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. 2026 Sep 23;11(39):58884–58895. doi: 10.1021/acsomega.6c03561

Cerium-Modified Diatom Biosilica for Lithium-Ion Anodes

Hongwei Liu †,‡,*, Can Liu †,‡, Jian Huang §, Hongchang Liu †,‡, Shimao Sun †,‡, Yuhang Fu †,‡, Li Yu †,‡
PMCID: PMC13647413  PMID: 42851855

Abstract

Diatom frustules offer hierarchically porous biosilica templates for sustainable electrode design; however, their low electrical conductivity and limited electrochemically accessible sites constrain lithium-storage performance. In this study, a two-stage cultivation strategy was used to introduce Ce-related species into Cyclotella cryptica frustules, followed by purification and pyrolysis to obtain Ce-functionalized biosilica/carbon anodes. Ce-200 provided the best balance among biological tolerance, cerium enrichment, structural retention, pore evolution, and electrochemical performance. SEM, ICP-MS, XPS, Raman, and nitrogen sorption analyses confirmed cerium enrichment, preservation of the frustule morphology, formation of defect-containing carbon, and an approximately 3-fold increase in specific surface area. In half-cells, DBS/C@Ce200 delivered 932.8 mAh g–1 after 200 cycles at 0.1 A g–1 and retained 527.0 mAh g–1 at 2 A g–1. CV, EIS, and postcycling SEM results were consistent with improved charge-transfer behavior and structural stability. This work demonstrates bioassisted cerium modification as a promising route for diatom-derived lithium-ion battery anodes.

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1. Introduction

The rapid development of new energy industries has driven a growing demand for high-performance energy storage devices. , Lithium-ion batteries are widely employed in electric vehicles, portable electronic devices, grid energy storage systems, owing to their high energy density, long cycle life, low self-discharge rate, and environmental friendliness. They are thus considered a key technology underpinning the modern energy transition. Within LIBs, the anode material is a critical factor that dictates the overall battery performance. Graphite is the conventional benchmark anode, but its theoretical capacity is only 372 mAh g–1, which cannot meet the increasing demand for high-performance energy storage systems. Among the various anode materials, , silicon has attracted considerable interest because of its exceptionally high theoretical specific capacity (≈4200 mAh g–1, more than ten times that of graphite), suitable working potential, and natural abundance. Nevertheless, silicon undergoes severe volume expansion (>300%) during lithiation/delithiation, leading to particle pulverization, repeated breaking and reformation of the solid electrolyte interphase (SEI) layer, and ultimately rapid capacity fade. The low intrinsic electrical conductivity further limits its rate performance.

To overcome these challenges, researchers have explored various strategies, including silicon nanostructuring, carbon compositing, , and porous structure engineering, , Among these approaches, , diatom biosilica-a naturally occurring porous nanomaterial, possesses a highly ordered nanoscale porous structure, This three-dimensional porous architecture can effectively buffer volume expansion and provide abundant channels for lithium-ion transport, , thereby significantly shortening ion diffusion pathways within the battery.

However, the primary component of diatom biosilica is silica, whose inherently low electrical conductivity poses a significant limitation on its electrochemical performance, previous studies have indicated that metal element doping can effectively enhance its physicochemical properties, for example, introducing metal ions with variable valence states and catalytic activity into semiconductor materials can improve electrical conductivity and structural stability.

Cerium (Ce), characterized by its unique 4f electronic configuration and the facile redox transition between Ce3+ and Ce4+, , has demonstrated considerable potential in catalysis, energy storage, and energy conversion. Furthermore, the derived CeO2 nanoparticles exhibit excellent oxygen storage capacity and catalytic activity, Therefore, incorporating cerium into the silica matrix of diatom biosilica is expected to enhance electrical conductivity through electronic structure modulation, while the presence of oxygen vacancies may help stabilize the electrode interface, thereby improving its suitability as an anode material for lithium-ion batteries.

Most existing methods for the metal modification of diatom frustules rely on post treatment approaches, such as pyrolysis or impregnation, However, these methods generally suffer from complex procedures, high energy consumption, potential environmental pollution, and difficulty in achieving uniform target-element doping, Previous studies have indicated, that metal elements can be incorporated into diatom frustules through the intrinsic biological metabolism of diatoms, research has shown that the natural biomineralization process of diatoms for in situ doping offers several advantages, including mild reaction conditions, environmental friendliness, the absence of secondary pollution, uniform elemental distribution, and effective integration with the biological structure. However, during this biological doping process, , the physiological toxicity of metal salts to diatoms poses a significant challenge for achieving high doping efficiency.

Studies have shown that dividing the diatom cultivation process into two stages can effectively alleviate the physiological stress caused by metal reagents. , Specifically, in the first stage, diatoms are cultured under optimal conditions without metal stress, thereby promoting normal growth and biomass accumulation. In the second stage, an appropriate amount of the target metal reagent is introduced into the culture medium. At this stage, the diatoms have entered an active biomineralization phase, exhibiting a significantly enhanced capacity for the uptake and incorporation of metal elements, together with improved tolerance to metal stress.

