Development of compact enzyme fuel cells based on carbon nanostructures

Earth’s oceans have absorbed more than 90% of the excess heat caused by global greenhouse gas (GHGs) emissions. The increase in greenhouse gases and the subsequent temperature increase is affecting marine ecosystems. Also, counting cyclone Tauktae that recently hit India's western coast, the Arabian Sea has averaged four consecutive years of severe pre-monsoon cyclones compared to 2 or 3 weak cyclones that it used to experience historically. These events have been attributed to ocean warming due to GHGs[1]. 
A hybrid enzyme fuel cell using an immobilised enzyme-based anode, a hydrogel-based separator and a glucose tolerant catalyst-based cathode has been fabricated and demonstrated in the first work. Multiwalled carbon nanotube-pyrene carboxylic acid (MWCNT–PCA) nanocomposite anode has been used for the electrostatic immobilisation of glucose oxidase (GOx) enzyme. The anode worked on the oxygen mediated oxidation of glucose. Agar-polyvinyl alcohol (PVA) hydrogel was made as an economic dual-purpose component in this fuel cell. In addition to acting as a separator between the  anode and the cathode, it also acted as a glucose store which supplied glucose to the anode for oxidation. Reduced graphene oxide-ceria (rGC) acts as the glucose tolerant non-enzymatic cathode. The electrodes used in this case were carbon cloth of area 4 cm2. A sandwiched construction was used to fabricate the device with which an open circuit potential of 140 mV has been recorded with a peak power density of 6.25 μW/cm2 and a short circuit current density of 60 μA/cm2. The fuel requirement per fuel cell in this case was 10 ml of 500 mM glucose solution and the material cost of building this device was calculated to be 4 $/ device.
The second work was based on electrochemical deposition of reduced graphene oxide (rGO) and electrochemical graphting of pABA (para-amino benzoic acid) for the anode and cathode. The anode of the fuel cell was constructed by electrographting pABA in neutral medium over rGO. This is done for anchoring a poly(1-vinylimidazole-co-allylamine) Os(bipy)2Cl polymer/GOx mixture in the anode. A change in orientation of pABA, during electrographting in acid medium is utilised for anchoring Prussian blue (PB) over rGO in the cathode. A membrane less hybrid enzyme fuel cell employing these electrodes is developed where oxidation of glucose and reduction of hydrogen peroxide take place at the anode and cathode respectively. Commercial 3D printing has been utilised for the construction of a low volume fuel cell container so as to house the electrodes and electrolyte. The assembled fuel cell is capable of delivering a power density of ca. 16 μW/cm2 with a short circuit current density of 300 μA/cm2 with the use of 1 cm2 anode and 4 cm2 cathode. The OCV produced by this device is 200 mV. This device was also found to work in a wide range of temperatures (from 5 °C to 55 °C) along with a good shelf life. The fuel requirement per fuel cell in this case was reduced to 3.5 ml of 100 mM glucose and material cost of building this device was estimated to be 1.3 $/ device.
A carboxylic acid functionalised MWCNT (MWCNTCOOH) supported zinc imidazolate framework (ZIF8) was used for GOx immobilisation in the last work. The anode works on benzoquinone (BQ) mediated oxidation of glucose. Cathode of the fuel cell works on high potential reduction of peroxide by horseradish peroxidase (HRP) adsorbed on gold nanoparticle modified MWCNT pyrene butyric acid (MWCNTPBA) composite. Anode and cathode of the fuel cell utilise 50 mM glucose and 10 mM peroxide respectively. The electrodes of this fuel cell were constructed using platinum (Pt) working electrode and graphite electrode for anode and cathode respectively. Construction of the fuel cell is based 1000 μl pipette tips. The device is capable of producing an OCV of 350 mV along with a power density of 55 μW/cm2 and a short circuit current density of 667 μA/cm2. The fuel cell can be repeatably constructed and can perform in a wide temperature range from 5 °C to 55 °C. Fuel requirement for this fuel cell is 1.2 ml of 50 mM glucose which enhances the cost effectivity. The cost for assembling this device was found to be 4.5 $/device.
 

Author(s): Arjun Ajith Mohan

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