Effect of Cobalt Content on the Microstructures and Electrochemical Performances of Cobalt-Based Prussian Blue Electrodes in a Sea Water Environment

Abstract

Cobalt-based Prussian blue hollow spheres (CoHCF HSs) with different Co contents were synthesized using a self-templated coprecipitation technology. The microstructure and electrochemical properties of CoHCF HSs were investigated. The results indicate that all samples exhibit a face-centered cubic crystal structure. With increasing cobalt content in the Prussian blue analogues, the X-ray diffraction peaks shift toward higher angles due to the reduction in interplanar spacing. Computer simulations revealed that Na⁺ ions exhibit higher adsorption energies (ΔE_a) at Co sites (ΔE_a = 1.45 eV) compared to Fe sites (ΔE_a = 1.18 eV), which enables Co sites to adsorb more Na⁺ ions, providing greater sodium storage capacity. With increasing cobalt content, the reduced aspect ratio of CoHCF HSs surface nanoscale protrusions decreases the specific surface area. Consequently, the overall average CoHCF HSs size decreases with increasing cobalt content, which predominates the increase in specific surface area, contributing to supplying more active sites. The best electrochemical properties showed an initial capacity of 121.16 mAh g⁻¹ at a current density of 0.2 A g⁻¹ but not at the largest specific surface area. These findings suggest that improving the electrochemical performance of CoHCF electrodes requires consideration of the synergistic effects between specific surface area and elemental composition.

1. Introduction

As an emerging electrochemical energy storage system, sodium-ion seawater batteries (SWBs) have attracted widespread attention [1,2,3]. The sodium-ion battery utilizes seawater as the electrolyte, which naturally provides a large amount of sodium ions for abundant sodium ions for charge carriers, offering several advantages, including resource abundance, low cost, high safety, and environmental friendliness. Consequently, they have demonstrated promising application potential in scenarios such as smart buoys, underwater sensors, and offshore energy storage systems [4,5,6]. However, SWBs face challenges due to their low energy density and lack of suitable cathode materials, which limits their practical application. The combined advantages of their open framework structure enabling high energy density and good cycling stability [7], along with aqueous synthesis compatibility that facilitates scalable production [7], collectively position Prussian blue analogues (PBAs) as promising cathode candidates for SWBs. For instance, electrodes fabricated with nickel hexacyanoferrate (NiHCF) not only enhance the output voltage of seawater batteries but also demonstrate virtually no capacity fading even under high-current-density cycling conditions [8]. The hybrid energy storage device featuring an amorphous MoO_x anode and a potassium copper hexacyanoferrate (KCuHCF) cathode delivers exceptional comprehensive performance: it achieves a specific energy of 41.2 Wh/kg at a high specific power of 519 W/kg while maintaining 76.8% energy efficiency [9]. As an ideal cathode material for seawater batteries, PBAs require critical enhancement of their specific capacity in marine environments. Current strategies to improve the specific capacity of PBAs primarily focus on increasing their specific surface [10]. For instance, self-templated core-shell hollow nanocubes composed of PBA and MOF demonstrate exceptional performance as aqueous battery cathodes, achieving 88 Wh kg⁻¹ energy density and 13.07 kW kg⁻¹ peak power density [11]. Similarly, Prussian blue-derived FeP cubes confined within a 3D MXene network (FeP@MXene) deliver a high reversible capacity of 444.1 mAh g⁻¹ with zero capacity fading over 500 cycles [12]. This composite design not only enhances specific capacity but also mitigates Jahn-Teller distortion during ion intercalation/deintercalation. The fundamental mechanism by which increased specific surface area enhances specific capacity lies in the exposure of more active sites. Therefore, simultaneously maximizing both the density of accessible active sites and their intrinsic sodium storage capability is essential for achieving superior specific capacity. Wu et al. [13] revealed a synergetic interaction between elemental composition and specific surface area. They synthesized Co-Ni Prussian blue analogues (PBAs) with varying Co:Ni molar ratios and discovered that specific capacity does not consistently increase with higher specific surface area. Consequently, selecting elements with superior inherent sodium storage properties and strategically optimizing both specific surface area and elemental composition through synthetic control enables the achievement of enhanced specific capacity. As a cathode material for sodium-ion batteries, high-quality Na_0.61Fe[Fe(CN)₆]_0.94 (HQ-PB NaFe) demonstrates outstanding electrochemical performance, delivering a high specific capacity of 170 mAh g⁻¹ coupled with exceptional stability, showing no noticeable degradation over 150 cycles [14]. In particular, cobalt hexacyanoferrate (CoHCF) exhibits the highest theoretical specific capacity (170 mAh g⁻¹) among all Prussian blue analogues for sodium-ion batteries [15]. Considering both elements comprehensively, cobalt-based Prussian blue analogues (CoPBAs) are highly suitable research subjects. Current studies on CoPBAs primarily focus on multi-element doping [16,17,18], while systematic investigations into the influence of cobalt content itself remain scarce. In particular, research addressing how cobalt concentration affects crystal structure, morphology, and electrochemical performance is notably insufficient. Crucially, during synthesis, the Co²⁺ ion concentration in precursor solutions significantly regulates crystal nucleation and growth processes, thereby inducing substantial variations in crystalline phase composition, surface elemental distribution, and the exposure degree of active sites [19,20].