This “growth-first, doping-second” two-stage sequential control strategy enables efficient in situ doping of metal elements while preserving the normal physiological state of the diatoms.

In this study, the marine diatom Cyclotella cryptica was selected as the biological template, and cerium ammonium nitrate was selected as the cerium source to systematically investigate the effects of the concentration of cerium on the physiological responses of the diatom, the structural evolution of its frustules, and its cerium doping behavior. Furthermore, the electrochemical performance of the resulting Ce-doped diatom biosilica/carbon composite was preliminarily evaluated when it was applied as an anode material for lithium-ion batteries.

The central hypothesis of this study is that controlled cerium addition during diatom cultivation can enable cerium incorporation into the developing biosilica framework while preserving the intrinsic porous architecture of the frustules. Instead of forming large phase-separated deposits, cerium is expected to remain in a dispersed, cerium-associated state, thereby modifying the local silica network and affecting the structural evolution of the material during pyrolysis. This bioassisted incorporation strategy may regulate the pore structure, carbon characteristics, and interfacial properties of the resulting biosilica/carbon composite, ultimately enhancing its lithium-storage performance.

Overall, this study seeks to clarify the role of cerium association during diatom biomineralization and to develop a bioderived strategy for engineering silicon-based functional materials for energy-storage applications.

2. Experimental Section

2.1. Diatom Strain and Cultivation

The diatom Cyclotella cryptica was isolated from the South China Sea near Wenchang, Hainan Province, China, and identified by 18S rRNA gene sequencing (99.59% identity to reference sequences in NCBI; Project No. PN20251113092, Personalbio Technology Co., Ltd., Shanghai, China). The strain has been maintained in axenic culture in our laboratory and is available from the corresponding author upon reasonable request.

The strain was cultured in sterilized f/2 medium prepared from artificial seawater supplemented with nutrients, trace elements, and vitamins. Cultivation was carried out in 1 L Erlenmeyer flasks at 24 °C under a 12 h light/12 h dark cycle (3000 lx) with an initial inoculation ratio of 10% (v/v) for 12 days.

To investigate the effects of cerium ammonium nitrate on diatom physiology and biosilica doping, a two-stage cultivation strategy was employed:

2.1.1. Stage I (Growth Stage)

Diatoms were first cultivated in the standard f/2 medium described above until they reached the early exponential growth phase (approximately 144 h), allowing sufficient biomass to accumulate.

2.1.2. Stage II (Doping Stage)

A predetermined amount of cerium ammonium nitrate solution was then added to the culture medium to achieve final concentrations of 100, 200, 300, and 400 μmol/L. Cultivation was subsequently continued until the end of the experiment. During this stage, the diatoms underwent physiological adaptation under cerium stress and progressively incorporated cerium ions into their biosilica frustules.

Note: The selected concentration range was determined based on preliminary experimental results (Figure S1), with the aim of achieving a significant doping effect while maintaining cell viability and metabolic activity.

2.2. Determination of Diatom Growth-Related Parameters

2.2.1. Diatom Growth Curve and OD Measurement

Diatom cell growth was monitored using both cell counting and optical density (OD) measurements. Cell counting was conducted using an inverted fluorescence microscope (NE910) equipped with a 20 × 20 phytoplankton counting chamber. For each sample, algal cells in nine randomly selected small grids were counted at 40× magnification. The average value was then used to calculate the cell density per unit volume based on the known volume of the counting chamber (eq S1).

The optical density of the culture at 680 nm (OD680) was measured using a UV–vis spectrophotometer to indirectly reflect the biomass, Preliminary wavelength scanning indicated that the maximum absorption peak of Cyclotella cryptica occurred at this wavelength, and the OD680 value strongly linearly correlated with the cell density.

2.2.2. Determination of Nitrogen, Silicon, and Phosphorus in the Diatom Culture Medium

The concentrations of silicate (SiO3 2–), total nitrogen (TN), and soluble reactive phosphorus (PO4 3––P) in the culture medium were determined using the molybdenum blue method, alkaline persulfate oxidation method, and molybdenum–antimony spectrophotometric method, respectively. Prior to analysis, all the samples were filtered through a 0.22 μm aqueous membrane filter. The analytical procedures were performed with reference to the Chinese National Standard Specifications for Oceanographic SurveysPart 4: Survey of Seawater Chemicals (GB 17378.4-2007), with minor modifications.

2.3. Diatom Frustule Extraction

The extraction and purification of diatom frustules were performed following a commonly used acid treatment method for biosilica, with minor modifications. At the end of cultivation, the cultures were harvested by centrifugation at 8000 × g for 15 min at. The pellet was subsequently washed three times with deionized water to remove residual salts. The sample was subsequently suspended in 0.6 mol L–1 hydrochloric acid solution and soaked at room temperature for 5 h to digest organic matter and metal oxides.

