Layer-by-layer assembling redox wood electrodes for efficient energy storage
Abstract
The exploration of redox-active organic materials and low tortuous thick-electrodes is attractive for energy storage. The in-situ valorized lignin on raw wood surface accompanied by layer-by-layer deposition of electro-active materials endow such spatially distributed wood electrodes with high specific capacitance. Here, we report a layer-by-layer assembled
Keywords
INTRODUCTION
The ever-developing market of wearable electronics, electric vehicles, and artificial intelligence has encouraged research on large-scale electrochemical energy storage (EES) devices. EES devices are facing growing demands for high energy density, fast energy storage, extended cyclic life, environment friendliness, and affordability[1,2]. Considerable research efforts have been made to modulate the crystalline structure and morphology of advanced electrode materials for high-performance EES devices over the past years[3-5]. Unique morphologies of nanomaterials have revealed extraordinary chemical and physical characteristics, dramatically different from their corresponding bulk components. Nanostructured electrode materials store a hefty amount of energy and charge/discharge faster if the electrode is thin and loaded with a small active mass[6]. However, the scaled-up hybrid nano-electrodes with high mass loading offer sluggish charge transport kinetics, low energy/power density, and poor structural/chemical stability - limiting their commercial applications. In most cases, an increase in the electrode thickness (ca., ≈100 µm) results in slower kinetics with hindered ion diffusions[7] and thus increased impedance and low energy storage performance[8]. This occurs due to the restricted availability of electrolyte ions to active sites and impeded electron transport inside a fractured structure[9]. Nowadays, these challenges are partially solved by designing three-dimensional (3D) electrodes featuring low tortuosity, fast electron transport structure, and high mass loading capability. The thick-electrode designs are attractive due to their high surface area, fast ion/electron transport and profligate electrochemical reactions offered by nanocrystalline subunits embedded into the highly porous scaffold. Such design also addresses some battery problems, i.e., the hierarchical porous structures accommodate large volume changes during charge/discharge to create stable and durable electrodes.
Natural selection, over billions of years, has endowed organisms with the ability to conduct efficient and optimal metabolic reactions that are impossible for artificial technology to match. Importantly, their energy transduction mechanisms are based on earth-abundant and eco-friendly elements. The “reverse engineering” of these biological entities (ca., important molecules, their products, or even themselves) provides good inspiration for developing next-generation EES devices[10]. For the metabolism, the energy transport from one complex to another in organisms is analogous to electrochemical reactions of EES devices where electrons and ions transport between two electrodes during the charge/discharge process. Such intrinsic redox reactions of organic molecules can also carry out energy transductions in EES devices[11-13]. Some studies have taken advantage of redox-active natural materials to design new electrodes, e.g., biomolecules extracted from biological sources used as active redox materials for EES with high gravimetric energy density[14-16]. Furthermore, their molecular structure is tunable to optimize redox activity and provides large potential for functionalization with other electrochemically active components. Unfortunately, the extraction and processing of redox-active molecules are time- and energy-consuming and require excess chemical utilization[17]. Researchers are keen to explore natural materials in their original form via in-situ modifications to make bioactive molecules environment-friendly and cost-effective. Natural wood (NW) possesses a hierarchical structure composed of aligned cellulose fibers embedded in the matrix of hemicellulose and redox-active lignin. The well-aligned microchannels, micro/nanopores and cellulose nanofibrils in wood offer non-competitive multi-phase transportation for liquids and gases[18]. Especially, the large hierarchical pores with charged surfaces and a swelled polymer matrix offer efficient ionic transference when filled with aqueous or gel electrolytes. The interconnected nanopores open in wide lumina, making wood an ideal low tortuous scaffold. Most importantly, the lignin contains redox-active molecules potentially beneficial in energy storage after slightly modifying the wood's surface.
