal−air batteries suffer from heavily decreased capacity and stability compared to those operated in pure O2, due to insulating carbonate formation from CO2 in real air. After the finding that nonaqueous Li−O2 batteries showed 3-fold enhanced capacity with a certain level of CO2 addition in the O2 supply gas,5 Archer’s group realized the first primary Li−CO2 batteries with temperature-depended discharge capacity.6 During discharge, CO2 was fixed to mainly Li2CO3 and C driven by Li dissolution accompanied by electricity output. Furthermore, Zhou’s group realized the first rechargeable Li−CO2 batteries with high discharge capacity at room temperature based on cathodes designed to promote insulating Li2CO3 decomposition.7 Afterward, a booming growth of rechargeable metal−CO2 batteries represented by rechargeable Li−CO2 batteries was seen with the participation of increasing groups.7−17 Current Li−CO2 batteries with advanced cathodes have achieved high operating voltage, high energy density, and stable cyclability over dozens of cycles.10 However, due to limited CO2 electrochemistry without the participation of proton, only limited discharge products have been realized in nonaqueous Li−CO2 batteries, including C, carbonate, oxalate, and CO. Moreover, hydrogenated carbon-based products, such as hydrocarbons, acids, and alcohols that form the basis of the modern chemical industry could not be produced from nonaqueous Li−CO2 batteries without proton-assisted CO2 electrochemistry. In theory, water is a perfect medium to provide proton to enable flexible CO2 electrochemistry. Actually, plenty of noble metal, transition metal, and metal-free carbon-based electrocatalysts have already been demonstrated to allow CO2

electrochemistry with tunable products in aqueous CO2 electrolysis.18,19 In addition, water is a lower-cost and more environmentally friendly solvent than organic ones. The aqueous Zn−CO2 battery system, which takes advantage of the proton-coupled electron transfer mechanism to promote flexible CO2 electrochemistry was thus proposed and realized. Based on cathodes designed to electrocatalyze CO2 reduction at low overpotentials, CO and HCOOH have been generated from aqueous Zn−CO2 batteries.20 Moreover, several models of secondary Zn−CO2 batteries have also been realized based on multifunctional catalyst cathodes, including noble metals, transition metals, and metal-free carbon materials.21−23 The primary objective of this Account is to motivate more studies on aqueous Zn−CO2 battery system to better promote low-cost, value-added, and tunable CO2 conversion via flexible CO2 electrochemistry in energy conversion and storage devices. The thermodynamics and kinetics of CO2 electrochemistry in nonaqueous Li−CO2 batteries are fundamentally different from those in aqueous Zn−CO2 batteries, leading to differences in battery reactions as well as different demands for electrolyte and catalyst cathode. In this Account, we are going to summarize recent developments in CO2 electrochemistry from nonaqueous Li−CO2 batteries to aqueous Zn−CO2 batteries, including battery reaction mechanisms and newly developed catalyst cathodes, aiming at shedding light on how CO2 electrochemistry works for value-added CO2 conversion in metal−CO2 batteries. We also outline the opportunities and challenges of value-added CO2 electrochemistry in aqueous Zn−CO2 batteries at very early stages, highlighting production rate and product selectivity that need to be optimized. B


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2. CO2 ELECTROCHEMISTRY IN Li−CO2 BATTERIES

promoting Li2CO3 decomposition and (2) producing other products with lower decomposition potentials. 2.2.1. Li2CO3 and C Production. When rechargeable Li− CO2 batteries are discharged with Li2CO3 and C products, they suffer a difficult CO2 electrochemistry because (1) both Li2CO3 and C are solid and (2) insulating Li2CO3 results in high charge voltage. Plenty of catalyst cathodes with high surface area to accommodate solid products and electrocatalytic activity for Li2CO3 decomposition have been developed in recent years. Rechargeable Li−CO2 batteries with metal-free graphene cathodes showed a charge voltage of ∼4.2 V, a discharge capacity of 14774 mAh g−1 (at 50 mA g−1), and stable cyclability over 20 cycles.25 Further Li−CO2 batteries with Cu dispersed nitrogen doped graphene cathodes achieved a discharge capacity of 14864 mAh g−1 (at 200 mA g−1) and a stable cyclability over 50 cycles. 26 Meanwhile, Li−CO2 batteries with Ru/Ni foam cathodes further increased the cyclability over 100 cycles with a charge voltage below 4.1 V.27 Though there is still discussion about how CO2 is activated and further reacts, it is an appealing consensus that CO2 in nonaqueous Li−CO2 batteries reacts as in aprotic CO2 electrolysis (eqs 3−6): one CO2 molecule is first reduced to CO2− with one-electron transfer and then reacts with another CO2−. The formed C2O42− further reacts with CO2, leading to finally C and Li2CO3 formation step by step.28