After the acid treatment, the solids were collected by centrifugation and washed with deionized water until the pH became neutral to terminate the reaction. The sample was then washed several times with anhydrous ethanol until the supernatant became colorless and transparent, enabling dehydration and solvent replacement. The purified diatom frustules were subjected to vacuum freeze-drying at −70 °C and stored in a desiccator for subsequent use.

2.4. Preparation of Biosilica/Carbon Anode Materials and Assembly of Lithium Half-Cells

Freeze-dried frustules obtained from untreated and Ce-treated Cyclotella cryptica cultures were converted into biosilica/carbon composites by pyrolysis. The sample was heated to 600 °C at a rate of 5 °C min–1 and maintained at this temperature for 3 h to achieve pyrolysis and carbonization, yielding a Ce-doped diatom biosilica/carbon composite (denoted as DBS/C@Ce). For electrode preparation, the obtained active material was mixed with conductive carbon black and polyvinylidene fluoride (PVDF) binder at a mass ratio of 7:2:1. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added as the solvent, and the mixture was ball-milled in a planetary ball mill at 500 rpm for approximately 3 h to form a homogeneous electrode slurry, The resulting slurry was uniformly coated onto copper foil using a doctor-blade method, with a coating thickness of approximately 50 μm. The coated electrodes were dried in a vacuum oven at 90 °C for approximately 12 h to remove residual solvent. After drying, the electrode sheets were punched into disc-shaped electrodes using a coin-cell cutter.

The electrode discs were then transferred into a vacuum glovebox for cell assembly. CR2032 coin-type half-cells were assembled using the prepared electrode discs as the working electrodes and lithium metal foil as both the counter and reference electrode. A microporous separator was placed between the electrodes, and a suitable electrolyte was added. A nickel foam spacer was used to ensure good contact between the electrode components. Finally, the cells were sealed using a coin-cell crimping machine to complete the assembly process (The electrolyte used was Lithium iron phosphate electrolyte (KLD-LFP01)).

2.5. Electrochemical Performance Measurements

Electrochemical measurements were carried out using CR2032 coin-type half-cells assembled in an Ar-filled glovebox, with lithium metal as both the counter and reference electrode. Galvanostatic charge–discharge tests and rate capability measurements were performed on a multichannel battery testing system (LAND CT3002A, China) within a voltage range of 0.01–1.5 V (vs Li+/Li).

Cyclic voltammetry (CV) was conducted on an electrochemical workstation in the voltage range of 0.01–1.5 V at scan rates of 0.1 and 0.5 mV s–1. Electrochemical impedance spectroscopy (EIS) measurements were carried out on a Gamry Interface 1010E workstation over a frequency range from 100 kHz to 10 mHz. The impedance spectrum was fitted using ZView software with an equivalent-circuit model.

2.6. Material Characterization Methods

The morphology and elemental distribution of the purified frustules were examined by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS, MIRA4, TESCAN, Czech Republic). Samples were mounted on conductive carbon tape and sputter-coated with gold before observation. An accelerating voltage of 5 kV was used for imaging and 15 kV for EDS analysis.

The elemental composition of the frustules was quantified by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7800, USA). Approximately 30 mg of sample was digested in concentrated HNO3, diluted with ultrapure water, and filtered prior to analysis.

Fourier transform infrared (FTIR) spectra were recorded on a Nicolet NEXUS 670 spectrometer (USA) using the KBr pellet method in the range of 4000–400 cm–1 with a resolution of 4 cm–1.

X-ray diffraction (XRD) patterns were collected on an Empyrean diffractometer (PANalytical, Netherlands) using Cu Kα radiation (λ = 1.5406 Å) over a 2θ range of 5–90°.

The morphology of the pyrolyzed materials and cycled electrodes was also examined by SEM. Raman spectra were collected to evaluate the carbon structure, and X-ray photoelectron spectroscopy (XPS) was used to analyze the surface elemental composition and chemical states of the pyrolyzed composites.

The thermal behavior of the pyrolyzed biosilica/carbon composites was evaluated by thermogravimetric analysis (TG, STA2500, Netzsch, Germany) under N2 at a heating rate of 10 °C min–1 from 30 to 740 °C. Nitrogen adsorption–desorption measurements were carried out on an automatic surface area and pore size analyzer (SAP2460, Micromeritics, USA). Before analysis, samples were degassed under vacuum at 300 °C for 8 h. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was derived using the Barrett–Joyner–Halenda (BJH) model.

2.7. Statistical Analysis

Experimental data are presented as the mean ± standard deviation (SD). All biological experiments were independently repeated three times (n = 3). Data analysis was performed using OriginPro 2025 software (OriginLab Corporation, USA).

Statistical differences among groups were evaluated using one-way analysis of variance (ANOVA). When a significant difference was observed (p < 0.05), Tukey’s post hoc multiple comparison test was applied. The value of p < 0.05 was considered to indicate statistical significance.

