Improving cycle stability and kinetics of rechargeable aluminum–CO₂ batteries using functional cathode materials

Abstract

The aluminum–carbon dioxide (Al–CO₂) battery has been demonstrated as a rechargeable system capable of delivering high discharge voltage and capacity. Unfortunately, this secondary battery faces challenges with low cycle stability. This work is focused on the enhancement of the Al–CO₂ battery performance through the optimization of gas diffusion electrodes (GDEs). We developed and characterized a novel GDE, designated as KBGR311 GDE, composed of three parts ketjenblack and one part graphene. By implementing high surface area and high conductivity carbon-based materials (ketjenblack and graphene), significant improvements were observed in current density and cycle stability of this battery. The investigation of hydrophobicity and active surface area between cathode–electrolyte interfaces created the fundamental knowledge necessary to optimize material properties, enhance electron mobility, and improve CO₂ diffusion. The contact between electrolyte–electrode and the high surface area of the material are crucial to reduce mass transport resistance. Significant improvements were observed in the current density and cycle stability of the Al–CO₂ battery when evaluating the new improved GDE design. During cyclic voltammetry tests, the new GDE demonstrated over five times increase in discharge current density and over four times improvement in cyclability. These results offer valuable insights into material properties and electrode design, demonstrating that the KBGR311 GDE offers a promising advancement optimizing both surface area and CO₂ diffusion kinetics for next-generation Al–CO₂ battery applications.

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Introduction

Ever increasing carbon dioxide (CO₂) emissions are contributing to global warming, leading to more extreme weather changes, food scarcity, and wildlife extinction.^1–3 CO₂ production has continued increasing through the years and even with the big break we gave Earth in 2020 during COVID-19, the production of CO₂ did not show a drastic reduction.^4–6 Greenhouse gases from fossil fuel combustion are blamed for being the main contributor to global warming. However, energy serves as a cornerstone for sustained economic growth and insufficient access to energy leads to poverty and lack of technological advancement. Thus, environmental issues such as global warming and ozone layer depletion are linked to economic expansion.^1,7 Data analysis reveals that the implementation of more ambitious targets significantly reduce emissions intensity, therefore we need to work on a solution for the capture and conversion of CO₂ from the atmosphere.^8–10 One promising technology for the electrochemical conversion of CO₂ is the rechargeable aluminum–CO₂ battery.^11,12 Aluminum is available in regions all over the world, and as the third most abundant element in the Earth's crust it is an attractive anode material for electrochemical conversion of CO₂.^13,14 Aluminum has a high specific energy density of 2980 A h per kg, which means that its electrochemical conversion by CO₂ may also be used to generate large quantities of electrical energy.^15–17 In comparison to other anode materials that can be used for battery technologies, including lithium, sodium, and potassium, electrochemical Al–CO₂ systems can be cheaper and safer due to the low cost and low reactivity of aluminum.^14,18–21 Therefore, rechargeable Al–CO₂ batteries can be a promising low cost and safe alternative.

The charging and discharging of this Al–CO₂ battery involves a chemical conversion reaction instead of an intercalation reaction that happens in the state-of-art lithium-ion batteries.^22,23 The discharge process involves aluminum oxidation at the anode and CO₂ reduction at the cathode to form aluminum oxalate (Al₂(C₂O₄)₃) as a discharge product. On the cathode side, CO₂ is dissolved in the electrolyte and with aluminum iodide (AlI₃) acting as a redox mediator, the reversibility of the battery is achieved.^11,17 However, this rechargeable Al–CO₂ battery experiences limited cycle stability due to the continuous buildup of the solid discharge product Al₂(C₂O₄)₃. Improving the cathode structure of the battery is a potential solution to increase its cyclability. Therefore, it is important to understand the fundamentals of CO₂ diffusion to design a gas diffusion electrode that can improve the performance of the aluminum–CO₂ battery.