In this work, hollow spherical cobalt-based Prussian blue (CoHCF HSs) with different Co contents was synthesized by adjusting the amount of Co-glycerate solid spheres (Co-glycerate SSs) precursor. According to crystal growth theory, modulating the concentration of a single element allows controlled growth of electrode materials, thereby regulating the material’s specific surface area and surface-active site distribution. When the amount of Co-glycerate solid spheres (Co-glycerate SSs) precursor was 140 mg, CoHCF HSs delivered a high reversible capacity of 121.16 mAh·g⁻¹ at 0.2 A·g⁻¹. Even under a high current density of 2 A·g⁻¹, the material retains 54.8 mAh·g⁻¹, exhibiting excellent rate capability along with good cycling stability. This study provides both theoretical insight and practical guidance for the application of Co-based PBAs as cathode materials in seawater batteries.

2. Methods

2.1. Synthesis of CoHCF HSs

Figure 1 illustrates the schematic synthesis route of CoHCF HSs (CoPBA hollow spheres, HSs). The synthesis method was adapted from the self-templating strategy based on facile anion exchange, as proposed by Zeng et al. [21].

Figure 1. Flowchart of the synthesis process of CoHCF HSs.

Synthesis of spherical Co-glycerate SSs:109 mg of Co(NO₃)₂·6H₂O (Meryer (Shanghai) Biochemical Technology Co., Ltd., Shanghai, China) was dissolved in a mixed solution of 8 mL glycerol (Shanghai Titan Scientific Co., Ltd., Shanghai, China) and 40 mL isopropanol (Shanghai Titan Scientific Co., Ltd., Shanghai, China), followed by stirring for 20 min. The resulting transparent solution was then transferred into a Teflon-lined stainless-steel autoclave and heated at 180 °C for 6 h. After the reaction, the autoclave was naturally cooled to room temperature. The resulting precipitate was collected by centrifugation and washed several times with ethanol to obtain spherical Co-glycerate SSs.

Synthesis of CoHCF HSs: A predetermined amount (x mg, x = 70 mg, 105 mg, 140 mg, 175 mg, and 210 mg) of Co-glycerate SSs was ultrasonically dispersed in 100 mL of ethanol for 15 min to form solution A. Simultaneously, 152 mg of Na₄[Fe(CN)₆]·3H₂O (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was dissolved in 60 mL of deionized water to form solution B. Under continuous stirring, solution B was slowly added to solution A. The resulting mixture was then transferred into a Teflon-lined stainless-steel autoclave and heated at 90 °C for 5 h. After the reaction, the product was collected by centrifugation and washed several times with deionized water and ethanol to obtain CoHCF HSs.

To investigate the effect of Co-glycerate SSs dosage on the morphological evolution of CoHCF HSs, the input amounts of Co-glycerate SSs were set to 70 mg, 105 mg, 140 mg, 175 mg, and 210 mg, respectively. The resulting samples were correspondingly denoted as CoHCF HSs-1, CoHCF HSs-1.5, CoHCF HSs-2, CoHCF HSs-2.5, and CoHCF HSs-3.