3. Results and Discussion

3.1. Effects of Ce Treatment on the Growth and Nutrient Dynamics of Cyclotella cryptica

This study employed a two-stage cultivation strategy to examine the physiological responses of the marine diatom Cyclotella cryptica to varying concentrations of cerium ammonium nitrate (0, 100, 200, 300, and 400 μmol/L) over a 12-day cultivation period. As shown in Figure a, during the first stage (days 1–6), all the experimental groups exhibited synchronized exponential growth, with cell density increasing from approximately 1.115 × 105 to 4.413 × 105 cells/mL. These results indicate that the standard F/2 medium, in the absence of cerium, provided optimal conditions for diatom growth. This uniform growth phase established a robust baseline biomass prior to cerium exposure, thereby minimizing initial physiological variability among individual cells.

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Time-dependent changes in diatom cell density and residual nutrient concentrations in the culture medium under varying concentrations of cerium ammonium nitrate (0, 100, 200, 300, and 400 μmol/L): (a) growth curve, (b) total nitrogen, (c) soluble reactive phosphorus, and (d) silicate. Note: Cerium was introduced on day 6 of cultivation. Samples were collected every 24 h throughout the 12-day cultivation period.

Following the addition of cerium ammonium nitrate on day 6, diatom growth exhibited a pronounced dose-dependent response. As presented in Table S1, the final cell densities on day 12 were 4.253 × 105 cells/mL for the control group, 8.273 × 105 cells/mL for Ce100, 5.253 × 105 cells/mL for Ce200, 4.827 × 105 cells/mL for Ce300, and 3.413 × 105 cells/mL for Ce400. Notably, growth in the Ce100 group was significantly enhanced, with a 94.5% increase in final cell density relative to that in the control group, , suggesting a hormetic effect in which low concentrations of cerium stimulate diatom proliferation. In contrast, higher cerium concentrations (Ce300 and Ce400) reduced final cell densities by 13.5% and 19.7%, respectively, suggesting that elevated cerium induces reversible oxidative stress, thereby inhibiting cell division and compromising membrane integrity, compared with the control group, the Ce200 group exhibited a moderate stimulatory effect, with a 23.5% increase in the final cell density, but this value was significantly lower than that of the Ce100 group, implying that the threshold for cerium toxicity lies between 200 and 300 μmol/L, beyond which inhibitory effects outweigh growth promotion.

Analysis of daily growth rates further clarified this pattern. Prior to cerium addition (days 1–6), all groups exhibited similar growth kinetics, peaking at ∼0.56 on days 1–2 and subsequently stabilizing between 0.05 and 0.17. After cerium addition (days 7–12), the Ce100 group maintained relatively high growth rates (e.g., 0.17 on days 6–7), whereas the Ce400 group showed negative growth during the later stages (e.g., 0.46 on days 9–10). This nonlinear inhibitory response aligns with known toxicity patterns of rare earth elements in algae, sublethal concentrations can trigger adaptive mechanisms, such as upregulation of antioxidant defense systems, , whereas excessive amounts of cerium disrupt core metabolic processes, including those involving photosynthetic reaction centers and nutrient transporters.

As a silicon-dependent organism, the primary component of diatom frustules is biosilica, whereas nitrogen (N) and phosphorus (P) serve as essential macronutrients for growth. Therefore, monitoring nutrient dynamics in the culture medium provides insight into nutrient reallocation mechanisms in diatoms under stress. As shown in Figure d, during the initial cultivation stage, the silicate consumption trends were similar across all the experimental groups, with concentrations decreasing from 3.26 mg/L on day 1 to 1.81 mg/L on day 6. Following the addition of cerium, silicate consumption accelerated in the high-concentration groups. At the end of the experiment, total silicate consumption reached 2.96 mg/L in the control group, 3.03 mg/L in Ce100, 2.68 mg/L in Ce200, 2.64 mg/L in Ce300, and 2.55 mg/L in Ce400. Notably, despite growth inhibition in the Ce400 group, its silicate consumption rate increased markedly after cerium addition (e.g., 0.62 mg/L·day from days 10 to 11), substantially exceeding that of the control group (0.19 mg/L·day). This observation suggests that diatoms prioritize frustule silicification through metabolic remodeling and incorporate cerium into the biosilica matrix as a strategy to mitigate cerium toxicity

A similar pattern was observed for total nitrogen. During the initial stage, the total nitrogen concentrations in all groups decreased from approximately 17.5–17.7 mg/L to 6.3 mg/L by day 6. After the addition of cerium, total nitrogen uptake increased in the high-concentration groups, with total consumption decreasing from 16.62 mg/L in the control group to 14.59 mg/L in the Ce400 group. Specifically, the peak nitrogen uptake rate of the Ce400 group was 1.19 mg/L·day (days 9–10), which was significantly greater than that of the control (0.70 mg/L·day). This enhanced nitrogen uptake likely reflects the synthesis of stress-responsive proteins, including those involved in cerium chelation and reactive oxygen species detoxification