In this work, we herein report the facile preparation of redox wood and its further layer-by-layer deposition of carbon nanotubes (CNTs) and polypyrrole (PPy) to generate hybrid electrodes for energy storage in supercapacitors. The chemical treatment on raw wood not only enhances the specific surface area by generating extra nanopores but unlocks molecular chains of lignin to enhance the redox-activity and functionalizability of treated wood (TrW). The CNTs were deposited on wood via sonochemical reaction to generate redox and conductive CNTs@TrW composite. Moreover, PPy nanorods growing in-situ on wood surfaces and inside channels afford high mass loading in thick-electrode design. The hierarchically interconnected pores opening in wide lumen provide low tortuosity electrolyte transport pathways [Figure 1].
Figure 1. (A) Mild delignification and layer-by-layer construction of hybrid wood composites. Optical images of the corresponding samples and the schematic illustration of morphology evolution inside wood cell walls. (B) NW, (C) TrW, (D) CNTs@TrW, (E) PPy@CNTs@TrW.
The as-prepared ultra-thick wood electrode exhibited a high areal capacitance of 1.46 F cm-2, an extraordinary energy density of 0.983 mWh cm-3 (3.68 Wh kg-1), and a remarkable power density of
EXPERIMENTAL SECTION
Materials and reagents
The balsa wood was purchased from a vendor at a local market in Pakistan. The ultra-dry dimethylformamide (DMF) with ≥ 99.8% purity, 4-dimethyl aminopyridine (DMAP) with ≥ 99.0% purity, pyrrole monomer with ≥ 97.0% purity, and FeCl3 catalyst (≥ 95.0% purity) were procured from Merck. Additionally, from Sigma Aldrich, we purchased non-carboxylated multi-walled CNTs with specifications of 6-9 nm diameter and 5 µm length (≥ 95% purity), NaOH (≥ 99.9% purity), Na2SO3 (≥ 98.0% purity), HNO3 (≥ 90.0% purity), and H2SO4 (≥ 95.0% purity). None of the compounds underwent extra purification before use.
Experimental procedure
Preparation of TrW, CNTs@TrW, and PPy@CNTs@TrW
The NW samples were boiled at 95 °C in a 150 mL solution of NaOH (2.5 M) and Na2SO3 (0.5 M) for 1.5 h for delignification. The samples were carefully washed with deionized (DI) water and ethanol and then frozen in a refrigerator for 30 min before being freeze-dried overnight to obtain TrW substrates. The CNTs were first functionalized in a round bottom flask by acidic treatment with HNO3 and H2SO4 (60:10 ml) for 48 h at 120 °C followed by careful washing with DI water and drying for 24 h at room temperature. The carboxylated CNTs were then dispersed into 50 mL of DMF at a concentration of 2 mg/mL by 1 h ultrasonication to obtain a well-dispersed CNT suspension. For the deposition of CNTs on TrW substrates, the samples were immersed into this suspension followed by the addition of a DMAP cross-linker. The reaction mixture was degassed and ultra-sonicated for 4 h to obtain CNTs@TrW electrodes. The procedure was repeated five times to obtain optimal CNT deposition on the TrW substrate, with wood samples being freeze-dried after each run. For the deposition of PPy on TrW and CNTs@TrW, the samples were further dipped into HCl solution (0.1 M, 80 mL) and degassed for 10 min followed by addition of pyrrole (2 g) monomer. The reaction mixture was cooled to 4 °C in an ice bath followed by the slow addition of catalyst FeCl3 (0.49 g) in aqueous solution (60 mL). After 12 h of polymerization, PPy@TrW and PPy@CNTs@TrW electrodes were collected by repeatedly washing with 0.1 M HCl, DI water and ethanol before being dried overnight at 60 °C in an oven.
Characterization
The scanning electron microscopy (SEM) analysis of the sample morphology was performed using a TESCAN/SOLARIS GMH instrument. The transmission electron microscopy (TEM) images were observed using JEOL JEM-F200 at 200 kV. The TEM samples were prepared on copper mesh by scratching powder from electrode surfaces. The X-ray diffraction (XRD) patterns were recorded using Cu Kα radiation
The CHI 760D electrochemical workstation was used for the electrochemical measurements. The electrochemical performance was evaluated in a three-electrode system in 1 M H2SO4 electrolyte, where PPy@CNTs@TrW (or CNTs@TrW, PPy@TrW) was employed as the working electrode, platinum sheet as the counter electrode, and Ag/AgCl as the reference electrode. The areal specific capacitance of electrodes is computed as
Or
where I, ∆t, ∆V, A, S, and dV are current, discharge time, electrode area, scan rate and potential window, respectively.
