Polypropylene elastomer composite for the all-vanadium redox flow battery: current collector materials

Abstract

ARTICLE This journal isIn this study, carbon-based polypropylene thermoplastic elastomer (PP-elastomer) composite for current collectors of all-vanadium redox flow battery (VRB) was successfully prepared. The volume resistivity of the PP-elastomer composite was 0.47 Ω·cm. Its tensile strength and elongation at break were 6.6 MPa and 250%, respectively. In addition, good flow property in processing makes this composite has the potential on mass industrial production of current collectors. The single cell and the cell stack of VRB battery equipped with the composite current collectors were assembled for battery tests, including cyclic voltammetry, long-term performance, long-term stability, and the oxidation corrosion. To evaluate the stability and the performance of the cell stack under a long-term operating condition, tests with more than 2300 charge-discharge cycles were carried out. The coulombic efficiency (CE) and voltage efficiency (VE) of the cell stack maintained around 93% and 80% during 2300 charge-discharge cycles, and energy efficiency EE hold around 75%. The results approved that VRB battery equipped the composite current collectors had a good stability and performance. Furthermore, long-term corrosion tests indicated that PP-elastomer composite could endure the strong corrosion of pentavalent vanadium and the concentrated sulfuric acid. The composite materials prepared in this study are more suitable to produce the current collectors. The corrosion resistance of composite materials is much better than that of the graphite, and the mechanism was also discussed. Positive electrode reaction (φ θ = 1.004 V):Negative electrode reaction (φ θ = -0.255 V):Overall electrochemical reaction of the cell (φ θ = 1.259 V):All-vanadium redox flow battery has a capacity on theoretically unlimited charging and discharging, which is superior to other types of batteries. However, excessive overcharging of the VRB can lead to degradation of the positive electrode and decreases the cycle life. The most critical issue for the development of VRB is the identification, characterization, and fabrication of suitable electrode materials with low cost, light weight, flexibility, and good stability under highly corrosive operating conditions, 21 especially for current collector materials. To date, several kinds of carbon-based electrode materials have been investigated for VRB application. 24 Graphite felt is an ideal VRB electrode material which provided active sites for the vanadium redox reaction due to its higher specific surface areas and enhanced three-dimensional network structures, excellent conductivity, and electrochemical stability. 25 Moreover, through surface treatments such as chemical etching, 26 nanoparticle decorating, 27, 28 thermal treatment, 29 the electrochemical reaction kinetics on graphite felt will be greatly improved.

Chemical Stability of Graphite-Polypropylene Bipolar Plates for the Vanadium Redox Flow Battery at Resting State

“…6,16,25 Electrochemical oxidation is caused by potential differences in the cells of a VRFB leading to damage of BPPs. 4 Corrosion may cause an increase in the porosity 7,16,21 leading to ion permeability, brittleness, loss in mechanical stability and a decreased electrical conductivity. It was reported that the shunt current might play the main role in this process whereas the main current is less dominant.…”

“…While vanadium redox reactions mainly occur on the surfaces of the porous electrodes, they could also occur unintentionally on the surfaces of the BPPs. [5][6][7] Therefore, a good BPP should be characterized by a high chemical and mechanical stability, high electrical conductivity and impermeability to preclude leakage. 8 Metallic BPPs are usually not used in VRFB as they corrode in acidic environments and would need a protective layer.…”

“…graphite, carbon black, carbon fibre and/or carbon nanotubes, provides the electrical conductivity. 5,12 Different composite materials were already investigated for the production of BPPs in the VRFB, including diverse graphite and carbon materials with polyethylene, 12,[14][15][16] polypropylene (elastomer), 6,7 fluoropolymer, 4 polyphenylene sulfide, 5 phenol formaldehyde resin, 17 as well as epoxy resin. 9,10,[18][19][20][21] Looking only at the single components, polymers like polyethylene, polypropylene and polytetrafluoroethylene exhibit high chemical resistance toward concentrated sulfuric * Electrochemical Society Member.…”

“…7,[23][24][25] Up to date there are several studies dealing with cyclic aging of BPPs for the VRFB. [4][5][6][7]9,10,14,16,18,20,21,26 For example Liu et al showed that carbon-polyethylene composite BPPs were aging when an anodic polarization potential of 2.26 V vs. standard hydrogen electrode (SHE) was applied for 6 h in 2 M VOSO 4 in 2 M H 2 SO 4 . Their results indicate that new oxygen functional groups (C=O and O=C-OH) were formed on the surface of the BPP, delamination processes commenced and material was lost by corrosion.…”

Bulk Aging of Graphite-Polypropylene Current Collectors Induced by Electrochemical Cycling in the Positive Electrolyte of Vanadium Redox Flow Batteries

“…[14][15][16] For example, after applying over 2300 charging/discharging cycles in a cell stack with BPPs containing carbon fibers/carbon black-polypropylene composite, the BPPs had developed local blisters and a rougher surface indicating slight corrosion of the plates during VRFB stack operation. 15 There are two main reasons for occurrence of corrosive overpotentials in the stack. Firstly, they can appear due to shunt or parasitic currents, for example through an electric contact of the positive electrolyte with BPPs of different potentials in the in-/outlet channels of the half-cells.…”