FIELD OF INVENTION
The present invention relates to the field of graphene based bioelectrodes for biofuel cells (BFCs).
BACKGROUND OF THE INVENTION
The alarming usage of fossil fuels and the growing demand for the renewable and green energy have promoted the development of novel methods for generating electricity. In this regard, biofuel cells (BFCs) are considered as green sources of energy. BFCs can be worked at low substrate content and can be accepted as the natural energy supply sources for biosensors and other bio-electronic devices. The two main kinds of BFCs are microbial-based and enzyme-based. It is recently reported by the transparency market research of the United States of America that the implantable therapeutic devices business grows at an average rate of 8% every year and is supposed to strike $73.9 billion by 2018. BFCs are devices that employ biocatalysts to transform chemical energy into electrical energy via an electrochemical reaction comprising biochemical processes. On the other hand, whole-cell bodies or systems acquire energy from (microbial-based biofuel cells) or utilize proteins from living organisms (enzymatic biofuel cells) in the electron-transfer series among the fuel electrode and the terminal surface. BFCs are entirely different from traditional energy devices regarding price and their performance in the market. The BFCs are producing electrical energy by utilizing natural materials viz. glucose, ethanol, etc. as fuel.
In the past few years, enzyme-based bioelectronics have increased because of its possible application as a power supply for transportable, implantable and non-invasive medical devices. The enzyme-based BFCs utilizing enzymes as a reaction initiator are worth consideration due to their capability to transform chemical energy directly into electrical energy. The vast numbers of enzymes can be used for the biocatalytic transformation of chemical energy connected with the fuel into electrical energy. In contrast, traditional energy devices employing metal inorganic catalysts such as Pd, Pt, or Ru which have high redox potential are not suitable for the reduction of particular fuel. Enzyme-based BFCs allow ease of configuration, moderate operational temperature, biocompatibility and an extensive range of available fuels compared to conventional fuel cells. Other advantages involve, enzymes are easily produced to a large scale and genetically modified by biotechnology techniques.
Despite advancement in BFCs, the practical applications of enzymatic BFCs remains a key issue because of their low power density, short lifetime, operational stability and the accumulation of unwanted materials by biological elements on the electrode surface. To overcome the aforementioned issues, efforts have been focused on protein functionalization strategies for achieving stable enzymes structures and for optimizing the approachability of their artificial active sites to improve the electronic charge transfer with electrode surfaces. Also, notable developments have been made by employing improved electrode materials and enzyme functionalization techniques. BFCs, in context, are of particular importance due to their ability to manufactured in small devices and precisely work very closer to the redox potential of the enzyme. Thus, BFCs play a paramount role in implantable devices, biosensors, drug delivery devices, nano-biobatteries, and bioelectronics devices.
Recently, there has been several attempts have been reported towards the use of graphene along with the electrode used in the BCFs. Graphene is an atom thick 2D semi-conducting carbon nanomaterial possessing honeycomb lattice arrangement of sp2 hybridized carbon atoms with very high aspect ratio. Few layered graphene is more suitable for device fabrication especially where on/off switchable property is desired. Graphene is an attractive electrode material for various electrochemical processes including energy storage and conversions, electrochemical sensors, fuel cells, bioreactors and many other devices. Consequently, graphene has become one of the most utilized material in the design and fabrication of electrode interfaces in the recent times, this is because it (graphene) is additionally cheap, non-toxic and compatible with all biomolecules. Therefore, the use of graphene along with electrode for BCFs held promise to fulfill a long standing and unmet need for development of an efficient process for manufacturing BFCs with desirable characteristics.
SUMMARY OF THE INVENTION
The present invention provides a solution to the problems associated with manufacturing, layered graphene on silicon carbide(SiC) which is used as electrode material, by developing a process and system for manufactureof layered graphene on silicon carbide (SiC) suitable for BCFs, the process comprising: (i) deposition of graphene on the Si-terminated face of 4H- or 6H- electrodes in an argon atmosphere at a temperature of 1700°C to 2000°C; and (ii) electrochemically modifying SiC with layered graphene obtained in previous step using poly(N-isopropyl acrylamide-co-diethylamino ethyl methacrylate) and glucose oxidase (GOx), wherein the pH during the process is in the range of 6 to 8 and wherein, the temperature is in the range of 20°C to 40°C.
