Membraneless hydrogen peroxide micro semi-fuel cell for portable applications

Abstract

This communication describes the first one-compartment hydrogen peroxide semi-fuel cell as a micro-power source for miniaturized applications. This power source is inspired by the high energy content of metals and producing a light-weight, adaptable design that is easy to package. Theoretical specific energy densities of 8600 and 17 000 W h kg⁻¹ are achievable for the power source by oxidation of magnesium (Mg) or aluminium (Al) at the anode side in combination with the reduction of hydrogen peroxide at the cathode side. We have fabricated a cell with Mg as the anode and Prussian blue as the cathode that had an open-circuit potential of 2.3 V and a net maximum power density of 7.5 mW cm⁻². These results are the highest ever reported for a one-compartment H₂O₂-based power source. This cell design can provide a platform for fabricating a new generation of power sources for portable and miniaturized applications.

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Portable microelectronic devices, including cyborg insects,¹ insect-scale robots² and medical point-of-care diagnostic sensors,³ require micropower sources for autonomous missions and operations. These micropower sources should be light-weight and have a high energy density.

Micro fuel cells have been investigated as micropower sources for portable applications.^4,5 However, some problems remain to be addressed. First, two reagents representing the fuel and the oxidant must be introduced to the anode and cathode compartments separately. This generally imposes the need for ancillary components, adding complexity to the design. Second, a membrane should be sandwiched between the electrodes to provide charge transport between them. This membrane-electrode-assembly increases the total weight of a micro fuel cell and adds complexity to miniaturization and packaging.

In order to address these problems, membraneless micro fuel cells with various configurations, have been investigated. Membraneless microfluidic fuel cells^6,7 usually exploit co-laminar flows containing liquid fuel and oxidant reagents in a micro channel. The fuel–oxidant interface functions as a virtual membrane to keep the reagents separated. Power densities of up to tens of milliwatts per square centimetre can be reached. However, microfluidic fuel cells generally need continuous pumping of two streams of liquid into the microchannel to provide a confined liquid–liquid interface and avoid fuel crossover. This results in a reduced net power generation and has prevented the deployment of the fuel cell in practical applications.

Ideally, membraneless fuel cells with selective electrodes should allow the anode and cathode to be present in a single compartment. This type of configuration may facilitate the delivery of reactants to the electrodes with less power. Tominaka et al.^8–10 introduced a one-compartment membraneless microfuel cell with a selective electrocatalyst for oxygen reduction in an air-breathing cathode. Each fuel cell could use methanol, ethanol or 2-propanol containing a supporting electrolyte in a reservoir fabricated in silicon or bendable polymeric substrates. Using full passive delivery of fuel and oxidant, a net power of 10 μW was achieved.

Furthermore, an ideal design for a membraneless microfuel cell should use only one reactant as both fuel (electron donor) and oxidant (electron acceptor) in a single-compartment design. This feature facilitates the fabrication of very compact fuel cells. Hydrogen peroxide has the unique characteristic of being a carbon-free energy carrier,¹¹ which can be exploited as both fuel and oxidant in a membraneless one-compartment fuel cell. Hydrogen peroxide benefits from easy transport and handling in the aqueous phase.¹²

Yamazaki et al.¹³ used hydrogen peroxide as both an electron donor and an electron acceptor in an alkaline one-compartment fuel cell. A maximum OCP of 0.13 V was achieved using Au, Ni, Pt and Pd wires as a selective anode and an Ag wire as a selective cathode.

Running a direct hydrogen peroxide fuel cell in acidic conditions is more desirable for hydrogen peroxide-based energy production and storage.¹⁴ Hydrogen peroxide can be formed under acidic conditions via the two-electron reduction of oxygen using solar power.¹¹ In addition, H₂O₂ is more stable in an acidic environment with lower decomposition kinetics, which increases the utilization efficiency of H₂O₂.¹⁵

Yamada et al.¹⁶ fabricated a direct acidic membraneless fuel cell running only on hydrogen peroxide. An open circuit potential (OCP) of 0.5 V with a maximum power density of 10 μW cm⁻² were achieved using [Fe^III(Pc)Cl] as a cathode material and Ni mesh as an anode. Both electrodes were immersed in a reservoir containing an acetate buffer with 0.3 M H₂O₂ at pH 3. Recently, we improved the OCP and maximum power density to 0.6 V and 1.55 mW cm⁻², respectively, using carbon-fibre paper coated with ferric ferrocyanide (Fe₄^III[Fe^II(CN)₆]₃) as a cathode material and a Ni mesh as an anode. A 0.5 M H₂O₂ solution containing 0.1 M HCl was used as both fuel and oxidant.¹⁴ More recently, Yamada et al. used carbon cloth coated with cyanide complexes as cathode materials and a Ni mesh as an anode, and achieved an OCP of 0.78 V and a maximum power density of 1.2 mW cm⁻².¹⁷

It is very desirable to improve the performance of one-compartment fuel cells for microelectronic portable applications. In this communication, we report a small-scale single-compartment semi-fuel cell using H₂O₂ in acidic conditions with the best ever power output reported for membraneless hydrogen peroxide fuel cells. The cathode comprised of carbon-fibre-based paper coated with Prussian blue (PB)¹⁴ and the anode was made of Al or Mg plates. Both the anode and cathode were immersed in a reservoir containing 0.5 M H₂O₂ with a supporting electrolyte of 0.1 M HCl, as shown in Fig. 1. A literature procedure for PB synthesis was used.¹⁸ Carbon-fiber paper was coated with PB by spraying a solution of PB containing Nafion.¹⁴ Both the Mg and Al plates were 0.25 mm thick, and were purchased from Alfa-Aesar and Good Fellow, respectively. The gap between the electrodes was maintained by a 1 mm-thick silicon rubber slab as an insulating spacer.