Several research studies have focused on understanding the fundamentals of CO₂ transport in GDEs in electrochemical cells. For example, Weng et al. proposed a multiphysics model to understand the transport and electrochemical reaction kinetics of CO₂ using a GDE. This study demonstrates how the use of GDEs can improve the mass transfer resistance by providing a high surface area for fast CO₂ reduction (CO₂R) and how wettability of the GDEs can affect the performance of the system.^24–27 Therefore, exploring different pore sizes, hydrophobicity, and surface area can lead to the fabrication of an optimized GDE that can increase the cycle stability and kinetics of the battery.^{25,26,28–30} Moreover, the understanding of the capillary effect based on the pore size and the role hydrophobicity plays during the battery performance can be used to identify the optimal conditions for the most efficient high surface area structure material for CO₂R and electron mobility.^24,31–33

The objective of the research is to show significant improvements in CO₂ kinetics while using high surface area, three-dimensional and two-dimensional materials to fabricate these GDEs.³⁴ In this study, we developed and characterized a novel GDE, designated as KBGR311 GDE, composed of three parts ketjenblack (KB) and one part graphene (GR). Multi-point BET surface area analysis revealed that the KBGR311 GDE achieved a surface area of 126.594 m² g⁻¹. Moreover, electrochemical surface area evaluations in symmetric cells further confirmed the superior active surface area of the KBGR311 GDE at 1186 cm², in comparison to the KBGR211 GDE and the KB GDE. Furthermore, scanning electron microscopy analysis provided insights into its morphology, highlighting the increased mass loading of the amorphous ketjenblack structure and the support layer structure of the graphene. This structural mixture between the amorphous ketjenblack and crystalline graphene underscores the design's success in balancing high surface area with the mechanical stability of the GDE. These results offer valuable insights into material properties and electrode design, demonstrating that the KBGR311 GDE offers a promising advancement optimizing both surface area and CO₂ diffusion kinetics for next-generation Al–CO₂ battery applications, paving the way for advancements in energy storage and carbon utilization. We found that the contact between electrolyte–electrode and the high surface area of the material are crucial to reduce mass transport resistance. Significant improvements were observed in the current density and cycle stability of the Al–CO₂ battery when evaluating the new improved GDE design. During cyclic voltammetry tests, the new GDE demonstrated over five times increase in discharge current density and over four times improvement in cyclability. Our work demonstrates that the fabrication of an efficient GDE that can support faster mass transport and kinetics of CO₂ will be achieved when utilizing porous cathode materials with high surface area.

Results and discussion

Initially, the hydrophobicity test with water using a goniometer/tensiometer (ramé-hart model 260) was performed on carbon paper without any coating, ketjenblack coated on carbon paper, and the ketjenblack–graphene mixture coated on carbon paper (Fig. 1a–c) showing contact angles Θ ≈ 151°, Θ ≈ 148°, and Θ ≈ 139°, respectively. Additional tests over the same surfaces were performed using the [EMIMCl] + AlCl₃ with 0.05M AlI₃ electrolyte (Fig. S2a–c) to identify indeed that the greatest surface wetting was achieved when the carbon paper substrate is coated with the ketjenblack–graphene mixture. Following that, two battery assembly configurations were tested using cyclic voltammetry to understand the contact impact between the electrolyte and cathode. Fig. 1d, the set-up where the ketjenblack coating faces the electrolyte (KB facing IL), reveals how electrolyte wetting promotes sufficient contact for CO₂ dissolution in the electrolyte and enhances the kinetics of the cathode active material (Fig. 1f and g).^26,35,36 For the second setup, the cathode was flipped, letting the ketjenblack coating face the CO₂ gas and the carbon paper face the electrolyte (Fig. 1e). This setup was tested to explore the possibility of a higher conversion of CO₂ happening due to higher surface area facing the cathode active material.^37,38 However in this case the high hydrophobicity of the carbon paper reduced the contact between the electrolyte and cathode, resulting in a decrease in the battery performance (Fig. 1f and g). It is observed that the current density at 1.28 V is lower when the carbon paper is facing the electrolyte (CP facing IL), in comparison to the KB facing IL profile (Fig. 1f). In addition, charge density calculations reveal a much larger electrical charge of 2394 coulombs when the ketjenblack coating is facing the electrolyte, indicating the importance of interfacial contact between the electrolyte and electrode (Fig. S3a and b). The electrochemical impedance spectroscopy (EIS) evaluations at the real part of the impedance (Re(Z)) also demonstrate a larger resistance at the low frequency point for the CP facing IL profile (Fig. 1g), suggesting that the hydrophobicity of the material affects the contact between electrolyte–electrode therefore increasing the interfacial resistance.