In another embodiment, the invention provides a process for manufacturing a layered graphene on silicon carbide (SiC), wherein the deposition of graphene is epitaxial.
Still in another embodiment, the invention provides a process for manufacturing a layered graphene on silicon carbide (SiC), wherein the deposition of graphene is done by silicon sublimation from silicon carbide (SiC).
In another embodiment, the invention provides a process for manufacturing a layered graphene on silicon carbide (SiC), wherein the deposition of graphene is carried out using 6H- electrodes.
Still in another embodiment, the invention provides a process for manufacturing a layered graphene on silicon carbide (SiC), wherein the electrochemical modification step is carried out at pH 8 and at a temperature of 40°C.
In another embodiment, the invention provides a process for manufacturing a layered graphene on silicon carbide (SiC), wherein the deposition of graphene comprises of: (i) placing a 6H and/or 4H-SiC wafer in a cylindrical graphite growth cell to act as substrate material; and (ii) graphitization of the material by drifting of SiC by difference in vapor pressure of Si and C using SiC platelet as a source material, wherein the distance between source and substrate is minimized to 0.1mm, wherein a growth spacer is used as a separator between source and substrate and wherein the deposition set up is kept in a thermally insulating coil.
Still in another embodiment, the invention provides a process for manufacturing a layered graphene on silicon carbide (SiC), wherein the growth spacer is Tantalum (Ta).
Still in another embodiment, the invention provides a process for manufacturing of zinc sulphate monohydrate powder, wherein the water solubility of the zinc sulphate monohydrate powder is 30 to 40 %
In another embodiment, the invention provides a process for granulation of zinc sulphate monohydrate powder.
Still in another embodiment, the invention provides a process for granulation of zinc sulphate monohydrate powder, wherein conveyer is provided to carry the granules collected from screening sieve to a storage silo and conveyer is provided to carry the granules of odd sizes to the grinder for recycling.
In another embodiment, the invention provides an enzyme modified layered graphene on silicon carbide (SiC) for BFCs.
Still in another embodiment, the invention provides a graphene on SiC having a current density of 5.70 mA/cm2.
In another embodiment, the invention provides a graphene on silicon carbide (SiC) having steps and edges for enhanced molecular reactions on the electrode surface.
Other aspects and salient features of the invention will be apparent to those skilled in the art from the detailed description taken in conjunction with the accompanying drawings of the present application.
DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Cyclic voltammograms of (a) modified 4H-SiC, and (b) modified 6H-SiC in blank solution (absence of glucose).
Figure 2: Cyclic voltammograms of graphitized 6H-SiC polytype modified with poly (NIPAAm-co-DEAEMA) and glucose oxidase (GOx) at the scan rate of 25 mVs-1.
Figure 3: (a) Effect of scan rate on modified 6H-SiC(blank) at 20ºC, and (b) Effect of scan rate on modified 6H-SiC(blank) at 40 ºC.
Figure 4: (a) Effect of different glucose concentration on modified 6H-SiC at 20ºC, and (b) Effect of different glucose concentration on modified 6H-SiC at 40ºC.
Figure 5: (a) Current density Vs glucose concentration, and (b) Power density Vs glucose concentration at 20ºC and 40ºC.
Figure 6: (a) AFM images of 6H polytypes before immobilization of enzyme, and (b) AFM images of 6H polytypes after immobilization of enzyme.
Figure 7: Schematic diagram of biofuel cell (BFC) using glucose as a fuel and ferrocene carboxylic acid Fc(COOH) as a mediator for half-cell oxidation reaction.
DETAILED DESCRIPTION
The various features with reference to the non-limiting embodiments and those illustrated by the accompanying drawings are explained herein in the following description. Descriptions of well-known components and process techniques are omitted so as not to unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein can be practiced and to further enable those skilled in the art to practice the embodiments described herein. Accordingly, the examples should not be construed as limiting the scope of the invention described herein.