Fig. 1 Schematic of the 2 mL one-compartment semi-fuel cell running on H₂O₂ with a supporting electrolyte of 0.1 M HCl. The anode consists of either a Mg or an Al plate. The cathode is made of carbon paper coated with PB.

The overall redox reactions happening in the Mg-PB and Al-PB cells were 4.15 and 3.43 V, respectively, as shown in reactions (1) and (2) (see ESI† for more details).

Mg + H₂O₂ + 2H⁺ → Mg²⁺ + 2H₂O; E⁰ = 4.15 V vs. SHE (1)

Al + H₂O₂ + 2H⁺ → Al³⁺ + 2H₂O + e⁻; E⁰ = 3.43 V vs. SHE (2)

Hydrogen peroxide has a specific energy of 478 W h kg⁻¹,¹⁹ whereas Mg–H₂O₂ and Al–H₂O₂ cells have specific energies of 8600 and 17 000 W h kg⁻¹, respectively. In comparison, lithium-ion batteries and lithium-air batteries have theoretical specific energies of less than 1000 and 12 000 W h kg⁻¹, respectively.²⁰

The OCP of two cells was monitored for 600 s before running other experiments. The Mg-PB cell produced an OCP of ca. 2.3 V, whereas the OCP of the Al-PB cell was ca. 1.25 V (see Fig. S1 in the ESI†). Both values are higher than the theoretical OCP of a membraneless hydrogen peroxide fuel cell (1.09 V). The actual cell open-circuit potentials are lower compared with the theoretical ones. This is attributed to the occurrence of parasitic reactions in the cell causing a significant drop in the cell potential.

A linear potential sweep voltammetry experiment was carried out to determine the potential–current characteristics of the device using only 2 mL of solution in a 13 mm-diameter container. As shown in Fig. 2, a maximum power density of ca. 4.9 mW cm⁻² at potential of 1.1 V was achieved using a Mg plate as the anode and PB as the cathode. The device using Al as the anode material with a PB cathode produced a maximum power density of 3.3 mW cm⁻² at ca. 0.58 V. The power densities produced by the Mg and Al anodes are three times and twice as high, respectively, as that produced by a membraneless hydrogen peroxide fuel cell using a nickel mesh anode and a PB-based cathode immersed in a 50 mL solution (1.55 mW cm⁻²).¹⁴ For the Mg anode with a PB cathode immersed in a beaker containing 50 mL of solution, a maximum power density of up to 7.5 mW cm⁻² was achieved. The higher power output was mainly because of the improved mass transport to the electrodes.

Fig. 2 Current–potential and current–power curves of the single-compartment H₂O₂ fuel cell/battery with Mg or Al plate anodes and carbon paper coated with PB as a cathode. Performance tests were carried out under acidic conditions using 2 mL of 0.1 M HCl and 0.5 M H₂O₂ solution.

To investigate the long-term performance and stability of the device, chronoamperometry tests were carried out for the cells containing the Mg and Al anodes. As shown in Fig. 3, the cell with the Mg anode showed a sharper drop in current density compared with the cell with the Al anode. The overall decrease in the current density for both cells may be attributed to the H₂O₂ consumption, degradation of the electrode electrochemical activity and losses due to mass transfer. In the case of the Mg anode, it was observed that a large area of the electrode was covered by bubbles, limiting H₂O₂ transport to the electrocatalytic active sites. The source of the bubbles could be either hydrogen or oxygen molecules produced from metal corrosion or hydrogen peroxide decomposition (see ESI† for corresponding chemical reactions).

Fig. 3 Chronoamperometry curves measured at 1 and 0.5 V for single-compartment H₂O₂ semi-fuel cells with Mg and Al anodes, respectively and carbon paper coated with PB as the cathode. Performance tests were carried out under acidic conditions using 0.1 M HCl and 0.5 M H₂O₂.

To investigate the prevailing mechanism responsible for the degradation of the performance of the cell with Al as the anode, the solution in the reservoir was replaced with a fresh solution after the first experiment. The subsequent current generation followed the same trend as the preceding experiment (see ESI† for experimental results). This implies that the cell performance degradation is mainly attributed to mass transfer, which is the result of a combination of bubble formation around the anode and H₂O₂ consumption.

Conclusion

Miniaturized one-compartment membraneless H₂O₂ semi-fuel cells with excellent performance using magnesium and aluminium as anode materials and PB as the cathode material were realized. The open-circuit potential and maximum power density were improved considerably compared with the previous one-compartment membraneless H₂O₂ fuel cells reported in the literature.

Since the membrane is removed from the structure of the device, there is no requirement for a tight membrane-electrode assembly with passive delivery of reagents to electrodes. In addition, this design benefits from ease of operation, with the possibility of fuel recirculation to improve performance. These features reduce the mass of inactive materials and allow light-weight assembly and packaging structures, ideal for portable applications. High specific energies with high theoretical OCPs, make H₂O₂ semi-fuel cells very appealing for further developments.

The performance of the fabricated device was mainly restricted by a combination of electrochemical kinetics and mass transport. More investigation should be focused on cell chemistry to determine the parasitic reactions that prevent the cell potential from achieving theoretical values (see ESI†). Nano-electrocatalysts with a three-dimensional structure could be explored in preference to thin metallic plates to enhance the electrocatalytic reactions. In addition, microfabrication techniques can be adopted for making the miniaturized electrodes, reservoir and assembly.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra00874j

‡ These authors contributed equally to this work.

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