Scientists create porous carbon electrodes with 3D printing that boost conductivity and design flexibility in flow batteries, offering new options for clean energy storage.
(Nanowerk Spotlight) The growth of solar and wind energy has placed increasing importance on scalable energy storage technologies that can balance power supply with demand. Among the options, vanadium redox flow batteries (VRFBs) stand out because they decouple energy and power capacity—allowing independent control over how much energy is stored and how quickly it can be delivered. This makes them particularly attractive for grid-scale storage.
Yet, despite their promise, VRFBs are still held back by limitations in core components, especially the electrodes responsible for driving the redox reactions that store and release energy.
Most commercial VRFBs rely on porous carbon felts—typically made from graphite—as electrodes. These materials are affordable, chemically stable, and reasonably conductive. But they come with significant limitations. Their geometry cannot be adjusted to optimize fluid dynamics, and their internal structure offers little control over how reactants flow or where reactions occur. Their composition is often variable, and their activity toward the redox reactions involved in vanadium flow chemistry is not optimal. Moreover, global supply of high-quality carbon felts is constrained to a few manufacturers, making scaling expensive and vulnerable to disruption.
To address these bottlenecks, researchers have explored various electrode modifications, including coatings, doping, and texturing. Others have turned to additive manufacturing—also known as 3D printing—to design electrodes with controlled shapes and internal structures that improve electrolyte flow and surface access. These efforts have had mixed success.
Early 3D-printed electrodes using polymers or metal structures often required additional coatings to function and suffered from poor conductivity or stability in acidic battery environments. Attempts to print carbon-based structures directly have struggled to combine mechanical strength with high electrical performance.
Their approach centers on custom carbon pastes that can be printed into 3D architectures and then heat-treated to create fully carbonized, mechanically robust structures. These electrodes perform well under real battery conditions and open new possibilities for tailoring electrode properties through material selection and design.
a) Grid-like unit cellmodel; b) L,W, H and D sizes (Figure 2a) of the model, as-printed and consolidated (post-processed) electrodes; c) image of a 3D-Gr consolidated electrode; the top-left inset shows an electrode before (i.e., as-printed, left) and after (i.e., consolidated, right) postprocessing; the shrinkage is negligible; d) scheme of the consolidated electrodes with different compositions. (Image: Reprinted from DOI:10.1002/advs.202417641, CC BY) (click on image to enlarge)
At the core of this method is a paste formulation that balances conductivity, printability, and structural integrity. The researchers combined graphite powder—a highly conductive form of carbon—with small amounts of either carbon nanotubes or short carbon fibers. These were blended with a fine particulate binder made from coal tar pitch, a material known to convert to carbon when heated. This binder melts during heat treatment, fusing the particles together into a solid structure.
Unlike traditional liquid binders that remain in the structure or leave behind unwanted residues, the pitch-based binder carbonizes into a conductive matrix, preserving the purity and integrity of the final electrode.
To support printing, the paste includes a carrier made from a thermoresponsive gel called Pluronic F127 dissolved in water. This gel behaves like a thick fluid when squeezed through a nozzle but holds its shape when deposited. The team tested four different formulations, each containing 51% solids, with variations in the type of added carbon material. Careful rheological testing—measuring how the paste flows under stress—was used to confirm that the pastes would extrude smoothly, maintain their shape, and support subsequent layers without sagging or collapse.
These rheological tests were essential for predicting how the paste would perform during printing. Each formulation was evaluated for its stiffness at rest, its ability to flow when stressed, and how quickly it recovered its structure after printing. The best pastes showed shear-thinning behavior (becoming less viscous when forced through the nozzle) and regained most of their original stiffness immediately after deposition. This behavior ensures that the printed layers don’t spread out or deform, maintaining the intended 3D structure.
After printing, the structures were dried and then heat-treated at 800°C in an inert atmosphere. During this step, the Pluronic gel decomposed cleanly, while the coal tar pitch melted and transformed into carbon, bonding the particles together. This process resulted in rigid, conductive structures with minimal shrinkage. The final composition included mostly graphite, with less than 10% of the weight coming from the carbonized binder. Raman spectroscopy showed that the binder formed a thin coating around the graphite flakes, helping maintain electrical contact between particles.
Mechanical testing showed that electrodes containing carbon nanotubes had the highest compressive strength, reaching 12 MPa—well above that of typical carbon aerogels and comparable to denser engineered carbons. Those with short fibers showed no strength improvement, though they maintained structural integrity.
Electrical conductivity tests revealed that electrodes made from graphite and CNTs achieved values as high as 8,500 S/m, approaching that of pure graphite. By contrast, formulations with oxidized carbon fibers had lower conductivity, likely due to poor bonding between the fibers and binder, which reduced the continuity of electrical pathways.
To evaluate electrochemical performance, the team ran a series of tests using both static and flow conditions. In cyclic voltammetry measurements, which track how the electrode responds to changing voltage in a controlled environment, electrodes with CNTs showed faster reaction kinetics and lower resistance in the positive half-cell of the battery. This improved behavior was attributed to the conductive network formed by the nanotubes and their high surface area.
However, when used on the negative side of the battery, CNT-containing electrodes increased unwanted side reactions—specifically the evolution of hydrogen gas, which lowers battery efficiency. In contrast, electrodes made with only graphite showed better selectivity and stability in the negative half-cell, making them more suitable for that role.
Impedance spectroscopy confirmed these trends. Electrodes with CNTs had lower charge transfer resistance, especially in the positive half-cell, improving energy conversion efficiency. But the same properties that made them reactive in the desired vanadium redox reactions also promoted side reactions under certain conditions, reinforcing the need to customize each electrode for its specific role in the cell.
To test the electrodes under real operating conditions, the researchers designed and built a custom flow battery cell to fit the printed electrodes. Using a graphite-CNT electrode on the positive side and a pure graphite electrode on the negative side, they performed repeated charge-discharge cycles at a current density of 15 mA/cm². The cell achieved stable operation with a voltage efficiency of 70% and a coulombic efficiency of 75%. These values are comparable to those obtained with commercial oxidized graphite felt.
However, the printed electrodes had significantly lower external surface area compared to the felt, limiting reaction rates. This surface area difference—roughly a factor of 17 based on geometric estimates—helps explain the lower voltage efficiency observed in the printed cell. The rigid nature of the 3D-printed electrodes also reduced their contact with current collectors, further contributing to performance loss. Still, the authors note that these trade-offs are addressable. Finer print resolution, more conductive binders, and optimized geometries could increase surface area and improve contact with electrical components.
Crucially, the ability to design and tune electrode geometry using DIW provides a degree of control not possible with felts or cloths. Custom layouts can be created to optimize electrolyte flow, minimize pressure drop, and improve mass transport, all of which impact battery performance and lifetime. The printed grid design used here already showed lower pressure drop than commercial felt at the same flow rates.
This method opens new possibilities for electrode design in flow batteries and other electrochemical systems. By adjusting the composition of the paste, researchers can modulate conductivity, reactivity, and mechanical properties. Other additives—not necessarily carbon-based—could be included to enhance wettability or suppress unwanted reactions. Moreover, printing finer features with more advanced equipment could bring surface areas closer to that of felts, while improving mechanical uniformity and reproducibility.
The study demonstrates that DIW is a viable path to producing all-carbon electrodes with high electrical performance and tailored structure. While further optimization is needed to match or exceed the performance of commercial materials in every respect, the design flexibility and material control it offers mark a significant step toward engineered electrodes for scalable energy storage.
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