In contrast, the differences in soluble reactive phosphorus (SRP) among the experimental groups were relatively minor. During the initial cultivation stage, the phosphate concentration decreased from 42.82 μmol/L to 10.71 μmol/L by day 6 across all groups. Following the addition of cerium, total phosphate consumption remained comparable among groups (approximately 35.6–35.8 μmol/L), with only a slight acceleration observed in the high-concentration treatments. For instance, the Ce400 group exhibited a consumption rate of 0.22 μmol/L·day from days 11–12, whereas the control showed 0.24 μmol/L·day, suggesting a minor phosphate release or reduced uptake in the control. The relatively subdued response of phosphate likely reflects its primary role in diatom energy metabolism rather than direct interactions with cerium. These results indicate that Cyclotella cryptica actively adapts to cerium stress, , exposure to cerium induces the upregulation of silicon and nitrogen transporters, facilitating the formation of cerium-doped siliceous cell walls to mitigate the adverse effects of growth inhibition. , This response aligns with established patterns of rare earth element-induced metabolic remodeling in microalgae: low doses stimulate proliferation via hormetic effects, whereas high doses, despite causing oxidative damage, redirect nutrient allocation toward detoxification and stress mitigation.

In summary, the results of the two-stage cultivation experiments demonstrated a dual effect of cerium ammonium nitrate on Cyclotella cryptica: low concentrations promote growth through physiological stimulation, whereas high concentrations inhibit growth via stress-induced nutrient reallocation. These findings provide a theoretical basis for the controlled functionalization of diatom biosilica.

3.2. Structural and Compositional Evidence of Ce-Doped Diatom Biosilica

The SEM–EDS elemental mapping results (Figure ) show that the Ce signal intensity increased progressively with increasing exogenous cerium concentration. with the most uniform and dense distribution observed in the DBS@Ce200 group, whereas the Si signal correspondingly decreased. Notably, compared with those in the control group, the microstructures of the frustules in all the doped groupsincluding nanoporous architecture, pore size distribution, and overall three-dimensional morphologyremained intact, and no nanoparticle agglomeration or formation of additional crystalline phases was detected. These observations indicate that the metabolic doping process is mild and biocompatible, with Ce primarily incorporated into the diatom frustule via the organism’s biomineralization processes, This strategy enables high-load Ce doping while preserving frustule structural integrity, providing a stable framework for potential applications as anode materials in lithium-ion batteries.

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SEM–EDS analysis of Ce-doped diatom frustules after 12 days of cultivation. Panels (a–e) show the EDS spectra of the frustules from the control group (0 μmol/L Ce) and samples treated with 100, 200, 300, and 400 μmol/L Ce, respectively. Panels (a1–e1) display the corresponding SEM images of the frustule morphology for each sample. Panels (a2–a4, b2–b4, c2–c4, d2–d4, e2–e4) present the elemental distribution maps of Si, Ce, and O, respectively, for the control, DBS@Ce100, DBS@Ce200, Ce-300, and Ce-400 samples in the same order. All scale bars represent 2.5 μm.

SEM–EDS, ICP-MS, FTIR, and XRD were employed to confirm cerium incorporation in the purified diatom frustules after the two-stage cultivation process. ICP-MS analysis provided quantitative evidence of cerium accumulation (Figure A and Table S2). The Ce content increased from 0.0444% in the control to 5.90% in DBS@Ce100 and reached 12.69% in DBS@Ce200, followed by slightly lower values in DBS@Ce300 (10.46%) and DBS@Ce400 (10.99%). It should be noted that the unusually low apparent Si content in DBS@Ce200 should be interpreted with caution. The ICP-MS values reported here represent bulk elemental mass fractions after acid digestion, dilution, and normalization, rather than the stoichiometric composition of the biosilica framework. Therefore, the decrease in Si wt % should not be interpreted as direct evidence for extensive substitution of Si by Ce within the silica network. This apparent decrease may be influenced by the combined effects of Ce enrichment, residual nonsilica components, sample heterogeneity, digestion/normalization procedures, and redistribution of the total elemental mass fraction.

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(A) ICP-MS-determined Ce and Si contents of frustules obtained under different cerium treatments. (B) FTIR spectra of purified frustules showing characteristic silica vibrations and treatment-induced changes in the local network environment. (C) XRD patterns of purified frustules, indicating the predominantly amorphous nature of the biosilica after cerium treatment.

This nonmonotonic trend suggests that cerium uptake was governed by diatom metabolism and biomineralization rather than simple adsorption. − The highest Ce incorporation observed at 200 μmol L–1 suggests that this concentration provided the most favorable balance between cerium availability and physiological tolerance.