2.2. Structure Characterization

The surface morphology of CoHCF HSs-X was observed and analyzed using field-emission scanning electron microscopy (FESEM, Hitachi S-4800, Hitachi, Ltd., Tokyo, Japan). Energy-dispersive X-ray spectroscopy (EDS, Oxford Xplore 30 Energy Dispersive Spectrometer, Oxford, UK) was performed at an accelerating voltage of 20 kV to conduct magnified elemental analysis in selected regions, enabling qualitative and semi-quantitative elemental identification. Phase composition and crystal structure were characterized by X-ray diffraction (XRD, Rigaku Corporation, Tokyo, Japan) using Cu Kα radiation (λ = 1.5418 Å) as the X-ray source. The scanning rate was 5°·min⁻¹ over a 2θ range of 5°–80°. The specific surface area of the samples was measured using nitrogen adsorption-desorption analysis (Micromeritics ASAP 2020, Micromeritics Instrument Corporation, Norcross, GA, USA). The adsorption temperature was −200 °C, with degassing performed at 120 °C for 12 h. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Nexsa, Thermo Fisher Scientific, Waltham, MA, USA) was employed to analyze the chemical states and bonding environments. Al Kα radiation (hν = 1.4 keV) was used as the excitation source, with a take-off angle of 50° (this was measured relative to the surface normal) and an analysis area of 200 μm. The applied voltage and current were 12 kV and 0.4 mA, respectively. The XPS spectra were calibrated using the work function method as described in references [22,23], with the C 1s peak set at 284.8 eV.

2.3. Electrochemical Measurements

In this study, the electrochemical performance of CoHCF HSs-X electrodes was evaluated at room temperature using a three-electrode testing system. The active material (CoHCF HSs-X), carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 7:2:1 and dispersed in N-methyl-2-pyrrolidone (NMP) to form a homogeneous slurry. The slurry was then uniformly coated onto a 1 cm × 0.5 cm carbon cloth substrate, with an effective coated area of 0.5 cm × 0.5 cm. The prepared electrodes were dried in a vacuum oven at 70 °C for 12 h. The mass loading of active material was approximately 9.6 mg·cm⁻². Electrochemical measurements were conducted using a electrochemical workstation (CHI 660E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) with a standard three-electrode setup in a 3.5% NaCl aqueous electrolyte. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests were performed within a potential window of 0.1–1.1 V.

3. Results and Discussion

The microstructure of the synthesized Co-glycerate SSs and CoHCF HSs samples was examined using scanning electron microscopy (SEM) (Figure 2). The precursor Co-glycerate SSs displayed a smooth, well-defined spherical morphology, with an average particle size of 500 ± 50 nm (Figure 2a). Figure 2b–f show SEM images of samples synthesized with varying Co-glycerate SSs dosages. The spherical morphology of the precursor is retained in all cases, with CoHCF nucleating on the surface of the Co-glycerate SSs and eventually encapsulating the entire sphere. This spherical structure increases surface exposure when stacked, thereby improving contact with the electrolyte solution. Nanoscale protrusions were observed on the spherical surfaces of the CoHCF samples, becoming thicker and shorter as the Co²⁺ concentration in the synthesis solution increased. According to classical crystal growth theory, low supersaturation leads to a slower nucleation rate, fewer nuclei, and slower growth, producing larger crystal particles. In contrast, high supersaturation causes rapid nucleation, a greater number of nuclei (often resulting in burst nucleation), and faster growth, leading to smaller crystals due to solute distribution across more particles [24]. In the CoHCF HSs-1 sample, a limited number of nucleation sites led to the sparse formation of nanoscale protrusions on the spherical surface, leaving parts of the underlying sphere exposed. This nanoscale protrusion-sphere composite had an average particle size of 850 ± 50 nm. In the CoHCF HSs-2 sample, higher nucleation density resulted in densely packed nanoscale protrusions, reducing the amount of exposed spherical surface, with an average particle size of approximately 770 ± 50 nm. With increasing Co²⁺ concentration, the particle size of CoHCF HSs progressively decreased, reaching 727 nm and 688 nm for the CoHCF HSs-2.5 and CoHCF HSs-3 samples, respectively. However, significant microstructural changes occurred in the CoHCF HSs-3 sample. The spherical surfaces became entirely covered with newly formed CoHCF, leading to overlap of nanoscale protrusions. Consequently, the original spherical morphology was no longer distinguishable, manifesting a stepwise crystallization pattern.

Figure 2. SEM images of (a) Co-glycerate SSs, (b) CoHCF HSs-1, (c) CoHCF HSs-1.5, (d) CoHCF HSs-2, (e) CoHCF HSs-2.5, and (f) CoHCF HSs-3.