The energy density (E) and power density (P) of electrodes are calculated as
where CA, ∆t, and ∆V are areal-specific capacitance, discharge time and voltage, respectively.
RESULTS AND DISCUSSIONS
Wood modification and characterization
The cellulose fibers of wood are naturally arranged in the polymer matrix of lignin and hemicellulose to form a highly hierarchical porous structure. The porosity of wood comes from the systematic assembly of nanoscale constituents, which arrange themselves in a way to prepare tube-shaped vessels, tracheids, and fibers[19]. These tube-shaped structures are further embedded with micro to macroscale porosity in the form of pits and nanopores within the lignin matrix. This hierarchical porosity is naturally designed in wood to support the transport of nutrients, water and ions in the entire tree. Here, we performed a mild delignification to remove part of lignin and hemicellulose from cell walls and middle lamella by boiling NW samples in NaOH/Na2SO3 solution for 1 h. The chemical treatment removed plenty of lignin and hemicellulose, leaving voids in the polymer matrix and lignin structure via cleavage of C-O bonds. The rest of the lignin matrix was left intact with some of the opened molecular structure of the polymer matrix and cellulose. The TrW was freeze-dried in the presence of NaOH/Na2SO3 to preserve porosity. The drying process helped to reassemble the lignin matrix through hydrogen bonding, and the presence of NaOH/Na2SO3 crystals facilitated porosity preservation.
The SEM images reveal the difference in the surface roughness before and after the delignification of wood samples. Upon closer inspection of the NW images, it is clear that the interior of the cell wall is completely smooth [Figure 2A-D]. At the same time, the roughness in the cell walls of TrW samples is visible in SEM images. The smoothness of the NW surface is due to uniform surface coverage by the lignin matrix and residual nutrients/wax left over the cell wall surfaces. The TrW images show exposed nanostructure inside the lumina, and the cell wall surface gives a rough and bumpy appearance [Figure 2E-H]. The natural 3D honeycomb structure of NW was unaffected by the mild delignification, while removing lignin/wax created the nanoscale pores. On the other hand, EDS analysis reveals about 17% decrease in C atoms (based on C, O, N, S, Na, and Fe being 100% cumulatively) due to lignin and hemicellulose removal after delignification
Figure 2. The microstructural analysis and elemental mapping of NW and TrW through SEM and EDS. (A) Top-view SEM image of NW showing open large pores - beneficial for cell wall surface modification and layer-by-layer assembly of nanomaterials, (B-D) Side-view SEM images of NW cell wall surface from low to high resolution showing vertical channels and smooth cell wall surface. (E and F) EDS elemental maps of NW showing Oxygen (O), Carbon (C), and Nitrogen (N) as major components and Sulfur (S), Sodium (Na), and Iron (Fe) as minor constituents of NW. (G and H) Followed by the top-view SEM images of TrW showing decrease in channel width and modified lignin at the surface. (I and J) Side-view SEM images of TrW with modified lignin at cell wall surface. (K and L) EDS elemental maps of TrW showing O, C, and N as major components and S, Na, and Fe as minor constituents of TrW.