2.1. Primary Li−CO2 Batteries

2.1.1. Li2CO3 and C Production. The first Li−CO2 battery with a pure CO2 supply was realized in primary mode, equipped with ionic liquid electrolyte and conductive carbon cathode. The primary Li−CO2 battery showed a discharge voltage of 2.6 V and a discharge capacity of 2500 mAh g−1 at moderate temperature (Figure 2a).6 From combined experimental results and theoretical analysis (Figure 2b), the discharge reaction for primary Li−CO2 batteries was proposed as eq 1. 4Li + 3CO2 → 2Li 2CO3 + C

(1)

Figure 2. Primary Li−CO2 batteries. (a) Galvanostatic discharge curves of Li−CO2 batteries at 0.05 mA cm−2. (b) The comparison of theoretical equilibrium potential with actual discharge potential. Reprinted with permission from ref 6. Copyright 2013 The Royal Society of Chemistry.

2.1.2. CO Production. At the same time, the authors ruled out the possibility of CO generation due to the unmatched voltage of the corresponding reaction (Figure 2b), which was conquered in one of our recent studies. This unique primary Li−CO2 battery with CO generation was realized with a designed porous fractal (PF)-Zn catalyst cathode in tetraethylene glycol dimethyl ether solvent (Figure 3).24 The

CO2 + e− → CO2−

(3)

CO2− + CO2− → C2O4 2 −

(4)

C2O4 2 − + CO2 → 2CO32 − + C

(5)

2CO32 − + 2Li+ → Li 2CO3

(6)

The battery reaction of rechargeable Li−CO2 batteries with Li2CO3 and C products could thus be described as eq 7. 4Li + 3CO2 ↔ 2Li 2CO3 + C

2.2.2. Oxalate Production. Oxalate was demonstrated to be formed in rechargeable Li−CO2 batteries with a designed Mo2C/CNT catalyst cathode.29,30 In these batteries, generated C2O42− was stabilized by the Mo2C catalyst rather than further reacted to form carbonate and C, leading to a rechargeable Li− CO2 battery with a charge voltage lower than 3.4 V and a stable cyclability of 40 cycles. The reaction was described as eq 8.

Figure 3. Li−CO2 batteries with PF-Zn catalyst cathodes. (a) Discharge voltages and (b) CO generation of the discharge process. Reprinted with permission from ref 24. Copyright 2018 The Royal Society of Chemistry.

2Li + 2e− + 2CO2 ↔ Li 2C2O4

(8)

In addition to Li−CO2 batteries, Al−CO2 electrochemical cells also showed oxalate formation with additive O2 activating CO2.31,32 Also, Li−CO2/O2 batteries that were supplied with a small amount of O2 were also reported with O2 activating CO2 while having carbonates as discharge products.15,33 In summary, CO2 electrochemistry in nonaqueous Li−CO2 batteries enables nonaqueous Li−CO2 batteries with high operating voltages and capacity as well as moderate cyclability. They go through reaction pathways without proton assistance, leading to limited products, including C, carbonate, CO, and oxalate, which inhibits the development of metal−CO2 battery techniques toward efficient CO2 utilization with value-added products.

designed PF-Zn catalyst cathode was able to electrocatalyze highly selective CO2-to-CO reduction. The assembled Li−CO2 batteries showed relatively high discharge voltages up to 1.9 V and selective CO generation up to 67% Faradaic efficiency. The discharge reaction in this novel Li−CO2 battery was described as eq 2. 2Li + 2CO2 → Li 2CO3 + CO