As illustrated in Figure 3, a schematic diagram depicts the effect of varying Co-glycerate SSs dosages on the crystal growth process of CoHCF HSs. At low Co-glycerate SSs dosages, crystal growth proceeds primarily outward from the surface of the spheres. Typically, the crystal growth rate (defined as the increase in crystal length per unit time) is a function of supersaturation—higher supersaturation yields a faster growth rate. Thus, growth occurs outward, perpendicular to the Co-glycerate SSs surface, toward regions of higher supersaturation in the Na₄[Fe(CN)₆] solution, resulting in the formation of elongated nanoscale protrusions. As the dosage of Co-glycerate SSs increases, the same amount of Na₄[Fe(CN)₆] must be distributed across more nucleation sites. This reduces the local supersaturation at the Co-glycerate surface, slowing the growth rate and yielding shorter, thicker nanoscale protrusions. When the dosage becomes excessive, CoHCF undergoes burst nucleation, rapidly covering the surface of the Co-glycerate SSs. This impedes the diffusion of Co²⁺ from the sphere interiors, forcing the system to utilize Co²⁺ from the external solution. As a result, cobalt concentration becomes higher at the outer surfaces of the spheres, while the externally formed nanoscale protrusions contain lower cobalt levels, as later confirmed by XPS analysis. The size of the nanoscale protrusions and the degree to which the original spherical surface remains exposed play a crucial role in determining the specific surface area of the samples, which in turn influences their electrochemical performance. The next paragraph examines the composition and specific surface area of these samples.

Figure 3. Schematic diagram of the effect of different Co-glycerate SSs incorporation amounts on the crystallization growth process of CoHCF HSs.

Energy-dispersive X-ray spectroscopy (EDS) analysis was performed on the synthesized CoHCF HSs samples, as shown in Figure 4a,b. The variation in Co²⁺ concentration in the synthesis solution significantly affected the cobalt content in the resulting products. When the original spherical surface of CoHCF HSs remained partially exposed, an increase in the dosage of Co-glycerate SSs correspondingly led to an increase in the cobalt content of the final product. When the protruding structures of CoHCF HSs fully covered the spherical surface, as in the CoHCF HSs-2.5 sample, the structure could no longer obtain Co²⁺ from its core. Instead, it relied on Co²⁺ ions released into the solution from other dissolved Co-glycerate SSs. As a result, it became more difficult for CoHCF crystals to acquire Co²⁺, leading to a decrease in cobalt content in the CoHCF HSs-2.5 sample. This issue was alleviated by subsequently increasing the Co²⁺ concentration, which led to a continued rise in cobalt content.

Figure 4. (a) Mass percentages of Na, Fe, and Co in different CoHCF HSs samples; (b) EDS spectrum of CoHCF HSs-2; (c) nitrogen adsorption-desorption isotherms of CoHCF HSs-1, CoHCF HSs-2, and CoHCF HSs-3.

The Co and Fe contents in the CoHCF HSs samples directly influence the electrochemical performance of the assembled electrodes. Therefore, it is necessary to investigate the Co and Fe contents in the samples. In the CoHCF HSs-2 sample, the average molar ratio of Co to Fe reached its maximum (as the relative atomic masses of Co and Fe are similar, the mass ratio approximates the molar ratio), with the sample having a chemical formula of NaCo₂Fe_0.4HCF. The total mass percentage of Co and Fe was second only to that of CoHCF HSs-1.5 (60.07% for HSs-1.5 vs. 59.47% for HSs-2). Figure 4b displays the EDS spectrum of the CoHCF HSs-2 sample, where Na, O, Fe, Co, C, and N elements were detected. No additional impurity elements were observed in the other samples; thus, their spectra are not shown. The EDS results confirm the successful synthesis of cobalt-based Prussian blue.

Brunauer–Emmett–Teller (BET) surface area analysis was performed on representative samples, as shown in Figure 4c. It was found that the nanoblock structures on the spherical surfaces significantly influenced the specific surface area, with CoHCF HSs-1, HSs-2, and HSs-3 exhibiting values of 61 m² g⁻¹, 180 m² g⁻¹, and 188 m² g⁻¹, respectively. A larger specific surface area provides more interfacial contact between the electrode material and the electrolyte, allowing for greater Na⁺ accessibility and thereby exposing more active sites. As previously discussed, an increase in Co content leads to wider surface nanoscale protrusions on the spheres, which in turn enhances the specific surface area.

