Research on the Influence of a Magnesium-Based Carbon Dioxide Battery System on CO₂ Storage Performance

Abstract

At present, the energy consumption and carbon emissions of maritime transportation have raised concerns about environmental issues. A potential way to reduce carbon emissions from vessels is the use of chemical-based carbon capture and storage (CCS) technology. However, this technology faces challenges such as high energy consumption, large space occupation, and high processing costs. Therefore, the development of a technology with low energy consumption and compact CO₂ storage is crucial to promote the advancement of CCS technology. This paper introduces a magnesium CO₂ battery system that converts CO₂ into new energy, in the form of hydrogen, while storing CO₂. By preparing highly efficient catalytic electrodes and testing the electrolyte and CO₂ flow rate on the battery performance, the optimal process parameters were determined to be Pd/CeO₂-oct for the electrodes, a 0.5 mol/L NaOH solution for the electrolyte, and a CO₂ flow rate of 1 L/h. The battery system demonstrated high cycling stability and conversion efficiency at a current density of 8 mA·cm⁻², with a stable cycling time of 600 min (20 cycles), a cathode hydrogen production of 10.135 mL, and a Faraday efficiency of 97.03%.

1. Introduction

With the rapid development of the global economy and the growth of the global population, anthropogenic energy consumption has increased dramatically. Much of the energy needed by humans is generated by chemical reactions, and much of the energy obtained through chemical reactions comes from fossil fuels. Fossil fuels are a primary energy source derived from the fossilized deposits of ancient organisms [1]. As fossil fuels are usually hydrocarbons, a large amount of carbon dioxide gas emissions are generated during the development and utilization of fossil energy, which causes increasingly serious environmental issues around the world, such as the greenhouse effect.

The transportation industry is a significant sector in society in terms of energy consumption and exhaust emissions. It has become the second-largest source of greenhouse gas emissions globally, with CO₂ emissions accounting for more than 20% of the world’s total carbon [2]. The demand for maritime transportation is increasing, leading to the expansion of the global shipping business. Consequently, the issue of vessel carbon emissions has become more prominent within the transportation industry. According to the statistics of the International Maritime Organization (IMO) [3], the global shipping CO₂ emissions reached 833 million tons in 2021, a 4.9% increase from the previous year, constituting 10.6% of the overall transport industry emissions. Therefore, vessel carbon emission control technology has become an inevitable trend in promoting green shipping [4,5]. Among these technologies, onboard CO₂ capture and storage (OCCS) technology is considered an important potential means of achieving large-scale CO₂ emission reductions in the future [6].

The OCCS system mainly consists of four units: capture, separation, liquefaction, and storage [7]. The liquefaction and storage units involve multiple process modules such as compression, cryogenic refrigeration, and storage tank. Hence, these two units take a lot of energy and space, accounting for 40% of the energy consumption and 60% of the space consumption for the entire CCS system. Moreover, when CO₂ trans-shipment is carried out during the port call of a ship, the liquid CO₂ stored during a voyage requires 10 h of trans-shipment time, which further cuts down on the convenience of the OCCS system. Therefore, the application of CCS based on the chemical absorption method [8] on short-haul small-sized ships, large container ships, and bulk carriers, where space for energy supply and arrangement is limited, has limited prospects. These challenges significantly impede the large-scale application of OCCS systems [9,10].

