Dual‐Functional Electrode Promoting Dendrite‐Free and CO2 Utilization Enabled High‐Reversible Symmetric Na‐CO2 Batteries

Sodium‐carbon dioxide (Na‐CO2) batteries are regarded as promising energy storage technologies because of their impressive theoretical energy density and CO2 reutilization, but their practical applications are restricted by uncontrollable sodium dendrite growth and poor electrochemical kinetics of CO2 cathode. Constructing suitable multifunctional electrodes for dendrite‐free anodes and kinetics‐enhanced CO2 cathodes is considered one of the most important ways to advance the practical application of Na‐CO2 batteries. Herein, RuO2 nanoparticles encapsulated in carbon paper (RuCP) are rationally designed and employed as both Na anode host and CO2 cathode in Na‐CO2 batteries. The outstanding sodiophilicity and high catalytic activity of RuCP electrodes can simultaneously contribute to homogenous Na+ distribution and dendrite‐free sodium structure at the anode, as well as strengthen discharge and charge kinetics at the cathode. The morphological evolution confirmed the uniform deposition of Na on RuCP anode with dense and flat interfaces, delivering enhanced Coulombic efficiency of 99.5% and cycling stability near 1500 cycles. Meanwhile, Na‐CO2 batteries with RuCP cathode demonstrated excellent cycling stability (>350 cycles). Significantly, implementation of a dendrite‐free RuCP@Na anode and catalytic‐site‐rich RuCP cathode allowed for the construction of a symmetric Na‐CO2 battery with long‐duration cyclability, offering inspiration for extensive practical uses of Na‐CO2 batteries.


Introduction
Given the escalating dilemma with fossil fuel supplies and the greenhouse effect, it is vital to transfer electricity production from conventional fossil fuels to renewable and clean energy sources such as wind and solar. [1]imultaneously, it is necessary to store and preserve renewable electricity via integration with battery storage technologies for enabling widespread applications.Rechargeable lithium-ion batteries (LIBs), as one of the most astounding modern battery storage technologies, have been widely used in portable electronic devices for the past 30 years. [2,3]However, it is challenging for LIBs to keep up with the rapidly developing requirements of electric vehicles and energy storage due to their comparatively high cost, scarce lithium resources, and limited theoretical energy density. [4]odium metal batteries, such as Na-O 2 , Na-S, and Na-CO 2 batteries that directly utilize lowcost, naturally abundant metallic sodium as the anode represent one of the most promising approaches to the aforementioned challenges. [5,6]Particularly, Na-CO 2 batteries, a brand-new energy storage system, hold great promise since they can efficiently utilize greenhouse gas (CO 2 ) and have a high theoretical energy density of 1125 Wh kg À1 (according to the reaction of 4 Na + 3CO 2 ↔ 2 Na 2 CO 3 + C). [7][8][9] Unfortunately, current Na-CO 2 batteries are still in their infancy, and their development is still hampered by numerous issues associated with Na metal anodes and CO 2 cathodes. [10,11]On the one hand, the endless formation and growth of dendrites on Na anodes during continuous discharge/charge might penetrate separators, triggering serious safety issues. [12]Additionally, frequent plating/stripping and unstable Na/ electrolyte interfaces may cause side reactions and fast loss of electrolyte and sodium, which will reduce battery capacity and shorten cycle life. [13]Various strategies have been attempted to strengthen the stability of the sodium/electrolyte interface by developing protective films on metallic Na or by using alloy Na anodes rather than metallic sodium, [13][14][15] but these strategies tend to result in low specific capacities and diverge from the goal of Na-CO 2 batteries with high energy density.By comparison, engineering "sodiophilic" substrates with 3D structures that can reduce local current density and modulate the initial Na nucleation and deposition behavior may be advantageous for achieving effective and secure sodium metal anodes. [16,17]On the other hand, at the CO 2 cathode, the CO 2 reduction reaction (CO 2 RR) and the decomposition reaction of Na 2 CO 3 (i.e., CO 2 evolution reaction, CO 2 ER) occur upon discharge and charge, respectively. [18]The high energy barrier of CO 2 (C=O, 750 kJ mol À1 ) and the broadband gap insulating Na 2 CO 3 cause the CO 2 RR/CO 2 ER kinetics to be sluggish. [19]esides, the accumulation of massive discharge products tends to be detrimental to electron transport and mass transfer (CO 2 , Na + , electrolyte diffusion), thus negatively affecting the reversible capacity and cycling performance of Na-CO 2 batteries. [18]To overcome these challenges, attempts have been made to build efficient CO 2 cathodes.[22] However, it is important to note that a large number of currently published investigations have concentrated solely on solving the sodium anode or cathode issues. [23]The systematization of anode and cathode, rather than a single electrode, is the key to fabricating practical and stable Na-CO 2 batteries.
Herein, free-standing carbon paper (CP) embedded with highly dispersed RuO 2 nanoparticles (RuCP) was designed as advanced electrodes for solving the aforementioned challenges on both the anode and cathode.RuCP offered the dense and homogeneous distribution of active sites intended to regulate initial Na metal nucleation and subsequent growth, as well as to promote CO 2 RR/CO 2 ER kinetics, ensuring dendrite-free and kinetics-enhanced Na-CO 2 batteries.To be precise, the following notable advantages can be found: 1) RuCP demonstrated powerful sodiophilic interphases that allowed for fascinating interactions between Na metal and RuO 2 nanoparticles, as well as S and N heteroatoms, greatly lowering the surface energy of commercial CP.This sodiophilic property was revealed to be preferable for Na nucleation with a low overpotential of 13 mV at 1 mA cm À2 , achieving homogeneous, dendrite-free reversible Na deposition with 1500 cycles.2) The interactive RuCP framework provided a 3D conductive structure, ensuring electron and mass transport, as well as abundant void space to accommodate Na 2 CO 3 in the cathode; meanwhile, the catalytic-site-rich RuCP exhibited enhanced electrocatalytic activity and durability for CO 2 RR and CO 2 ER.A long-term cycling capability of 350 cycles was obtained in an asymmetric Na-CO 2 battery.Given the above benefits, the symmetric Na-CO 2 battery with the RuCP@Na anode and RuCP cathode exhibited good cycling stability (120 cycles), demonstrating the viability of dual-functional RuCP electrodes.

