Spider nest shaped multi-scale three-dimensional enzymatic electrodes for glucose/oxygen biofuel cells

Highlights

• A unique spider nest shaped multi-scale three-dimensional substrate is fabricated.

• The bioelectrodes show high enzymatic loading density and catalytic efficiency.

• The glucose oxidase on bioanodes shows great enzymatic activity.

• High power output and good stability of the biofuel cell is obtained.

Abstract

A spider nest shaped multi-scale three-dimensional substrate consisting of reduced graphene oxide (RGO) and nickel foam is fabricated for enzymatic electrodes and biofuel cells. According to the excellent conductivity and large electroactive surface area of this special structure, the enzymatic electrodes show large enzymatic loading density, low electron transfer resistance and high electrocatalytic efficiency. The strong forces in the spider nest shaped structure and the excellent enzymatic embedding method ensure the stability of the glucose oxidase bioanodes and laccase biocathodes. The Michaelis–Menten constant value for the bioanodes to glucose is calculated to be 2.24 mM, which is close to the Michaelis–Menten constant for free glucose oxidase, implying a remarkably high enzymatic activity. Employed the obtained bioelectrodes with great properties, the relative glucose/oxygen biofuel cell has an open-circuit voltage of 0.70 V, with a high output power performance, and a maximum output power of 7.05 ± 0.05 mW cm⁻², owe to the high enzyme loading and low electron transfer resistance of the electrode based on the spider nest shaped structure. After 60 days of periodic storage experiments, the performance of the biofuel cell still maintained 84.2%, showing a good long-term stability.

Introduction

Enzymatic biofuel cells (EBFCs) can utilize various enzymes as catalysts to convert chemical energy into electrical energy [1,2]. Because of its mild operating conditions and good biocompatibility, EBFCs have drawn extensive attention for their great potential in implantable devices, wearable devices, medical detection etc. [[3], [4], [5], [6]] Especially, glucose/oxygen biofuel cells have most been concerned, since the oxidation of glucose provides life-sustaining energy for many organisms, including human beings [7]. Commonly, glucose oxidases (GOx) and laccases (Lac) are employed in glucose/oxygen biofuel cells, due to that they are high-efficiency enzymatic electro-catalyst for glucose and oxygen [[8], [9], [10], [11]].

Although vast amounts of works in this field have been carrying on in recent years, the output performance of the glucose/oxygen biofuel cells is unsatisfactory for actual need [[12], [13], [14]].

Enlarged the surface enzyme loading density on electrodes and deceased electron transfer resistance of electrodes are main strategies for saving the poor output properties of EBFCs at present [15,16]. Employed three-dimensional structure with high conductivity in bioelectrodes could obviously increase the enzyme loading density due to the great surface area of the structure, meanwhile the structure could remain the electron transfer resistance at a low level. Based on the ideas above, the nickel foam and various carbon nanomaterials with three-dimensional structure have been attracted a great attention recently [[16], [17], [18]]. Yin Song and coworkers fabricate a glucose/oxygen biofuel cells based on three-dimensional carbon micropillar arrays coated with reduced graphene oxide, carbon nanotubes and a biocatalyst composite, showing a maximum power density of 196.04 μW cm⁻² [17]. Arman Amani Babadi and partners employ three-dimensional graphene into glucose/oxygen biofuel cells for enhancing the maximum power density to 164 μW cm⁻² [18].

The three-dimensional structure of carbon nanomaterials results in the large maximum power density of the EBFCs in the researches above. However, the maximum power density could be remarkably larger if three-dimensional nickel foam is utilized as substrate. Based on our previous work, the maximum power density of the glucose/oxygen biofuel cell with gold-coated nickel foam can reach to 2.32 ± 0.07 mW cm⁻², which is much larger than the output performances of EBFCs based on carbon materials, due to the smaller electron transfer resistances of nickel foam [19].

However, the through-holes of nickel foam (diameter is about 100–120 μm) is excessively large for enzymatic immobilization, therefore the enzymes can only be loaded on the skeleton of nickel foam, suggesting the waste of space of the holes [19]. If a conductive three-dimensional structure with smaller scale could fill in the holes of nickel foam, greater enzymatic loading density, larger catalytic current and excellent output properties may occur.

Except the electron transfer resistances bringing conductive three-dimensional structure, the electron transfer resistances between enzymes active sites and the conductive structure are also a problem which deserved consideration, because the active sites of enzymes are usually buried inside the non-conductive protein matrices, such as glucose oxidase (GOx) [4]. To enhance the electron transfer rate, employing the direct electron transfer (DET) system by using low-resistance materials (such as graphene and nickel) is a good strategy [13,17,19].

In addition, unsatisfactory stability is another critical effect for hindering the practical application of EBFCs [15]. The method for enzymatic immobilization can greatly affect the stability of EBFCs [20]. Specifically, the enzymatic embedding on electrodes by polymer is a good method so far, owe to that it has negligible negative impact on enzymatic activity [20,21]. And the method can firmly fix the enzymes onto electrode surface [20,21]. Among the polymers with good biocompatibility, the poly (ethylene glycol diglycidyl ether) (PEGDGE) has been selected to immobilize enzymes recently [22]. Zafar research group utilize PEGDGE to embed cellobiose dehydrogenase onto the surface of graphite electrode for fabricating a biofuel cell with outstanding stability [22]. PEGDGE can wrap the enzyme firmly by steric effect, allowing the substrate molecules to go through the polymer and contact with the enzymes, remaining the long-term enzymatic activity.

In this work, we decorate reduced graphene oxide (RGO) onto the nickel foam to fabricate substrate with spider nest shaped multi-scale three-dimensional structure. Specifically, the three-dimensional RGO on smaller scale fill in the holes of the larger three-dimensional nickel foam. After immobilized GOx and Lac onto the electrodes by PEGDGE respectively, the obtained bioanodes and biocathodes are assembled to a member-less glucose/oxygen biofuel cell. The morphology, chemical structure and electrochemical properties of the electrodes with spider nest shaped structure and the performances of the bioanodes, bio-cathodes and obtained biofuel cells are evaluated. The relationship between the spider nest shaped structure and the performances of bioelectrodes and biofuel cells are investigated.