To solve the problems of the large-size and high energy consumption of an OCCS system, a type of CO₂ battery storage technology was introduced. A CO₂ battery utilizes the electrochemical reaction principles between metals and CO₂ to store CO₂ in a solid state without an external energy supply. During the CO₂ storage process, the CO₂ battery can produce H₂, which might be helpful for powering ships [11,12,13]. At present, the research and application of a metal–CO₂ battery in the vessel field are still in their initial stages [14], and many scholars are working to improve the performance of the CO₂ conversion rate, the battery stability, and the durability. They have shown that the direct factors that affect these properties are the cathode and anode materials of the battery [15]. Usually, the cathode/anode materials used for catalysis are mainly from precious metal Pt/C [16], among which is Pt, with scarce production rates and a high price. Palladium (Pd) is also an element of group VIII, like Pt, and it shares a similar electronic structure and adsorption energy for H⁺. It is possible to use Pd catalysis to replace Pt catalysis [17,18]. In order to reduce the palladium metal content and improve its catalytic activity, a composite catalytic design has been widely used in many catalytic reactions [19,20]. Current research efforts have found that the synergistic effect between a precious metal and the metal oxide interface may play an important role in catalysts’ electrocatalytic activity and stability. Therefore, metal oxides (such as transition metal oxides and others [21,22]) are used as a carrier to support the metal palladium, which has also become a significant research direction for the preparation of catalytic materials for hydrogen reactions [23,24]. Among the various metal oxides, cerium dioxide (CeO₂) is commonly used as a palladium carrier. On the one hand, cerium dioxide has a large number of hole structures and vacancies, which increase the specific surface area of the catalyst and the active site, affect the valence state of surface palladium and the electron cloud [25], and produce significant synergistic effects; on the other hand, cerium dioxide is insoluble in water and chemically stable, so it can achieve better stability in alkaline environments [26]. The design studies of palladium-based composites provide an effective strategy for optimizing the performance of battery cathode materials, thus improving the performance and stability of metal–CO₂ molecules.

In this paper, we introduce a Mg-CO₂ membrane-free cell system: the cathode is composed of Pd-based compounds, the anode is made of the metal Mg, and the electrolyte is an alkaline NaOH solution. During the operation of the battery system, the discharge reaction employs CO₂. The conversion efficiency and discharge capacity of the Mg-CO₂ membrane-free battery for CO₂ were determined through electrochemical performance tests. The cathode material was prepared using a simple hydrothermal method and impregnating a Pd/CeO₂ composite catalytic material. Microstructure, composition, and surface activity detection were used to examine the catalytic mechanism of Pd/CeO₂, which was combined with the Mg-CO₂ battery system to achieve a technology with low energy consumption and efficient CO₂ absorption and conversion. These are useful for the OCCS technology’s applications and promotion of technical methods and theoretical research.

2. Materials and Methods

The cavity of the Mg-CO₂ battery system was designed to be 240 × 110 × 140 mm, and it contained 2 L of the NaOH electrolyte. The two electrodes were designed to have a size of 70 × 70 × 4 mm and were encapsulated on a polypropylene plate in the electrolytic cell, maintaining at least 50 mm between the two electrodes. The cathode was a Pd/CeO₂ composite catalyst and the anode was a Mg metal both of them connected to the auxiliary reference electrode. One end of the panel contained the CO₂ air inlet, and the other end was the H₂ outlet, as shown in Figure 1. An external electrochemical workstation collected the performance indicators of the Mg-CO₂ battery, such as the constant flow discharge curve (CP), the open-circuit voltage–time curve (OCPT), and cyclic voltammetry (CV).

2.1. The Reaction Mechanism of the Mg-CO₂ Battery

To gain a comprehensive understanding of the Mg-CO₂ battery’s performance, this section introduces the reaction mechanism of the Mg-CO₂ battery. According to the chemical properties of each component in the Mg-CO₂ battery (mainly the chemical properties of the cathode, anode, and CO₂) and the existing research literature, the main electrochemical reactions during the discharge process of the Mg-CO₂ battery are the CO₂ dissolution reaction [27,28,29], the magnesium oxidation reaction, and the hydrogen precipitation reaction, and the details of each reaction are as follows:

CO₂ dissolution reaction: When CO₂ is continuously passed through the electrolyte solution, CO₂ will first dissolve and then undergo multi-step hydrolysis in the electrolyte solution. The specific steps and equilibrium constants are shown below (where ${\mathrm{k}}_{\mathrm{H}}$ is the phase equilibrium constant of the CO₂ dissolution process, ${\mathrm{p}}_{{\mathrm{k}}_{\mathrm{a}1}}$ is the dissociation constant when the first dissociation occurs, and ${\mathrm{p}}_{{\mathrm{k}}_{\mathrm{a}2}}$ is the second dissociation constant when the second dissociation occurs).

