Gassing side reactions can turn charging current into gas instead of stored chemical energy. In a vanadium redox flow battery (VRFB), excessive polarization during charging can trigger hydrogen evolution reaction (HER) at the negative electrode and oxygen evolution reaction (OER) at the positive electrode. The resulting bubbles block electrode pores and flow paths, consume applied current, increase charging time, and can distort the results of cell-performance testing.
The central issue is not simply gas production—it is the loss of effective electrochemical and hydraulic performance. Engineers can reduce gassing by controlling temperature, limiting charge-voltage excursions, improving electrode and flow-field design, and monitoring cell potential and operating conditions closely.
Why HER and OER Occur During VRFB Charging
The intended charging reactions
During normal charging, electrical energy drives the vanadium redox reactions that restore the different oxidation states in the negative and positive electrolytes.
The applied cell voltage includes the thermodynamic reaction potential plus activation, ohmic, and mass-transfer overpotentials. As these losses increase, the electrode potentials can move into ranges where water-splitting reactions become competitive.
Hydrogen evolution at the negative electrode
The hydrogen evolution reaction occurs at the negative side when the local cathodic potential is sufficiently reducing:
[ 2H^+ + 2e^- \rightarrow H_2 ]
In practical VRFB testing, HER is promoted by excessive negative-electrode polarization, high temperature, electrode surface chemistry, and operating conditions that create local current-density peaks.
Oxygen evolution at the positive electrode
The oxygen evolution reaction occurs at the positive side when the local anodic potential becomes sufficiently oxidizing:
[ 2H_2O \rightarrow O_2 + 4H^+ + 4e^- ]
OER can become significant when charging continues beyond the useful vanadium reaction range or when mass-transfer limitations force the required polarization higher.
How Gassing Reduces Cell Performance
It blocks electrochemically active surface area
Gas bubbles adhere to carbon-felt fibers and mask active reaction sites. This reduces the area available for vanadium redox reactions and increases the local current density on the remaining wetted surface.
The resulting polarization can further accelerate gas evolution, creating a negative feedback loop between poor electrode utilization and additional overpotential.
It disrupts electrolyte flow and mass transfer
Bubbles obstruct pores and alter the intended flow distribution through the porous electrode. This can create dry or poorly wetted regions, channeling, and local reactant depletion.
The effective transport of vanadium species and protons is therefore impaired. In testing, the cell may appear to have higher mass-transfer resistance than the electrode or flow-field design would produce under bubble-free conditions.
It consumes charging current
Current diverted to HER or OER does not contribute to the intended vanadium redox reactions. This lowers charging efficiency and prolongs the time required to reach the voltage or state-of-charge cutoff.
Persistent gas evolution can also produce electrolyte imbalance because hydrogen and oxygen are generated through different reactions and may leave the electrolyte compartments at different rates.
It reduces effective electrolyte inventory
Gas occupies volume that would otherwise be filled by liquid electrolyte. This reduces the effective liquid volume available for reaction and can affect reservoir level, pump operation, and electrolyte circulation.
The effect is especially important in small laboratory cells, where even modest gas accumulation can materially change hydraulic behavior.
It distorts test results
Gassing can appear in measurements as increased charge voltage, reduced voltage efficiency, longer charge time, lower accessible capacity, and poorer apparent stability.
If gas is not identified, engineers may incorrectly attribute these changes to membrane resistance, electrode degradation, electrolyte imbalance, or an inadequate flow rate.
How Engineers Identify Gassing During Cell Testing
Monitor cell voltage and electrode potentials
A rising cell voltage during charging is not, by itself, proof of gassing. Engineers should combine cell-voltage data with individual electrode or half-cell potential measurements where possible.
Potential monitoring helps identify whether the negative side is approaching HER conditions or the positive side is approaching OER conditions. It also distinguishes a side reaction from a purely ohmic or mass-transfer limitation.
Track charge efficiency and charge duration
A growing gap between charge and discharge behavior can indicate that an increasing fraction of current is being consumed parasitically.
Unexpectedly long charging times, declining coulombic efficiency, or a mismatch between delivered capacity and the expected electrolyte state of charge should prompt inspection for gas evolution and electrolyte imbalance.
Inspect the hydraulic system
Visible bubbles in tubing, gas accumulation in reservoirs, unstable pump flow, and irregular pressure behavior are direct warning signs.
However, the absence of visible bubbles does not prove that the cell is gas-free. Bubbles can remain trapped inside the porous electrode or membrane-adjacent regions.
Control test conditions
Temperature, flow rate, current density, state of charge, and voltage cutoff should be logged with sufficient resolution to correlate performance changes with operating conditions.
Repeat tests under controlled conditions are essential because small differences in temperature or cutoff voltage can produce large differences in parasitic reaction rates.
Practical Mitigation Strategies
Maintain precise temperature control
Higher temperature generally accelerates electrochemical reaction kinetics, including HER and OER. It can therefore increase the rate at which charging current is diverted into gas evolution.
Use accurate thermal management and monitor the electrolyte temperature near the cell rather than relying only on ambient laboratory temperature. Thermal control also improves the repeatability of comparative electrode and membrane tests.
