Research progress on gas-liquid two-phase flow and electrical properties in microgravity fuel cells

Characteristics of electrical properties of PEMFC under different gravity conditions

A fuel cell is a power generation device that continuously converts the chemical energy of a fuel and an oxidant into electric energy through an electrochemical reaction, and has a characteristic of high specific power, high specific energy, fast response, high power, and long-time power supply. Applications in the field of aerospace are increasingly valued.

However, in the space microgravity environment, the two-phase flow accompanied by electrochemical reactions inside the fuel cell will exhibit characteristics that are very different from the ground gravity environment. This not only poses new challenges to the gas-liquid management of the fuel cell, but also has a direct impact. The electrical performance of the fuel cell and its stable operation have become one of the key technical issues to be solved in the development of aerospace fuel cell technology.

On the other hand, even for fuel cell applications in the ground-based constant-gravity environment, gravity also has an important effect on its internal two-phase flow characteristics and thus on its electrical properties. Therefore, it is of great academic significance and significant application value to study the mechanism of gravity in the internal process of fuel cell.

The Institute of Mechanics, Chinese Academy of Sciences, and Prof. Guo Hang of Beijing University of Technology and their research team cooperated to use the 100-meter falling tower of the Institute of Mechanics to take the lead in pro-active exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). The characteristics of gas-liquid two-phase flow with electrochemical reactions and changes in the electrical properties of related fuel cells were studied in a short-term microgravity experiment. Compared with ground-based gravity experiments, gravity was used as a controllable variable to make up for the past. The ground method of studying the gravity effect by changing the gradient of the flow channel in the conventional gravity environment has obvious methodological deficiencies.

Recently, the joint research team reported the characteristics of gas-liquid two-phase flow in PEMFC cathode flow channels (single-channel single channel, cross-sectional area 2×2 mm2) under different gravity conditions and their effects on the electrical properties of PEMFC. Compared with the gravity environment, the decrease of the gravity level leads to great changes in the water-gas two-phase flow pattern in the cathode flow path, which directly affects the electrical performance of the PEMFC (see figure).

The experiment also found that the influence of gravity on the water and gas two-phase flow and its electrical properties in the PEMFC is related to the operating state of the battery: In the high current density, high water production area, the liquid water generated by the electrochemical reaction in the PEMFC cathode flow path is under normal gravity conditions. It tends to converge in the vertical upward flow path, causing flooding of the flow channel, obstructing the effective supply of reactants and timely discharge of the product; while in microgravity conditions it tends to move with the flow and is easy to discharge, thus effectively suppressing the flow channel The flooding was flooded and the corresponding electrical properties increased by 4.6% under microgravity conditions compared to normal gravity.

In the low current density, low water production area, micro-gravity dispersed water droplets converging to block the flow channel, resulting in about 6.6% lower electrical performance than normal gravity. For details see Guo H, Liu X, Zhao JF, Ye F, Ma CF. Experimental study of two-phase flow in a proton exchange membrane fuel cell in short-term microgravity condition. Applied Energy, 2014, 136: 509-518.

The research work was funded by the National Natural Science Foundation and the Open Project of the Key Laboratory of Microgravity.