$${\mathrm{CO}}_2\left(\mathrm{gas}\right)\rightleftharpoons { \mathrm{CO}}_2\left(\mathrm{dissolved}\right), {\displaystyle \frac{\left[{\mathrm{CO}}_2\right]}{{\mathrm{p}}_{{\mathrm{CO}}_2}}}={\displaystyle \frac{1}{{\mathrm{k}}_{\mathrm{H}}}}, \left({\mathrm{k}}_{\mathrm{H}}=29.76 \mathrm{L}·\mathrm{atm}·{\mathrm{mol}}^{-1}\right)$$

(1)

$${\mathrm{CO}}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\rightleftharpoons {\mathrm{H}}_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)\left({\mathrm{k}}_{\mathrm{H}}=1.70\times {10}^{-3}\right)$$

(2)

$${\mathrm{H}}_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{HCO}}_3^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\left({\mathrm{p}}_{{\mathrm{k}}_{\mathrm{a}1}}=6.35\right)$$

(3)

$${\mathrm{HCO}}_3^{-}\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{CO}}_3^{2-}\left(\mathrm{aq}\right)+{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\left({\mathrm{p}}_{{\mathrm{k}}_{\mathrm{a}2}}=10.33\right)$$

(4)

Magnesium oxidation reaction: During the discharge process, the magnesium metal on the anode is continuously oxidized to divalent magnesium ions due to the continuous loss of electrons. The reaction, as well as its reaction potential, is shown below (where ${\mathrm{E}}_{\mathrm{a}}$ represents the overpotential of the anodic reaction).

$$\mathrm{Mg}\left(\mathrm{s}\right)\to {\mathrm{Mg}}^{2+}\left(\mathrm{aq}\right)+2{\mathrm{e}}^{+}\left({\mathrm{E}}_{\mathrm{a}}=-2.37\mathrm{V} \mathrm{vs}. \mathrm{SHE}\right)$$

(5)

Hydrogen precipitation reaction: During the discharge process, the electrolyte solution near the cathode precipitates hydrogen due to the continuous gain of electrons. The reaction and its reaction potential are shown below (where ${\mathrm{E}}_{\mathrm{c}}$ represents the overpotential of the cathode reaction).

$$2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+2{\mathrm{e}}^{+}\to {\mathrm{H}}_2\left(\mathrm{g}\right),\left({\mathrm{E}}_{\mathrm{c}}=-0.44\mathrm{V} \mathrm{vs}. \mathrm{SHE}\right)$$

(6)

Combining the magnesium oxidation reaction and the hydrogen precipitation reaction [30], the total reaction of the entire discharge process of the Mg-CO₂ battery as well as the theoretical potential of the battery can be obtained as follows (where ${\mathrm{E}}_{\mathrm{theo}}$ represents the potential difference in the positive and negative electrode reactions):

$$\mathrm{Mg}\left(\mathrm{s}\right)+2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{HCO}}_3^{-}\left(\mathrm{aq}\right) \to {\mathrm{H}}_2\left(\mathrm{g}\right)+\mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2 \left(\mathrm{aq}\right)$$

(7)

$${\mathrm{E}}_{\mathrm{theo}}={\mathrm{E}}_{\mathrm{c}}-{\mathrm{E}}_{\mathrm{a}}=-0.44\mathrm{V}-\left(-2.37\mathrm{V}\right)=1.93\mathrm{V}$$

(8)