Scheme of RuCP for the Symmetric Na-CO 2 Battery
Figure 1a illustrated the schematic synthesis process of RuCP by a typical galvanostatic pulse electrodeposition technique [24] combined with pyrolysis treatment.First, hydrous RuO x N y S z is deposited onto the pretreated CP in an acidic solution containing Ruthenium (III) nitrosylsulfate ([Ru(NO)] 2 (SO 4 ) 3 ) by a galvanostatic pulse technique.The pulse lasted 10 s and was followed by a 30-s rest interval to ensure that the porous structure/electrolyte interface remained stable and equilibrated.Subsequently, the obtained RuO x N y S z -containing CP was annealed in the N 2 atmosphere to obtain highly dispersed RuO 2 nanoparticles that were encapsulated in 3D carbon substrates, namely, RuCP.Details on the preparation can be found in the Experimental Section and Figures S1 and S2, Supporting Information.Figure 1b described the configuration of the RuCP@Na || RuCP symmetric Na-CO 2 battery and the bifunctional role of RuCP as a sodium metal host and CO 2 cathode.RuCP anodic scaffold with a 3D porous structure can provide a sufficient interface for sodium deposition, lowering the local current density.The homogeneous and highly dispersed RuO 2 nanoparticles allow for reducing the barriers to sodium nucleation, while the doping of nitrogen and sulfur heteroatoms also promotes CP to achieve the transition from "sodiophobic" to "sodiophilic", thus optimizing the Na nucleation process and modulating the sodium deposition behavior to achieve a dendrite-free and smooth Na metal anode.For the RuCP cathode, the 3D porous structure allows for fast CO 2 transport and makes large space available for product deposition.Moreover, the presence of RuO 2 nanoparticles and heteroatoms N and S in the CP framework is beneficial in providing better active sites in the CO 2 batteries, which stimulates the fast CO 2 RR/CO 2 ER kinetics.