(7)

(2)

2.2. Rechargeable Li−CO2 Batteries

Due to the high decomposition potential of insulating Li2CO3, two solutions with diverse CO2 electrochemistry have been developed to realize rechargeable Li−CO2 batteries: (1) C


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Figure 4. Schematic of an aqueous Zn−CO2 battery with bipolar membranes and a catalyst cathode. (a) During discharge Zn anode dissolution provides the driving force for CO2 reduction on cathode. (b) During charge, Zn deposits on the anode and CO2 or O2 forms on the cathode with input energy.

3. FUNDAMENTALS TO REALIZE FLEXIBLE CO2 ELECTROCHEMISTRY IN METAL−CO2 BATTERIES Compared to nonaqueous solvents that allow only electron transfer pathways, aqueous electrolytes provide a protoncoupled electron transfer pathway to implement variable CO2 reduction to value-added carbon-containing products, including CO, C2H4, HCOOH, and C2H5OH (eqs 9−12).34,35 In addition, water media is more accessible, greener, and lower cost. Therefore, aqueous metal−CO2 batteries become a promising system to realize flexible CO2 electrochemistry. Taking (1) spontaneous reaction between Li and water and (2) mature aqueous Zn−air battery technology into consideration, an aqueous Zn−CO2 battery system was thus proposed as a candidate to realize metal−CO2 batteries with flexible CO2 electrochemistry. CO2 + 2e− + 2H+ → CO + H 2O

molecule further reacts. For example, CO2 reduction to C2H5OH is considered to go through 12 elementary reactions, each of which contains a proton-coupled electron transfer and has an individual overpotential. Besides overpotential, product selectivity is another kinetics problem. Competition with hydrogen evolution (eq 16) causes an initial selectivity barrier, and a more complicated barrier comes from competition among various CO2 reduction products due to scaling relationships. CO2 + e− → CO2•−

2H+ + 2e− → H 2

E θ = − 0.11 V

E θ = − 0.20 V (10)

2CO2 + 12e− + 12H+ → C2H4 + 4H 2O E θ = 0.07 V

(11)

2CO2 + 12e− + 12H+ → C2H5OH + 3H 2O E θ = 0.08

V

Zn(OH)4 2 − + 2e− → Zn + 4OH−

Eθ = 0 V

(15) (16)

In order to address these kinetics problems to realize flexible CO2 electrochemistry in aqueous Zn−CO2 batteries, cathodes of aqueous Zn−CO2 batteries are primarily required to electrocatalyze aqueous CO2 reduction with (1) low overpotential to guarantee the spontaneous discharge and (2) high selectivity to value-added CO2 conversion in aqueous Zn−CO2 batteries. Numerous electrocatalysts for aqueous CO2 reduction have been developed in recent decades accompanied by mechanism studies on the relationship of the structure and properties, which facilitate the design of catalyst cathodes for aqueous Zn−CO2 batteries.18,36 Second, when aqueous Zn− CO2 batteries are designed to be rechargeable, the cathode is required to have one more catalytic property to enhance the final energy efficiency and the safety of batteries. To realize rechargeable Zn−CO2 batteries with O2 evolution reaction during charge, bifunctional catalyst cathodes toward CO2 reduction and O2 evolution at low overpotentials are desired. Specifically, to realize reversible Zn−CO2 batteries, bifunctional catalyst cathodes toward CO2 reduction and evolution at low overpotentials are required. In addition, conductivity, porosity, and hydrophobicity are basic demands of cathodes to assemble aqueous Zn−CO2 batteries. Additionally, anodic Zn dissolution prefers a basic solution for higher solubility while CO2 intermediately reacts with alkali. Actually, aqueous CO2 electrolysis commonly occurs in neutral solution to avoid not only side reactions with alkali in basic solution but also enhanced hydrogen evolution in acidic solution. This contradiction could be conquered by applying a bipolar membrane, which presents proton and hydroxyl to two sides separately. Thus, not only the stability of both basic anolyte and neutral catholyte but also the inner conductivity between the two compartments of aqueous Zn−CO2 batteries could be ensured.37

