Carbon Nanotube PtSn Nanoparticles for Enhanced Complete Biocatalytic Oxidation of Ethylene Glycol in Biofuel Cells

Abstract

We report a hybrid catalytic system containing metallic PtSn nanoparticles deposited on multiwalled carbon nanotubes (Pt₆₅Sn₃₅/MWCNTs), prepared by the microwave-assisted method, coupled to the enzyme oxalate oxidase (OxOx) for complete ethylene glycol (EG) electrooxidation. Pt₆₅Sn₃₅/MWCNTs, without OxOx, showed good electrochemical activity toward EG oxidation and all the byproducts. Pt₆₅Sn₃₅/MWCNTs cleaved the glyoxilic acid C-C bond, producing CO₂ and formic acid, which was further oxidized at the electrode. Concerning EG oxidation, the catalytic activity of the hybrid system (Pt₆₅Sn₃₅/MWCNTs+OxOx) was twice the catalytic activity of Pt₆₅Sn₃₅/MWCNTs. Long-term electrolysis revealed that Pt₆₅Sn₃₅/MWCNTs+OxOx was much more active for EG oxidation than Pt₆₅Sn₃₅/MWCNTs: the charge increased by 65%. The chromatographic results proved that Pt₆₅Sn₃₅/MWCNTs+OxOx collected all of the 10 electrons per molecule of the fuel and was able to catalyze EG oxidation to CO₂ due to the associative oxidation between the metallic nanoparticles and the enzymatic pathway. Overall, Pt₆₅Sn₃₅/MWCNTs+OxOx proved to be a promising system to enhance the development of enzymatic biofuel cells for further application in the bioelectrochemistry field.

Figure 1

XRD data of the (A) Pt/MWCNT and (B) Pt₆₅Sn₃₅/MWCNTs nanoparticles prepared by the microwave method.

Figure 2

Cyclic voltammetry curves (A) and current peak (B) of EG in the presence of Pt₆₅Sn₃₅/MWCNTs+OxOx (blue/circle), Pt₆₅Sn₃₅/MWCNTs (red/square), or pure enzyme (carbon vitreous electrode in the presence of 0.03 U mL^–1 OxOx) (green/circle). The solid lines refer to the presence of EG; the dashed lines refer to the absence of EG; supporting electrolyte of 150 mmol L^–1 citric acid/phosphate buffer (pH = 5.5), [EG] = 100 mmol L^–1, [OxOx] = 0.03 U mL^–1, scan rate = 10 mV s^–1.

Figure 3

Chronoamperometric assay performed in the presence of Pt₆₅Sn₃₅/MWCNTs+OxOx (blue line), Pt₆₅Sn₃₅/MWCNTs (red line), and control experiment (carbon vitreous electrode in the electrolyte) (gray dash line). Successive additions of 20 mmol L^–1 ethylene glycol (a), glycolic acid (b), glyoxylic acid (c), and oxalic acid (d) at a fixed oxidation potential (0.900 V vs Ag/AgCl). Supporting electrolyte of 150 mM citric acid/phosphate buffer (pH = 5.5), [OxOx] = 0.03 U mL^–1.

Figure 4

Amperometric i–t curves that were measured during long-term bulk EG electrolysis in the presence of Pt₆₅Sn₃₅/MWCNTs+OxOx (blue line), Pt₆₅Sn₃₅/MWCNTs (red line), and pure enzyme (carbon paper electrode in the presence of 0.03 U mL^–1 OxOx) (green line) and control experiment (Pt₆₅Sn₃₅/MWCNTs+OxOx in the electrolyte) (gray dashed line). Supporting electrolyte of 150 mmol L^–1 citric acid/phosphate buffer (pH = 5.5), [EG] = 100 mmol L^–1, Eap = 0.5 V vs Ag/AgCl, [OxOx] = 0.03 U mL^–1.

Figure 5

Distribution of products from EG oxidation (100 mmol L^–1) after electrolysis for 12 h at Pt₆₅Sn₃₅/MWCNTs+OxOx (blue) and Pt₆₅Sn₃₅/MWCNTs (red). Support electrolyte of 150 mmol L^–1 citric acid/phosphate buffer (pH = 5.5), [OxOx] = 0.03 U mL^–1, Eap = 0.5 V versus Ag/AgCl.

Figure 6

Proposed cascade mechanism for ethylene glycol oxidation at the hybrid electrode system Pt₆₅Sn₃₅/MWCNTs+OxOx. Oxidation mediated by Pt₆₅Sn₃₅ (red line) and bioenzymatic oxidation mediated by OxOx (black line).

Figure 7

Power density curves of ethylene glycol/O₂ biofuel cell employing the hybrid system (Pt₆₅Sn₃₅/MWCNTs+OxOx) (blue line) and the metallic system (Pt₆₅Sn₃₅/MWCNTs) (red line) (100 mmol L^–1 EG). Inset: profile in the absence of EG (green, Pt₆₅Sn₃₅/MWCNTs; black, Pt₆₅Sn₃₅/MWCNTs+OxOx). Supporting electrolyte of 150 mmol L^–1 citric acid/phosphate buffer (pH = 5.5), scan rate of 1 mV s^–1, [OxOx] = 0.03 U mL^–1.