Characterizations of RuCP
The crystal structure of the as-prepared sample was analyzed by X-ray diffraction (XRD) in Figure S3, Supporting Information.The peaks located at 27.89°, 34.95°, 39.86°, 44.79°, 54.03°, 57.63°, 59.26°, 65.22°, 66.73°, 69.20°, and 73.81°were noticed, corresponding to (110), ( 101), ( 200), ( 210), ( 211), ( 220), ( 002), ( 310), ( 112), (301), and (202) lattice planes of RuO 2 (JCPDS No.65-2824), respectively.The scanning electron microscopy (SEM) image of the CP scaffold was shown in Figure 2a, revealing a three-dimensional electrically conductive structure formed by the intersection of carbon fibers with a clean surface.As illustrated in Figure 2b,c, the SEM images of RuCP revealed that RuCP inherited the framework of CP without structural collapse and changes.Additionally, the confinement effect enabled the efficient encapsulation of the RuO 2 nanoparticles inside the carbon pores of CP, implying the highly dispersed and uniform nature of RuO 2 nanoparticles.The transmission electron microscopy (TEM) image indicated that the RuO 2 nanoparticles were highly dispersed on the carbon matrix (Figure 2d).The well-defined lattice interlayer spacing of 0.316 nm was observed in the high-resolution TEM image, corresponding to the (110) plane of RuO 2 (Figure 2e).High-angle annular dark-field scanning TEM (HAADF-STEM) and corresponding mapping were further used to disclose the elemental distribution of RuO 2 nanoparticles in RuCP (Figure 2f-i).In addition to ruthenium and oxygen, we also observe the presence of elemental sulfur on the surface of RuO 2 nanoparticles.Interestingly, the energy-dispersive Xray spectroscopy (EDS) mapping demonstrated a uniform distribution of Ru, S, and N throughout the porous carbon fiber host (Figure S4, Supporting Information).The findings in Figure 2f-i and Figure S4, Supporting Information indicated that the N species were mainly distributed on the carbon fiber, while S species derived from [Ru (NO)] 2 (SO 4 ) 3 were distributed on the surface of the Ru nanoparticles, which could also be evidenced by X-ray photoelectron spectroscopy (XPS).
XPS also provided further evidence for the existence of Ru, N, and S in RuCP, with the corresponding elemental contents given in Table S1, Supporting Information.As demonstrated in the Ru 3d + C 1s spectrum of RuCP (Figure S5, Supporting Information), the binding energy of the Ru 3d 5/2 core-line located at 280.1 eV would be comparable to the Ru (IV) oxidation state. [25]Additionally, the peak at approximately 284.4 eV was more likely related to the C-C bonds, while the peak near 284.8 eV might be attributed to the disruption of Ru 3d 3/2 . [25,26]ergy Environ.Mater.2024, 7, e12626 Through analysis of the Ru 3p spectrum, it was determined that the surface chemical state of Ru was predominately Ru 0 and Ru 4+ (Figure 2j).Specifically, the deconvolution of Ru 3p spectra exhibited peaks at 461.5 and 466.0 eV, which could be assigned to Ru 0 3p 3/2 and Ru 4+ 3p 3/2 , and those peaks at 483.6 and 487.7 eV originated from Ru 0 3p 1/2 and Ru 4+ 3p 1/2 , respectively. [27]The S 2p spectrum presented two distinct forms (Figure 2k): as oxidized sulfur groups (ÀC-SO x -C-, x = 2-4, at 167.5-171.5 eV) originating from such as sulfate or sulfonate, [28,29] and as sulfide groups (ÀC-S-C-, at ~163.6 eV).Three peaks positioned at 399.7, 402.1, and 405.2 eV were ascribed to pyrrolic-N, graphitic-N, and oxidized-N (Figure 2l). [18,19]The doping of carbon substrates with heteroatoms such as N and S can not only activate CO 2 RR/CO 2 ER, but is also suggested to be useful for achieving sodiophilic behavior, which thus is favorable to the electrochemical process.