Fig. 1 Contact angle measurements, DI water on the substrate. (a) Carbon paper without any coating, (b) ketjenblack coated on carbon paper, and (c) ketjenblack–graphene mixture coated on carbon paper. Schematic of Al–CO₂ battery components in two different set-ups: (d) schematic of the battery when the cathode with the ketjenblack coating faces the electrolyte and (e) schematic of the battery when the cathode with non-coated carbon paper is facing the electrolyte. (f) Cyclic voltammetry (CV) profiles from 0.5 to 1.6 V at 0.1 mV s⁻¹. (g) Electrochemical impedance spectroscopy (EIS), inset: impedance model.

Cyclic voltammetry tests with a 0.1 mV s⁻¹ scan rate from 0.5 V to 1.6 V were performed to compare the cycle stability and current densities of the Al–CO₂ battery when using various GDEs (Fig. 2a). The original setup, ketjenblack coated on carbon paper (KB coated), displayed current densities of 0.14 mA cm⁻² and 0.016 mA cm⁻² during charge and discharge respectively for 7 cycles. The ketjenblack and graphene coated on carbon paper (KBGR221 coated) profile, displayed current densities of 0.19 mA cm⁻² and 0.03 mA cm⁻² during charge and discharge respectively for 15 cycles. However, further improvement was obtained by casting the ketjenblack and graphene on both sides of the carbon paper (KBGR221 double sided coating), resulting in almost four times discharge improvement of the current density (0.06 mA cm⁻²) and over two times cyclability (15 cycles) improvement against the original profile (Fig. S5a). The observed increase in current peak intensity and shift in peak position indicate enhanced electrochemical activity and faster reaction kinetics associated with CO₂.³⁹ SEM characterization was conducted to further investigate the structure of this newly developed and the pristine GDE. Fig. 2b illustrates that the amorphous structure of the ketjenblack only gets deposited on the carbon fibers, generating many voids in the GDE. In contrast, the mixture of ketjenblack and graphene coated on carbon paper (Fig. 2c) demonstrates that graphene fills the voids, creating a structural foundation for ketjenblack deposition. Graphene serves as a robust support layer in the GDE, providing structural integrity and preventing the degradation and detachment of the ketjenblack throughout the charge–discharge cycle (Fig. S7). This stability enhances electron transport and maintains CO₂ diffusion, ultimately improving the efficiency and durability of the electrochemical CO₂ reduction reaction and contributing to superior battery performance.

Fig. 2 (a) Cyclic voltammetry examinations of an Al–CO₂ battery using various GDEs. The blue profile corresponds to a battery with a ketjenblack coated GDE, the black profile corresponds to a battery with a ketjenblack and graphene coated GDE, and the red profile corresponds to a battery with ketjenblack and graphene coated on both sides of the GDE. Scanning electron microscopy (SEM) characterization: (b) carbon paper coated with ketjenblack and (c) carbon paper coated with ketjenblack and graphene.