(9)

CO2 + 2e− + 2H+ → HCOOH

E θ = − 1.90 V

(12)

E θ = −1.20 V (13)

From the theoretical perspective, the metallic Zn anode provides a driving force of 1.2 V under standard conditions (eq 13), and standard potentials of CO2 reduction to CO, HCOOH, C2H4, and C2H5OH are all around 0 V, enabling a thermodynamic possibility of spontaneous discharge of aqueous Zn−CO2 batteries, in which metallic Zn dissolution at the anode drives CO2 reduction at the cathode (eq 14): θ θ θ Etheo = Ecθ − Eaθ = ECO − EZn = 0 − (− 1.2 V) = 1.2 V 2

(14)

From the kinetics perspective, however, the CO2 molecule holds a huge Gibbs free energy for activation, and thus a large overpotential is required to activate the CO2 molecule with one electron when no catalyst is applied (eq 15).35 Moreover, additional overpotentials occur when the activated CO2 D


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Accounts of Chemical Research Therefore, flexible CO2 electrochemistry in metal−CO2 batteries is available in both theory and practice in the form of aqueous Zn−CO2 batteries. In order to realize this proposal, rationally designed catalyst cathodes are essential.

4. CO2 ELECTROCHEMISTRY IN Zn−CO2 BATTERIES According to the above-mentioned principles and considerations, we could assemble a concept device for aqueous Zn− CO2 batteries (Figure 4) based on experiences with aqueous Zn−air batteries, nonaqueous Li−CO2 batteries, and aqueous CO2 electrocatalysts.24,38−40 By design of targeted single or multifunctional catalyst cathodes to tune CO2 electrochemistry, the aqueous Zn−CO2 battery system realized value-added CO2 conversion and several operating modes that could be potentially applied under diverse conditions. 4.1. Primary Aqueous Zn−CO2 System

Figure 5. Reversible aqueous Zn−CO2 batteries with Pd catalyst cathode. (a) Partial current density of HCOO− with competitive H2 during three-electrode CO2 reduction on Pd catalyst cathode. (b) Cyclic voltammetry curves of neutral formic acid oxidation on Pd catalyst cathode with comparisons of bare carbon paper (CP). Results without HCOONa addition are also shown. (c) Discharge voltages and corresponding Faradaic efficiency of HCOO− formation in batteries. (d) Cyclability of batteries at 0.56 mA cm−2. The geometric area was applied in calculating current densities. Reproduced with permission from ref 23. Copyright 2018 Wiley.

Primary aqueous Zn−CO2 batteries carry out only the discharge process with CO2 reduction on a single-functional catalyst cathode. A Cu3P/C catalyst for selective CO2-to-CO conversion was used to assemble such a primary Zn−CO2 battery with a maximum power density of 2.6 mW cm−2 at 10 mA cm−2.20 Furthermore, primary Zn−CO2 electrochemistry was recently applied in a hybrid redox-medium-assisted system to implement electrochemical CO2 reduction driven by Zn dissolution, resulting in a discharge voltage of 0.2 V and a high Faradaic efficiency for CO generation of ∼90% at 5 mA cm−2.41 The key nano-Au catalyst grown on carbon paper was responsible for selective CO2-to-CO conversion at low overpotentials. In both cases, the battery reaction during discharge can be described as eq 17:

