What happens when a methanol fuel-cell catalyst is engineered as a three-part team—and then heated? This study finds that performance rises with temperature, but only up to a specific sweet spot: 323 kelvin. Fuel cells convert the chemical energy in fuel into electrical energy through an electrochemical process, and direct methanol fuel cells are regarded as appealing because of their energy density, safe storage, ease of operation, safe handling, and minimal environmental impact.

The commercial problem is that direct methanol fuel cells mostly use expensive platinum-based anode catalysts, while high costs and partial methanol conversion impede wider adoption. That creates an urgent need for cost-effective alternatives to precious-metal catalysts, and palladium is regarded as a substitute for platinum because it has lower cost and comparable efficacy in methanol electrooxidation.

Composite formation, functionalisation, and surface modification have been used to lower electrocatalyst cost and enhance durability, while incorporating a secondary metal into palladium can diminish the binding strength of palladium carbon monoxide and improve resistance to intermediate poisoning.

Manganese dioxide is attractive because of its redox shuttling and oxygen-binding sites, and it can provide active hydroxyl species while improving noble-metal nanoparticle dispersion. But manganese dioxide has low intrinsic electrical conductivity and poor crystallinity, which limit interfacial contact with palladium.

Exfoliated boron instead offers a two-dimensional architecture for nanoparticle dispersion, rapid electron transport, and electrolyte penetration. The proposed ternary composite assigns primary methanol-oxidation sites to palladium, oxygen-containing species to manganese dioxide, and conductivity and structural stability to exfoliated boron; the study focuses especially on operating temperature.

The Pd/MnO2/exfoliated boron ternary composite was made in three steps: solvent-assisted exfoliation of boron, hydrothermal synthesis of the manganese-dioxide and exfoliated-boron composite, and ultrasonic integration of palladium. For exfoliation, 75 milligrams of commercial boron powder was dispersed in 20 millilitres of acetone and continuously ultrasonicated for 24 hours, then centrifuged at 4500 revolutions per minute for 10 minutes.

The exfoliated boron nanosheets were collected from the supernatant and dried at 80 degrees Celsius overnight, then 5 milligrams was dispersed in 20 millilitres of deionised water before potassium permanganate was added. The mixture was held in a Teflon-lined autoclave at two hundred degrees Celsius for twenty-five hours.

Finally, commercial palladium powder was mixed with the manganese-dioxide and exfoliated-boron composite at a one-to-three weight ratio and ultrasonicated for three hours. The electrochemical performance was evaluated in a conventional three-electrode cell using a glassy-carbon working electrode, an silver chloride reference electrode, and a platinum-wire counter electrode.

The catalyst ink used 10 milligrams of electrocatalyst, ethanol, and Nafion, and 5 microlitres was drop-cast onto the glassy-carbon electrode at a loading of approximately 6.43 milligrams per square centimetre. Methanol oxidation was tested by cyclic voltammetry in two molar sodium hydroxide, with and without two molar methanol, while impedance spectroscopy examined charge-transfer characteristics.

To isolate temperature effects, cyclic voltammetry was performed under identical conditions at 303, 313, 323, and 333 kelvin. The diffraction pattern is dominated by sharp reflections from crystalline manganese dioxide, confirming hydrothermal growth of the oxide phase.

Additional diffraction peaks at approximately 40, 46, 68, and 82 degrees are assigned to face-centred cubic metallic palladium, confirming its incorporation into the composite. Distinct exfoliated-boron peaks are not observed, which the text attributes to low loading, reduced crystallinity after exfoliation, and homogeneous dispersion within the manganese-dioxide matrix.

In panel b, the spectrum shows distinct elemental signals for carbon, oxygen, manganese, and palladium, with the manganese peak near six kiloelectronvolts especially prominent. The inset enlarges the higher-energy region and reveals a small additional feature marked B, providing visual evidence for boron in the composite.

Transmission electron microscopy reveals agglomerated polyhedral nanoparticles about 100 to 300 nanometres across, uniformly anchored on electron-transparent sheet-like structures. The transparent sheets are consistent with ultrathin exfoliated-boron nanosheets and provide a conductive substrate for manganese-dioxide nanoparticle growth.