To compare the surface area of the various gas diffusion electrodes, multi-point BET surface area and N₂ adsorption isotherms were recorded on the ketjenblack coated carbon paper (KB GDE) and the double-sided ketjenblack and graphene coated sample (KBGR221). The surface area for the KB GDE is 75.210 m² g⁻¹ and the surface area for the KBGR221 GDE is 35.762 m² g⁻¹ (Fig. S4a and b). Building on the fundamental studies of hydrophobicity and the surface area characteristics of the amorphous structure of ketjenblack and the crystalline structure of graphene, a new GDE was developed with an increased mass loading of ketjenblack to enhance the material's surface area while maintaining the support layer properties of graphene. This new GDE, composed of three parts ketjenblack and one part graphene, will hereafter be referred to as the KBGR311 GDE. In comparison to the other two samples, the KBGR311 GDE exhibits the highest surface area of 126.594 m² g⁻¹ (Fig. S4c). The N₂ adsorption isotherms follow the same pattern as the BET surface area results, confirming the consistency of the surface area measurements, showing a surface area of 87.981 m² g⁻¹, 34.559 m² g⁻¹, and 139.539 m² g⁻¹ for the KB GDE, KBGR221 GDE, and the KBGR311 GDE respectively (Fig. 3a–c).^40,41 In addition, electrochemical surface area evaluations in symmetric cells highlighted the active surface area of the GDEs, demonstrating that the KBGR311 GDE indeed possesses a high surface area of 1186 cm², compared to 215 cm² for the KBGR211 GDE and 580 cm² for the KB GDE.^42–44 These quantitative results were obtained by using CV data (Fig. 3d) and equation which correlates the relationship between current and voltage across a system. When a potential is applied, a non-faradaic current flows due to the charging or discharging of this double layer capacitance; this is represented as I, and corresponds to the average charge and discharge current peaks. The capacitive current I is related to the rate of change of the scan rate ; this relationship allows for the estimation of the double-layer capacitance by measuring the capacitive current at a given scan rate. Furthermore, to identify the electrochemical active surface area (ECSA), equation was used,^45–48 where C_dl represents the double layer capacitance and C_s represents the specific capacitance of carbon. To further investigate the morphology and structure of the new KBGR311 GDE, SEM was conducted, confirming that the increased mass loading of the amorphous ketjenblack was successfully achieved (Fig. 3e). Moreover, the reduced content of graphene in the material continued to function as a support layer for the ketjenblack amorphous structure, enabling the formation of larger ketjenblack deposits within the GDE (Fig. 3e and f). These high surface area results from the BET and ECSA are consistent with the SEM results, indicating how the increased mass loading of ketjenblack can improve the surface area of the material.

Fig. 3 (a) N₂ adsorption and desorption isotherms of the ketjenblack coated sample (KB). (b) N₂ adsorption and desorption isotherms of the ketjenblack and graphene 2 : 2 : 1 coated sample (KBGR221). (c) N₂ adsorption and desorption isotherms of the ketjenblack and graphene 3 : 1 : 1 coated sample (KBGR311). (d) Electrochemical active surface area involving the ketjenblack coated sample (KB), ketjenblack and graphene 2 : 2 : 1 coated sample (KBGR221), and ketjenblack and graphene 3 : 1 : 1 coated sample (KBGR311). (e and f) Scanning electron microscopy (SEM) characterization for the ketjenblack and graphene 3 : 1 : 1 coated sample at different magnifications.

To investigate the conductivity of the GDEs, four-point probe evaluations were implemented. The true sheet resistance for all samples was adjusted using the National Bureau of Standards Technical Note 199.^49,50 The KB, KBGR221, and KBGR311 GDEs exhibited conductivities of 10.138 S m⁻¹, 11.071 S m⁻¹, and 10.909 S m⁻¹ respectively. These conductivity measurements revealed distinct differences attributed to the GDE material compositions, highlighting how the KBGR221 GDE possesses a higher electrical conductivity in comparison to the KBGR311 and the KB GDE. These conductivity differences are attributed to the appropriate graphene content in the GDEs, allowing the improvement of charge transport properties in the cathode. It is hypothesized that the amorphous structure of ketjenblack mixed with the crystalline structure of graphene allows for CO₂ diffusion and improvement of the current density of the material.