From the theoretical potential of the Mg-CO₂ battery during discharge and the theoretical mass capacity of the magnesium metal (2205 mAh·g⁻¹) [31], the theoretical energy density (${\mathrm{ED}}_{\mathrm{Mg}}$) of the magnesium metal in the Mg-CO₂ battery can be calculated as follows:

$${\mathrm{ED}}_{\mathrm{Mg}}={\mathrm{C}}_{\mathrm{Mg}}·{\mathrm{E}}_{\mathrm{theo}}=2205 \mathrm{mAh}·{\mathrm{g}}^{-1}\times 1.92\mathrm{V}=4255 \mathrm{Wh}·{\mathrm{kg}}^{-1}$$

(9)

$${\mathrm{C}}_{\mathrm{Mg}}=2205 \mathrm{mAh}·{\mathrm{g}}^{-1}$$

(10)

2.2. Preparation of CO₂ Battery Materials

The chemicals used for the Mg-CO₂ battery are listed in

Table 1. The chemicals used for Mg-CO₂ batteries.

No.	Item	Chemistry	Specifications	Manufacturer
1	Palladium nitrate	Pd(NO3)2·2H2O	17.27 wt%	Guyan chem., Kunming, China
2	Cerium(III) nitrate	Ce(NO3)3·6H2O	AR	Macklin, Shanghai, China
3	Potassium hydroxide	KOH	95%	Macklin, Shanghai, China
4	Sodium hydroxide	NaOH	95%	Macklin, Shanghai, China
5	Carbon dioxide	CO2	99.999%	Peric, Handan, China
6	Nafion perfluorinated resin solution	-	5 wt%	Macklin, Shanghai, China
7	Ethanol	CH3CH2OH	99.5%	Macklin, Shanghai, China
8	Porous carbon nanotube	C	97.5%	Macklin, Shanghai, China
9	VulcanXC-72 Carbon Black	C	99.5%	CABOT, Boston, MA, USA
10	Magnesium flake	Mg	-	Huabei Metal, Wuxi, China
11	Nickel Foam Sheet	Ni	-	Suzhou Metal, Suzhou, China
12	Polyvinylpyrrolidone	PVP	AR	Macklin, Shanghai, China
13	Trisodium phosphate	Na3PO4	96%	Macklin, Shanghai, China

.

The Pd/CeO₂ cathode’s preparation was divided into three steps: the CeO₂ carrier’s preparation, the Pd/CeO₂ catalyst’s preparation, and the Pd/CeO₂ cathode’s coating.

(1)

CeO₂ carrier preparation

Ce(NO₃)₃·6H₂O, PVP, and NaOH were dissolved in 10 mL of deionized water in a molar ratio of 1:1:10, respectively. The Ce(NO₃)₃·6H₂O solution and the PVP solution were mixed homogeneously [30]. Then, the excess NaOH was added dropwise until a precipitate was formed. The mixture was stirred for 15 min to ensure a sufficient reaction. The mixture was transferred to a Teflon reactor, and the reactor was placed in an oven. The nanocubes of CeO₂ (CeO₂-cube) were obtained by reacting the solution at 180 °C for 12 h, and the nanorods of CeO₂ (CeO₂-rod) were obtained by reacting the mixture at 120 °C for 12 h. The Na₃PO₄ was added to the mixture at the molar ratio of PVP: Na₃PO₄ = 1:1 and reacted at 180 °C for 24 h to obtain the nano-octahedral CeO₂ (CeO₂-oct). The product was washed more than three times by filtration with the deionized water to remove the remaining ions and unreacted substances. The product was dried in an oven at 110 °C for 12 h to remove water and any residual organic matter. The products were calcined in a muffle furnace for 4 h at 450 °C.

(2)

Pd/CeO₂ catalyst preparation

A certain amount of CeO₂ carrier (CeO₂-cube, CeO₂-rod, and CeO₂-oct) powder was weighed. The corresponding mass of precursor Pd(NO₃)₂·2H₂O was weighed according to 1 wt% of the mass of CeO₂ by Pd. The CeO₂ carrier and the Pd(NO₃)₂·2H₂O precursor [31] were dissolved in the deionized water to form a mixture with continuous stirring for 12 h. Stirring was continued at 85 °C until a thick substance was formed, followed by drying the thick substance in an oven at 110 °C for 12 h. The dried material was removed from the oven for grinding and subsequently calcined in a muffle furnace at 450 °C for 2 h. Three different morphologies of Pd/CeO₂ catalysts (Pd/CeO₂-cube, Pd/CeO₂-rod, and Pd/CeO₂-oct) were obtained as nanocubes, nanorods, and nano-octahedra.