The present application discloses an enzyme modified layered graphene on silicon carbide (SiC) for the biofuel cell (BFC) and a method for producing the same. The method disclosed in present application have an improved electrochemical performance and produces a bioelectrodes with enhanced enzyme steadiness suitable for enzymatic biofuel cells for enhanced power output and life of electrode. Initially, epitaxial deposition of graphene was conducted on the Si-terminated face of 4H- and 6H- electrodes by silicon sublimation from SiC in an argon atmosphere at a temperature of 2000°C. Later, the SiC with layered graphene was electrochemically modified using poly(N-isopropylacrylamide-co-diethylaminoethylmethacrylate) and glucose oxidase (GOx).The electrochemical deposition was carried out at different pH values and at different temperatures. Atomic force microscopic investigated the surface morphology of the 4H and 6H SiC substrates before and after enzyme modification. A modified micro-Raman setup confirmed the formation of single layer and few layers graphene on SiC for reflectance mapping. The resultant electrode was applied as a bioanode for enzymatic BFC. Further on, both 6H and 4H-SiC polytypes were compared. The bioanode with graphitized 6H-SiC showed a higher cyclic voltammetry (CV) response in comparison with 4H-SiC with a higher sensitivity to temperature and pH of the solution which may be due to the difference in stacking structure and step bunching during heating to create more active sites for immobilization. The fabricated bioanode was used for the study of ferrocene carboxylic acid mediated half-cell oxidation reaction in the presence of glucose. The current densities of the systems were calculated to be 4.12 mA/cm2 and 5.70 mA/cm2 at 20°C and 40°C in a pH 6, respectively. The electrochemically deposited enzyme could act as a new geometrical electrode on the layered graphene/SiC that contains steps and edges for enhanced molecular reactions on the electrode surface for efficient bioenergy devices.
Fabrication of SiC
The 6H and 4H-SiC wafer surfaces were used as substrate material. This wafer was kept in a cylindrical graphite growth cell. SiC platelet was used as a source material. The source to substrate distance was kept quite small (0.1mm) because the decrease in the distance eliminates the influence of graphite walls in the crucible. Growth spacer was used as a separation between source and substrate. We used Tantalum (Ta) as the growth spacer so that it can work as carbon getter. This set up was kept in a thermally insulating coil (i.e. graphite foam). Graphite foam was heated using a RF generator at approximately ~40 KHz. Growth pressure was varied from 1 to 50 mbar and growth time was 1 hour. Growth temperature was ranged from 1700-2000°C and temperature gradient was applied. (i.e. source has the higher temperature as compared to substrate). Si and C species evaporate and transport from source towards substrate. Because of the difference in vapor pressure of Si and C, Si drifts away from SiC and causes graphitization of the material.
Electrochemical Characterization of the Anode
Suitability of graphitized 4H-SiC and 6H-SiC as the electrode in EBFCs is evaluated by the cyclic voltammetry analysis of the modified bioanode with and without glucose. Figure1a and 1b show cyclic voltammograms (CVs) of the graphitized 4H-SiC and 6H-SiC polytypes electrodes, which were electrochemically modified using poly(NIPAAm-co-DEAEMA) at two different pH valuesi.e. 6 and 8 and at two different temperatures of 20°C and 40°C, in the absence of glucose oxidase (GOx) at the scan rate of 25 mVs-1, respectively.
It has been observed that the oxidation current of the bioanode electrodes based on graphitized 4H-SiC and 6H-SiC polytypes increased significantly with increase in pH and temperatures. Since poly (NIPAAm) is a thermo responsive polymer and the polymer undergoes a reversible phase transition with a rise in temperature. Beneath its lower critical solution temperature (LCST) this polymer exists in the coiled form and these coils unwind upon increase in the temperature and thereby increase the oxidation current.
On the other hand, DEAEMA is a pH responsive polymer and hence an increase in current is observed with a rise in pH values of the solution. The increase in current from the modified electrodes in the absence of glucose has been observed in 6H polytype as compared to 4H polytypes SiC-graphene, which may be due to the presence of graphene bilayer on the surface of 6H polytypes. The bioanode with graphitized 6H-SiC showed a higher CV response in comparison to 4H-SiC with higher sensitivity towards temperature and pH of the solution. The fabricated bioanode with graphitized 6H-SiC was used to study ferrocene carboxylic acid (Fc(COOH)) mediated half-cell oxidation reaction in the presence of glucose.