The structural characteristics of Prussian blue analogues (PBAs), including lattice parameters, interaxial angles, and potential phase transitions, can significantly influence the intercalation behavior of alkali metal ions. As shown in Figure 5a, the XRD pattern of the synthesized cobalt-glycerate solid sphere sample exhibits excellent agreement with previously reported data, as evidenced by a descending trend within the 13° to 20° range and a distinct protrusion between 20° and 30°, confirming the successful synthesis of the precursor. Figure 5b presents the XRD patterns of different CoHCF HSs samples. Compared to the standard card of Fe₄[Fe(CN)₆]₃ (PDF#73-0687), the diffraction peaks of the samples exhibit slight shifts toward lower angles, which can be attributed to the occupation of octahedral voids in the crystal lattice by Na⁺ ions, leading to an increase in interplanar spacing. The diffraction peaks observed at 2θ = 17.1°, 24.6°, and 34.6° correspond to the (200), (220), and (400) planes of the CoHCF phase, respectively, indicating that the synthesized samples possess a face-centered cubic (FCC) crystal structure. The peak splitting observed at the (220) plane may result from rhombohedral distortion of the face-centered cubic lattice induced by the intercalation of Na⁺ ions into the crystal structure [25,26,27]. A magnified examination of the (200) peaks revealed a systematic shift among the different CoHCF HSs samples. Fe²⁺ and Fe²⁺ possess different ionic radii. This inherent size difference determines that any substitution is necessarily accompanied by systematic alteration of lattice parameters, thus leading to shifts in XRD diffraction peaks. It is therefore speculated that the observed peak shifts may be attributed to the substitution of larger Fe²⁺ ions by smaller Fe²⁺ ions during the coprecipitation process, resulting in reduced interplanar spacing. Based on this observation, it is inferred that the cobalt content in the samples follows a similar trend. For instance, CoHCF HSs-2 exhibits the largest rightward shift of the (200) peak, suggesting the highest degree of Fe²⁺ substitution by Co²⁺, and consequently, the highest Co content. This inference is consistent with the EDS results shown in Figure 4a, further supporting the conclusion of maximum cobalt incorporation in this sample. According to Bragg’s Law (Equation (1)), a decrease in the interplanar spacing d leads to an increase in the diffraction angle θ, which explains the peak shift observed in the XRD patterns [28]:

$$2\mathrm{d}\mathrm{s}\mathrm{i}\mathrm{n}\theta =\mathrm{n}\lambda$$

(1)

Figure 5. (a) XRD patterns of Co-glycerate SSs and (b) CoHCF HSs-X, along with the XRD magnified view of their (200) peaks.

In the following section, the relationship between the chemical composition and crystal structure of the samples will be further analyzed in conjunction with energy-dispersive X-ray spectroscopy (EDS) results.

To further investigate the surface chemical states of the CoHCF HSs, X-ray photoelectron spectroscopy (XPS) was conducted. The XPS analysis confirmed the presence of C, O, N, Co, Fe, and Na elements, consistent with the earlier EDS results. Figure 6a–c show the high-resolution fitted spectra of C 1s, O 1s, and N 1s for the CoHCF HSs-2 sample. In the C 1s spectrum (Figure 6a), four deconvoluted peaks are observed at 284.79 eV, 286.27 eV, 288.88 eV, and 289.7 eV, corresponding to C-C, C≡N, C-O, and C=O bonds, respectively [29]. The O 1s spectrum (Figure 6b) shows two distinct peaks at binding energies of 531.65 eV and 533.33 eV, attributed to C=O and C-O bonds, respectively [30]. As shown in Figure 6c, the N 1s peak appears at 397.89 eV and is assigned to the C=N configuration [31]. Figure 6d presents the Co 2p spectra of the CoHCF HSs-X series (X = 1, 2, 3). In the CoHCF HSs-1 sample, the main Co 2p_3/2 and Co 2p_1/2 peaks are located at 781.15 eV and 797.22 eV, respectively. Deconvolution reveals the coexistence of Co³⁺ (at 787.22 eV and 800.58 eV) and Co²⁺ (at 781.02 eV and 797.20 eV) oxidation states [32]. No significant peak shift was observed in the Co 2p spectra of the CoHCF HSs-2 and HSs-3 samples, indicating that Co is in a similar chemical environment in both cases. In the Fe 2p spectrum (Figure 6e), the CoHCF HSs-1 sample exhibits Fe 2p_3/2 and Fe 2p_1/2 peaks at 708.45 eV and 721.31 eV, respectively. The fitted spectrum confirms the presence of both Fe²⁺ (708.43 eV and 721.29 eV) and Fe³⁺ (713.3 eV and 723.85 eV) oxidation states [33]. According to the full spectrum XPS analysis results shown in Figure 6f, the atomic ratios of Na, Fe, and Co were calculated. Among all samples, CoHCF HSs-2 exhibited the highest surface atomic ratio of Co, in agreement with the EDS results. However, the Co to Fe atomic ratio in CoHCF HSs-2 was determined to be 1.65, which is significantly lower than the molar ratio of 4.38 obtained from EDS measurements. Considering the comparable atomic masses of Fe and Co, the atomic ratio approximates the molar ratio, suggesting that the content of Co in the surface nanoblock protrusions is relatively lower than that on the original spherical surface. In summary, cobalt is more concentrated in the interior of the particles, whereas iron and sodium are preferentially enriched in the surface nanostructures.

