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Keywords:

  • biomass;
  • energy conversion;
  • gold;
  • nanoparticles;
  • nanotubes

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

The viability of employing brown macroalgae as a future renewable energy and chemical resource has so far proven to be challenging because of the low efficiency of the energy extraction process from alginate, which forms the principle, difficult-to-degrade component. In contrast to currently employed alginate-metabolizing microbial approaches, we here extract energy from macroalgae, for the first time, through a fuel cell system that exploits the electrochemical oxidation of alginate by gold nanoparticle-decorated functionalized carbon nanotubes without any external input of energy. The analyses suggest that the electrochemical oxidation process induces partial oxidation of alginate and, in addition to bioenergy, also yields valuable chemicals, which paves the way for the future production of energy and feedstock materials from inedible biomass.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

In the quest for renewable energy and chemical resources, lignocellulosic biomass such as corn and sugarcane are currently exploited.13 The food-versus-fuel issue has, however, sparked significant controversy over the future availability and applicability of lignocellulosic biomass. Consequently, there is growing interest in the exploration of inedible materials that can produce biofuels at an efficiency equal to that obtained for lignocellulosic materials. Among inedible lignocellulosic materials, marine macroalgae (i.e., seaweed) stand out as they require no fresh water, fertilizer, or arable land and do not interfere with the human food chain.46 To extract energy from macroalgae (Figure 1), the key step is to release sugars from the algae cell walls, which mainly contain alginate, mannitol, and glucan. Importantly, macroalgae do not contain lignin, which facilitates a higher saccharide yield compared to lignocellulosic materials through a relatively simple process. The second step involves energy extraction from the algae-derived sugars by currently available energy conversion processes. To date, most studies focus on an appropriate microbial platform that converts these sugars to ethanol, which is then used as a fuel in combustion engines and/or direct fuel cell systems.4, 5 Unfortunately, although glucan and mannitol can be almost completely degraded by using recently discovered microbes, the straightforward degradation of alginate remains a formidable challenge.

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Figure 1. Conventional and proposed energy extraction pathways for brown macroalgae.

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In general, to degrade alginate, a polysaccharide composed of 1[RIGHTWARDS ARROW]4 linked β-D-mannuronic acid and α-L-guluronic acid units, into its oligomers, microbes need to be engineered to secrete alginate lyases, which requires a complicated process that involves expensive biomolecules (e.g., antigen 43).4, 7 Alternatively, alginate can be partially oxidized by strong oxidants, or gamma, ultrasonic, or UV irradiation to produce alginate-derived biopolymers for cell and tissue engineering.810 Most of these methods, however, require further purification and are costly and energy intensive, which accounts for their strongly reduced potential for realistic energy generation. Herein, we sought to extract bioenergy from alginate by direct electrochemical oxidation by using fuel cell catalytic materials that are conventionally used for ethanol and monosaccharide oxidation (Figure 1). Although full alginate oxidation is nearly impossible because of the nature of the polymer, we aim to enhance the energy extraction efficiency of the whole biomass-to-energy conversion process through partial oxidation, which simultaneously produces high-value chemicals for further applications.810

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Characterization of the alginate oxidation reaction on catalytic materials

Based on our previous studies on the catalytic oxidation of cellulose, we first investigated alginate oxidation on a conventional Au electrode as alginate and cellulose share the common polysaccharide structure with polyol groups.11 The alginate oxidation reaction was examined by using an Au-coated glassy carbon (GC) electrode in 0.2 % sodium alginate dissolved in different media, which ranged from acidic to basic. Different from basic and neutral solvents, the use of an acidic solution resulted in the precipitation of sodium alginate, thus the measurement could not be performed. Cyclic voltammograms of Au-coated GC in 0.5 M NaOH featured the oxidation peak at 0.43 V in the forward scan and the reduced reduction peak of gold oxide at 0.06 V in the backward scan. This reduction indicates a strong adsorption of alginate and/or intermediates on the Au surface under alkaline conditions (Figure S1 A in the Supporting Information). In contrast, the solution of 0.2 % alginate in neutral solvent (phosphate-buffered saline (PBS), pH 7) did not induce significant changes in the forward and backward peaks, which indicates that alkaline conditions are preferred over neutral or acidic conditions (Figure S1 B). This is in agreement with previous studies on direct biomass-derived fuel oxidation on Au surfaces.1113 Compared to pretreated cellulose, alginate oxidation is advantageous in that it does not require any energy-consuming pretreatment.11 Nevertheless, the fact that the alginate oxidation peak occurred at a positive potential indicated a relatively low power output of this direct alginate-based fuel cell.

