Performance of a vanadium redox flow battery with a VANADion membrane

Highlights

• Performance of the VANADion membrane in flow batteries is evaluated.

• The battery with present membrane shows good rate capability.

• The battery with present membrane offers good capacity retention.

• The high performance and low cost make the membrane promising in VRFBs.

Abstract

Conventional vanadium redox flow batteries (VRFBs) using Nafion 115 suffered from issues associated with high ohmic resistance and high capital cost. In this work, we report a commercial membrane (VANADion), consisting of a porous layer and a dense Nafion layer, as a promising alternative to Nafion 115. In the dual-layer structure, the porous layer (∼210 μm) can offer a high ionic conductivity and the dense Nafion layer (∼20 μm) can depress the convective flow of electrolyte through the membrane. By comparing with the conventional Nafion 115 in a VRFB, it is found that the change from the conventional Nafion 115 to the composite one results in an increase in the energy efficiency from 71.3% to 76.2% and an increase in the electrolyte utilization from 54.1% to 68.4% at a current density of as high as 240 mA cm⁻². In addition, although two batteries show the comparable cycling performance at current densities ranging from 80 mA cm⁻² to 240 mA cm⁻², the composite membrane is estimated to be significantly cheaper than the conventional Nafion 115 due to the fact that the porous layer is rather cost-effective and the dense Nafion layer is rather thin. The impressive combination of desirable performance and low cost makes this composite membrane highly promising in the VRFB applications.

Introduction

Unlike state-of-the-art solid-state rechargeable batteries, redox flow battery (RFB) architecture allows the battery’s power and capacity to be sized independently. The system scalability of RFBs, along with other unique characteristics, provides a promising solution to the energy storage challenge of various renewable energy systems which are intermittent in nature, such as wind and solar power [1], [2], [3], [4]. In particular, the all-vanadium redox flow battery (VRFB) has been recognized as the most promising technology, primarily because it uses the same element in both half-cells, which avoids cross-contamination between the two half-cell electrolytes [5], [6].

Despite its remarked merits, however, the VRFB technology has not made significant progress in market presence. One significant factor preventing its widespread commercialization is the high capital cost, mainly attributed to the high cost of raw materials (highly stable ion-exchange membranes, typically Nafion membrane) and the unsatisfactory performance of power packs [7], [8], [9], [10], [11], [12], [13]. Therefore, developing cost-effective materials and improving the cell performance are urgently needed to address the cost issue. In this regard, development of reliable, inexpensive membranes that offer high conductivity is critically important [7], [8], [11]. As mentioned before, the most widely used membranes in VRFB systems are perfluorinated ion-exchange membranes (IEMs), typically Nafion membrane [17], [18]. Compared with other types of IEMs, Nafion membrane shows excellent chemical stability and good ionic conductivity. Previously, Nafion 115 was often chosen among the Nafion series as a result of tread-off between mechanical strength, vanadium ion permeability and ohmic resistance [19], [20], [21]. The thick membranes have the advantages of higher mechanical strength and lower vanadium crossover rate, however, the use of thick membranes induces high ohmic resistance and large amount of precious Nafion ionomer involved, leading to low performance of the power packs and the high material cost.

To address the cost and conductivity issues, porous separators, which are widely used in the lithium-ion battery and flooded lead-acid batteries, have been tested and applied as alternative membranes in VRFBs [22], [23], [24], [25], [26], [27], [28]. Unlike Nafion and most hydrocarbon membranes, the porous separators have much larger pore sizes, which are usually at submicron-size level. The porous separators usually have no inherent conductivity, but become highly conductive in strong acid electrolyte environment of VRFB systems. The high ionic conductivity combined with the remarkably low cost makes it promising for VRFB applications. However, there are two main disadvantages in the application of porous separator in VRFB systems [27], [28], [29]. The first is its large hydraulic permeability, which means that even a small pressure differential between the two sides will result in a substantial convective flow. The second is the lack of selectivity, i.e. the proportions of the ionic current carried by the various ions in the electrolyte filling the pores are no different than those proportions in the electrolyte itself. Due to these two reasons, porous separators showed lower cell efficiency and cycling performance than Nafion membranes. Interestingly, both of these disadvantages can be overcome by the incorporation of an IEM coating layer, including Nafion. Hence, a composite membrane, which is composited of a porous layer and a Nafion layer, will probably exhibit excellent performance. Due to the fact that the pores of the microporous layer are fully saturated with liquid electrolyte and the Nafion layer in this case can be very thin, the composite membrane will provide much lower ionic resistance and lower cost than the conventional thick Nafion membranes. In addition, the mechanical strength can be ensured by the porous layer.

The VANADion membrane is a newly developed composite membrane, which is comprised of a porous layer and a thin Nafion layer [29], as shown in Fig. 1. It is expected that this dual-layered structure can combine the attributes of porous separator (high conductivity and low cost) and IEMs (high selectivity), offering excellent performance in VRFBs. The objective of this work is to employ the VANADion membrane as a membrane for a VRFB system and conduct a comparison study with the Nafion-115 system through investigating area resistance, vanadium ion permeability, cell efficiency and cycling performance. It is demonstrated that the area resistance of a VANADion membrane is less than 1/3 of that of the Nafion 115 membrane, indicating that the VANADion membrane is highly conductive in the VRFB system. The VANADion membrane-based VRFB demonstrates an energy efficiency of 76.2% and an electrolyte utilization of 68.4% at the current density of 240 mA cm⁻², as opposed to an energy efficiency of 71.3% and an electrolyte utilization of 54.1% seen in a Nafion 115-based VRFB.