4.3. Rechargeable Aqueous Zn−CO2 Batteries

Rechargeable aqueous Zn−CO2 batteries can be realized with bifunctional catalyst cathodes that catalyze CO2 reduction reaction during discharge and catalyze O2 evolution reaction during charge. Though these two reactions have been studied on numerous catalysts in half cells separately, there was rare study focused on a bifunctional catalyst toward these two reactions. We developed a series of bifunctional catalyst cathodes, including noble metal, transition metal, and metalfree carbon-based materials, for rechargeable aqueous Zn− CO2 batteries. 4.3.1. Noble-Metal-Based Catalyst Cathode. A noblemetal-based catalyst cathode, Ir@Au, synthesized via a chemical-coupled electrochemical deposition method increased the benefits and suppressed the shortcomings of both iridium and gold.21 With abundant tips and porous structure, Ir@Au showed high catalytic activity for both selective CO2-to-CO conversion and neutral O2 evolution at low overpotentials in KHCO3 solution (Figure 6a,b). In contrast, the single-metal Au catalyst showed poor O2 evolution, and the single-metal Ir one favored HER under CO2 reduction conditions. Rechargeable aqueous Zn−CO2 batteries with Ir@Au catalyst cathodes showed superior CO generation up to 90% Faradaic efficiency at a discharge current of 1.5 mA (Figure 6c) accompanied by the maximum discharge voltage of 0.74 V at 0.01 mA during discharge and achieved up to 68% energy efficiency. Also, the battery showed a stable cyclability over 90 cycles (Figure 6d). 4.3.2. Transition-Metal-Based Catalyst Cathode. A transition-metal-based catalyst cathode, Ni and P codoped graphene (NiPG), synthesized via a two-step doping process enhanced the advantages and suppressed the shortcomings of both Ni−N groups and P,N-codoped carbon materials.22 NiPG presented similarly high catalytic properties toward selective CO2-to-CO conversion to those of Ni−N group

Zn + 4OH− + CO2 + 2H+ → Zn(OH)4 2 − + CO + H 2O

(17)

4.2. Reversible Aqueous Zn−CO2 Batteries

When bifunctional catalyst cathodes are designed to catalyze a reversible reaction, namely, CO2 reduction during discharge and CO2 evolution during charge, reversible aqueous Zn−CO2 batteries could be realized. We reported such a reversible aqueous Zn−CO2 battery based on a key interconversion between CO2 and HCOOH on a bifunctional Pd catalyst cathode.23 Based on three-dimensional dendrite structure derived from porous nanosheets, the Pd catalyst cathode synthesized by hydrogen evolution-assisted electrodeposition promoted not only selective CO2-to-HCOO− conversion (∼90%) but also neutral formic acid oxidation to CO2 with high activity and selectivity, both of which occurred at low overpotentials (Figure 5a,b). Reversible aqueous Zn−CO2 batteries with Pd catalyst cathode exhibited stable and high Faradaic efficiencies of HCOO− production during battery discharge (Figure 5c). Also, the battery showed a stable cyclability over 100 cycles and a high energy efficiency of 81.2% (Figure 5d). The battery reaction could be described as eq 18. Zn + 4OH− + CO2 + 2H+ ↔ Zn(OH)4 2 − + HCOOH (18)

Particularly, reversible aqueous Zn−CO2 batteries provide not only a long-term chemical production process (discharge) but also a highly efficient energy storage process (charge). E


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Figure 6. Rechargeable aqueous Zn−CO2 batteries with Ir@Au catalyst cathode. (a) Partial current density of CO and competitive H2 during three-electrode CO2 reduction on Ir@Au. The single metal Au and Ir are also shown as comparison. (b) Neutral O2 evolution linear sweep voltammetry curves of Ir@Au with Ir/C and Au as comparison. (c) CO and competitive H2 generation in batteries. (d) The cyclability of batteries at 5 mA cm−2. The geometric area was applied in calculating current densities. Reproduced with permission from ref 21. Copyright 2019 Wiley.

Figure 7. Rechargeable aqueous Zn−CO2/O2 batteries with NiPG catalyst cathode. (a) Partial current density of CO and competitive H2 during three-electrode CO2 reduction on NiPG. (b) O2 evolution linear sweep voltammetry curves of NiPG in 1 M KOH. (c) O2 reduction linear sweep voltammetry curves of NiPG in 0.1 M KOH. NiG and PG are shown as comparison. (d) CO and competitive H2 generation during discharge of rechargeable aqueous Zn−CO2 batteries with NiPG catalyst cathode. (e) Cyclability of Zn−CO2 batteries, discharging at 0.5 mA and charging at 0.25 mA. (f) Cyclability of Zn−O2 batteries at 0.2 mA. The geometric area was applied in calculating current densities. Reprinted with permission from ref 22. Copyright 2019 Royal Society of Chemistry.