To overcome this hurdle, we utilized Au nanoparticles (AuNPs) conjugated to functionalized multiwalled carbon nanotubes (AuNPs/MWCNTs) to enhance both the catalytic activity and total active surface area.14 For a better correlation between the respective geometric surface areas and for comparison to conventional electrodes, we compared AuNPs/MWCNT-modified carbon sheets (AuNPs/MWCNTs/C) with Au sheets (instead of Au on glassy carbon). We considered the structure–activity relationships of AuNP-catalyzed monosaccharide oxidation12 and tuned the size of the AuNPs to find the catalytic system with the highest homogeneity and activity for the alginate oxidation reaction by utilizing carbon nanotubes (Figure 2) of different sizes and loading amounts (Figure S2).

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Figure 2. The effect of the carbon nanotube diameter on the sintered-AuNPs size (upon thermal treatment at 400 °C) and the corresponding alginate oxidation current. A) SEM images of AuNPs on i, i′) small MWCNTs (S_MWCNTs, diameter of 3–20 nm), ii, ii′) medium MWCNTs (M_MWCNTs, diameter of 40–70 nm), and iii, iii′) large MWCNTs (L_MWCNTs, diameter of 110–170 nm). B) Summary of the sintered-AuNP diameter data shown in part A.

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Medium MWCNTs induced the smallest and most homogeneous AuNPs upon thermal treatment (Figure 2, Scheme S1). Both small and large MWCNTs resulted in a bigger AuNP size and less homogeneous distribution than that from medium MWCNTs, which led to a lower total surface area of AuNPs. The optimal system consisted of 6.9 nm AuNPs deposited on medium-sized functionalized carbon nanotubes (40–70 nm in diameter) at loadings of 0.45 mg cm−2 Au and 0.5 mg cm−2 CNTs.

To study alginate oxidation by the catalytic materials, we first performed cyclic voltammetry (CV) within a potential window of −0.4 to 0.8 V versus Ag/AgCl in the absence and presence of alginate in 0.5 M NaOH. The CV scans of the AuNPs/MWCNTs/C electrode resulted in a clear alginate oxidation peak at 0.108 V followed by a gold oxide formation peak at 0.264 V (Figure 3 A).15 In contrast, the Au sheet electrode only demonstrated a small shoulder at around 0.3 V, which indicated the enhanced reaction kinetics on AuNPs owing to the 3D carbon nanotubes.12, 14 Importantly, the current density of the AuNPs/MWCNTs/C electrode was ∼53 times higher than that of the Au sheets, even though the active surface area of Au (calculated based on the Au oxide reduction peak on the backward scans) of AuNPs/MWCNTs/C was ∼24 times larger than that of the Au sheet. In both cases, the alginate oxidation starts at E>0.0 V versus Ag/AgCl, which suggests that the primary reaction is the oxidation of secondary alcohols on a surface with a high OH coverage rather than the oxidation of aldehyde (or hemiacetal) groups at the head of the polymer chain. To further elucidate the peak assignment, we performed the same CV experiment on AuNPs/MWCNTs/C by using 0.2–5 % glucuronic acid in 0.5 M NaOH, the monosaccharide that has a similar structure to the alginate monomer (Figure S3). At the same concentration (0.2 %), the CV of glucuronic acid featured the oxidation peak of secondary alcohol groups at 0.171 V near the alginate oxidation peak, albeit at a higher current density and earlier onset potential. If we increased the concentration of glucuronic acid to 5 %, a clear oxidation peak of hemiacetal groups at negative potential (−0.256 V) was observed, which is not the case of alginate.

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Figure 3. Electrochemical characterization of alginate oxidation on AuNPs/MWCNTs/C and Au sheets. A) For each sample, data was acquired in the first cycle at a scan rate of 10 mV s−1. The electrolyte was 0.5 M NaOH with (/) and without (••••/••••) 0.2 % sodium alginate. B) Chronoamperometry was performed at a potential of 0.1 V. The electrolyte was 0.2 % sodium alginate in 0.5 M NaOH. C) Effect of oxidation current density on sodium alginate concentration. D) Nyquist plots of Au sheet and AuNPs/MWCNTs/C (inset) at different applied potentials in 0.2 % sodium alginate in 0.5 M NaOH.