FTIR spectra of the purified frustules (Figure B) showed the characteristic bands of amorphous silica, , including the Si–O–Si asymmetric stretching band near 1080–1090 cm–1, the symmetric stretching band around 800 cm–1, and the Si–O bending vibration at 460–480 cm–1. Compared with the control, cerium-treated samples exhibited changes in band intensity and slight peak shifts. especially in the low-wavenumber region and near the main Si–O–Si band. These changes indicate that cerium altered the local chemical environment of the silica network during biosilicification. Although FTIR cannot definitively assign specific Ce–O–Si bonds, the observed spectral variations support cerium interaction with the biosilica framework rather than simple surface adsorption.

XRD patterns of all samples (Figure C) displayed a broad diffuse hump centered at ∼22°, characteristic of amorphous silica, with no sharp peaks attributable to crystalline cerium-containing phases. This result confirms that cerium treatment did not alter the predominantly amorphous nature of the frustules. The absence of distinct crystalline reflections suggests that cerium existed in a highly dispersed or low-crystallinity state within the biosilica matrix rather than as separate crystalline aggregates.

Overall, these results demonstrate that cerium was successfully incorporated into diatom biosilica through a mild bioassisted mineralization process while preserving the characteristic porous frustule structure. Among the tested conditions, DBS@Ce200 showed the highest incorporation efficiency and the most homogeneous elemental distribution, making it the optimal precursor for subsequent carbonization and electrochemical evaluation.

3.3. Structural and Chemical Evolution of Pyrolyzed Ce-Modified Biosilica/Carbon Composites

To evaluate the structural robustness of the Ce-modified biosilica/carbon composites after pyrolysis, SEM images of the carbonized frustules were first examined (Figure S3). All samples retained the characteristic disc-like morphology and ordered surface perforation pattern of Cyclotella cryptica, with no obvious large-scale collapse or severe fragmentation observed after heat treatment. This result indicates that the natural hierarchical frustule framework was largely preserved during pyrolysis.

The pore structure of the pyrolyzed materials was further characterized by N2 physisorption (Figure A, Figure S5, and Table S4). All samples exhibited type-IV adsorption–desorption isotherms with H3-type hysteresis loops, indicating predominantly mesoporous characteristics. Compared with DBS/C, the Ce-modified samples showed substantially increased adsorption capacity and broader pore-size distributions in the mesopore region. The BET surface area increased from 12.37 m2 g–1 for DBS/C to 23.69, 28.65, 36.71, and 23.83 m2 g–1 for DBS/C@Ce-100, DBS/C@Ce-200, DBS/C@Ce-300, and DBS/C@Ce-400, respectively, with the maximum value corresponding to an approximately 3-fold increase over the control. The total pore volume also increased markedly, reaching a maximum of 0.123 cm3 g–1 for DBS/C@Ce-200, compared with 0.0206 cm3 g–1 for DBS/C. In addition, the average pore diameter increased from 6.66 nm in DBS/C to 16.94–17.17 nm in the Ce-100 and Ce-200 samples. These results indicate that appropriate cerium incorporation promoted the development of a more open mesoporous network during pyrolysis, whereas excessive cerium loading led to partial loss of this advantage, as reflected by the reduced surface area and pore volume in DBS/C@Ce-400.

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Textural and thermal characterization of pyrolyzed biosilica/carbon composites. (A) N2 adsorption–desorption isotherms of DBS/C and Ce-modified DBS/C samples. (B) Thermogravimetric curves of DBS/C and Ce-modified DBS/C samples recorded under N2 atmosphere.

Thermogravimetric analysis further showed that cerium modification altered the thermal decomposition behavior of the composites (Figure B, Figure S4, and Table S3). All samples displayed a minor initial weight loss below 150 °C, which is attributed to desorption of physically adsorbed water and residual volatiles. The main mass-loss event occurred in the range of 300–600 °C, corresponding to decomposition of the carbonaceous fraction. Compared with DBS/C, the Ce-modified samples exhibited markedly lower mass loss in this region and higher residual yields at 700 °C. Specifically, the residue increased from 54.15% for DBS/C to 76.51%, 74.23%, 73.28%, and 71.40% for DBS/C@Ce-100, DBS/C@Ce-200, DBS/C@Ce-300, and DBS/C@Ce-400, respectively. The DTG curves also show that the main decomposition peak shifted to slightly lower temperature after cerium treatment, indicating that cerium changed the local decomposition environment of the carbon-containing matrix. Overall, these data demonstrate that cerium incorporation increased the thermally stable inorganic fraction of the pyrolyzed materials and improved their residual structural stability.

Taken together, the SEM, N2 physisorption, and TG results demonstrate that bioassisted cerium incorporation preserved the intrinsic frustule architecture during pyrolysis while significantly improving the pore structure and thermal stability of the resulting biosilica/carbon composites. Such structural optimization provides a plausible materials basis for the improved electrochemical performance of the Ce-modified samples.