Plating Na Behaviors on RuCP as Anode Host
Asymmetric Na cells were assembled by applying RuCP and CP hosts as working electrodes, Na foil as the counter electrode, and 1 M sodium hexafluorophosphate (NaPF 6 ) in diglyme as the electrolyte to investigate the plating and stripping behaviors of Na on RuCP and CP electrodes via galvanostatic measurements, and the surface morphologies evolution were characterized using SEM (Figure 3). Figure 3a displayed the voltage-time curves of Na plating/stripping on RuCP and CP electrodes at a current density of 1 mA cm À2 with a fixed area capacity of 20 mAh cm À2 , all electrochemical states marked in Figure 3a correspond to the following SEM images at different stages.Figure 3b1,b2, h1,h2 illustrated SEM images of the CP and RuCP electrodes after initial nucleation (1 mAh cm À2 of Na plating).As a result of the affinity between the Na and the carbon fibers and the confinement of the 3D scaffold structure, the deposition of Na gradually expanded along the one-dimensional direction of the carbon fibers.The different morphologies of Na deposited on RuCP and CP electrodes were clearly observed (Figure 3b,h).The CP electrode presented large-size Na nuclei with uneven distribution, while the diameter of nucleated sodium on RuCP was smaller and consistently dispersed over the carbon fibers.It could be attributed to the higher affinity of RuCP for sodium compared to pure CP, since highly conductive RuO 2 nanoparticles uniformly encapsulated on the CP fibers served as sodiophilic sites for the uniform deposition of Na, and also the doping of nitrogen and sulfur heteroatoms facilitated the transition from "sodiophobic" to "sodiophilic" behavior, with consequential improvements in the nucleation process. [12,30]Mediated by the sodiophilic interphase, the initial homogeneous nucleation of Na ensured that Na was uniformly plated in the subsequent growth phase and uniformly covered the entire RuCP electrode when the loading was increased to 3 mAh cm À2 (Figure 3j).Even at a rather high Na loading of 10 and 20 mAh cm À2 , the RuCP electrode exhibited a smooth and homogeneous morphology without dendrites (Figure 3k,l).In contrast, the CP electrodes demonstrated an inhomogeneous plating behavior, with surface voids or interstitial spaces even at a load of 5 mAh cm À2 and a rough and uneven surface at higher Na loads (10, 20 mAh cm À2 ).From the cross-sectional view (Figure 3f2), the CP electrode showed a large number of cavities and a thick sodium plating layer of about 180 lm, stemming from a poor sodium affinity.The increased sodium affinity resulted in denser sodium deposition on RuCP, with a sodium layer thickness of ~150 lm (Figure 3l2).Some residual sodium ("dead Na") was still retained on the CP surface that was not stripped at the end of the stripping process (Figure 2g).Contrastingly, there was virtually no residual Na deposition at the RuCP electrode (Figure 2m) because of its excellent reversibility, which corresponded to a high CE of 96.5% in the first plating/stripping cycle (the CE of CP electrode was 95.1%, Figure 2a).