Figure 2 shows the CVs obtained by graphitized 6H-SiC polytype modified with poly(NIPAAm-co-DEAEMA) and glucose oxidase (GOx) at similar pH and temperature values. The redox peak current linearly increases with an increase in pH and temperature values. Interestingly, catalytic behaviour of glucoseoxidase (GOx) modified substrate also increases with an increase in pH and temperature of the solution. The anodic current starts to increase from more negative potential, which has substantial benefits for the BFC applications.
It is an established fact that more negative potential will increase the cell voltage, which in turn increase the output current of the BFC.Thus, the graphitized 6H-SiC polytype modified with poly(NIPAAm-co-DEAEMA) and glucose oxidase (GOx) electrode showed effective and improved electrocatalytic ability towards mediated electro-oxidation of glucose in the presence of ferrocene carboxylic acid a mediator for half-cell oxidation reaction.
The effect of different scan rate was also investigated with modified 6H polytypes at various temperatures. Figure 3a and 3b show the CVs of graphitized 6H-SiC polytype modified with poly(NIPAAm-co-DEAEMA) and GOx at 20°C and 40°C with different scan rates, respectively. The redox peak currents linearly increased with the scan rates from 25 to 100 mVs-1, reflecting a surface-controlled process. In both cases with the rise in scan rate, the anodic current is increasing linearly. This is because by changing the scan rate more oxidizing species are generated in the probe solution which diffuses through the diffusion layer and hence increase in current is observed.
Figure 4 shows the CV curves for the graphitized 6H-SiC polytype modified with poly(NIPAAm-co-DEAEMA) and glucose oxidase (GOx) electrode in the presence of 0.5 to 20 mM glucose at (a) 20°C and (b) 40°C. The anodic currents of bioelectrodes increased with increasing potential between 0.0 to +0.6 V, reaching a moderately high current at 40°C, however at 20°C bioelectrode displayed relatively lower current response in the chosen glucose concentration range. This indicates high electrocatalytic activity of GOx at the electrode surface aided by the temperature. The catalytic current at 0.3 V are shown as a function of temperature in Figure 4a and 4b.
Biofuel Cell Assembly and Performance
The current density was calculated by dividing the current with the estimated surface area of the modified electrode. CVs was done by using glucose as the probe solution, with an increase in glucose concentration more enzymes is exposed for reaction and hence increase in current is observed. With the rise in temperature, the coiling of poly(NIPAAm) uncoils leading to increasing in oxidation current.
In Figure 5a, linear response is observed in current density on the increase in glucose concentration. This linear response is more in case of 40°C as compared to 20°C. The current densities of the systems were calculated to be 4.12 mA/cm2 and 5.70 mA/cm2 at 20°C and 40°C in pH 6, respectively.
The power density was calculated by using the formulae,
W = P/A ………. (1)
where, P = Vcell× I ………. (2)
Power densities were calculated at 20°C and 40°C for modified 6H polytypes. Power density calculated is 1.45 mW/cm2 and 2.00 mW/cm2 at 20°C and 40°C respectively (Figure 5b).
In Figure 6 (a) it is visible that the surface contains steps and edges for enhanced molecular reactions. After electrodepositing the enzyme and polymer (Figure 6b), surface provides new geometrical dimensions, enzyme covers the steps and edges and hence leads to increase in current.
Also, in biofuel cells oxygen is favoured cathodic electron acceptor and for the determination of complete effectiveness for generation of power reduction kinetics of oxygen can be a vital part. For oxygen reduction reaction kinetics numerous studies have been described with the construction of noble catalysts based on graphene. Process of oxygen reduction involves multi electron reaction by way of four electrons or two electron pathways. Oxygen reduction reaction by 4 electron pathway produces water by direct conversion of oxygen into water, do not form any intermediate and very less amount of energy loss during the process.