doped graphene (NiG) and meanwhile apparently higher catalytic properties toward O2 evolution and O2 reduction than P,N-codoped graphene (PG) (Figure 7a−c). In contrast, NiG showed worse catalytic properties toward O2 reaction, and PG showed little catalytic benefit for CO2 reduction but enhanced H2 evolution under CO2 reduction conditions. A dual-model rechargeable aqueous Zn−CO2/O2 battery was thus realized with the NiPG catalyst cathode. When CO2 was supplied, the battery worked under the Zn−CO2 model with Faradaic efficiency for CO2-to-CO conversion higher than 50% over a wide range of discharge current density (Figure 7d). Also, the battery showed stable cyclability over 60 cycles (Figure 7e). When the supply gas was switched to air or O2, the battery converted to the Zn−O2 model and also presented stable cyclability over 200 cycles (Figure 7f). Particularly, this dual-model rechargeable Zn−CO2/O2 battery shows strong promise for applications in changeable and complicated scenarios. 4.3.3. Metal-Free Carbon-Based Catalyst Cathode. A newly developed metal-free catalyst cathode, Si and N codoped carbon (SiNC), synthesized by self-sacrificial template-assisted pyrolyzation is an excellent bifunctional catalyst superior to Sidoped carbon (SiC) and N-doped carbon (NC).42 With the highest surface area, electrochemical active surface area, and inherent activity of active sites, SiNC showed the highest activity toward both CO2-to-CO conversion and neutral O2 evolution at low overpotentials (Figure 8a,b). Rechargeable aqueous Zn−CO2 batteries with SiNC catalyst cathodes showed selective CO generation with over 50% Faradaic efficiency among discharge current densities from 0.4 to 1.2 mA cm−2 (Figure 8c). In addition, the battery exhibited stable cyclability over 15 h (Figure 8d). Particularly, the bifunctional metal-free carbon-based catalyst cathodes realize low-cost rechargeable aqueous Zn−CO2 batteries. Furthermore, a tridoped SiNFC catalyst cathode was further synthesized by self-sacrificial template-assisted pyrolyzation.43

Figure 8. Rechargeable aqueous Zn−CO2 batteries with metal-free SiNC catalyst cathode. (a) Partial current density of CO and competitive H2 during three-electrode CO2 reduction on SiNC. (b) Neutral O2 evolution linear sweep voltammetry curves of SiNC. SiC and NC are also shown as comparison. (c) CO generation and voltage during the battery discharge. (d) Cyclability of batteries discharging at 0.75 mA and charging at 0.25 mA. The geometric area was applied in calculating current densities. Reproduced with permission from ref 42. Copyright 2018 Wiley.

F


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Accounts of Chemical Research With silicon, nitrogen, and fluorine tridoping, SiNFC showed ultrahigh and stable selectivity toward CO2-to-CO conversion at a wide range of low overpotentials (Figure 9a). At the same time, SiNFC exhibited much enhanced activity toward neutral O2 evolution at low overpotentials (Figure 9b).

Charge process: Zn(OH)4 2 − + H 2O → Zn + 1/2O2 + 2H+ + 4OH− (20)

Overall battery reaction: CO2 → CO + 1/2O2

(21)

5. CONCLUSIONS AND PERSPECTIVES In this Account, we have highlighted the development of CO2 electrochemistry from nonaqueous Li−CO2 batteries to aqueous Zn−CO2 batteries. Moreover, we demonstrated how electrocatalyst cathodes influenced CO2 electrochemistry in metal−CO2 batteries. By reviewing selected studies that systematically designed electrocatalyst cathodes, we have presented a number of models for aqueous Zn−CO2 batteries that possibly drive practical metal−CO2 batteries under diverse conditions with value-added and low-cost CO2 conversion via flexible CO2 electrochemistry. However, metal−CO2 batteries and their flexible CO2 electrochemistry are still at the very early stage. In order to promote practical aqueous Zn−CO2 batteries with flexible CO2 electrochemistry toward a higher value-added process, we provide several strategies toward challenges in the following: • Product diversity of Zn−CO2 batteries. In the reviewed examples, CO and HCOOH have been achieved. However, higher value-added chemical feedstocks, including CH3OH, C2H5OH, and C2H4, are desired to be directly generated from metal−CO2 batteries.31 The product selectivity mostly depends on the catalytic properties of catalyst cathodes. Cu-based catalysts show the most promise because Cu moderately binds with the key intermediates CO and H.44,45 In addition, several non-Cu catalysts also succeeded though with limited current densities.18