Electrochemical Properties of Na Deposition on RuCP Host
The behavior of plating/stripping Na on RuCP hosts prompted us to evaluate the electrochemical characteristics of Na deposition on RuCP hosts further.Activation treatments were carried out prior to assessing the stability of the Na deposition with a view to removing surface contamination and establishing the initial SEI layer (Figure S6, Supporting Information).Impressively, RuCP delivered an extraordinarily stable plating/stripping process (Figure 4a,b) approaching 1500 cycles with a high average coulombic efficiency (CE) of 99.5% at a current density of 1 mAh cm À2 with a deposition capacity of 1 mAh cm À2 , and corresponding voltage profiles showing stability and low voltage hysteresis upon cycling.For the control experiments, the CP exhibited distinctly limited stability (~800 cycles) and relatively large voltage hysteresis.In the case of CP, the worsening cycle performance could be attributed to the lack of active sites on the electrode surface, resulting in an initial heterogeneous Na nucleation with unstable SEI layers, followed by accelerated electrolyte decomposition and formation of a large number of "dead Na" due to dendrite growth, which eventually leads to deteriorated cycle performance. [31,32]urther rate capability tests for Na plating/stripping were carried out at various current densities and deposition capacities (Figure 3c and Figure S7, Supporting Information).The RuCP electrodes exhibited the steady voltage plateau and high CE at the current density ranging from 1 to 5 mA cm À2 with plating capacities from 1 to 5 mAh cm À2 .
Additionally, when the current density was reduced from 5 to 1 mA cm À2 , the RuCP electrode continued to provide steady cycling over 2000 h, while the CP demonstrated inferior cycling stability for ~1200 h.In addition, it was noted that there was less voltage hysteresis for the RuCP electrode in comparison to the CP electrode (Figure 4f),  .SEM images of b-g) CP and h-m) RuCP electrodes were corresponding to the states marked in the voltage profiles after Na plating process for 1, 3, 5, 10, 20 mAh cm À2 , and stripping process for 20 mAh cm À2 , respectively.b1 and h1) show the corresponding enlarged SEM images of b2 and h2).f2 and l2) exhibit the cross-sectional view SEM image of f1 and l1).
Energy Environ.Mater.2024, 7, e12626 suggesting that RuCP offers sufficient sodium nucleation sites and fast sodium transport kinetics to effectively diminish the local current density during Na plating/stripping at high current rates.
Figure 4d,e displayed the voltage profiles of Na deposition on RuCP and CP electrodes at different current densities.Observations of Na deposition overpotentials can be categorized as nucleation overpotentials (g n ) which is the potential difference between the steady plateau and lowest voltage drop, and mass transfer-controlled plating overpotentials (g p ) associated with Na growth after initial nucleation. [33]oth g n and g p values of the RuCP electrode were significantly lower than those of the CP electrode at various current densities (Figure 4g, h), confirming the favorable sodiophilicity of the RuCP electrode, which corresponds to the flat and smooth morphology of the SEM findings mentioned above.Especially, it can be observed that the RuCP electrode was capable of stable plating at the current density of 1-5 mA cm À2 , even at 5 mA cm À2 , the g n was only 29 mV, while the plating plots of PC exhibited higher voltage overpotential and even nonuniform curve at 5 mA cm À2 .The low overpotential of RuCP hosts seems likely to originate from the synergistic effect of doping with N and S, as well as the anchored RuO 2 nanoparticles acting as nucleation seeds for Na metal plating to decrease the overpotential.Furthermore, electrochemical impedance spectroscopy (EIS) also gave further evidence for the low overpotential of RuCP hosts.As shown in Figure 4i, the RuCP electrode exhibited remarkably low interfacial resistance, indicating fast Na + , electron, and mass transport capabilities.The RuCP electrode exhibited small nucleation barriers and fast charge transfer during nucleation and growth, which is advantageous to enhancing Na plating quality and lowering energy consumption.
Figure 4g and Figure S8, Supporting Information demonstrated the cyclic voltammetry (CV) curves of asymmetric Na cells with RuCP and CP hosts at À0.2 to 1 V with a sweep rate of 0.1 mV s À1 .The two pairs of redox peaks located at ~0.6/0.8 and À0.2/0.2V correspond to Na + insertion/extraction and Na plating/stripping processes, respectively.The peak overlap of the Na plating/stripping process on the RuCP electrode after the first cycle was well-established, and the corresponding peak current was higher than that of the CP electrode, indicating that the RuCP electrode was able to accelerate the charge transfer kinetics of Na deposition, which further demonstrates the excellent cycling stability and low overpotential as above.Aurbach CE tests [34] were also performed to evaluate the efficiency of Na cycling on RuCP and CP hosts, which further prove the benefit of the RuCP design by showing a low voltage hysteresis of 26 mV and substantially improved CE of 99.2% compared with CP (36 mV and 97.1%) revealing that the RuCP was useful for preventing the irreversible consumption of Na metal and stabilizing the electrode-electrolyte interface.