Diverse oxygen reduction reaction pathways are showed below:
Four electron pathways:
O2 + 4H+ + 4e- ? 4H2O, E° = 1.229 V vs. NHE ………. (3)
Two electron pathways:
O2 + H2O + 2e- ? HO2- + OH-, E° = -0.065 V vs. NHE ………. (4)
HO2- + H2O + 2e- ? 3OH-, E° = 0.867 V vs. NHE ………. (5)
2HO2- ? 3OH- + O2 ………. (6)
Hydrogen peroxide (H2O2) which is highly reactive is formed as intermediate in two electron pathway of oxygen reduction reaction kinetics and due to high reactivity, it causes damage to electrodes. Expensive platinum electrodes can be replaced by some graphene-based oxygen reduction reaction catalysts. The complete performance of biofuel cells can be altered by cathode materials of graphene because they can tackle with the activation, mass transport and ohomic losses. Moderate kinetics of oxygen reduction reaction causes activation loss (?act) but struggle in the transport of ion at catalysis sites causes ohomic loss (?ohomic) and deficient supply of reactants to the catalysis sites causes mass transport (?conc) loss. Materials based on graphene can be appropriate cathodic catalysts as they can decrease the activation over potential of O2 (498 kJ m-1) improving the electro kinetics of oxygen reduction reaction. Also, graphene materials rise the oxygen-oxygen double bond distance to 2.89 Å spreading the number of active edges and defect sites. Also, to big superficial area, tunnel and hollow structure, graphene easily put up O2 facilitating cleavage of oxygen-oxygen double bond because of bridge manner motion. The action of oxygen reduction reaction catalysts can be increased by the electrons swift transport which is promoted by robust interconnectivity between the pores due to constant pore walls of graphene. Unrestricted electrons movement between molecular O2 and electrode due to huge electrical conductivity of graphene materials decreases the core resistance of electrode. Furthermore, graphene materials offer oxidative stability, biocompatibility, and hydrophobicity with low cost for their effective application in biofuel cells as cathodic catalysts. A typical scheme of biofuel cell (BFC) using glucose as a fuel and ferrocene carboxylic acid Fc(COOH) as a mediator is described in the Figure 7.
In this system ferrocene carboxylic acid oxidizes to lose an electron, simultaneously oxidized ferrocene accepts an electron from gluco-reductase to form glucose oxidase, this reacts with glucose to form gluco-reductase and gluconic acid (Figure 7).
The foregoing description completely discloses the general nature of the embodiment/aspect claimed herein that others can, by applying current knowledge, readily modify and/or adapt for various applications and such specific embodiments without departing from the generic concept, and therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiment. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not for limitation.

Claims:We claim,
1. A process for manufacturing layered graphene on silicon carbide (SiC) suitable for biofuel cell (BFC) applications, the process comprising:
i. deposition of graphene on the Si-terminated face of 4H- or 6H- electrodesin an argon atmosphere at a temperature of 1700°C to 2000°C; and
ii. electrochemically modifying SiC with layered graphene obtained in previous step using poly(N-isopropyl acrylamide-co-diethylamino ethyl methacrylate) and glucose oxidase (GOx), wherein
the pH during the process is in the range of 6 to 8 and wherein,
the temperature is in the range of 20°C to 40°C.
2. The process as claimed in claim 1, wherein the deposition of graphene is epitaxial.
3. The process as claimed in claim 1 to 2, wherein the deposition of graphene is done by silicon sublimation from SiC.
4. The process as claimed in claim 1 to 3, wherein the deposition of graphene is carried out using 6H- electrodes.
5. The process as claimed in claim 1 to 4, wherein the electrochemical modification step is carried out at pH 8 and at a temperature of 40°C.
6. The process as claimed in claim 1 to 5, wherein the deposition of graphene comprises of:
i. placing a 6H and/or 4H-SiC wafer in a cylindrical graphite growth cell to act as substrate material; and
ii. graphitization of the material by drifting of SiC by difference in vapor pressure of Si and C using SiC platelet as a source material, wherein
the distance between source and substrate is minimized to 0.1mm, wherein
a growth spacer is used as a separation between source and substrate and wherein
the deposition set up is kept in a thermally insulating coil.
7. The process as claimed in claim 1 to 6, wherein the growth spacer is Tantalum (Ta).
8. An enzyme modified layered graphene on silicon carbide (SiC) suitable for biofuel cell (BFC) applications produced using the process as claimed in claims 1 to 7.
9. The graphene on silicon carbide (SiC) as claimed in claim 8, wherein having a current density of 5.70 mA/cm2.
10. The graphene on silicon carbide (SiC) as claimed in claims 8 and 9, wherein having steps and edges for enhanced molecular reactions on the electrode surface.