Figure 9. Rechargeable aqueous Zn−CO2 batteries with metal-free SiNFC catalyst cathodes.43 (a) Faradaic efficiency of CO and competitive H2 during three-electrode CO2 reduction on SiNFC. (b) Neutral O2 evolution linear sweep voltammetry curve of SiNFC in 0.1 M KHCO3. SiNC is also shown as comparison. (c) CO and competitive H2 generation in batteries. (d) The cyclability of batteries at 0.5 mA. (e) Schematic of solid-state Zn−CO2 battery with metalfree SiNFC catalyst cathode. (f) Voltage and CO generation during battery discharge. The geometric area was applied in calculating current densities.

• Production rate of Zn−CO2 batteries. In the reviewed examples, aqueous Zn−CO2 batteries often discharge at current densities ≤10 mA cm−2 to ensure high efficiency of CO2 reduction, leading to limited production rate. To achieve a higher production rate, the key is to realize high-rate and high-efficiency CO2 reduction on the catalyst cathode at low overpotential. Newly developed abrupt interface construction with sufficient CO2 supply and concentrated electrolyte would facilitate this process.46,47

Rechargeable aqueous Zn−CO2 batteries with SiNFC catalyst cathodes showed ultrahigh CO generation with Faradaic efficiency over 85% at discharge current ranging from 0.1 to 1.5 mA cm−2 (Figure 9c). Also, the battery showed stable cyclability over 7 h (Figure 9d). Moreover, SiNFC allows us to assemble a solid-state Zn−CO2 battery (Figure 9e). The solid-state battery could realize discharge spontaneously with over 50% Faradaic efficiency of CO generation at discharge current up to 1 mA cm−2 (Figure 9f). Bifunctional metal-free carbon-based catalyst cathodes further realize a safer, low-cost, and flexible solid-state Zn−CO2 batteries for value-added CO2 conversion. In total, with these bifunctional catalyst cathodes toward CO2 reduction and O2 evolution, the reaction of the rechargeable aqueous Zn−CO2 battery system could be described as eq 19−21: Discharge process:

Beyond those mapped in this Account, there are more factors influencing CO2 electrochemistry in metal−CO2 batteries that should be taken into consideration, particularly for aqueous Zn−CO2 batteries. Overall, we claim that the aqueous Zn−CO2 battery system provides a more practical approach for low-cost, value-added, and controllable CO2 utilization via flexible CO2 electrochemistry.

■

Corresponding Author

*E-mail: [email protected].

Zn + CO2 + 2H+ + 4OH− → Zn(OH)4 2 − + CO + H 2O

AUTHOR INFORMATION

ORCID (19)

Yaobing Wang: 0000-0001-6354-058X G


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Accounts of Chemical Research Author Contributions

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The manuscript was written through contributions of both authors. Both authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Jiafang Xie received her Ph.D. degree from the University of Science and Technology of China (P. R. China) in 2016. She is now an Associate Professor at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (P.R. China). Her research is focused on electrocatalysts for CO2 reduction and their applications in energy conversion and storage devices. Yaobing Wang is a Professor at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (P.R. China). He received his Ph.D. degree from the Institute of Chemistry, Chinese Academy of Sciences (P.R. China), in 2008. His research is focused on design and synthesis of novel electrocatalysts and their application in energy conversion and storage, etc.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21872147, 21501173, 21805277, and 21601190), the Natural Science Foundation of Fujian Province (No. 2018J05030), National Key R&D Program of China (No. 2016YFB0100100), and the Strategic Priority Research Program, CAS (No. XDB20000000).

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