The effect of chemical treatment on specific surface area and porosity was quantified through adsorption-desorption isotherms of CO2. Both factors affect the functionalizability of the wood scaffold and microfluidic transport characteristics, wherein improved specific surface area and porosity are favorable. All the adsorption-desorption isotherms [Supplementary Figure 1A and B] showed typical type IIB behavior wherein micropores filled at low relative pressure (P/Po = 0-0.2)[20]. Remarkable mesoporous adsorption was evidenced by continuous adsorption in the intermediate relative pressure range. The steep adsorption in a high relative pressure range indicates exceptional adsorption by macropores of treated and untreated NW. The freeze-dried TrW samples showed enhanced adsorption in the micro and mesoporous region
The chemical surface modification of wood was also evaluated with FTIR and XPS. Bands of Most NW become more apparent after the wax is removed by chemical treatment. The results suggest a slight modification in the wood surface with mainly preserved wood components
The mild delignification removal of wax from the wood surface makes the surface functionalities clearer, as indicated by the XPS spectra [Figure 3A and B]. This implies that chemical treatment made the surface lignin dissolve in the form of debris and enriched the material surface, resulting in excess lignin coverage. The NW exhibits the main elements as C (285 eV) and O (532 eV) and a small amount of N (399 eV) from the XPS full survey scan. The O/C ratio can be used to assess the composition of the outer surface of NW, including carbohydrate, lignin and extractive content [Supplementary Table 2]. The O/C ratios of NW and TrW samples remained close to the theoretical values of lignin (0.33), implying that lignin is the outer surface of scaffolds[34]. The surface coverage by lignin in the TrW sample remained 100% despite its total amount being reduced. Based on the carbon atoms in wood, the C1s peak [Figure 3C and D] is deconvoluted into four subpeaks: C1 containing C-C/C-H groups mainly corresponding to lignin; C2 and C3 having C-O/C-OH and C=O/O-C-O based on carbohydrate; C4 referring to O-C=O groups present in carboxylic acids and other substances[34]. The mild delignification decreased the degree of lignin polymerization, increasing the amount of C1 and C4 while decreasing the relative contents of C2 and C3. More phenolic hydroxyl radicals and lignin degradation products are exposed, which can enhance redox activity and functionalization of the TrW scaffold. The O1s peak for TrW is deconvoluted into three subpeaks: O1 containing O-C=O associated with lignin; O2 containing C-O of cellulose and hemicellulose; O3 containing C-OH associated with lignin
Figure 3. XPS survey spectra of (A) NW and (B) TrW. Deconvoluted C1s spectra of (C) NW and (D) TrW. Deconvoluted O1s spectra of (E) NW and (F) TrW.
Lay-by-layer assembling thick-wood hybrid
The highly porous and hydroxy-enriched TrW substrates were ultra-sonicated in carboxylated CNT dispersion in the presence of DMAP. The chemical immobilization of CNTs onto TrW cell walls via esterification was accelerated under the catalysis of DMAP. The freeze-thaw process then dried the samples, and such a two-step procedure was repeated five times. SEM images reveal a well-organized porous CNT network on the cell wall surface [Figure 4A-C]. Several CNTs have arranged themselves into lengthy bundles - clusters of CNTs self-assembled into a conductive network layer. The CNT networks exhibited high mesoporosity with cross-linking manners. The obtained CNTs@TrW electrode thus presents a good hierarchical structure with implanted conductivity, where cell walls with cellulose fibers embedded in lignin matrix are coated with highly conductive layers and hollow tube-shaped lumen channels open for electrolyte transport [Figure 4D]. The conducting layers facilitate redox reactions of surface lignin as a porous interface and pathways for electron transport. The cell walls can be saturated with electrolytes transported by cellulose fibers. Electrolyte-filled lumens accompanied by micropores form low-tortuous channels, forming a multi-phase transport system to efficiently shuttle ions across thick-wood electrodes.
Figure 4. (A) Top-view and (B) cross-sectional SEM images of CNTs@TrW. (C) High magnification view of CNTs@TrW cell wall. (D) Schematics of the charge transport mechanism in thick CNTs@TrW hybrid. The cell walls have cellulose fibers embedded in the lignin matrix, where the cell walls can be saturated with electrolytes transported from cellulose fibers. The cell walls are coated with CNTs to enable electron transport. The lumen filled with electrolytes makes low tortuosity channels for transporting ions to neighboring micropores.
The PPy@CNTs@TrW hybrids were further prepared by in-situ redox polymerization of PPy on CNTs@TrW substrate. The slow deposition of PPy on the cell wall surface afforded a wool-like, highly porous structure. The PPy started aggregation around CNTs and spread three-dimensionally to make a highly porous nanostructure [Figure 5A-G]. The intercellular space pores in cell walls and ray cells were partially filled with nanorods. The PPy nanorods are aggregated into nanofibers to form an interconnected fiber mesh in the lumen connecting the cross-section of cell walls. The conductive structure is highly porous at the microscale and resembles soft wool-like composites. At the nanoscale, the well-separated PPy nanorods construct a highly porous structure. For reference, we also prepared PPy@TrW without CNTs by following similar redox polymerization of pyrrole. The resultant binary composite reveals the growth of PPy nanoball on the cell wall surface and lumina to make a full coverage [Figure 5H-M]. Compared to PPy@CNTs@TrW SEM images, less mass loading was observed on cell walls of PPy@TrW, indicating the positive role of CNTs in guiding the self-assembly of pyrrole to polymerize in-situ on cell walls.