The XPS survey spectra show that, besides Si, O, and C, clear Ce signals are detected in all Ce-treated samples but not in control, indicating that cerium remained associated with the materials after purification and pyrolysis (Figure A,D). Si 2p spectra exhibits subtle changes after cerium treatment, suggesting that the local silicon chemical environment was affected to some extent. More importantly, the Ce 3d spectra can be deconvoluted into contributions assigned to both Ce3+ and Ce4+related species (Figure F), indicating the presence of mixed-valence cerium on the surface of the pyrolyzed composites. Combined with the predominantly amorphous XRD features and the absence of obvious large surface agglomerates in SEM, these results are consistent with cerium existing mainly as highly dispersed cerium–oxygen-related species associated with the biosilica/carbon matrix, rather than as large phase-separated deposits.

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XPS and Raman characterization of pyrolyzed Ce-modified biosilica/carbon composites. (A) XPS survey spectrum of DBS/C. (B) High-resolution Si 2p spectrum of DBS/C. (C) Ce 3d region of DBS/C. (D) XPS survey spectra of DBS/C and Ce-modified samples. (E) High-resolution Si 2p spectra of DBS/C and Ce-modified samples. (F) High-resolution Ce 3d spectra of Ce-modified samples. (G) Raman spectrum and peak fitting of DBS/C. (H) Raman spectra and peak fitting of Ce-modified samples. (I) Comparison of Raman D and G bands and corresponding ID/IG values for DBS/C and Ce-modified samples.

Raman spectra of all samples display the typical D and G bands of carbon (Figure G–I). Compared with DBS/C, the Ce-treated samples show reduced ID/IG values, with the lowest value observed for DBS/C@Ce-200, suggesting that moderate cerium treatment may promote the formation of a relatively more ordered carbon structure during pyrolysis. Together with the increased surface area and mesoporosity revealed by N2 physisorption, this structural evolution is consistent with improved electrolyte accessibility and more favorable overall electrochemical transport behavior.

It should be noted, however, that the current evidence still cannot unambiguously distinguish true lattice-level Ce incorporation within the silicate framework from surface-associated Ce species or highly dispersed Ce-containing oxide domains. Therefore, the present results are more appropriately interpreted as evidence of Ce enrichment associated with the frustule-derived biosilica/carbon composite, rather than definitive proof of uniform lattice incorporation into the silica framework.

Overall, the XPS and Raman results complement the SEM, BET, TG, FTIR, and XRD analyses by showing that cerium treatment is associated with Ce-related surface chemical states and carbon-structure evolution in the pyrolyzed composites. These features, together with the preserved frustule architecture and optimized pore characteristics, may collectively contribute to the improved electrochemical performance.

3.4. Electrochemical Performance and Interfacial Kinetics of Ce-Modified Biosilica/Carbon Anodes

The lithium-storage performance of the Ce-modified biosilica/carbon composites was evaluated by galvanostatic cycling and rate tests (Figure ). As shown in Figure A, all electrodes exhibited low initial Coulombic efficiencies, which is typical for Si-based composite anodes and mainly arises from irreversible lithium consumption during the initial formation of the electrode/electrolyte interphase. After the first few cycles, the Coulombic efficiencies rapidly approached ∼100%, indicating improved reversibility upon cycling.

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Electrochemical performance of Ce-modified biosilica/carbon anodes in Li half-cells. (A) Cycling performance and Coulombic efficiency of DBS/C and Ce-modified DBS/C electrodes at 0.1 A g–1 over 200 cycles within 0.01–1.5 V (vs Li+/Li). (B) Rate performance of DBS/C and Ce-modified DBS/C electrodes at different current densities from 0.1 to 2 A g–1.

Compared with the undoped DBS/C electrode, the Ce-modified samples showed markedly improved cycling stability. Among them, DBS/C@Ce200 delivered the best overall performance and maintained the highest reversible capacity after 200 cycles. Its gradually increasing capacity during cycling suggests a progressive activation process, likely related to improved electrolyte penetration and gradual utilization of active sites within the porous framework. In contrast, the control sample showed obvious capacity fading, indicating less stable structural and interfacial behavior. The postcycling SEM images (Figure S5) further support this trend, showing that the Ce-modified electrodes, especially DBS/C@Ce200, better preserved the electrode morphology after prolonged cycling than the undoped sample.

The rate capability results are shown in Figure B. DBS/C@Ce200 again exhibited the best performance over the full current-density range and showed good capacity recovery when the current density returned to 0.1 A g–1, indicating favorable kinetic reversibility. Ce300 and Ce400 also outperformed the control, but their recovered capacities were lower than their capacities in continuous cycling because the rate test involved prior high-current polarization. Therefore, the capacities at 0.1 A g–1 in Figure B should not be directly compared with those in Figure A. Overall, the improved cycling and rate performance of DBS/C@Ce200 is consistent with its preserved frustule framework, enhanced mesoporosity, and more favorable carbon structure.

