This paper presents the continuous flow operation of membraneless sodium percarbonate fuel cell (MLSPCFC) using acid/alkaline bipolar electrolyte. In the acid/alkaline bipolar electrolyte, percarbonate works both as an oxidant as well as reductant. Sodium percarbonate affords hydrogen peroxide in aqueous medium. The cell converts the energy released by H2O2 decomposition with H+ and OH− ions into electricity and produces water and oxygen. At room temperature, the laminar flow based microfluidic membraneless fuel cell can reach a maximum power density of 28 mW/cm2 with the molar ratio of [Percarbonate]/[NaOH] = 1 as fuel and [Percarbonate]/[H2SO4] = 2 as oxidant. The paper reports for the first time the use of sodium percarbonate as the oxidant and reductant. The developed fuel cell emits no CO2 and features no proton exchange membrane, inexpensive catalysts, and simple planar structure, which enables high design flexibility and easy integration of the microscale fuel cell into actual microfluidic systems and portable power applications.
The advancement of portable power electronics and the continual integration of functionality into a single all encompassing device has created an increased demand on energy supply. These portable applications include not only common appliances such as cell phones, laptop computers, and personal organizers, but also more specialized devices such as clinical and diagnostic tests, microanalytical systems for field tests, and global positioning systems [
A microfabrication method—fabrication inside capillaries using multistream laminar flow—provided the idea for a novel type of fuel cell, which eliminates several of the technical issues related to the use of polymer electrolyte membrane fuel cells (PEMs), such as fuel crossover [
In this paper, the specific power source of a fuel cell is studied that utilizes a unique property of fluid flow at the microscale, multistream laminar flow (Figure
The side view of the E-type planar membraneless microfuel cell.
The dimensions and operating conditions of the microfluidic fuel cells discussed here are such that fluid flow is pressure driven and characterized by a Reynolds number (Re) less than 10. Two different aqueous streams, one containing fuel and the other with oxidant, are introduced into the same channel at
Since the diffusion process at the liquid/liquid interface determines the performance of the membraneless fuel cell, the two key parameters are the Reynolds number
Fuel and oxidant react at the electrodes while the two liquid streams and their common liquid-liquid interface provide the required ionic conductance to complete the fuel cell chemistries. All the aforementioned membrane-related issues can be avoided in the MLSPCFCs studied here. The flexibility and the performance implications of operating MLSPCFCs under “dual-media,” that is, one electrode is acidic and the other one is alkaline condition, will be the focus of this study.
In this communication, first time we introduce sodium percarbonate (
Many experimental studies had been performed in developing membraneless microfuel cells. Recently, Da Mota et al. reported a different membraneless, microfuel cell concept at room-temperature that also lacks a PEM [
Compared to hydrogen, methanol, and formic acid, hydrogen peroxide has a higher overall theoretical open circuit potential and maximum efficiency as indicated in Table
Summary of thermodynamic data for different fuels.
Fuel | Reaction |
|
|
|
|
Max efficiency (%) |
---|---|---|---|---|---|---|
Hydrogen | H2 + 1/2O2 |
2 | 286 | 237.3 | 1.23 | 82.97 |
Methanol | CH3OH + 1/2O2 |
6 | 726.6 | 702.5 | 1.21 | 96.68 |
Formic acid | HCOOH + 1/2O2 |
2 | 270.3 | 285.5 | 1.48 | 105.6 |
Hydrogen peroxide | H2O2 + |
2 | 138 | 176 | 1.83 | 127.54 |
The MLSPCFC has some advantages, such as sodium percarbonate being a cheap, nontoxic, large scale industrial chemical used primarily in detergents and as a mild oxidant. The cell is more environmentally friendly than the DMFC because there is no CO2 emission and the sodium percarbonate fuel can be handled more simply than hydrogen, as it is well-known fact that sodium percarbonate solution is a widespread safe disinfectant. On the performance side, the MLSPCFC generates electric power comparable to a typical air-breathing DMFC when operating in a microchemical channel at room temperature. In addition, the MLSPCFC requires no membrane-electrode assemblies (MEAs). Thus, the cost for the materials is low and the structure of the cell is simple. In this study, a new branch of simplified architectures that is unique from those that have been reported in the literature has been developed by eliminating and integrating the key components of a conventional MEA.
The performance of MLSPCFC using a fuel stream of 0.75 M percarbonate + 0.75 M NaOH and an oxidant stream of 0.75 M percarbonate + 0.375 M H2SO4 was investigated. In aqueous medium percarbonate affords H2O2.
H2O2 is usually known as an oxidant. In an acid electrolyte, the electron-gaining reaction of the oxidant proceeds as follows [
The acid/alkaline bipolar electrolyte illustrated in Figure
Operating principle of membraneless sodium percarbonate fuel cell (MLSPCFC) using acid/alkaline bipolar electrolyte.
