Efficient stacking of glucose/oxygen microfluidic biofuel cells using a single-streamflow channel

The stacking of microfluidic enzymatic biofuel cells (EBFCs) is a lucrative approach to achieve the targeted power requirements of microelectronic devices. Co-laminar flow based microfluidic EBFCs favour easy and cost-effective microfabrication owing to membrane-less architecture. However, stacking of these unit cells in large number is inefficient due to cross-mixing issues, especially in the cells located downstream of the stack. In this article, we present two-dimensional stacking of four single-streamflow-based EBFCs in a cascade-style serpentine-shaped microfluidic channel. A novel attribute of the device is the identical fuel utilisation in all cells due to mixed single electrolyte stream, which results in stable connection efficiencies. In addition, a novel composite material was used as an air-breathing biocathode implementing the cathodic enzyme covalently immobilised onto carboxyl multiwalled carbon nanotubes (MWCNTs). Negatively charged carboxyl MWCNTs were conformally attached to the highly porous structure of Toray carbon paper (TCP) using positively charged polyethyleneimine (PEI) by electrostatic layer-by-layer (LBL) deposition. At the bioanode, the enzyme and mediator were immobilised on the MWCNTs by a cross-linking method. The experimental results showed that enzyme immobilisation increased proportionally with the number of PEI/MWCNTs bilayers on TCP. When the bioelectrode was analysed individually with five layers of MWCNTs via cyclic voltammetry (CV), the catalytic performance was increased by 4.4-fold. Moreover, the unit cells in the microfluidic stacking device were connected in various configurations i.e., all in series, all in parallel, and series/parallel combined configuration. The connection efficiencies obtained for all the configurations were stable, thereby validating the potential of stacking single-stream microfluidic EBFCs in large number to satisfy the power requirements of small-scale electronic devices.