To further clarify the effect of cerium incorporation on the electrode reaction kinetics, CV and EIS measurements were carried out (Figure and Table S5). In the first cathodic scan, all samples exhibited a broad reduction feature at ∼0.6–0.8 V, which is attributed to electrolyte decomposition and initial SEI formation. This feature became much weaker in subsequent cycles, indicating that the major irreversible interfacial reactions mainly occurred during the initial cycle. The cathodic peak near 0.1–0.2 V and the anodic peak around 0.3–0.5 V correspond to the alloying/dealloying reactions of Si with Li. Compared with the control, the Ce-modified electrodes showed better overlap of the subsequent CV curves, suggesting improved reversibility and interfacial stability.

7.

7

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Cyclic voltammograms of (A) DBS/C@Ce-100, (B) DBS/C@Ce-200, (C) DBS/C@Ce-300, (D) DBS/C@Ce-400, and (E) DBS/C (control) electrodes. (F) EIS Nyquist plots of the corresponding electrodes, together with the equivalent circuit used for fitting and an enlarged view of the high-frequency region.

The Nyquist plots (Figure F) further reveal the kinetic advantage of moderate cerium incorporation. According to the fitted parameters in Table S5, DBS/C@Ce200 exhibited the lowest SEI-film resistance (6.74 Ω) and charge-transfer resistance (5.30 Ω), both markedly lower than those of the control electrode (14.31 and 16.11 Ω, respectively). Although DBS/C@Ce400 showed the lowest ohmic resistance, its interfacial resistances remained higher than those of DBS/C@Ce200, indicating that low bulk resistance alone does not ensure optimal electrode kinetics. These results demonstrate that an appropriate cerium content, particularly 200 μmol L–1, is most effective in stabilizing the interfacial structure and facilitating charge transfer, which is consistent with the superior cycling and rate performance of DBS/C@Ce200.

3.5. Analysis of Study Limitations

Although the present results demonstrate that cerium treatment enabled Ce enrichment in diatom-derived biosilica/carbon composites and improved their lithium-storage performance, several limitations should be acknowledged.

First, the current structural evidence cannot unambiguously distinguish lattice-level Ce incorporation within the biosilica framework from surface-associated Ce species or highly dispersed cerium-containing oxide domains.

Second, although the two-stage cultivation strategy supports a bioassisted incorporation pathway, the absence of a nonbiological Ce-containing control prepared by conventional impregnation or post-treatment means that electrochemical improvement cannot be attributed exclusively to biological incorporation. Future work should include such control samples to separate the effects of biological uptake, surface deposition, and postpyrolysis chemical evolution.

Third, the electrochemical evaluation was performed in lithium half-cells. Further studies using full cells, higher mass loading electrodes, and extended cycling under practical testing conditions are needed to assess the application potential of these materials more rigorously. In addition, advanced characterization methods such as transmission electron microscopy, synchrotron-based X-ray absorption spectroscopy, and in situ/ex situ interfacial analysis would help clarify the local coordination environment of Ce and its role during lithiation/delithiation.

4. Conclusions

In this work, a bioassisted strategy was developed to enrich Ce-related species in association with the biosilica frustules of Cyclotella cryptica, followed by pyrolysis to obtain Ce-functionalized biosilica/carbon composites for lithium-ion battery anodes. Among the tested conditions, Ce200 provided the best overall balance among biological tolerance, Ce enrichment, structural retention, and electrochemical performance. Structural and compositional analyses indicated preserved frustule architecture, enhanced mesoporosity, increased thermal residue, and Ce-related surface species together with improved carbon ordering after pyrolysis.

As a result, DBS/C@Ce200 delivered the best electrochemical performance, with a reversible capacity of 932.8 mAh g–1 after 200 cycles at 0.1 A g–1 and 527.0 mAh g–1 at 2 A g–1. CV and EIS results further suggested improved interfacial kinetics and cycling stability. It should be noted, however, that the absence of a nonbiological Ce-containing control prevents the observed performance enhancement from being attributed exclusively to a uniquely proven biological incorporation pathway.

Overall, this work demonstrates that bioassisted Ce functionalization is a promising route for tuning diatom-derived biosilica/carbon anodes.

Supplementary Material

ao6c03561_si_001.pdf (1.1MB, pdf)

Acknowledgments

This work was supported by the Scientific and Technological Project of the Yunnan Precious Metals Laboratory (Grant No. YPML-20240502044, YPML-2023050213).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c03561.

  • Calculation of diatom cell density; cerium ammonium nitrate concentration exploration; growth curves and growth kinetics of Cyclotella cryptica; ICP-MS analysis of Ce and Si contents; SEM images of pyrolyzed diatom frustules; DTG curves and thermal decomposition parameters; pore-size distribution curves and textural properties; SEM images of electrodes after cycling; electrochemical impedance fitting parameters; abbreviations and sample labels (PDF)

The authors declare no competing financial interest.

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