(3)

Pd/CeO₂ cathode coating

A total of 20 mg of Pd/CeO₂ catalyst (Pd/CeO₂-cube, Pd/CeO₂-rod, and Pd/CeO₂-oct) powder was taken. A total of 60 μL of Nafion and 600 μL of CH₃CH₂OH were added to the powder. Then, the mixture was ultra-sonicated for 1 h at 100 W to form a uniformly dispersed and turbid catalyst slurry. The catalyst was pipetted onto the 20 mm × 20 mm × 1 mm Ni foam electrode drop by drop, and the Ni foam electrode was irradiated with an infrared lamp at the same time. The coated Ni foam electrode was placed in an oven and dried at 65 °C for 720 min. Three Pd/CeO₂ cathodes with three different morphologies with a palladium loading of 50 μg/cm² were produced.

2.3. Characterization

The X-ray diffraction (XRD) patterns of the studied materials were analyzed using a Bruker D8 advance X-ray diffractometer with a Cu K radiation source (=1.54178 Å)(Bruker, Karlsruhe, Germany). The surface morphology, composition, elements, and their contribution were analyzed by transmission electron microscopy (TEM) with 120 kV JEOL JEM-2100F (Akishima, Tokyo, Japan). The Brunauer–Emmet–Teller (BET) spectrum was obtained with a Bruker Vertex 70 spectrophotometer within the range of 4000–400 cm⁻¹ (Bruker, Karlsruhe, Germany). The components of the produced gas were observed using a gas chromatograph (GC) with GC-2014 (Shimadzu, Kyoto, Japan).

3. Results and Discussion

3.1. The Effect of the Catalytic Electrode on the Conversion Efficiency of Mg-CO₂ Batteries with Molecules

The open-circuit voltage–time curve and constant discharge curve of Pd/CeO₂ cathode Mg-CO₂ batteries with three different morphologies (nanocubes, nanorods, and nano-octahedra) were determined at a CO₂ flow rate of 1 L/h and a NaOH solution concentration of 0.05 mol/L, as illustrated in Figure 2.

The operating time difference in the Mg-CO₂ battery systems with three different morphologies of Pd/CeO₂ catalytic electrodes as positive electrodes in the high-voltage range was not significant, all measuring around 2200–2500 s. However, the voltage peak of the nanorod Pd/CeO₂ cathode (Pd/CeO₂-rod) was significantly lower than that of the nanocube (Pd/CeO₂-cube) and nano-octahedral (Pd/CeO₂-oct) morphologies, while the voltage peak of the Pd/CeO₂-oct cathode was slightly higher than that of the Pd/CeO₂-cube. At the same time, the Pd/CeO₂-cube cathode had the fastest voltage response time, which was about half of that of the other two materials (Pd/CeO₂-rod and Pd/CeO₂-oct).

Under conditions of 50 mA and 75 mA constant current discharge (Figure 2b), the potential changes in the three cathodes were small, indicating that the electrocatalytic activity and stability of the three electrodes were relatively good. Among them, Pd/CeO₂-cube and Pd/CeO₂-rod had certain potential fluctuations [32]. In general, the Pd/CeO₂-oct electrodes had the fastest voltage response speed and the lowest open-circuit voltage, showing better battery performance. It is worth noting that, in subsequent experiments, the Pd/CeO₂ oct electrodes were used to study the performance of CO₂ batteries.

