The core problem the researchers set out to solve is one that has haunted the clean energy and water treatment fields for years. Electrochemical water splitting, which uses electricity to pull water molecules apart into hydrogen and oxygen, is one of the most promising routes to green hydrogen, but it demands large amounts of extra energy because the half-reactions at the electrodes are sluggish. Meanwhile, photocatalytic advanced oxidation processes, which use light-activated catalysts to generate reactive molecules that shred organic pollutants, are hampered by the fact that excited electrons and holes tend to recombine almost instantly, wasting the absorbed light energy before it can do useful chemistry. Catalysts also tend to degrade over time, undermining long-term operation.

To attack both problems simultaneously, the team synthesized a family of composite materials by coupling vanadium pentoxide, a well-known transition metal oxide, with graphitic carbon nitride in two forms: an unmodified version abbreviated CN and a sulfur-modified version abbreviated SCN. By varying the amount of the sulfur-modified carbon nitride, they produced three composites containing 10, 20 and 30 weight percent of the modified polymer, labeled VSCN-1, VSCN-2 and VSCN-3 respectively. The idea behind this interfacial engineering is that when two semiconductors with different electronic structures are brought into intimate contact, the junction between them creates an internal electric field that sweeps charge carriers apart, keeping electrons and holes separated long enough for them to drive chemical reactions.

The star performer was VSCN-2, the composite with 20 weight percent sulfur-modified carbon nitride. When deposited on nickel foam, a common conductive support for electrodes, and tested in a concentrated potassium hydroxide electrolyte, the material required an overpotential of just 250 millivolts to drive the oxygen evolution reaction at a current density of 10 milliamperes per square centimeter. For the hydrogen evolution reaction, the overpotential was an impressively low 89 millivolts at the same current density. Overpotentials are the extra voltage beyond the thermodynamic minimum that must be applied to make a reaction proceed at a useful rate, so lower values translate directly into lower energy consumption and cheaper hydrogen.

Equally important for real-world deployment is how the catalyst behaves over time, and here the results were equally striking. The measured Tafel slopes, which describe how quickly current rises as voltage is increased, came in at 68 millivolts per decade for the oxygen reaction and 109 millivolts per decade for the hydrogen reaction, indicating favorable reaction kinetics on the modified surface. The electrode also operated stably for 60 hours without significant loss of activity, a duration that rules out the rapid degradation that plagues many experimental catalysts. When two identical VSCN-2 electrodes were paired into a full water-splitting electrolyzer, the device needed only 1.55 volts to sustain 10 milliamperes per square centimeter and kept running steadily for 80 hours, edging close to the 1.48-volt thermodynamic benchmark that defines an ideal electrolyzer.

The same material proved remarkably adept at a completely different task: destroying tetracycline, a widely used antibiotic that escapes into waterways through agricultural runoff and pharmaceutical wastewater. Residual antibiotics in the environment are a serious concern because they promote the evolution of drug-resistant bacteria, so finding efficient ways to degrade them is a growing research priority. Under irradiation from ordinary light-emitting diodes, the VSCN-2 composite achieved 99 percent removal of tetracycline through a process called sonophotocatalysis, which combines ultrasonic agitation with light activation. The ultrasound generates microscopic cavitation bubbles in the water that collapse violently, producing additional reactive species and improving mass transport, while the illuminated catalyst generates electron-hole pairs that attack the pollutant. The synergy between the two energy inputs pushes degradation rates well beyond what either light or sound alone can achieve.

Durability carried over to the photocatalytic application as well. The composite retained its degradation activity across repeated use cycles, suggesting that the interfacial structure is robust rather than a fleeting artifact of fresh synthesis. To understand which reactive species were actually doing the work of shredding the tetracycline molecule, the researchers performed radical-scavenging experiments, in which specific chemicals are added to selectively neutralize particular reactive species, alongside electron spin resonance spectroscopy, which detects short-lived radicals directly. Both approaches pointed to superoxide radicals and hydroxyl radicals as the dominant destructive agents, which in turn supported the proposed charge transfer mechanism operating across the vanadium pentoxide and sulfur-modified carbon nitride interface.