The measurements were carried out at room temperature with external resistances and PC controlled digital multimeters, keeping the molar ratio of [Percarbonate]/[H2SO4] = 2 as oxidant and [Percarbonate]/[NaOH] = 1 as fuel. The percarbonate concentration was varied from 0.075 to 0.75 mol/L, a range which is lower than the concentration of normal percarbonate disinfectant. These acid and alkaline solutions with the same percarbonate concentration flowed over the cathode and the anode, respectively. The open circuit voltage,
The polarization curves recorded from these membraneless fuel cells have the same characteristic shape of typical fuel cells with the kinetically limited, ohmic, and mass transport limited regions (Figures
Efficiency of the membraneless sodium percarbonate fuel cell (MLSPCFC).
Fuel cathode |
Fuel anode |
Catalyst | Potential (V) | Current density (mA/cm2) | Electrochemical efficiency % |
---|---|---|---|---|---|
[Percarbonate]/[H2SO4] | [Percarbonate]/[NaOH] | Pt-black | 0.799 | 127.73 | 43.71 |
The molar ratio of [Percarbonate]/[NaOH] = 1 as fuel and [Percarbonate]/[H2SO4] = 2 as oxidant. The electrochemical efficiencies were calculated by dividing the measured potential by the maximum achievable potential.
Variations of cell performance with five assigned fuel concentrations of sodium percarbonate in sodium hydroxide solution: (a) the polarisation curves; (b) the corresponding power density curves.
The lines indicate that the current density increases by increasing concentration of oxidant from 0.075 M to 0.75 M
The curved lines indicate that the maximum power density is almost same for the variation of fuel concentration from 0.75 M to 0.075 M
Variations of cell performance with five assigned oxidant concentrations of sodium percarbonate in sulphuric acid solution (a) the polarisation curves. (b) The corresponding power density curves.
The lines indicate that the current density increases by increasing the concentration of oxidant from 0.075 M to 0.75 M
The curved lines indicate that the power density increases by increasing the concentration of oxidant from 0.075 M to 0.75 M
As shown in Figures
In this experiment, we examined the thermodynamic properties for the overall reaction of the MLSPCFC by using chemical thermodynamic data [
Using the
In addition to the experiment with a single cell, we have been carrying out series and parallel integrations of the cell on a microchemical chip. This simple structure can allow an easier integration process than a micro-DMFC [
A microscale membraneless sodium percarbonate fuel cell (MLSPCFC) was fabricated and its operating behaviour characterized for the first time. Standard microfabrication techniques were used to create the device. The novel microfuel cell utilized a membraneless dual-electrolyte design with sodium percarbonate as both reductant and oxidant. At room temperature, the laminar flow-based microfluidic fuel cell produced a maximum power density of 28.09 mW/cm2. The electrochemical performance of the membraneless fuel cell was examined in detail with respect to several critical system parameters.
The results demonstrated that the performance of the developed membraneless fuel cell enhanced profoundly if the concentration of oxidant in cathodic stream is 10 times larger, and the current density is also increased approximately ten times. But a variation of concentration for [Percarbonate]/[NaOH] at the anode produced only a small influence on the cell performance. Thus, the present experimental results have confirmed that this membraneless microfuel cell is cathodic limited and suggest that it is a crucial factor in improving cell performance to increase the concentration of oxidant in the cathodic stream. In this experiment, we also examined the thermodynamic properties for the overall reaction of the MLSPCFC.
The flexibility of membraneless fuel cells to function with different media allowed the successful operation of mixed alkaline and acidic fuel cells. The membraneless microfuel cell system investigated in this study seems to be a good candidate for feasible application because its performance is comparable to an air-breathing DMFC without CO2 emission. In addition, the development of metal catalysts to accelerate the efficiency of MLSPCFC is in progress. Some further experimental works towards the microchannel and the flow rate of MLSPCFC will be beneficial to verify the present predictions and fulfill the practical utilization in portable power sources.
The MLSPCFC has the advantages of no CO2 emission, use of aqueous fuel, and good cost-efficiency. Furthermore, percarbonate is a cheap, nontoxic, stable, easily handled, environmental friendly, large-scale industrial chemical and is a convenient source of hydrogen peroxide. We expect that the MLSPCFC may be a promising candidate for practical fuel cells to establish a clean and sustainable energy future.
In the MLSPCFC configuration, we used an E-shaped laminar flow channel with catalyst-coated graphite plates of 1 mm (Graphite India, poco grade EDM-3, 0.0001 in. particle size) employed to act as electrodes [
Subsequent electrodeposition of catalyst to the cathode and anode, the E-shaped microfluidic channel structure is molded with PDMS poly(dimethylsiloxane), typically 1–10 mm in thickness, and finally sealed with a solid substrate such as 2 mm thick pieces of PMMA poly(methylmethacrylate) to provide rigidity and supportive strength to the layered system (Figure
Schematic of the E-shaped membraneless laminar flow-based fuel cell with graphite plates molded with PDMS poly(dimethylsiloxane) and sealed with PMMA poly(methylmethacrylate).
Silicon tubing (Instech Solomon PE = 205, I.D. 1.0 mm) is placed to guide the fuel and oxidant into the E-shaped channel systems at the top and to guide the waste stream out at the bottom of the channel. The tubing is inserted into holes that are punched exactly at the three ends of the E-shaped channel design and glued into place [
The fluid flow is regulated using a syringe pump with typical flow rates of ~17
All experiments were conducted using sodium percarbonate