Figure 6. XPS spectra of CoHCF HSs-2: (a) C 1s, (b) O 1s, and (c) N 1s high-resolution spectra; XPS spectra of CoHCF HSs: (d) Co 2p, (e) Fe 2p, and (f) survey spectrum; (g) schematic diagrams of elemental distribution for different samples.

Figure 6g presents a schematic diagram of elemental distribution across different samples based on XPS and EDS data alongside growth mechanisms, where CoHCF HSs-1 exhibited reduced surface cobalt content due to insufficient cobalt addition during synthesis, while CoHCF HSs-2 achieved optimal cobalt exposure through stoichiometrically balanced reaction conditions that maximized surface-active sites; however, CoHCF HSs-3 ultimately displayed diminished surface cobalt exposure because excessive cobalt addition during early reaction stages led to encapsulation of active sites by subsequent crystal growth.

To evaluate the electrochemical performance of CoHCF HSs-X electrodes in a 3.5% NaCl electrolyte, a three-electrode system was employed. Figure 7a presents the cyclic voltammetry (CV) curves of the CoHCF HSs electrodes at a scan rate of 50 mV·s-1. Distinct redox peaks are observed at 0.46/0.19 V and 0.97/0.77 V, corresponding to the Co²⁺/Co³⁺ redox couple (Na₂Co²⁺Fe³⁺(CN)₆ ↔ Co³⁺Fe³⁺(CN)₆ + Na⁺ + e⁻) and the Fe²⁺/Fe³⁺ couple (Na₂Co²⁺Fe²⁺(CN)₆ ↔ NaCo²⁺Fe³⁺(CN)₆ + Na⁺ + e⁻), respectively [29,34]. Among the samples, CoHCF HSs-2 exhibits the largest CV curve area and higher redox potentials for both the Co²⁺/Co³⁺ and Fe²⁺/Fe³⁺ couples, indicating superior electrochemical performance. Figure 7b displays the galvanostatic charge-discharge (GCD) curves of CoHCF HSs-X electrodes at a current density of 0.2 A·g⁻¹. It is evident that CoHCF HSs-1 and CoHCF HSs-2 deliver significantly higher specific capacities of 91.64 mAh·g⁻¹ and 104.08 mAh·g⁻¹, respectively, compared to the other samples. The increased cobalt content enhances the specific capacity of the electrodes, while the microstructural characteristics also play a crucial role in sodium storage performance. In both CoHCF HSs-1 and HSs-2, the original spherical surface remains partially exposed, with Co enrichment in the core and Fe enrichment in the surface nanoscale protrusions—consistent with the findings reported by Zeng et al. [21]. Additionally, as shown in Figure 7b, the Co-related redox reaction contributes to an extended voltage plateau, leading to a higher energy density. Although CoHCF HSs-3 possesses the largest specific surface area, it exhibits a lower capacity due to the reduced cobalt content in the surface nanoblocks. Moreover, a noticeable voltage hysteresis in the Co²⁺/Co³⁺ redox plateau was observed, which gradually diminished with increasing Co content. Figure 7c presents the rate performance of CoHCF HSs-1, HSs-2, and HSs-3 at various current densities. CoHCF HSs-1 delivered an average capacity of 92.2 mAh·g⁻¹ at 0.2 A·g⁻¹, which retained 46.5 mAh·g⁻¹ at 2 A·g⁻¹. Upon returning to 0.2 A·g⁻¹, 90% of the initial capacity was recovered. CoHCF HSs-2 exhibited an average capacity of 113.5 mAh·g⁻¹ at 0.2 A·g⁻¹, reaching a maximum of 121.16 mAh·g⁻¹. Even at a high current density of 2 A·g⁻¹, it retained 54.8 mAh·g⁻¹, with a 77% recovery upon returning to the initial current. In contrast, CoHCF HSs-3 showed inferior rate performance, delivering 69.4 mAh·g⁻¹ at 0.2 A·g⁻¹ and dropping to 30.8 mAh·g⁻¹ at 2 A·g⁻¹, with a recovery ratio of 81% after the current density was reduced. CoHCF HSs-2 exhibited rate characteristics comparable to HSs-1 under high current conditions, which may be attributed to the Co-rich spherical surface. However, the capacity degradation at low current density was similar to that of HSs-3, likely related to the nanoscale protrusions’ surface morphology. Figure 7d shows the coulombic efficiency of the CoHCF HSs-2 electrode over 100 cycles at a current density of 0.2 A·g⁻¹. The initial specific capacity was 104 mAh·g⁻¹. From the 15th cycle onward, the coulombic efficiency stabilized at 100%. However, after 100 cycles, the capacity declined to 46 mAh·g⁻¹, indicating that the cycling stability and reversibility require further improvement.