The team went further than simply confirming that the antibiotic disappeared. Using liquid chromatography coupled with mass spectrometry, they identified the intermediate products formed as the tetracycline molecule was progressively broken apart, and then used the ECOSAR predictive software to estimate the toxicity of those fragments. Encouragingly, the analysis indicated that the degradation pathway leads toward products with lower predicted toxicity than the parent antibiotic, meaning the process genuinely detoxifies the water rather than merely converting one harmful compound into another. This combination of mechanistic spectroscopy, product identification and toxicity prediction represents the kind of thorough evidence chain that regulators and engineers need before such technologies can move from the laboratory toward practical water treatment systems.

What makes this study notable in a crowded field of catalyst papers is the deliberate dual-function design. Most catalysts are optimized for a single reaction and a single application, but the underlying physics that limits both technologies, namely the wasteful recombination of charge carriers and sluggish interfacial charge transfer, is shared. By engineering a heterostructure in which the sulfur modification tunes the electronic structure of the carbon nitride and the intimate junction with vanadium pentoxide accelerates charge separation, the researchers addressed both limitations with one material. The sulfur atoms in the carbon nitride framework are thought to adjust the band positions and introduce active sites, while the oxide-polymer interface acts as a highway for electrons migrating in one direction and holes in the other, depending on whether the driving force is an applied voltage or absorbed light.

The implications stretch across the clean energy and environmental technology landscape. A catalyst that can lower the voltage needed for hydrogen production could reduce the cost of green hydrogen, which remains a key stumbling block for the hydrogen economy, while the same chemistry applied to polluted water offers a low-energy route to eliminating pharmaceutical contaminants using nothing more than LEDs and ultrasound. The work was supported by the National Research Foundation of Korea through its Basic Science Research Program, with additional funding from King Saud University in Saudi Arabia. As laboratories worldwide race to design multifunctional materials that squeeze more value from every input of energy, this sulfur-modified heterostructure stands as a compelling demonstration that clever interfacial engineering can turn two hard problems into one elegant solution.

Subject of Research: Sulfur-modified g-C3N5/V2O5 heterostructure catalysts for electrochemical water splitting and sonophotocatalytic antibiotic degradation

Article Title: Interfacial engineering of sulfur-modified g-C3N5/V2O5 heterostructures for enhanced overall water splitting and sonophotocatalytic tetracycline degradation

Article References: Kumaravel, S., Durai, M., Gnanasekaran, L., Kumaravel, S., Erusappan, E., Shanmugapriya, D., Afzal, M., Paskalis, S. M. K., & Lee, D. S. (2026). Interfacial engineering of sulfur-modified g-C3N5/V2O5 heterostructures for enhanced overall water splitting and sonophotocatalytic tetracycline degradation. Advanced Composites and Hybrid Materials. https://doi.org/10.1007/s42114-026-02085-y

Image Credits: AI Generated

DOI: 10.1007/s42114-026-02085-y

Keywords: electrocatalysis, water splitting, hydrogen evolution, oxygen evolution, photocatalysis, sonophotocatalysis, tetracycline degradation, g-C3N5, V2O5, heterostructure, sulfur modification, advanced oxidation processes

The starting materials belong to a family of cyanometallate coordination polymers with the general framework T[Fe(CN)₅NO], where T is a transition-metal ion. In the study, T was cobalt, nickel, or copper. Pyridine molecules bind to the outer metal centers and change the architecture from a three-dimensional bulk structure into layered two-dimensional sheets. This transformation is important because the parent nitroprussides have limited surface area and disperse poorly, whereas thin layers can be distributed across a conductive carbon support. Pyridine is therefore not introduced primarily as an oxygen-evolution promoter. Its principal role is structural: by selectively replacing axial metal–cyanide connections, it enables the layered morphology and helps stabilize neighboring sheets through pi–pi and dipole–dipole interactions. The resulting materials were combined with reduced graphene oxide in a one-to-one weight ratio. The graphene network provides pathways for electron transport and helps expose the coordination-polymer domains to the alkaline electrolyte.

