Resistance Breakdown of a Membraneless Hydrogen–Bromine Redox Flow Battery

Abstract

A key bottleneck to society’s transition to renewable energy is the lack of cost-effective energy storage systems. Hydrogen–bromine redox flow batteries are seen as a promising solution, due to the use of low-cost reactants and highly conductive electrolytes, but market penetration is prevented due to high capital costs, for example due to costly membranes to prevent bromine crossover. Membraneless hydrogen–bromine cells relying on colaminar flows have thus been investigated, showing high power density nearing 1 W/cm 2 . However, no detailed breakdown of resistance losses has been performed to-date, a knowledge gap which impedes further progress. Here, we characterize such a battery, showing the main sources of loss are the porous cathode, due to both Faradaic and Ohmic losses, followed by Ohmic losses in the electrolyte channel, with all other sources relatively minor contributors. We further develop and fit analytical expressions for the impedance of porous electrodes in high power density electrochemical cells to impedance measurements from our battery, which enabled the detailed cell resistance breakdown and determination of important electrode parameters such as volumetric exchange current density and specific capacitance. The insights developed here will enable improved engineering designs to unlock exceptionally high-power density membraneless flow batteries.

Pathways to High-Power-Density Redox Flow Batteries

“…This was also seen in optimistic calculations of cell and electrode ASR by Milshtein et al, showing the porous electrodes dominated cell resistance by contributing ∼150 mΩ·cm 2 each . This was further confirmed by Alfisi et al via a detailed resistance breakdown in a membraneless hydrogen–bromine cell, where it was found that the porous positive (bromine) electrode contributed ∼60% of the total cell ASR . The latter work also found only a small contribution (<10%) of the negative (hydrogen) electrode to cell ASR.…”

“…Thus, breakthrough work for achieving yet higher power density may rest on reducing porous electrode ASR via either improved ion transport or catalytic capability. Both Chen et al and Alfisi et al identified that the largest contribution to losses in a porous bromine electrode were Faradaic losses due to electron transport across the solid–liquid interface, accounting for ∼50% of the electrode’s ASR in both works. , Thus, one promising strategy is to improve the catalytic capability of the porous electrode via, for example, doping or surface chemistry modifications.…”

“…Lower insets show the high-frequency intercept equivalent circuit for the case of negligible and significant solid-phase resistance. Analytical expressions used to plot porous electrode impedance are provided in Alfisi et al, and parameters used include R mem = 18 mΩ·cm 2 , R L ′ = 2600 mΩ·cm 2 , R F ″ = 6.6 mΩ·cm 3 , and C DL ′ = 9000 F/cm, as well as the ASR of the electrode’s liquid phase, ASR L = 180 mΩ·cm 2 . Contact resistances, such as those between the electrode and current collector, were neglected.…”

“…This suggests that, for electrodes with gas-phase reactants, where fast electrochemical reactions occur at nearly planar interfaces decorated with catalyst nanoparticles, the contribution to cell ASR may be relatively small . This is in contrast to liquid-filled 3D porous electrodes, where even with kinetically fast electrochemical reactions, ion transport through relatively long pores and the presence of distributed reaction zones can couple to dominate cell ASR. − Following this line of thought, it may be easier to reach <250 mΩ·cm 2 for batteries with only one liquid-solvated reactant, which is thus far borne out by the data in Figure . In the figure, we observe that the optimized hydrogen–bromine battery has achieved ∼46% higher power density than the optimized AQDS–bromine cell, , despite operating at lower temperature (room temperature vs 40 °C).…”

“…To help guide efforts to further reduce the ASR associated with porous electrodes, it is important to recognize that operation at regimes of extremely low ASR (<250 mΩ·cm 2 ) leads to different considerations for battery characterization. For example, at such ASR values, transport of electrons through the porous electrode can no longer be neglected in linear circuit models. , In classical implementations of the transmission line circuit model representing porous electrodes, , the resistance of the solid phase was justifiably neglected, as it was generally over an order of magnitude smaller than that associated with the electrolyte phase. As batteries push into regimes of extremely high power density, this assumption may no longer be valid.…”