Optimize charge-voltage cutoffs
Avoid charging substantially beyond the voltage required for the intended vanadium redox reactions. Excessive voltage cutoff is one of the most direct ways to increase polarization-driven gassing.
The appropriate cutoff depends on electrolyte composition, state of charge, current density, temperature, electrode configuration, and system design. It should be established experimentally using potential monitoring rather than copied without qualification from another cell.
Reduce polarization through flow optimization
Insufficient flow can increase concentration gradients and mass-transfer polarization, pushing electrode potentials toward HER or OER conditions.
Optimize flow rate and flow distribution together. Increasing flow may reduce concentration polarization, but the benefit must be balanced against pumping power, pressure drop, and possible mechanical effects on the cell.
Improve flow-field and electrode wetting
A well-designed flow field should distribute electrolyte uniformly across the porous electrode and limit stagnant regions where bubbles can accumulate.
Electrode compression, thickness, pore structure, and wetting behavior also matter. The goal is to maintain reliable liquid contact and provide pathways for bubbles to leave without causing excessive bypass flow or uneven reaction distribution.
Evaluate electrode materials and surface chemistry
Carbon felt and related electrode materials can differ in their tendency to promote vanadium reactions versus water-splitting reactions.
Screen electrode treatments and materials for both catalytic activity toward the desired redox reactions and resistance to corrosion or unwanted gas evolution. A treatment that lowers vanadium reaction overpotential may be beneficial, but it should be evaluated for its effect on HER, OER, durability, and electrolyte compatibility.
Use appropriate flow-cell test assemblies
Laboratory flow cells should provide stable compression, reproducible flow distribution, reliable sealing, and effective thermal management.
The assembly should also allow engineers to observe or remove accumulated gas where practical. Otherwise, a gas-management problem can be mistaken for a material or electrochemical problem.
Use conservative test protocols during screening
When comparing materials, begin with operating conditions that minimize avoidable gassing. Excessively aggressive current densities, high temperatures, or extended high-voltage holds can mask the intrinsic performance of the component being evaluated.
Once baseline behavior is established, more demanding tests can be used to assess operating limits and degradation mechanisms.
Understanding the Trade-offs
Lower voltage cutoffs can limit usable capacity
Reducing the charge cutoff generally decreases the opportunity for HER and OER, but it may also prevent the electrolyte from reaching the intended state of charge.
The correct cutoff is therefore a controlled compromise between capacity utilization, efficiency, cycle life, and gas-generation risk.
Higher flow rates are not free
Higher flow can reduce concentration polarization and improve bubble removal, but it increases pumping energy and system pressure drop.
A higher flow rate is not automatically the best solution if the underlying problem is excessive voltage, poor flow-field distribution, or an unsuitable electrode surface.
More catalytic electrodes can have mixed effects
Improving catalytic activity for the vanadium reactions can lower polarization and reduce the voltage required for charging. However, a surface that also catalyzes HER or OER can increase gassing once the relevant potential is reached.
Electrode selection must therefore consider reaction selectivity, not only general electrochemical activity.
Temperature changes affect more than gassing
Lowering temperature may suppress HER and OER, but it can also slow desired redox kinetics and alter electrolyte transport properties.
Temperature should be optimized within the validated operating range rather than minimized without regard to total cell performance.
Gas removal does not eliminate the root cause
Venting or removing bubbles can restore some hydraulic performance, but it does not recover the current already consumed by HER or OER.
Gas management is useful, but it must accompany control of the electrochemical conditions that generate the gas.
Making the Right Choice for Your Goal
Use a combined electrochemical, thermal, and hydraulic approach rather than relying on a single mitigation measure.
- If your primary focus is accurate cell-performance testing: Control temperature and flow tightly, monitor electrode potentials, and record gas-related symptoms alongside voltage, current, capacity, and efficiency.
- If your primary focus is charging efficiency: Optimize the voltage cutoff and reduce polarization through appropriate flow distribution, electrode compression, and operating current density.
- If your primary focus is electrode or catalyst development: Compare materials for vanadium-reaction performance, HER/OER tendency, corrosion resistance, and durability under realistic VRFB conditions.
- If your primary focus is long-duration cycling: Track electrolyte imbalance, gas accumulation, capacity retention, and changes in charge efficiency rather than evaluating only the initial polarization curve.
- If your primary focus is laboratory repeatability: Use a flow-cell assembly with stable thermal management, reproducible compression, controlled pumping, and reliable potential monitoring.
When engineers treat gassing as both an electrochemical side reaction and a hydraulic failure mode, they can obtain more reliable test data and preserve VRFB performance.
Summary Table:
| Side Reaction | Location | Impact on Performance | Primary Causes | Mitigation Strategies |
|---|---|---|---|---|
| Hydrogen Evolution Reaction (HER) | Negative electrode | Blocks active sites, consumes current, increases polarization, reduces efficiency | Excessive negative polarization, high temperature, poor wetting | Optimize voltage cutoff, control temperature, improve electrode design |
| Oxygen Evolution Reaction (OER) | Positive electrode | Forms gas bubbles, blocks pores, consumes current, lowers capacity | High positive potential, mass-transfer limitations | Limit charge voltage, enhance flow distribution, use selective catalysts |
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