Several complementary measurements confirmed that the metal identity altered both the structure and the electronic environment of the composites. X-ray diffraction showed layered materials with characteristic basal-plane reflections for the cobalt and nickel systems, while the copper composite displayed different stacking behavior. Electron microscopy revealed average particle sizes of approximately 133 nanometers for the cobalt material, 47 nanometers for the nickel material, and 214 nanometers for the copper material. Infrared and Raman spectroscopy detected the cyanide, nitrosyl, pyridine, and graphene-related signatures expected from the composite design. The cyanide stretching frequency shifted systematically as the metal changed from cobalt to copper, reflecting differences in the cations’ polarizing power. Raman measurements also indicated a defect-rich reduced graphene oxide support. The disorder can be useful in electrocatalysis because defects may create additional sites for oxygen-related reactions, although the measured surface area alone did not determine which composite performed best.

Electrochemical testing revealed a clear division of labor among the three materials. For OER in alkaline solution, the nickel composite showed the lowest onset potential and the strongest current response, reaching 10 milliamperes per square centimeter at an overpotential of about 398 millivolts. The cobalt and copper versions required approximately 437 and 518 millivolts, respectively, under the same benchmark. Nickel also exhibited the lowest charge-transfer resistance across the tested potential range. Cobalt, however, produced the lowest fitted OER Tafel slope, 61 millivolts per decade, compared with 74 for nickel and 156 for copper. The researchers caution that a lower Tafel slope by itself does not establish the fastest overall reaction; onset potential, overpotential, resistance, and operating conditions must be considered together. For ORR, the pattern reversed. The copper composite delivered the lowest onset potential and the highest limiting current, indicating the most favorable oxygen-reduction behavior among the tested materials.

Rotating-disk experiments provided further information about the oxygen-reduction pathway. Koutecky–Levich analysis indicated that all three composites predominantly followed a four-electron route, which is desirable in zinc-air batteries because it converts oxygen more directly and limits peroxide formation. The estimated electron-transfer numbers were approximately 3.95 for cobalt, 3.63 for nickel, and 3.97 for copper. The nickel value suggests that a mixture of four-electron and two-electron pathways may occur, potentially producing some hydrogen peroxide. The authors note that rotating-ring-disk measurements would be needed to investigate that possibility directly. Together, the half-cell results show why optimizing a rechargeable zinc-air battery is difficult: nickel favors the charging reaction, copper favors the discharging reaction, and neither is automatically the best complete-device material. The measurements instead point toward a compromise in which both reactions remain sufficiently active.

That compromise emerged when the composites were assembled into working zinc-air batteries. The cobalt-based cathode produced the highest peak power density, 54 milliwatts per square centimeter, narrowly exceeding the nickel system at 53 and clearly surpassing the copper system at 39. It also maintained higher discharge-voltage plateaus, particularly as the current density increased. During galvanostatic cycling at 10 milliamperes per square centimeter, the cobalt battery began with a voltage gap of 0.99 volts between charge and discharge and reached 1.18 volts after 24 hours. The corresponding gaps for copper were 1.05 and 1.29 volts, while nickel showed 1.43 and 1.36 volts. Initial round-trip energy efficiencies were 52 percent for cobalt, 50 percent for copper, and 34 percent for nickel. After the cycling period, they were 46, 42, and 33 percent, respectively. The cobalt device also retained a relatively stable impedance and delivered an average energy above 8.5 watt-hours per square meter per cycle.

The study’s central chemical finding is that the synthesized nitroprussides do not remain unchanged during battery operation. Post-cycling analysis of the best-performing cobalt electrode showed that the characteristic cyanide signal disappeared, while hydroxyl and metal–oxygen signatures emerged. X-ray diffraction and Raman spectroscopy likewise indicated conversion into a cobalt hydroxide or oxyhydroxide phase. The original material is therefore a precursor rather than the final catalyst. Importantly, X-ray photoelectron spectroscopy detected a pyridinic nitrogen signal after cycling, suggesting that the pyridine ligand remained associated with the reconstructed active phase instead of being completely lost. The researchers propose that the retained molecules act as structural pillars and electronic modifiers, helping prevent the newly formed oxyhydroxide sheets from agglomerating. The same alkaline environment that drives the battery reactions promotes this reconstruction. Similar hydroxide or oxyhydroxide phases are considered likely for the nickel and copper composites, although direct post-cycling structural confirmation was performed most extensively for cobalt.