3.2. The Effect of the Electrolyte Concentration on the Conversion Efficiency of the CO₂ Molecules

The electrolyte concentration change was one of the factors affecting the ion transfer rate in the solution, and it affected the ability to transform the material [33]. To explore the trend in the effect of different electrolyte concentrations on the battery performance, a 0.5 mol/L NaOH solution at a high concentration, a 0.1 mol/L NaOH solution, a 0.05 mol/L NaOH solution, and a 0.1 mol/L NaCl solution without NaOH were configured (with NaCl enhancing the solution’s conductivity) for the CO₂ battery charge and discharge test. To obtain the NaHCO₃ time via calculation, as shown in

Table 2. The rates of CO₂ absorption by different concentrations of electrolyte.

NaOH Concentration (mol/L)	CO2 Flow Rate (L/h)	Complete Reaction Generation NaHCO3 Time (min)
0.5	1	107.52
0.1		21.50
0.05		10.75

, 0.1 mol/L and 0.05 mol/L NaOH quickly produced NaHCO₃ due to the low ion concentration.

Combined with the open-circuit voltage curve of the CO₂ battery test, as shown in Figure 3a, the incoming CO₂ and NaOH reacted at the beginning, the solution’s pH value was high, and the open-circuit voltage was low. The open-circuit voltage corresponding to the NaCl solution increased at the beginning. After a period of time, the pH value gradually decreased as NaOH transformed into NaHCO₃, with the open-circuit voltage increasing. And, the higher the NaOH concentration was, the longer the duration of its high-voltage phase. However, the above lowered the voltage value at the highest point of the curve. Therefore, in order to achieve the best battery performance, the concentration of the electrolyte should not be too high. Combined with the analysis of the constant discharge test in Figure 3b, the discharge curve of 0.5 mol/L NaOH was close to zero, and the discharge voltage was too low. Hence, it was not selected for further comparisons. In the reaction process of the primary battery, the function of NaOH was to provide the alkaline environment [34,35], accelerate ion transport, and improve CO₂ absorption. The hydrogen evolution reaction was the main reaction that occurred at the anode in the CO₂ battery system:

$$2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2\left(\mathrm{g}\right)$$

(11)

When the pH value of the solution was high, the concentration of hydrogen ions was very low, leading to the difficulty in discharging hydrogen ions and reducing the discharge voltage. Therefore, under the same current, the discharge curve of 0.05 mol/L NaOH showed the highest discharge voltage, meaning that it was the optimal electrolyte concentration of CO₂. The rates of CO₂ absorption by different concentrations of electrolyte are listed in Table 2.

3.3. The Effect of CO₂ Flow Change on the Conversion Efficiency of CO₂ Molecules

In the process of CO₂ battery conversion, the change in CO₂ flow had a certain impact on the stability of the battery and the duration of the high-voltage phase. Therefore, a CO₂ flow of 0.5 L/h, a CO₂ flow of 1.0 L/h, and a CO₂ flow of 1.5 L/h were selected to study the impact of flow on battery performance. The battery discharge curve in Figure 4 was thus obtained, calculating the battery’s NaHCO₃ generation time, as shown in

Table 3. The CO₂ absorption rates for different flows.

CO2 Traffic (L/h)	NaOH Potency (mol/L)	Complete Reaction Time to Generate NaHCO3 (min)
0.5	0.05	21.5
1		10.75
1.5		7.17

.

Contrary to the effect of the NaOH concentration on battery performance, it took longer for the NaOH reaction to produce NaHCO₃ when the flow rate of CO₂ was low. Additionally, the low decreasing rate of Ph increased the opening voltage. Hence, the battery performed better at a certain concentration of Ph, when it was in the high open-circuit voltage phase. At the same time, when the flow rate of CO₂ increased, it also caused battery voltage instability, to a certain extent. Due to the fast flow rate of CO₂, a large number of bubbles blocked the surface active site of the electrode, resulting in a decrease in the reaction rate. With the current increase, this phenomenon was more noticeable. It can be seen from the calculation results in Table 3 that the complete reaction time of the 1.5 L/h CO₂ flow was close to that of the 1 L/h CO₂ flow. Combining the battery’s voltage stability and the conversion rate, the CO₂ flow rate was finally determined to be 1 L/h.