Electrochemical Properties of RuCP as CO 2 Cathode for Na-CO 2 Battery
Inspired by these findings, we evaluated the electrochemical performances of RuCP cathodes in a Na-CO 2 battery with Na foil as the anode.[37] Commercial carbon paper (CP) is used directly as a collector in metal-CO 2 batteries, and its contribution to battery performance is often overlooked due to its insignificant contribution.However, as a control experiment for RuCP, we carried out a number of activation treatments on CP in this study (see Experimental Section for details), which are able to increase the reactive sites on the CP surface and contribute to the performance of Na-CO 2 batteries and cannot be disregarded.Herein, the CP cathode was also used as a comparison.Figure 5a showed the cyclic voltammetry (CV) curves that were obtained in the 1.8 to 4.5 V range under a CO 2 environment.The reduction onset potential that occurred at around ~2.3 V could well be related to the consumption of CO 2 toward the generation of Na 2 CO 3 , and the oxidization onset potential that emerged at approximately 3.6 V at the subsequent anodic scanning was caused by the decomposition of Na 2 CO 3 to release CO 2 .The battery with RuCP cathode demonstrated more pronounced reduction and oxidation peaks as well as considerably higher peak currents than the battery with CP cathode, revealing that the RuCP cathode has superior catalytic activity towards CO 2 RR/ CO 2 ER reactions than the CP cathode.
To further evaluate the activity and reversibility of the RuCP cathode, the discharge-charge behaviors of the Na-CO 2 batteries with RuCP and CP cathode were also investigated at 100 lA cm À2 .As demonstrated in the initial discharge-recharge curves in Figure 5b, the RuCP cathode exhibited an obviously larger discharge capacity of 2788 lAh cm À2 than that of the CP cathode (736 lAh cm À2 ).During the subsequent recharging process, the RuCP cathode delivered remarkable reversibility with a superior Coulombic efficiency (CE) of ~100%, while the CP cathode only displayed an inferior CE of merely ~55%.In addition, the charge voltage plateau of the RuCP cathode was also much lower than that of the CP cathode, indicating superior catalytic activity of the RuCP cathode for the decomposition of Na 2 CO 3 , which might be the result of active site enrichment.What is more, following the initial discharge and charging process, prolonged discharge-charge testing was also carried out at the same current density with a fixed 2 h per cycle (Figure 5c).It could be observed that the CP cathode exhibited poor sustained cycling stability due to the lack of an effective catalytic active site to facilitate CO 2 RR and CO 2 ER, causing significantly greater polarization and even cell death at a later stage.In contrast, the battery with RuCP cathode exhibited better reversibility, offering tremendous promise for high performance.
The EIS analysis of Na-CO 2 batteries with RuCP and CP cathodes at different stages was illustrated in Figure 5d,e, respectively, and the insets presented the corresponding equivalent circuit models.The charge transfer resistance (R ct ) after fitting was summarized in Table S2, Supporting Information.The R ct in Na-CO 2 batteries with CP cathode after 50th discharge (5913 Ω) was extremely higher than that at the pristine stage (1925 Ω) due to its surface coverage by continuous accumulation of wide bandgap insulating Na 2 CO 3 layer.Even after charging, charge transfer resistance remained as high as 2865 Ω, signifying low reversibility.In marked contrast, the Na-CO 2 battery based on CP cathode with a low R ct of 744 Ω at the pristine stage displayed a drastically decreased transfer impedance during the 50th charge process (from 4392 to 948 Ω), thus indicating the excellent catalytic activity of RuCP cathode for the decomposition of Na 2 CO 3 .
Furthermore, the galvanostatic cyclic discharge-charge tests were investigated to further evaluate the long-term durability of the catalytically active sites of RuCP cathode, as shown in Figure 5f and Figure S9, Supporting Information.The discharge end voltage of the CP-based Na-CO 2 battery dropped sharply to below 1.8 V after 300 h (150 cycles, Figure S9, Supporting Information), while, even more significantly, the RuCP-based Na-CO 2 battery showed markedly improved durability over 700 h (>350 cycles).The chosen partial discharge-charge curves of the RuCP-based Na-CO 2 battery depicted in Figure 5g demonstrated that after 350 cycles, the charge-discharge voltage gap was only ~0.5 V larger than the original one, demonstrating exceptional cyclability.This durable cyclability is noticeably Energy Environ.Mater.2024, 7, e12626 improved than most of the majority of previously reported nonaqueous Na-CO 2 batteries (Table S3, Supporting Information).