Magnetic measurements offered an unusual window into the reconstructed electrodes, indicating that the active hydroxides are locally disordered rather than simple, perfectly ordered crystals. Measurements of magnetic susceptibility showed metal-dependent interactions, including antiferromagnetic correlations, weak ferromagnetic-like behavior, metamagnetic transitions, and magnetic relaxation. The cobalt and nickel hydroxide phases displayed low-temperature features associated with competing or complex interactions, while the copper material showed behavior consistent with antiferromagnetic correlations and a distorted local environment. These observations matter because local disorder can change the electronic states available for binding oxygen intermediates. Valence-band X-ray photoelectron spectra placed the approximate d-band centers in the order nickel below cobalt below copper. Within the commonly used d-band framework, a higher d-band center can strengthen adsorption of oxygen intermediates, helping ORR but potentially making OER less favorable. The results fit that qualitative picture: copper was strongest for ORR, nickel for OER, and cobalt, with an intermediate electronic position, offered the best balance. The authors emphasize that the d-band values are approximate descriptors, not proof of a single causal mechanism.

The work presents coordination polymers as tunable molecular precursors for practical oxygen electrocatalysis, while also highlighting the limits of laboratory battery demonstrations. The cobalt composite approached a zinc-specific capacity of 809.2 milliampere-hours per gram at 5 milliamperes per square centimeter, close to the theoretical value of 820, but the researchers caution that this result reflects a relatively low current density, a high-purity zinc anode, and a particular concentrated alkaline electrolyte. Further studies will be needed to assess operation at higher rates, longer lifetimes, different electrolyte compositions, and larger electrode areas. In situ spectroscopy and density-functional calculations could also test how the retained pyridine, disordered hydroxide structure, and d-band position cooperate during cycling. Even with those questions unresolved, the comparison demonstrates a useful design principle: the best bifunctional catalyst is not necessarily the material that wins either half-reaction independently. In this series, deliberately balancing electronic structure, conductive architecture, and electrochemical reconstruction allowed the cobalt-based pre-electrocatalyst to deliver the most practical zinc-air battery performance.

The precursor strategy also addresses a practical materials challenge beyond catalytic activity. Conventional oxygen electrocatalysts often rely on platinum for reduction or iridium- and ruthenium-based oxides for evolution, but the scarcity and cost of these elements complicate deployment at scale. Nitroprusside coordination polymers offer a different platform because their metal sites, cyanometallate framework, and organic coordination environment can be varied systematically. The cobalt, nickel, and copper comparison therefore functions as a controlled probe of how first-row transition-metal chemistry affects a common molecular architecture, rather than as a simple search for a single universally optimal composition.

That architecture may be especially valuable for studying catalyst reconstruction under realistic operation. Because the electrochemically generated hydroxide is the working phase, evaluating only the pristine powder could obscure the properties that govern a battery’s behavior. The reported combination of spectroscopy, diffraction, microscopy, and magnetic susceptibility illustrates why identifying active materials requires measurements before and after polarization. It also suggests a broader design opportunity: coordination ligands can serve as temporary structure-directing components while influencing the local environment retained after conversion. Determining how much pyridine survives, where it is located, and whether its effect persists over extended cycling will be important for distinguishing a genuine electronic contribution from a short-lived synthesis advantage. Such questions are relevant to translating these composites from proof-of-concept electrodes into durable, scalable air-cathode materials.

Subject of Research: Transition-metal nitroprusside and reduced graphene oxide pre-electrocatalysts for rechargeable zinc-air batteries

Article Title: Pyridine-based nitroprussides coupled to reduced graphene oxide as bifunctional pre-electrocatalysts for zinc-air batteries

Article References: Quintanilla-Serrano, E. A., Acevedo-Peña, P., Ávila, Y., González, M., & Reguera, E. (2026). Pyridine-based nitroprussides coupled to reduced graphene oxide as bifunctional pre-electrocatalysts for zinc-air batteries. Discover Electrochemistry, 3(1), Article 77. https://doi.org/10.1007/s44373-026-00164-9

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00164-9

Keywords: zinc-air batteries, electrocatalysis, reduced graphene oxide, nitroprussides, oxygen reduction, oxygen evolution, cobalt hydroxide, d-band center, Pyridine-based, coupled, reduced, graphene