|Active Area||50 × 50 mm (25 cm2)|
|Flow Channels||Number: 4|
|Size: 2 mm × 2 mm|
|Spacing: 2 mm × 2 mm|
|Minimum Reactant Concentration||726 mol m−3|
|3D Printed Parts||Material: ABS|
|Charging Began at||1.40 V|
|Discharging Began at||1.50 V|
|Average Resistance||0.015 Ω|
|Energy Efficiencies||Optimized design: 81.4%|
|Unoptimized design: 79.0%|
|Flow Rate||50 mL min−1|
A shift to renewable energy sources is undoubtedly the path to ease the worsening climate crisis the world is facing. Yet adequate technology required for large-scale energy storage is non-existent, making it one among the world's most sought challenges.
Compared to conventional batteries, flow batteries can store energy in liquids, known as electrolytes, which can be carried in separate tank reservoirs. These longer-lasting batteries can last for 20 years and be customized to contain more energy for a broader range of applications. However, they attract certain drawbacks that limit their wider usage, such as high cost of materials and low energy density electrolytes—making systems large and limited for stationary use.
The demand for affordable and robust test equipment in the flow battery technology brings a wide array of work, helping to make research on flow-electrochemistry more accessible. Some focus areas include cell topologies and manifold designs for a performance boost and developing eco-friendly electrolytes abundant in nature at a low cost.
The use of 3D printing has shown value in attaining such goals. The FDM 3D-printing cells provide a significantly lower cost option than other options available. Its use can carry out daily tasks on research about flow batteries, opening doors to more investigative cell designs and electrolytes. This open-source work makes it accessible for researchers to conduct their studies in the field of flow-electrochemistry.
The cell design features two 3D-printed components with channels for electrolyte flow and a cavity for the redox reaction site. The assembly uses simple brass and graphite materials connected to an electrical circuit via gaskets and O-rings, preventing electrolyte leakage. This 3D-printing application showcases an improved performance even in real-world testing, manufacturing a variety of designs tested in the nick of time. Results also show favorable performance comparisons against similar commercial test cells.
A flow-through design, having an active area of 50x50mm, answers the call for low-cost yet easy-to-manufacture flow cells. For optimization, a couple of electrochemical-CFD models improve the reactant distribution while reducing the concentration overpotential of the 3D-printed cells. The examination of the flowing electrolyte allows for cell designs that have the potential to investigate improvements in the reactant distribution.
The compact cell design results in a shorter printing time while using fewer materials at a low cost. The simple manifold design avoids large overhangs that can lead to difficulty in printing, consisting of four 2x2mm flow channels placed in the middle of the electrode compartment and the outlets.
The manifold design achieves an even velocity profile in most of the electrode area, although regions of higher and lower velocity exist in the outlet channels. This profile near the inlet is critical as it can dramatically affect the distribution of the vanadium(III) (V3+) concentration throughout the working area. The reduction of V3+ to V2+ upon entering the cell during charging results in fewer V3+ ions moving from inlet to outlet, causing a loss of distribution uniformity. Thus a reduced concentration overpotential through an evenly distributed flow in the cell must be attained to have a higher output power.
The refined manifold design now consists of a diffuser from the outlets to the electrode area. The diffuser uses 20 evenly spaced 2x2mm pillars in every manifold, reducing overhang sizes and limiting design complexity. The design increases the cross-section, resulting in a decelerated flow from the inlet. The more even velocity profile eliminates the disturbances occurring in the initial manifold design, providing a more uniform distribution and increasing minimum reactant concentration from 720 mol m-3 to 726.
Optimizing the design also resulted in a pressure drop from 2.2 kPa to 1.82 kPa, about a 17% reduction. This number is significant when considering larger commercial-scale cells where cell area and flow rates also scale up, reducing pumping losses compared to other systems. Such an improvement highlights its future application in iterative refinement via rapid prototyping, likewise on CFD-electrochemical modeling.
Print Parameter Optimisation for Flow Electrochemistry
The 3D-printed components suffer demanding conditions, where corrosive fluids flow through the porous media while other parts experience back pressure. Preventing such leakages is important as any may ruin lab results while posing health hazards in real-life scenarios. Optimizing the print parameters results in a leak-proof cell featuring ABS cells insusceptible to electrolyte permeation. Increasing the flow rate or k-value also reduces electrolyte ingress and porosity, occurring during cooling where polymer retraction leaves small voids up for electrolyte filling.
Different slicing settings are also mounted for a good surface finish around the O-ring channels, where components are imported to Ultimaker Gura as two different parts. This slicing software changes the path calculated for printing. These areas, printed more slowly, have a lower flow rate among the cell parts. Given these optimized parameters, the cells have not recorded leaks for over a month of long-duration testing.
The higher overpotentials of commercially available cells than the 3D-printed cell are seen at 42% compression, where charging of the former begins at 1.45V compared to 1.4V and discharging at 1.44V against 1.50V. The commercial cell reaches the cut-off voltage 20 min sooner during charging and 23 min for discharging, at a smaller capacity of 485 mAh on charging and 438 on discharging. This overpotential of the commercially available cell has potential explanations, such as the 0.015 Ω resistance of the 3D-printed cells compared to 0.035 of the commercial one,
Regarding energy efficiency, the 3D-printed cell tallies at a similar level to commercial cells, reaching 81.4% efficiency upon optimization from 79% unoptimized, while commercial cells have 78%. These are satisfactory figures given the lack of optimization among the membrane and electrodes of the cells. The efficacy of the cell also extends for a longer lab duration, recording no leaks for 31 days at 42% compression and a flow rate of 50 mL per minute.