To further investigate the Al–CO₂ battery performance with the mixed ketjenblack and graphene GDEs, a cyclic voltammetry test was conducted to compare the redox peaks and cyclability between KBGR311 GDE and KBGR221 GDE. CV profiles of the KBGE311 GDE revealed an improvement in charge and discharge current densities in comparison to the KBGR221 GDE, having current densities of 0.23 mA cm⁻² for the charge and 0.08 mA cm⁻² for discharge (Fig. 4a). The difference in the oxidation peak position trends observed in Fig. S5 between the batteries with KBGR311 and KBGR221 GDEs is attributed to the extent of cathode clogging by the discharge product Al₂(C₂O₄)₃ (Fig. S7d–f). In the case of the KBGR221 GDE (Fig. S5a), more pronounced clogging likely occurs due to its lower surface area, which can hinder mass transport and lead to greater accumulation of Al₂(C₂O₄)₃. This buildup affects the electrochemical environment at the electrode surface, resulting in a noticeable shift in the oxidation peak. In contrast, the KBGR311 GDE possesses a much higher surface area facilitating a better discharge product removal, leading to a more stable peak position throughout the 31 cycles (Fig. S5b), which is double the cyclability of the KBGR221 GDE. These CV results using this newly developed cathode demonstrate that improving the mass loading of ketjenblack in the GDE can improve the battery performance by improving the kinetics of CO₂, while the lower content of graphene serves as a support layer, allowing longer cyclability of the battery. Moreover, cycle stability tests conducted at a high current density of 100 mA g⁻¹ over 45 cycles demonstrated distinct performance characteristics among the KBGR221 GDE and the KBGR311 GDE. The KBGR311 GDE exhibited superior discharge capacity and stability, maintaining consistently high capacities throughout the cycles, which are attributed to its high surface area that enhances CO₂ dissolution and conversion efficiency. The KBGR221 GDE exhibited moderate stability in comparison to the KBGR311 GDE, displaying much faster discharge capacity decay throughout the 45 cycles (Fig. 4b–d). Both KBGR221 and KBGR311 GDEs showed suppressed discharge capacities in the first cycle, a phenomenon linked to electrochemical and structural stabilization of the cathode and electrolyte (Fig. 4b). The KB GDE exhibited complete failure at a high current density of 100 mA g⁻¹ due to defoliation of the ketjenblack material, rendering it unsuitable for any sustained performance. The incorporation of an external CO₂ tank in the battery assembly ensured a consistent and ample CO₂ supply, which was critical for supporting the high discharge capacities observed in the KBGR221 and KBGR311 GDEs over the 45-cycle test period. These results highlight the KBGR311 GDE's robustness for high-current electrochemical applications, driven by its optimized cathode structure and enhanced CO₂ utilization. Ultimately, the electrochemical evaluations indicate how increasing the surface area can improve CO₂ diffusion, increasing the performance of the Al–CO₂ battery with respect to current and cyclability.

Fig. 4 (a) Cyclic voltammetry test comparison between KBGR221 and KBGR311. (b) Discharge capacity vs. cycle number for 45 cycles using KBGR221 and KBGR311 GDEs. (c) Galvanostatic charge–discharge curves at 100 mA g⁻¹ using the KBGR221 GDE. (d) Galvanostatic charge–discharge curves at 100 mA g⁻¹ using the KBGR311 GDE.

Experimental

Gas diffusion electrode preparation

A slurry is prepared utilizing a ball mill machine to mix ketjenblack EC-600JD, graphene nanopellets, and polyvinylidene fluoride (PVDF) in weight ratios of 2 : 2 : 1 and 3 : 1 : 1, respectively. Then this slurry is cast on both sides of a Toray carbon fiber composite paper (030 – TGP-H-030) with a 15-micrometer thickness. As a last step, the GDE is placed in a vacuum oven for 12 hours at 120 degrees Celsius. Then, the cathodes are cut to a diameter of 1.27 cm using an arch punch. The main reason for utilizing graphene in this study was to exploit the high surface area and high conductivity of the material.