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To investigate the long-term stability of the material towards oxidation, we conducted chronoamperometry in sodium alginate at 0.1 V (Figure 3 B), which is close to the oxidation potential observed in the voltammogram shown in Figure 3 A. In the first 10 s, the Au sheet exhibited a drastic decrease in current density owing to poisoning from the alginate oxidation. In contrast, AuNPs/MWCNTs/C showed a monotonous decay and reached a steady state after approximately 2000 s, which indicated that poisoning was significantly reduced. Accordingly, the current density of the Au sheet reached its maximum saturated value at 0.1 % alginate concentration (data not shown), whereas that of the AuNPs/MWCNTs/C exhibited a linear increase up to 0.5 % alginate at a much higher current density (Figure 3 C). Furthermore, under the same measurement conditions, the Au sheet demonstrated an impedance that was more than 10 times higher than that of AuNPs/MWCNTs/C, which indicates that charge transfer occurred at a higher rate for AuNPs/MWCNTs/C (Figure 3 D). As the Au loading of the AuNPs/MWCNTs/C (0.45 mg cm−2 Au) was much lower than that of the Au sheet, the increased current density and stability is credited to the enhanced reaction kinetics, which in turn result from the unique architecture of the AuNPs/MWCNTs.

Product analysis of the alginate oxidation reaction on catalytic materials

To explore possible reaction pathways, the products that resulted from the alginate oxidation were analyzed. Based on previously reported studies on alginate oxidation by enzymes, irradiation,10, 16, 17 and strong chemical oxidants,1821 we suggest two possible pathways (Scheme 1). The first pathway is similar to irradiation-induced and enzymatic oxidation processes and involves glycosidic bond cleavage and β-elimination, followed by oxidation at the O[BOND]C site. The second pathway resembles the chemical oxidation mechanism and involves cleavage of the C2[BOND]C3 bond in the monosaccharide unit, followed by electrochemical oxidation to form a binding platform for cell and tissue engineering.21

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Scheme 1. Illustration of the two possible reaction pathways.

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To differentiate between these mechanisms, we subjected 0.2 % sodium alginate to the electrolysis process, which was performed at the AuNPs/MWCNTs/C electrode in comparison with the Au sheet electrode in 0.5 M NaOH. The potential program consisted of the oxidation plateau, which was set at 0.108 V for 60 s, following by a cleaning pulse at −1.2 V for 1 s. This two-potential-step-process was repeated for 12 h then the final solution was collected and analyzed by HPLC, UV/Vis spectrophotometry, and attenuated total reflectance (ATR) FTIR spectroscopy.

The HPLC data depicted in Figure 4 A confirm that after 12 h of electrolysis, the apparent molecular weight of alginate was almost unchanged as the peak position did not move appreciably. However, the decrease of the peak intensity suggests two cases: (1) the alginate concentration was decreased upon electrochemical oxidation followed by glycosidic bond scission and (2) the alginate molecule was oxidized and formed a new molecule that has a molecular weight close to that of the original substance. As there is no new peak in the chromatogram, the latter case seems to happen. Analysis by using a UV detector at 264 nm revealed a new peak in the case of the oxidized sample compared to the original alginate, and NaOH control samples further confirmed this suggestion. To quantify the possible changes in the functional groups of the alginate molecule, UV/Vis spectra were acquired immediately after oxidation. The UV/Vis spectra of all samples demonstrated a band at 264 nm that corresponds to an n[RIGHTWARDS ARROW]π* transition (Figure 4 B). The intensity of this peak, which represents the n[RIGHTWARDS ARROW]π* transition of C[DOUBLE BOND]O functional groups,9 increased slightly in the case of the Au sheet and strongly in the case of AuNPs/MWCNTs/C. This peak might result from the oxidation of the secondary OH groups at C2 and C3 in the monomer ring, which is similar to the mechanism of the oxidation process by strong chemical oxidants.

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Figure 4. Product analysis of the electrochemical oxidation of alginate. A) HPLC analysis of alginate before and after electrolysis. Calibration standard: pullulan (Shodex), column: Shodex OHpak SB-802.5 HQ, detector: refractive index (RID) and UV (inset), flow rate: 0.5 mL min−1, eluent: 0.2 M phosphate buffer (pH 7), concentration: 0.2 %. B) UV/Vis spectra. C) ATR-FTIR analysis of dried sodium alginate in 0.5 M NaOH before and after electrolysis compared to commercial sodium alginate (without NaOH). The molecular weight of P5-800 pullulans are 0.59×104, 2.11×104, 4.71×104, 20.0×104, and 70.8×104, respectively.

