Dual-mode charge storage achieved in laser-induced graphene supercapacitors

In a development that could reshape how flexible energy storage devices are made, researchers in Iran have shown that a modest, low-power blue diode laser can convert ordinary polyimide tape into graphene-based supercapacitor electrodes with performance competitive with devices made using far more energy-hungry infrared lasers. The work, published in the Journal of Materials Science, also uncovers a surprise in how the resulting material stores charge: unlike laser-induced graphene made with carbon dioxide lasers, which behaves almost purely as an electrical double-layer capacitor, the blue-laser material displays a measurable pseudocapacitive contribution, pointing to a hybrid storage mechanism that its infrared-made counterparts do not share.

Laser-induced graphene, or LIG, first burst onto the scene in 2014 when researchers at Rice University showed that a commercial CO2 laser could scrib directly onto polyimide film and leave behind a porous, conductive graphene foam. Since then, the technique has become a darling of the flexible electronics community because it requires no catalysts, no chemical precursors, no wet processing and no cleanroom, just a polymer substrate and a laser. Supercapacitors, strain sensors, gas sensors, microfluidic devices and antennas have all been patterned this way. Yet almost all of that progress has relied on infrared CO2 lasers, typically operating around 10.6 micrometers, whose photons are strongly absorbed by the carbon-hydrogen bonds in polyimide and whose power levels often run to tens of watts.

The new study, led by Hedieh Pazokian of the Photonics and Quantum Technologies Research School at the Nuclear Science and Technologies Research Institute in Tehran, together with Reza Ghanbari, Amir Soleimani and Leila Irannezhad, takes a different tack. The team used a compact 445-nanometer diode laser, a device small enough to sit on a benchtop and cheap enough for almost any laboratory, operating at just 1.75 watts. That is a fraction of the power demanded by conventional infrared systems. Building on a parametric study the group had previously published, they scanned the beam across polyimide tape at a writing speed of 10 millimeters per second and a line density of 230 laser pulses per inch, parameters they had already identified as optimal for producing high-quality material with a visible-wavelength source.

The physics behind the conversion is worth pausing on. Polyimide is an aromatic polymer rich in carbon, and when a laser deposits enough heat into its surface, the polymer undergoes photothermal conversion: the imide rings break, non-carbon atoms vent away as gases, and the remaining carbon reorganizes into disordered graphitic layers. A 445-nanometer blue laser interacts with the polymer differently than an infrared one. Visible photons at this wavelength are absorbed less strongly by the polymer backbone, so the process runs at milder effective thermal conditions, and the team suspected this gentler treatment might leave a different chemical fingerprint on the resulting graphene.

To find out, the researchers subjected their material to a battery of characterization techniques. Raman spectroscopy revealed a hierarchically porous graphene network with a moderate defect density, quantified by the ratio of the disorder-induced D peak to the graphitic G peak, which came out at 0.76. That value sits in the range typical of LIG produced by other routes and indicates a carbon lattice that is well graphitized but still contains enough edge sites and defects to be electrochemically interesting. The hierarchical porosity matters because supercapacitor performance depends heavily on ion-accessible surface area: the more pores of the right size, the more electrolyte ions can reach the charge-storing surfaces.

The electrochemical story is where the study becomes genuinely distinctive. Using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy, the team measured an areal specific capacitance of 34.48 millifarads per square centimeter for a single LIG electrode in a three-electrode configuration. For symmetric solid-state devices, pairing two identical LIG electrodes with a solid electrolyte, the device delivered 9.45 millifarads per square centimeter, a maximum energy density of 1.31 microwatt-hours per square centimeter and a power density of 125 microwatts per square centimeter. Those figures place the 445-nanometer material firmly in the conversation with CO2-derived LIG supercapacitors reported in the literature, despite the dramatically lower laser power involved.

The pivotal discovery came from analyzing the shape of the cyclic voltammograms. If a carbon electrode stores charge purely through the electrical double layer, ions simply accumulate at the electrode-electrolyte interface and the current-voltage curve takes on a near-rectangular, box-like shape. But the blue-laser LIG showed deviations from that ideal rectangular profile, and the analysis of how peak currents scaled with scan rate revealed a measurable pseudocapacitive contribution. Pseudocapacitance arises when charge is stored not just at the surface but through fast, reversible faradaic reactions, typically involving heteroatoms or functional groups on the material. The researchers attribute the effect to oxygen-containing functional groups, carbonyls, hydroxyls and related moieties, that survive the synthesis. In infrared processing, the intense and prolonged heating tends to strip most of these oxygen species from the carbon surface, leaving a nearly pristine, purely capacitive graphene. The milder thermal environment of visible-laser conversion, by contrast, apparently preserves a population of these oxygen groups, which then participate in charge storage alongside the double-layer mechanism.

The team describes this as a mixed-mode charge storage mechanism, and it carries real consequences. Oxygen functionality on carbon electrodes is well known to enhance wettability, improve ion access into the pore network and add extra capacitance through redox-active sites, and the literature on graphene oxide and modified carbons documents how surface chemistry can dominate charge storage behavior. If visible-laser LIG intrinsically retains useful surface chemistry without any post-processing, that removes a fabrication step and opens a route to electrodes whose pseudocapacitive contribution can be tuned simply by adjusting laser parameters.

The practical implications extend beyond electrochemistry. The diode laser used in this work costs a small fraction of a CO2 laser system, draws far less power, and integrates easily into compact, scalable fabrication setups. For applications in wearable electronics, soft sensors, medical patches and Internet of Things devices, where energy storage must bend, flex and conform to the body, the ability to write supercapacitor electrodes directly onto flexible polyimide with an inexpensive blue laser is a genuinely attractive proposition. The authors position visible-wavelength diode lasers as an energy-efficient, accessible and until now underexplored pathway for flexible energy storage fabrication, and their results substantiate that framing with quantitative electrochemical data rather than mere proof-of-concept.

There are, of course, questions still to be answered. The areal capacitance of the single electrode, while respectable, will need to climb for many practical applications, and the team’s own prior work has shown that batch-to-batch variation in LIG electrodes remains a real concern across the field, a problem tied to the sensitivity of laser conversion to power, speed and substrate conditions. Whether the oxygen functionality that drives the pseudocapacitance is stable over thousands of charge-discharge cycles, and whether it survives in gel-electrolyte devices under mechanical strain, will determine how far this mixed-mode storage can be pushed. The researchers also note that the exact balance between double-layer and pseudocapacitive storage can be quantified more rigorously, and the field has recently debated the best analytical frameworks for separating those contributions.

Even so, the study broadens the palette of tools available to the rapidly growing LIG community. It demonstrates that the choice of laser wavelength is not merely a matter of cost and convenience but a genuine chemical lever that changes the surface chemistry and therefore the electrochemical character of the product. A blue diode laser costing a few hundred dollars can now be said to produce not just a cheaper imitation of infrared LIG, but a material with its own distinct storage behavior. For laboratories and startups working on flexible and wearable power sources, that distinction may prove to be the most important part of the story: the cheapest laser on the bench may, in some respects, make the more interesting electrode.