Functional Design of the Gradient Carbon Fiber Electrode for Flow Batteries
Authors: Fan Xinzhuang 
Published: May 2020

To promote the universal application of renewable energy has become an important strategy for the energy security and sustainable economic development around the world. The proportion of renewable energy in the overall energy structure is greatly increasing in recent years, however, it is intermittent and unstable for the renewable energy generation, and the direct grid connection of the renewable energy generation will affect the stable operation of the power grid system. Supporting high-efficiency energy storage batteries to ensure the continuity and stability of power generation and power supply is an important way to achieve renewable energy development strategies. Thereinto, flow batteries (FBs) have a broad application prospect and huge market potential due to their safety, easy scale, flexible structure design and environmental friendliness.
As the platform of the electrochemical reaction for FBs, the electrode is one of the key materials to determine its performance and cycle life. Currently the widely used electrode material of vanadium flow battery (VFB) is the polyacrylonitrile-based carbon felt (CF), and its low price and high stability make it the first choice of the electrode materials of FBs. However, the relatively lower electrochemical activity and specific surface area, in addition to higher electron resistance and transmission resistance of CF greatly limit the improvement of the battery performance and the reduction of the energy storage cost. For example, the power density of VFB is only 0.6 W cm-2, and its operating cost is about 650 $ kWh-1. In general, the basic requirements for the electrode materials of VFB as follows: i) high fluid permeability; ii) high specific surface area; iii) high electron conductivity; iv) low cost; v) high chemical and electrochemical stability. In order to achieve high electrolyte permeability, the conventional CF often possesses an average fiber diameter of up to 10 μm, resulting in a low electrochemical surface area. Therefore, only thicker electrodes can be used in VFB to ensure the electrochemical reaction interface, but this will result in a substantial increase in the ohmic resistance. In addition, the electrochemical activity of the electrode often depends on the oxygen-containing group on the surface of CF. However, the introduction of the oxygen-containing group also destroys the conductive network of the original graphite fiber simultaneously, thereby causing the loss of electronic conductivity of the electrode. The above several factors are coupled to each other, resulting in a very low power density of VFB.
Fig. 1 Schematic diagram of the gradient carbon fiber electrode for FBs.
Generally, the electron conductivity of the electrode is much larger than the ion conductivity of the electrolyte, and the ion migration path on the membrane side is short for FBs, so the electrochemical reaction tends to occur at the membrane side, which is often called the reaction zone; meanwhile the ion migration path at the plate side is long, which is not conducive to the reaction, so it is often caller the current collecting area. Thereinto, the former should has higher electrochemical activity, while the latter should possess higher conductivity. Therefore, the carbon fiber electrode for FBs should be constructed according to its specific structure characteristics. Based on this, a gradient carbon fiber electrode is proposed to resolve the contradiction among the conductivity, activity and transmission. Firstly, the fiber density exhibits a gradient distribution in the thickness direction of the electrode, the plate side with higher fiber density is used to reduce the contact resistance, while the membrane side with lower fiber density is used to reduce the mass transfer polarization. In addition, electrolyte will tend to flow through the membrane side due to its lower transmission resistance, which could ensure the supply of the active reactant. Secondly, the nano fibers are also gradiently distributed in the thickness direction of the electrode. The plate side of the electrode is mainly composed of graphite fibers with good conductivity, while the membrane side mainly consists of the composites of graphite fibers/nano fibers with good activity. And such gradient structure could result in an excellent electrochemical activity, a favorable conductivity and a preferable transmission performance simultaneously, successfully resolving the efficient coordination problem of charger transfer, electron conductivity and ion transport in FBs.

Acknowledgement: This study was funded by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) and HKUST fund of Nanhai (Grant No.FSNH-18FYTRI01).

Figure 1