Electrochemical Properties of Symmetric Na-CO 2 Battery
Furthermore, we studied the cycle stability of the symmetric Na-CO 2 battery.As shown in Figure 6, CP@Na || CP battery displayed relatively poor cycling stability and large overpotential (~2.2 V), with a discharge terminal voltage of only 1.6 V after 120 cycles at 100 lA cm À2 .The main reasons for the potential decreases so obviously after 100 cycles can be attributed to the low CO 2 RR/CO 2 ER catalytic activity of CP itself and the sodiophobic interphases of CP leading to sodium dendrite generation.As anticipated, the RuCP@Na || RuCP battery exhibited satisfactory rechargeability, its discharge terminal voltage was >2.1 V for 120 cycles.Moreover, the RuCP@Na || RuCP battery demonstrated Energy Environ.Mater.2024, 7, e12626 smooth, stable, and highly over-lapped charge-discharge voltage profiles with low overpotential (about 1.6 V) in Figure 6c.The low overpotential and improved cycling performance can be explained by the high catalytic activity and stable interface of the RuCP electrode, as evidenced by the low internal resistance (Figure S10, Supporting Information).Furthermore, the surface morphologies of the sodium deposited after cycling gave further evidence of the interfacial stability of RuCP anodes.As shown in Figure 6d, the Na-deposited CP anode was found with a large amount of dead sodium and sodium dendrites on the surface after 120 cycles, which is the reason why the performance of the CP battery deteriorated rapidly after 100 cycles.Instead, the surface of the RuCP anode exhibited slight cracking but remained flat and compact after 120 cycles (Figure 6e), which confirms the excellent cycling stability of the symmetric RuCP@Na || RuCP battery.In light of the above results, it is reasonable to propose that the RuCP electrode not only has a sodiophilic interphase that effectively guides Na nucleation, stabilizes the interface, and inhibits dendrite growth, but also has a good electrocatalytic activity for the electrochemical reduction of CO 2 and carbonate decomposition that improve the kinetic properties of the discharge-charge process, thus optimizing the cycling performance of Na-CO 2 batteries.

Conclusions
In summary, a "one stone two birds" strategy was presented by using carbon paper embedded with RuO 2 nanoparticles as dual-functional electrodes for both the Na anode and CO 2 cathode.As an anode host, the RuCP with remarkable sodiophilicity could dominate Na nucleation behavior and achieve uniform Na deposition/dissolution with dendrite-free morphology.Meanwhile, the RuCP afforded high catalytic activity for catalyzing the formation/decomposition of Na 2 CO 3 in the CO 2 cathode.As a result, the Na was homogeneously deposited on the RuCP anode, showing a low nucleation overpotential of only 13 mV

Figure 1 .
Figure 1.a) Scheme for the fabrication process of RuCP.b) Schematic illustration of the RuCP@Na || RuCP symmetric Na-CO 2 battery configuration and the role played by RuCP in Na deposition and catalytic CO 2 RR/CO 2 ER processes.

Figure 2 .
Figure 2. SEM images of a) CP and b, c) RuCP.d) TEM, e) HRTEM, and e) HRTEM images of RuCP.f) HAADF-STEM and elemental mapping images of g) Ru, h) O, and i) S. High-resolution XPS spectra of j) Ru 3p, k) S 2p, l) N 1s.

Figure 3 .
Figure3.Na plating/stripping morphology evolution on RuCP and CP electrodes.a) Galvanostatic plating/stripping curves at a current density of 1 mA cm À2 with a fixed area capacity of 20 mAh cm À2 .SEM images of b-g) CP and h-m) RuCP electrodes were corresponding to the states marked in the voltage profiles after Na plating process for 1, 3, 5, 10, 20 mAh cm À2 , and stripping process for 20 mAh cm À2 , respectively.b1 and h1) show the corresponding enlarged SEM images of b2 and h2).f2 and l2) exhibit the cross-sectional view SEM image of f1 and l1).

Figure 4 .
Figure 4. Electrochemical characterization of the Na metal anodes with RuCP and CP hosts.a, b) Long-term cycling and the corresponding voltage profiles at 1 mAh cm À2 and 1 mA cm À2 , respectively.c) Rate performances at various areal current densities, only the first 110 h of the long-term cycle are presented here for clarity.Nucleation potentials of Na metal plating on the d) RuCP and e) CP hosts at various current densities.Histograms of f) voltage hysteresis, g) Na nucleation overpotentials, and h) plating overpotentials.i) Nyquist plots.j) CV curves at 0.1 mV s À1 between À0.2 and 1.0 V. k) Aurbach CE tests for calculating average coulombic efficiencies.

Figure 5 .
Figure 5. Electrochemical performances of asymmetric Na-CO 2 battery with RuCP cathode.a) CV profiles of CP and RuCP-based battery under CO 2 atmosphere.b) The initial discharge-recharge curves at 100 lAÁcm À2 .c) Prolonged galvanostatic discharge-charge curves after the initial discharge-recharge process.d and e) The EIS analysis of RuCP and CP-based Na-CO 2 battery at pristine, after the 50th discharge and 50th recharge processes, respectively.f) Long-time cycling performance and g) stability of RuCP-based Na-CO 2 battery at 100 lA cm À2 .