Dixon, D.

Ehrenberg, H.

Langner, J.

Melke, J.

Roth, C.

KITopen

KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association Diagnostics in All-Vanadium Redox-Flow Batteries J. Langner1, J. Melke1, D. Dixon1, H. Ehrenberg1, C. Roth2 1Karlsruher Institut für Technologie (KIT), Institut für Angewandte Materialien (IAM), 76344 Eggenstein-Leopoldshafen, Germany 2Freie Universität Berlin, Fachbereich Biologie, Chemie, Pharmazie, 14195 Berlin, Germany Motivation:  • Redox-flow batteries (RFB) are a potential energy storage technology for electronic grid integration of renewables • In RFB energy and power can be scaled independently • All-Vanadium redox-flow  batteries (VRB) offer the additional advantage  that cross-over is not a problem and the energy is completely stored in the liquid electrolyte • Currently used carbon based electrodes still show high overpotentials • E/i-curves can give valuable information to distinguish the  source of losses 11,251,50 20 40 60 80 100 120Cell Potential [V] Current [A] activation loss ohmic loss transport loss open circuit potential E/i-Curves  • With E/i-curves it is possible to determine the major losses of an electrochemical cell. • Activation loss arises from reaction mechanism itself. • Ohmic losses come from charge transfer through the electrode and the membrane. • Transport loss arises from diffusion limitations Porous electrode Proton exchange membrane Current collector from storage tanks to storage tanks Cell Scheme  The VRB consists of two storage tanks for the electrolyte and the electrochemical cell. The latter consists of two carbon based porous electrodes, which are separated with a proton exchange membrane (Nafion 117). They are encased in graphite current collectors. Those can be used either with or without a flow field for the even distribution of the electrolyte. -0,75-0,250,250,751,251,75-75 -50 -25 0 25 50 75Voltage [V] \\ Potential [V] vs. Ag/AgCl Current density [mA/cm²] Cell neg. Side pos. Side Membrane-0,4-0,200,20,4-75 -50 -25 0 25 50 75Overpotential [V]  \\ Resistance (Membrane) [Ohm] Current density [mA/cm²] Cell [V] neg. Side [V] pos. Side [V]Membrane [V] Membrane [Ohm] HFR [Ohm]Determination of the Ohmic Losses  One approach is the High Frequency Resistance (HFR) at 10 kHz. Another one is to use reference electrodes on both sides of the membrane. By subtracting both half-cell overpotentials from the cell overpotential, the ohmic and diffusion losses remain. The results are in good agreement. Differences arise from uncertainties of both approaches (e.g. inhomogeneous potential distribution over the electrode, further potential losses over the membrane (diffusional, Donnan). Influence of State of Charge (SOC) 𝑽𝟐+   𝒅𝒊𝒔𝒄𝒉𝒂𝒓𝒈𝒆𝒄𝒉𝒂𝒓𝒈𝒆  𝑽𝟑+ + 𝒆− 𝑽𝑶𝟐+ + 𝟐𝑯+ + 𝒆−   𝒅𝒊𝒔𝒄𝒉𝒂𝒓𝒈𝒆𝒄𝒉𝒂𝒓𝒈𝒆   𝑽𝑶𝟐+ + 𝑯𝟐𝑶 0,60,811,21,41,61,8-250 -200 -150 -100 -50 0 50 100Cell Voltage [V] Current density [mA/cm²] 𝐸 = 1,26 𝑉 +𝑅𝑇𝐹 𝑙𝑛𝑐 𝑉𝑂2+  𝑐 𝑉2+𝑐 𝑉𝑂2+  𝑐 𝑉3+∙𝑐+ 𝐻+12∙𝑐+ 𝐻+𝑐− 𝐻+  The support by BMBF, Heraeus and Freudenberg under grant number 03EK3011A is gratefully acknowledged. Summary  • Proposed setup with reference electrodes enables another approach to distinguish different potential losses • Performance of the cell depends strongly on the SOC • Different loss mechanisms are a function of the SOC • Thermal treatment improves mainly the negative half-cell reaction • Electrochemical characteristics can be improved by other carbon materials   Composite electrode Future Plans  • Evaluate different carbon felts and effects of chemical or thermal oxidation (wettability, catalytic improvements) (XPS, BET, NEXAFS) • Evaluate further materials (Carbon Black, Graphite, CNT) for composite electrodes • Design composite electrodes • Enhance surface area, better electrochemical behavior • Investigate the composite electrodes 3D structure  (Hg-Porosimetry, SEM, FIB-SEM) The open circuit potential of the VRB depends highly on the SOC (i.e. Nernst equation).   This leads not only to a shift of the polarization curve to lower potentials but also increases the transport losses due to starvation.  At 50% SOC (dotted curve) the charge and discharge curve are similar as expected.   At lower SOC the charging process does not become as efficient as the discharge process at the corresponding higher SOC. Thermal Treatment  Oxidation of the carbon felt leads to an increase of the wettability towards the electrolyte and a performance gain.  Reference electrodes show that both half-cells contribute to this effect  Negative half-cell shows the higher improvement Electrolyte: 0.5M VOSO4 in 3M  H2SO4 Flow rate: 100 mL/min Membrane: Nafion 117 Electrode: Freudenberg [carbon nonwoven] Single Cell: 25 cm² [flow through] -1,5-1-0,500,511,50 50 100 150 200Overpotential [V] Current [mA/cm²] Cell [pristine] Cell [thermal] pos. Side [pristine]pos. Side [thermal] neg. Side [pristien] neg. Side [thermal]00,511,520 50 100 150 200Voltage [V] Current denstiy [mA/cm²] pristine electrodes thermal treated electrodes   Introduction    Conclusions    Experimental Different Oxidation states of Vanadium Single Test Cell -0,004-0,00200,0020,0040,5 1 1,5Current [A]  Potential [V] vs NHE Heraeus LG2N Carbon feltElectrochemical Activity  Cyclic Voltammetry in a three electrode setup was used to evaluate the activity of different carbon based materials. Used Electrolyte was 0.1M VOSO4  in 3 M H2SO4  Carbon felt (used electrode material) has to be improved  LG2N from Heraeus shows very good electrochemical characteristics  and is a promising candidate for a composite electrode material  

Diagnostics in all-vanadium redox-flow batteries

https://publikationen.bibliothek.kit.edu/220092745/3816360

Diagnostics in all-vanadium redox-flow batteries

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