Figure 5. (A) In-situ redox polymerization of pyrrole on CNTs@TrW. (B-D) Top-view SEM images of PPy@CNTs@TrW at different magnifications, showing uniform PPy coating on CNTs surface. (E-G) side-view SEM images of PPy@CNTs@TrW at different magnifications, showing a high amount of PPy nanorods aggregated on the cell wall surface. (H-J) Top-view and (K-M) side-view SEM images of PPy@TrW showing uniform PPy coating on the wood surface. The images show a limited amount of PPy aggregation on the TrW surface.
The TEM and EDS analyses further give insight into structure and composition of PPy and PPy@CNTs deposited over wood surfaces. The PPy, depicted as uniform solid nanoballs with an average of 80 nm in diameter, aggregates at the wood surface to form a conductive coating [Figure 6A-C]. Meanwhile, PPy@CNTs samples show a wool-like nanostructure wrapping around CNTs and extending towards the substrate surface for interfacial coating between wood and CNTs [Figure 6D-F]. The results indicate that the interaction between the CNT surface and PPy during polymerization significantly influenced the wool-like nanostructure formation of PPy, as observed in SEM. In this core-shell structure, the CNTs provide chemical and mechanical stability and excellent conductivity, while porous structure of PPy takes lead in providing active sites for energy storage through high surface area. On the other hand, the EDS analysis shows C as the dominant element followed by N in both structures with O also being part of it - indicating the presence of oxygen functionalities in the structure as well. Here, the presence of Fe is less than 0.07 % in both cases, showing complete washing of the FeCl3 catalyst from the structures.
Figure 6. (A-C) TEM and EDS analysis of PPy nanostructure, after scratching powder from PPy@TrW electrode. (D-F) TEM and EDS analysis of PPy@CNTs nanostructure, after scratching powder from PPy@CNTs@TrW electrode. (G) FTIR plots in 850-1,800 cm-1 region for PPy@TrW and PPy@CNTs@TrW. (H) XRD plots of TrW, PPy@TrW and PPy@CNTs@TrW.
The deposition of PPy on CNTs@TrW or TrW can be revealed from FTIR analysis
Electrochemical performance
The energy storage properties of our TrW-based composites as electrodes were first evaluated in three-electrode electrochemical systems. The cyclic voltammetry (CV) of CNTs@TrW and CNTs@NW electrodes exhibit clear deviance from the typical rectangular trace corresponding to CNTs, where a well-defined pair of redox peaks are observed at 0.5 and 0.35 V versus Ag/AgCl, respectively, in 1 M aqueous H2SO4. The electric double layer capacitor (EDLC) behavior, in combination with the Faradaic reaction, ensures enhanced charge transport [Figure 7A and B]. The faradaic redox reaction can be assigned to the Q/QH2 pair from raw lignin (QH2
Figure 7. (A and B) CV plots of CNTs@NW and CNTs@TrW at 100 mv s-1 showing distinct redox peaks. Enhanced redox peaks were observed in CNTs@TrW samples as compared to CNTs@NW. (C) Schematic elaboration of redox reactions supported by redox-active lignin in supercapacitor electrodes.