3.4. CO₂ Battery System Performance Analysis

The process parameters of the CO₂ battery were determined by the rate of CO₂ absorption conversion and battery performance of the catalytic electrode [36], the NaOH concentration, and the CO₂ flow of different nanostructures. In order to further determine the stable charging, discharging, and CO₂ storage capacity of the CO₂ battery, the CO₂ long charging and discharging cycle test was carried out. As shown in Figure 5, the battery circulated for 600 min (20 charge and discharging cycles) at an 8 mA·cm⁻² current density, showing excellent cycle performance, with an almost constant charge and discharge voltage clearance.

A further test was conducted on the gas outlet composition of the CO₂ batteries under constant current discharge over 600 min. The total flow rate of the exported gas was 53.90 cm³/h, which was a mixture of CO₂ and H₂. Hydrogen gas accounted for 1.88% of the total gas volume obtained by gas chromatography, as shown in Figure S1, so the battery produced 10.135 cm³ hydrogen gas, and the Faraday efficiency was calculated as follows [37]:

$$\mathrm{FE}={\displaystyle \frac{\mathrm{nF}}{\mathrm{Q}}}$$

(12)

where $\mathrm{n}$ is the mole number of the electron transfer in the electrochemical reaction (expressed in terms of the electron coefficient in the electrochemical equation), $\mathrm{F}$ is the Faraday constant, about 96,485 C/mol, and $\mathrm{Q}$ is the amount of charge converted through the electrochemical reaction in Coulomb [38].

The Faraday efficiency was calculated according to the results of the hydrogen production of the Mg-CO₂ battery under certain constant current discharge conditions. The calculation formula was provided as follows:

$$\mathrm{FE}={\displaystyle \frac{2\frac{{\mathrm{V}}_{{\mathrm{H}}_2}}{{\mathrm{V}}_{\mathrm{m}}}\mathrm{F}}{\mathrm{Q}}}$$

(13)

${\mathrm{V}}_{{\mathrm{H}}_2}$ was the volume of the hydrogen produced over a certain time, ${\mathrm{V}}_{\mathrm{m}}$ was the molar volume of gas, $\mathrm{F}$ was the Faraday constant, and $\mathrm{Q}$ was the discharge amount of the constant current discharge of the battery at a certain time. Then, the Faraday efficiency was calculated to be 97.03%, indicating that the Mg-CO₂ battery had excellent charge transfer rate and CO₂ conversion ability.

3.5. Characterization of Catalyst Structure

To further understand the Mg-CO₂ battery’s performance, various characterization analyses of the Pd/CeO₂ catalysts with different morphologies were carried out, containing the composition and crystal structure of the Pd/CeO₂ catalysts, the surface morphology, the compositional distribution, and the elemental valence. These analyses could help reveal the mechanism of the hydrogen precipitation reaction catalyzed by the Pd/CeO₂ catalysts from the structural level and evaluate the performance of the Pd/CeO₂ catalysts in terms of the electrocatalytic activity and stability [39,40].

The TEM images of nanocube, nanorod, and nano-octahedral Pd/CeO₂ catalysts are shown in Figure 6. It can be seen that the grain sizes and structures are different. In terms of grain size, the size of the nanocube CeO₂ is around 50~100 nm, the length of the nanorod CeO₂ is about 40~200 nm, and the size of the nano-octahedral CeO₂ is about 20~50 nm. In terms of structure, the Pd/CeO₂-cube exposes the (100) crystal plane of CeO₂, the Pd/CeO₂-rod exposes the (111) and (110) crystal planes of CeO₂, and the Pd/CeO₂-oct exposes the (111) crystal plane of CeO₂. It can also be observed that the Pd species are more inclined to form nanoclusters or particles on the surface of the nanocubic and nano-octahedral CeO₂, while, for the nanorod CeO₂, the Pd species are distributed on the surface, in a highly dispersed state. Combined with the results of the battery performance tests in the previous section, this indicates that the Pd species has higher catalytic hydrogen precipitation activity and stability when forming nanoparticles or clusters compared to when it is highly dispersed on the surface.