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ATR-FTIR spectroscopy was employed to (1) investigate the interaction between NaOH and the untreated sodium alginate and (2) to determine the functional groups involved and generated in the electrochemical oxidation. Upon dissolution in 0.5 M NaOH and subsequent drying, a crystalline structure resulted, which gave rise to drastic changes in the IR vibrations (Figure 3 C). The most significant changes occurred at 3200, 1590, and 1020 cm−1, which corresponds to the ν(OH), ν(COO)asym, and ν(COC)+ν(CCO)+δ(CC) bands (i.e., bands that involve the alcohol group and the glycosidic ring).22, 23 After crystallization, these peaks were strongly diminished, which indicates that the Na+-bound [BOND]OH and COO groups of alginate might induce the dissociation of H-bonded [BOND]OH to form a monodentate coordination with Oδ− atoms. As a result, the initially broad OH peak was sharpened, whereas the strong ν(COO)asym vibration decreased considerably. In contrast, the associated peak at 1410 cm−1, which corresponds to ν(COO)sym, increased as a result of the change in pH.23 If we consider the IR vibrations before and after oxidation, the C[BOND]H peak at 3000 cm−1 decreased after chemical oxidation, which indicates the alteration of the [BOND]CH[BOND]OH bond as a result of the oxidation process. The oxidized sample further showed an increase in the intensity of ν(COO)asym, which indicates that there might be an additional formation of C[DOUBLE BOND]O groups upon the oxidation of [BOND]OH at C2 and C3 followed by C2[BOND]C3 scission (i.e., the aldehyde or ketone groups), which is in agreement with the UV/Vis spectroscopy, HPLC, and electrochemical results. As the products are similar to the reported oxidized alginate used for cell and tissue engineering purposes,21 our data further suggest that the electrochemical oxidation process is not only capable of inducing partial oxidation to generate bioenergy but also yields valuable chemicals (Figure 1).

The performance of direct alginate fuel cells

A fuel cell assembly with an air-cathode chamber and a 1 mL Si anode chamber was employed to evaluate the performance of the as-prepared AuNPs/MWCNTs/C electrode as the anode in comparison with an Au sheet by using 0.2 % alginate in 0.5 M NaOH at room temperature. The AuNPs/MWCNTs/C electrode was freshly synthesized and the Au sheet was cleaned thoroughly by boiling in Piranha solution followed by hot ultrapure water. A cleaned Pt mesh served as the cathode and was pressed on to the opposite side of a Nafion membrane and mounted inside the fuel cell as illustrated in Figure 5. As expected from the electrochemical measurements, the direct alginate fuel cell (DAlgFC) that comprised AuNPs/MWCNTs/C as the anode exhibited a maximum power density of 53.7 mW m−2 (5.37 W m−3), which is 2.1 times higher than that of the DAlgFC with an Au sheet as the anode (Figure 5 A). In addition, although the open-circuit voltage of the Au sheet-based fuel cell (0.62 V) was 0.1 V higher than the AuNPs/MWCNTs/C-based fuel cell (0.53 V), which suggests less activation loss in the case of the Au sheet (Figure S4), the operating voltage steeply decreased with increasing current density. This result indicates that the Au sheet electrode induces a higher internal resistance and mass transfer limitation because of the insufficient alginate active adsorption on the Au surface compared to the AuNPs/MWCNTs/C electrode. Although this is still a factor of ∼20 lower than a direct glucuronic fuel cell (Figure S5), it does fall in the acceptable power range of microbial fuel cells and enzymatic fuel cells that do not consume any external energy (e.g., thermocontrolled bioreactors, pumps for continuously supplied fuels) and function in air-breathing mode with similar reactant concentration.2428 Notably, the testing concentration of alginate was 0.2 %, whereas in the case of glucuronic acid the concentration was 5 %. Furthermore, because the alginate molecule is a hundred times larger than other compared monosaccharides, this difference in power density is expected and acceptable. As this is the first report on direct alginate fuel cells, further improvements both in the catalysts and in the design are needed to advance this type of fuel cell to the stage of practical application.