Figure 7. (a) Comparative cyclic voltammetry (CV) curves of CoHCF HSs electrodes at a scan rate of 50 mV·s⁻¹; (b) galvanostatic charge-discharge (GCD) curves of CoHCF HSs electrodes at a current density of 0.2 A·g⁻¹; (c) rate capability of CoHCF HSs electrodes at various current densities; (d) cycling performance and coulombic efficiency of the CoHCF HSs-2 electrode at 0.2 A·g⁻¹ over 100 cycles.

As shown in Figure 8a, to further understand the sodium storage mechanism of the CoHCF HSs-2 electrode, cyclic voltammetry (CV) tests were conducted at scan rates ranging from 0.2 to 1 mV·s⁻¹ to analyze the electrochemical kinetics of Na⁺. Two pairs of symmetric redox peaks observed in the CV curves correspond to the insertion and extraction processes of Na⁺ within the CoHCF HSs lattice. The relative contributions of diffusion-controlled and capacitive processes in the current response can be described by the empirical relationship [35]:

$$i=a{v}^b$$

(2)

where i is the current, v is the scan rate, and a and b are adjustable parameters. A b value close to 0.5 indicates a diffusion-controlled process, while a value near 1.0 suggests a capacitive-controlled mechanism. As shown in Figure 8b, the linear fitting of log(i) versus log(v) yielded b values of 0.60 and 0.52 for the Co²⁺/Co³⁺ redox peaks and 0.38 and 0.34 for the Fe²⁺/Fe³⁺ peaks. These results indicate that the electrochemical process of CoHCF HSs-2 is primarily diffusion-controlled, with the Fe redox reaction being more severely limited by ion diffusion. The b values for Co fall between 0.5 and 1.0, implying that the sodium storage behavior is governed by a combination of diffusion-controlled and surface capacitive processes. In addition, the relative contributions from capacitive and diffusion-controlled processes can be quantified using Dunn’s equation [36,37]:

$$i=k_{1}v+{k_{2}v}^{{\displaystyle \frac{1}{2}}}$$

(3)

where k₁v represents the capacitive contribution and k₂v^1/2 corresponds to the diffusion-controlled portion. As shown in Figure 8c, at a scan rate of 0.2 mV·s⁻¹, the capacitive contribution accounts for 49% of the total current. The current distribution also indicates that the redox peaks are predominantly governed by ion diffusion, which is consistent with the prior b value analysis. Taken together, the b values and capacitive contribution analysis suggest that Co ions exhibit more favorable electrochemical kinetics in this material compared to Fe ions, indicating faster redox dynamics.

Figure 8. (a) CV curves at different scanning rates of CoHCF HSs-2, (b) the corresponding calculated value of b, (c) surface capacitance percentage at 0.2 mV s⁻¹, and (d) computational model of CoHCF HSs-2 ((d₁) represents the structural model of Na₂CoFe(CN)₆, (d₂) represents the model depicting Na binding at the Co site within this structure, and (d₃) represents the model depicting Na binding at the Fe site within this structure).