Swagelok battery cell assembly

The aluminum–CO₂ cells are assembled in an argon environment using a glovebox. The components of the battery are a 0.25 mm thickness mechanically polished aluminum foil anode, a glass microfiber separator, a 100 μL aluminum chloride 1-ethyl-3-methylimidazolium chloride [EMIMCl] + AlCl₃ with 0.05 M aluminum iodine (AlI₃) electrolyte, and a ketjenblack and graphene GDE. After the battery is assembled, CO₂ is injected into the Swagelok cell through a mass flow controller connected to a CO₂ gas cylinder. The CO₂ purging time is 35 minutes, the gas residence time process is necessary to ensure that CO₂ is present as an active cathode material in the system. Then, a resting period of 12 hours is applied prior to testing the battery.

Characterization

Hitachi S-5200, Helios NanoLab 650, and JEOL JSM-IT800HL Scanning Electron Microscopes (SEMs) were used to understand the morphology of the mixed material (ketjenblack and graphene) in comparison with the original material (ketjenblack). In addition, contact angle measurements were performed using a goniometer/tensiometer (ramé-hart model 260) to understand the hydrophobicity of these materials, and then electrochemical tests were conducted to identify performance degradation with respect to the contact between the cathode and electrolyte. An Autosorb iQ High Vacuum Physisorption instrument was utilized to determine the surface area of the various GDEs.

Electrochemical measurements

A Biologic electrochemical potentiostat VSP3 was employed to perform Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). CV measurements were conducted to assess the reversibility, current densities, and cycle stability of the battery with the new GDE, using 0.1 mV s⁻¹ scan rates in a voltage window from 0.5 V to 1.6 V. EIS was used to evaluate system resistance in relation to the material's hydrophobicity, using a frequency range from 200 kHz to 100 mHz. In addition, using symmetric cells in 0.5 M sulfuric acid electrolyte, electrochemical active surface area measurement was performed to evaluate the active surface area of the various GDEs, using a scan rate of 50 mV s⁻¹ and a carbon double layer capacitance of 20 μF cm⁻².

Conclusions

We fabricated and tested a cathode or gas diffusion electrode for Al–CO₂ batteries. We found that the KBGR311 GDE can improve the cycle stability of the rechargeable Al–CO₂ battery. To understand some of the fundamental mechanisms of the cathode, multiple tests were then conducted to evaluate the cathode and identify improvements in the battery performance. Hydrophobicity evaluations conclude that the hydrophobicity of a material reduces the current density of the battery due to less contact between the electrolyte and cathode. Additionally, it identified that the wettability of the cathode is necessary to promote CO₂ diffusion and enhance kinetics of the cathode active material. The integration of a ketjenblack–graphene double-sided coating has shown remarkable improvements in current density and cycle stability of the Al–CO₂ battery. SEM characterization revealed the structural advantage of the graphene allowing filling on the voids and creating a robust foundation for ketjenblack deposition. This study demonstrates the successful development of the KBGR311 GDE, an optimized gas diffusion electrode designed to enhance the performance of Al–CO₂ batteries. By increasing the mass loading of ketjenblack and while maintaining graphene as the structural support, the newly developed electrode achieved a significantly higher surface area and improved electrochemical activity within the Al–CO₂ system. Moreover, multi-point BET surface area analysis and electrochemical active surface area measurement confirmed that the novel KBGR311 GDE achieved the highest surface area in comparison to the other two GDEs (the KB GDE and the KBGR221 GDE). Furthermore, SEM analysis revealed that the increased mass loading of the amorphous ketjenblack, supported by a reduced yet effective graphene layer, enables greater ketjenblack deposition while maintaining structural integrity. These results highlight the KBGR311 GDE as a significant advancement in electrode design, offering high surface area to improved CO₂ diffusion while maintaining a graphene support layer to enhance electrode mobility. These findings are valuable insights into material properties, electrode design, and battery performance for promising next-generation Al–CO₂ battery applications, initiating future interest in energy storage and CO₂ utilization.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI.

Supplementary information: Electrolyte fabrication, leak testing, contact angle measurements, multi-point BET surface area analysis, four-point probe, defoliation test, Fig. S1–S7, and references. See DOI: https://doi.org/10.1039/d5se00608b.

Acknowledgements

This work was supported by the National Science Foundation: Award No. 2119688, and the New Mexico Consortium.

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