A solid hydrogel electrolyte-based micro fuel cell integrated with fuel/electrolyte storage and supply function

Abstract

<p indent="0mm">The development of modern electronic equipment has promoted the requirement of portable energy conversion devices. Fuel cell technology provides clean energy, which is pollution-free and environmentally friendly. It has several advantages, including instant charge and discharge, high energy conversion efficiency, and easy integration. However, some disadvantages, such as high membrane cost, membrane aging degradation, water management, and extra ohmic resistance, always accompany the conventional proton exchange membrane fuel cells. In addition, the need for fuel tanks increases the system cost and complexity and is unfavorable for miniaturization. Meanwhile, the consumption of additional pump power to supply fuel can reduce the net output power. Hydrogel is a solid-state electrolyte crosslinked via a three-dimensional (3D) polymer network. The porous structure possesses the abilities of water absorption and retention and enables fast mass transport on a large surface. To simplify the fuel cell system and improve its portability, a miniature fuel cell integrated with the fuel/electrolyte is proposed, complemented by the water retention and ionic conductivity of the hydrogel. Polyvinyl alcohol (PVA) hydrogel is prepared via cyclic freezing and thawing processes. The effects of the fabrication parameters, such as swelling and conductivity of the hydrogel electrolyte, are discussed, and the fuel cell equipped with hydrogel is set up. The micromorphology and element distribution of PVA hydrogel are evaluated via physical characterization. PVA hydrogel has an abundant 3D-network porous structure. PVA polymer chain is uniformly crosslinked, and C and O elements are evenly distributed on the hydrogel skeleton. Fourier transform infrared results reveal numerous hydrophilic groups in PVA hydrogel, including –OH and C–O, contributing to the high aqueous electrolyte absorption ability. The effect of fabrication parameters on the swelling properties of PVA hydrogel is studied. The high concentration and efficient repetitive freezing-thawing can improve the crosslinking density of the PVA hydrogel, firmness of the hydrogel skeleton, and swelling equilibrium. The PVA hydrogel in 5% can reach the maximum swelling ratio of 45%. The ionic conductivity of the PVA hydrogel after swelling the electrolyte is studied via the two-probe AC impedance method. The various freezing-thawing cycles have an evident influence on the crosslinking density and pore size of the hydrogel, contributing to the absorption ability of the sulfuric acid solution and hydrogel conductivity. The ionic conductivity of the PVA hydrogel in 5% by three freeze-haw cycles is up to 173.61 mS/cm, facilitating the hydrogen ion conduction in the fuel cell. Moreover, the effects of hydrogel thickness, fuel concentration, and electrolyte concentration on the fuel cell performance are studied. Consequently, when the hydrogel electrolyte thickness is <sc>6 mm</sc> using <sc>6 mol/L</sc> HCOOH and <sc>1 mol/L</sc> H₂SO₄, the maximum power density is <sc>2.5 mW/cm²</sc>, and the limiting current density is <sc>15.2 mA/cm².</sc> The results suggest that the fuel cell can be used to power miniature portable electronic devices.