The galvanostatic charge/discharge (GCD) process was further performed at an increasing current density from 0.4 to 5.0 mA cm-2 in the potential window of 0 to 1.0 V [Supplementary Figure 4]. Unlike symmetrical triangle GCD traces observed for most carbon electrodes, the CNTs@TrW electrode exhibits distinct tailing in the discharge process at current densities below 1.0 mA cm-2. Similar but smaller tailing GCD traces are found for both CNTs@NW and PPy@TrW electrodes, further confirming the redox behavior of raw lignin
In contrast, PPy@CNTs@TrW thick-electrodes exhibit combined EDLC and pseudocapacitive behavior. The CV curves showed a nearly rectangular behavior in the potential window of -0.1 to 0.8 V at a wide scan rate increasing from 10 to 100 mV s-1 [Figure 8A]. The GCD curves of PPy@CNTs@TrW are nearly triangular in current densities ranging from 1 to 50 mA cm-2 [Figure 8B]. Significantly, PPy@CNTs@TrW electrodes exhibited high areal capacitance of 1.46, 1.36, 1.067, and 0.61 F cm-2 at the current density of 2, 5, 10, and 50 mA cm-2, respectively [Figure 8C]. Compared to PPy@TrW, these electrodes showcased 30 times higher capacitance. This improved capacitance indicates the importance of the conductive network between the cell wall and electrochemically active material. The porous CNT networks templated the PPy network to grow in mesh form providing high surface area and ensuring fast charge transport to each active site. Combining a conductive network and spatially distributed nanomaterial makes an ideal design of 3D thick-electrodes.
Figure 8. Electrochemical profile of PPy@CNTs@TrW thick electrode. (A) CV curves of PPy@CNTs@TrW at scan rate increased from 10 to 100 mV s-1. (B) GCD curves of PPy@CNTs@TrW at different current densities. (C) Correlation between areal and volumetric specific capacitance and current density of CNTs@TrW and PPy@CNTs@TrW. (D) EIS profile of CNTs@TrW, PPy@TrW, and PPy@CNTs@TrW having full-scale graph and zoomed one to show semicircle. (E) Ragone plot of our thick PPy@CNTs@TrW electrodes with typical carbon electrodes in the literature. (F) Capacitive retention over charge-discharge cycles and plot of gravimetric energy density against gravimetric power density.
Let us analyze Nyquist plots by considering electrode (Re), electrolyte (R∞), and internal resistance
The PPy@CNTs@TrW electrode delivered a maximum volumetric energy density of 0.983 Wh cm-3
CONCLUSIONS
In conclusion, we successfully in-situ valorized raw lignin of wood scaffold via C-O bond cleavage to use it as a redox-active material in low-tortuous thick-electrode. The electrodes were prepared via simple chemical treatment of raw wood followed by layer-by-layer deposition of electroactive nanomaterials. The chemically treated wood is a one-of-a-kind mesoporous structure with oxygen functionalities in its rich surface lignin and multi-phase ion transport system composed of cell walls running along thickness and micropores running horizontally. The as-prepared electrodes exhibited excellent redox activity in 1M H2SO4 electrolyte - storing protons in lignin structure. The layer-by-layer assembled PPy@CNTs@TrW. The resulting PPy@CNTs@TrW electrode with a thickness of 1.5 mm and a high mass loading of 20 mg cm-2 delivered a high areal capacitance of
DECLARATIONS
Acknowledgement
We gratefully acknowledge the financial support from the National Natural Science Foundation of China, Tan Kah Kee Innovation Laboratory, and the Key R&D Program of Natural Science Foundation of Jiangsu Province. Tanveer F thanks China Scholarship Council and Nanjing Municipal Government for the Scholarships.
Authors’ contributions
Methodology, investigation, and writing manuscript: Farid T
Project administration and funding acquisition: Tang W
Discussed the whole paper: Razaq A, Hussain S
Performed the measurements: Wang Y
Conceptualization and supervision: Tang W
Availability of data and materials
The data supporting our work can be found in the Supplementary Materials.
Financial support and sponsorship
This work is supported by the financial support from the National Natural Science Foundation of China (Grant Nos. 22375170 and 51861145401), Tan Kah Kee Innovation Laboratory (HRTP-[2022]-45), and the Key R&D Program of Natural Science Foundation of Jiangsu Province (BE2019733).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
Supplementary Materials
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Farid, T.; Wang, Y.; Razaq, A.; Hussain, S.; Tang, W. Layer-by-layer assembling redox wood electrodes for efficient energy storage. Energy Mater. 2024, 4, 400041. http://dx.doi.org/10.20517/energymater.2023.96
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