There is a growing demand for energy devices that can be used to store power for microscaled devices, including sensors, transmitters, and switches. Electrolyzers are devices that can store power (generated by solar cells) through the formation of H₂. However, a conventional electrolyzer/ fuel cell requires flow plates, liquid/gas diffusion layer, and membranes, which induces fabrication complexities and requires stringent operating conditions of temperature, pressure, and humidity. A complex, state-of-art water electrolyzer/ fuel cell was condensed as a chip small enough to fit on an adult’s fingertips by exploiting the benefits of microfluidics.¹ A standard soft lithography and lift-off technique was used to fabricate the microchannels and microelectrodes. The microfluidic device is membraneless and demonstrates flexibility in operation concerning the choice of pH. The microfluidic device was characterized by a low Reynolds number and a high Peclet number, which implies laminar flow and negligible diffusion of species by dissolution across the microelectrodes.

Hydrogen generation in the microfluidic electrolysis cell was investigated in acidic and alkaline pH. Improvement in the kinetics of water splitting was attained by using an asymmetric electrolyte configuration (acidic catholyte and alkaline anolyte). The water splitting kinetics enhancement was attributed to the excess energy from electrochemical neutralization arising from the pH difference between the catholyte and anolyte.² A fluid dynamic approach was used to optimize the electrolyte flow rate to attain the separation of H₂ and O₂ in the membraneless configuration. The crossover of gas products across the microelectrodes was negligible, as confirmed by gas chromatography. The flexibility in the choice of pH allows for enhanced catalyst stability. The earth-abundant transition metal catalysts can be incorporated as an anode that is stable in alkaline electrolyte and promotes oxygen evolution reaction. Catalyst stable in the acidic electrolyte can be used as a cathode for hydrogen evolution reaction.^3–7

The hydrogen generated in the microfluidic electrolysis cell can be utilized to generate power by operating a microfluidic fuel cell in tandem. The micropower produced in the tandem operation can provide on-demand power for modern miniaturized microelectronics ranging from sensors to transmitters. Two pairs of microelectrodes for the microfluidic electrolysis cell and microfluidic fuel cell were encompassed in a double Y-shaped microchannel. The microfluidic electrolysis cell induces a two-phase flow under an applied voltage, resulting in H₂ and O₂ gases. The gas products were convectively transported to the microfluidic fuel cell and consumed to generate power. The tandem microfluidic electrolysis cell – fuel cell generated a peak power density of 14.68 mW cm^-2.⁸ The fuel utilization efficiency attained by the tandem device was up to 50%, with the microfluidic electrolysis cell operating at 99.98% energy conversion efficiency. The membrane-less microfluidic energy conversion devices exhibit potential as a disruptive future technology, allowing the deployment of small electronic devices in remote areas.

References

1. B. S. De et al., ACS Appl. Energy Mater., 4, 9639–9642 (2021).
2. B. S. De, A. Singh, A. Elias, N. Khare, and S. Basu, Sustain. Energy Fuels, 4, 6234–6244 (2020).
3. A. Singh, R. Tejasvi, S. Karmakar, and S. Basu, Mater. Today Commun., 27 (2021).
4. A. Singh, S. K. Sarma, S. Karmakar, and S. Basu, Chem. Eng. J. Adv., 8, 100142 (2021).
5. A. Singh, S. Karmakar, and S. Basu, Int. J. Hydrogen Energy, 46, 39868–39881 (2021).
6. R. J. Dixit, K. Bhattacharyya, V. K. Ramani, and S. Basu, Green Chem., 23, 4201–4212 (2021).
7. R. J. Dixit, A. Singh, V. K. Ramani, and S. Basu, React. Chem. Eng., 2342–2353 (2021).
8. B. Samir De et al., Appl. Energy, 305, 117945 (2022).