The XRD spectra of the CeO₂ carriers and the Pd/CeO₂ catalysts with different morphologies are shown in Figure 7. It can be seen that the various catalysts show the same characteristic peaks of CeO₂, and there is no relevant peak of the Pd species, which indicates that Pd is highly dispersed on the surface of the CeO₂ carrier and that there is no noticeable agglomeration. According to Xiele’s formula [41], after calculating the grain size based on the diffracted peak with the largest intensity, it was found that the grain sizes of Pd/CeO₂ became smaller, compared to those of its CeO₂ carriers. This indicated that the interaction between Pd and CeO₂ triggered lattice distortion and that the XRD patterns of the Pd/CeO₂ catalyst were smaller than those of the CeO₂ carrier. The diffraction half-peak width of Pd/CeO₂ was widened, compared to that of the CeO₂ carrier in the XRD spectra.

The BET analysis results of the Pd/CeO₂ catalysts with different morphologies are shown in Figure 8. As can be seen from the left panel in Figure 8, the N₂ isothermal adsorption and desorption curves of all three catalysts show H₃-type hysteresis loops with IV-type adsorption isotherms, which indicates that the three catalysts have similar pore structures, such as microscopic flat-plate slits, cracks, and wedges [42]. The right panel in Figure 8 shows that the Pd/CeO₂-rod catalyst has the smallest pore size and the largest pore volume, which means that d/CeO₂-rod can accommodate the largest number of reactive molecules. As such, it might achieve a better catalytic performance. In addition, the Pd/CeO₂-rod has the largest specific surface area, followed by Pd/CeO₂-oct and Pd/CeO₂-cube, which means that the Pd/CeO₂-rod can provide the most active sites for the catalytic reaction and the Pd/CeO₂-rod has an excellent catalytic performance.

However, according to the results of the battery test in the previous section, the electrocatalytic hydrogen precipitation activity and stability of the Pd/CeO₂-rod catalyst were the worst. This indicated that the main factor affecting the activity and stability of electrocatalytic hydrogen deposition was not the pore structure of the Pd/CeO₂ rod catalyst but the difference in the exposed CeO₂ crystal planes supported.

4. Conclusions

This study examined the impact of catalytic electrode materials, electrolyte concentration, CO₂ flow rate, and other components and process parameters on the discharge capacity and CO₂ absorption and utilization efficiency of Mg-CO₂ batteries. The findings revealed the successful preparation of high-surface-area nano catalytic materials, including nanocube CeO₂, nanorod CeO₂, and nano-octahedral CeO₂ structures. Notably, the nano-octahedral CeO₂ structure of Pd/CeO₂-oct with small grains of 20–50 nm exhibited the best catalytic conversion activity and stability. The Mg-CO₂ battery, utilizing an electrolyte comprising a 0.05 mol/L NaOH solution and a CO₂ flow rate of 1 L/h, demonstrated an outstanding performance, showcasing a high cycling stability at a current density of 8 mA·cm⁻² for 600 min (20 charge–discharge cycles). The battery’s hydrogen production measured 10.135 cm³, with a Faraday efficiency of 97.03%, indicating the feasibility of CO₂ storage and utilization.

Using metal–CO₂ batteries as rechargeable energy storage systems, carbon dioxide (CO₂) was reduced to carbon-containing chemical products during discharge, leading to CO₂ solidification and the consumption of metal elements. Metal salts were reduced by applying external energy during charging. In comparison to the CO₂ compression-cooling storage process, the CO₂ battery storage technology stored CO₂ in solid form. The battery did not require an external energy supply. Additionally, during the storage period, CO₂ batteries could discharge voltage to the outside, meeting the electricity needs of the system and some ships. Therefore, this CO₂ battery storage technology could fundamentally address the issues of large system volumes and high energy consumption, laying the groundwork for achieving efficient carbon reduction in ships.