To further elucidate the superior Na⁺ storage performance of the CoHCF HSs-2 sample, first-principles calculations based on density functional theory (DFT, Simulation and calculation were performed using Materials Studio 2023 software) were conducted to evaluate the Na⁺ adsorption energies (ΔE_a) at Co and Fe sites [38,39,40]. Based on the experimental data, a 4 × 4 × 4 supercell model of Na₂Co[Fe(CN)₆] was constructed, into which Na⁺ ions were inserted to form Na-Co and Na-Fe bonding configurations (Figure 8d). The calculated Na⁺ adsorption energy at the Co site (ΔE_a = 1.45 eV) was higher than that at the Fe site (ΔE_a = 1.18 eV), indicating a stronger binding affinity of Na⁺ to Co atoms. This suggests that a higher Co content favors enhanced Na⁺ storage capacity in CoHCF HSs anodes.

Table 1 lists the capacities of relevant Prussian blue materials, demonstrating that the electrode material synthesized in this work exhibits exceptional sodium storage capacity and promising application prospects.

Table 1. Comparison of specific capacities of different PBA materials in sodium-ion electrolyte.

Formula	Year	Specific Capacity at Current Density (mAh/g@mA/g)	Electrolyte	Ref.
NiHCF	2021	57.97@4 (mA cm−2)	East China Sea seawater	[8]
Co0.86Ni0.14HCF	2023	76.5@100	3.5% NaCl	[13]
KCuHCF	2024	66.5@100	Chinese seawater	[9]
Flowerlike PB	2021	72.1@100	1M NaCl	[41]
NaxFeFe(CN)6	2021	64.6@100	1 M NaClO4	[42]
Na1.74Fe[Fe(CN)6]0.94·3.3H2O	2018	120.4@200	1 M NaClO4	[43]
Na1.59Fe[Fe(CN)6]0.95	2017	100@100	1.0 M NaPF6	[44]
Na1.56Fe[Fe(CN)6]0.90·2.42H2O	2024	99.2@70	1 M NaClO4	[45]
NiHCF	2016	76.2@100	0.5 M Na2SO4 and 50 mM ZnSO4	[46]
Na1.05Co[FeCN)6]0.81·2.68H2O	2023	122@100	1 M Na2SO4	[47]
Na2Ni0.2Co0.5Mn0.3Fe(CN)6	2020	120@8.5	wt% FEC	[48]
NaCo2Fe0.4HCF	-	121.16@200	3.5% NaCl	This article

4. Conclusions

In this study, spherical CoHCF materials were successfully synthesized via a self-templated method combining coprecipitation with a facile anion exchange process. Experimental results demonstrate that the Co concentration in the reaction solution significantly influences both the surface morphology and particle size of CoHCF materials. As the dosage of Co-glycerate SSs increases, the corresponding rise in Co²⁺ concentration within the synthesis environment induces a morphological evolution wherein the nanoscale protrusions on CoHCF HSs transition from slender to stubby configurations, ultimately forming a continuous coverage across the spherical surface. Mechanistic analysis reveals that this morphological transformation is accompanied by progressive particle size reduction, wherein the dominant size effect results in a continuous increase in specific surface area—theoretically expected to enhance specific capacity. However, studies identified a compositional gradient across the spherical surface, with nanoscale protrusion-rich regions exhibiting lower Co content. As these protrusions progressively envelop the entire sphere, the overall Co proportion declines. Computational simulations further demonstrate that Co sites (ΔE_a = 1.45 eV) possess higher Na⁺ adsorption energy and greater capacitive contribution compared to Fe sites (ΔE_a = 1.18 eV), enabling superior sodium storage capacity and enhanced Na⁺ reaction kinetics. Consequently, the surface Co concentration fundamentally governs the electrochemical performance. Optimization of specific capacity requires not only increased surface area but also precise regulation of surface elemental composition. When employing 140 mg of Co-glycerate SSs precursor, the resulting CoHCF HSs-2 delivers the highest initial specific capacity of 121.16 mAh·g⁻¹ at 0.2 A·g⁻¹. This study concludes that the electrochemical performance of CoHCF materials is governed by the synergistic interplay between specific surface area and elemental composition, providing an effective strategy for optimizing sodium storage properties through structural and compositional control, thereby offering valuable insights and theoretical guidance for developing high-performance cathode materials for seawater batteries.