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Figure 5. Performance of the direct alginate fuel cell that used AuNPs/MWCNTs/C as the anode in comparison with that with an Au sheet anode. A) Polarization and power density curves of direct alginate FCs fuelled with 0.2 % sodium alginate in 0.5 M NaOH. B) Photograph of our fuel cell prototype.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Our findings on the direct electrochemical oxidation of alginate are complementary to the traditional microbial approach and allow the simultaneous production of bioenergy and oxidized alginate feedstock material. As far as we are aware, this is the very first report. The proposed AuNPs/MWCNTs/C anode outperformed the conventional Au sheet electrode, which makes the practical application of alginate fuel cells more attractive. Analysis of the oxidized product suggests a reaction pathway similar to chemical oxidation process by strong oxidants, which results in oxidized alginate that is usable for cell and tissue engineering. Furthermore, in contrast to the energy-costly chemical method, the electrochemical oxidation of alginate provides an economical route to the production of value-added oxidized alginate products and may contribute to the future production of energy and feedstock from inedible biomass.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Materials

Carbon sheets, Au sheets (1.153 g, 25×25 mm), 1,2,3,4-tetrahydronaphthalene (tetralin, 95–98 %), tert-butylamine borane (TBAB), and sodium alginate were purchased from Sigma–Aldrich, USA. Multiwalled carbon nanotubes (MWCNTs), hydrogen tetrachloroaurate(III) tetrahydrate, oleylamine, hexane, phosphate buffer, concentrated sulfuric acid, and ethanol were obtained from Wako Co., Japan.

Fabrication of sintered gold-nanoparticle-decorated carbon nanotubes

AuNPs were synthesized by a burst nucleation method by using TBAB as the reducing agent. Briefly, HAuCl4⋅3 H2O (10 mg), oleylamine (1 mL), and TBAB (1 mL) were mixed to obtain a precursor solution, which was stirred at room temperature for 10 min. A reducing solution was then prepared by mixing oleylamine (100 μL), tetralin (100 μL), and TBAB (0.1 mmol), and slowly added to the precursor solution. The reaction mixture was stirred at room temperature for 1 h. Ethanol (10 mL) was added into the reaction mixture to precipitate the AuNPs, which were collected by centrifugation at 8000 rpm for 8 min. The supernatant was removed and 2 mL hexane followed by 10 mL ethanol were added to the redispersed AuNPs followed by further centrifugation. This procedure was repeated until the supernatant was colorless. Finally, hexane was added to the AuNP solution to avoid aggregation. The final solution contained 15 mg HAuCl4 in 1 mL hexane.

MWCNTs were surface-oxidized by heating to reflux in concentrated HNO3 for 24 h at 140 °C. The acid-treated MWCNTs (2 mg) were sonicated and dispersed in ethanol (1 mL). The mixture was cast onto a cleaned carbon sheet, which, after drying, was used as the substrate for further fabrication of AuNPs/MWCNTs hybrid electrodes (Scheme S1). AuNP solution (100 μL) was slowly dropped onto the surface of the MWCNT-modified carbon sheets to obtain the AuNPs/MWCNTs-modified carbon sheet (2 cm2). The AuNPs/MWCNTs/C was subjected to thermal treatment at 400 °C for 2 h to remove the stabilizing agents on the AuNP surface and firmly immobilize the AuNPs on the MWCNTs.

Characterization of the electrocatalytic materials

The morphology of the AuNPs/MWCNTs was investigated by SEM by using a DB 235 microscope (FEI Co.). All electrochemical measurements were performed by using an AUTOLAB potentiostat–galvanostat PGSTAT12 (EcoChemie, Netherlands) controlled by GPES software (EcoChemie). A three-electrode cell was used for the electrochemical measurements with a platinum rod as the counter electrode and an Ag/AgCl reference electrode (BASi, 3.0 M NaCl, +0.209 V versus NHE at 298.2 K). The as-prepared AuNPs/MWCNTs/C acted as the working electrode. All potentials are reported with respect to Ag/AgCl.

Characterization of the alginate oxidation reaction

Products that resulted from the alginate oxidation were collected and characterized by using UV/Vis spectrophotometry (Shimadzu Co., Japan), HPLC (LC-10 A, Shimadzu, Japan), and ATR-FTIR spectroscopy (Horiba FT-720, Japan).

Fuel cell characterization

For fuel cell characterization, the anode consisted of AuNPs/MWCNTs on carbon sheet (or Au sheet), whereas Pt mesh and Nafion 117 (Dupont Co., USA) functioned as the cathode material and electrolyte membrane, respectively. The fuel cell was operated in air-breathing mode at room temperature. A solution of 0.2 % alginate in 0.5 M NaOH (1 mL) was delivered to the anode chamber by using a syringe pump. The cathode was open to the ambient air. The open-circuit voltages and polarization curves were obtained by using a Solartron analytical 1280C Fuel Cell Testing system controlled by CorrWare (Scribner Associates, Inc.).

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

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