High-resolution microscopy shows lattice fringes with an interplanar spacing of approximately 0.31 nanometres, assigned to the two-hundred plane of tetragonal manganese dioxide, agreeing with the powder X-ray diffraction results. Discrete palladium nanoparticles are not readily distinguished in the transmission electron micrographs, although X-ray diffraction and energy-dispersive spectroscopy confirm their incorporation.

Integrated palladium-oxide reduction responses give estimated electrochemically active surface areas of approximately 25.01 square metres per gram of palladium for Pd/MnO2/eB and 11.52 for Pd/C. The exfoliated-boron sheets may disperse electroactive palladium, while the palladium–manganese-dioxide interface may limit palladium agglomeration and keep more palladium sites accessible to the electrolyte.

Because the ternary composite has a larger electrochemically active surface area, it is expected to deliver an enhanced methanol-oxidation response compared with palladium on carbon. However, these values are estimated mass-normalised areas rather than absolute bulk-normalised values, because the composite's palladium mass was estimated from semi-quantitative energy-dispersive spectroscopy.

Impedance spectroscopy shows a depressed high-frequency semicircle followed by a steep low-frequency line, with the latter corresponding to the Warburg diffusion region and electrolyte-ion diffusion into the electrode structure. The composite has a solution resistance of 9.42 ohms, indicating good ionic conductivity, minimal ohmic losses, efficient ionic transport, and satisfactory electrical contact between the catalyst layer and current collector.

Its charge-transfer resistance is 361.42 ohms; because charge-transfer resistance reflects electron transfer across the electrode–electrolyte interface, this relatively low value indicates favourable charge-transfer kinetics and efficient electron transport. The reduced charge-transfer resistance is attributed to interaction among conductive palladium nanoparticles, electrochemically active manganese dioxide, and conductive exfoliated boron.

In two molar sodium hydroxide containing two molar methanol, both catalysts show pronounced anodic oxidation peaks, but Pd/MnO2/eB has a substantially stronger forward oxidation response than commercial Pd/C. The composite reaches a forward peak mass activity of approximately 4.50 milliamperes per milligram of palladium, compared with 2.75 for Pd/C, an approximately one-point-six-four-fold enhancement.

This improved methanol-oxidation response is consistent with the electrochemically active surface-area analysis, because the larger accessible palladium surface provides more sites for methanol adsorption and dehydrogenation. The lower reverse oxidation current density for Pd/MnO2/eB suggests better oxidation of intermediate species and good resistance to surface poisoning.

The proposed bifunctional mechanism assigns surface hydroxyl formation to manganese dioxide, oxidative removal of poisoning intermediates to those hydroxyl species, and rapid electron transport and palladium dispersion to conductive exfoliated boron. In methanol-containing electrolyte, the charge-transfer resistance is 325 ohms, slightly below the 361.42 ohms measured without methanol, indicating that methanol facilitates electron transfer across the interface.

Across scan rates from 2 to 40 millivolts per second, the peak currents increase while the voltammetric profile remains preserved, and the linear dependence on the square root of scan rate indicates predominantly diffusion-controlled methanol oxidation. Temperature-dependent cyclic voltammetry was performed from 303 to 333 kelvin, and the electrocatalytic response increased strongly as temperature rose.

The onset potential shifted from approximately negative 0.285 volts at 303 kelvin to negative 0.335 volts at 333 kelvin, indicating that oxidation begins at a lower overpotential and suggesting a reduced activation barrier. The forward oxidation peak current density increased from 303 to 323 kelvin, reaching 1.689 milliamperes per square centimetre, before decreasing slightly at 333 kelvin.

That decline shows that excessively high temperatures do not continuously improve activity and indicates an optimum operating temperature around 323 kelvin, beyond which competing processes influence performance. The backward-current column rises across all four rows, from 0.207 to 0.573 milliamperes per square centimetre, while the corresponding ratio column changes from 4.928 to 2.617.

The peak-potential columns also show the forward and backward values moving closer together by the final row. The pseudo-Arrhenius plot has good linearity, with an R-squared value of 0.990, indicating Arrhenius behaviour across the investigated temperature range at the fixed potential.

The apparent activation energy calculated from the Arrhenius slope is 47.9 kilojoules per mole, representing the energy barrier for overall methanol oxidation on the ternary composite. The Eyring analysis gives an activation enthalpy of 45.5 kilojoules per mole, and the positive value indicates that activated-complex formation is endothermic and requires energy before electron transfer and bond cleavage.

The activation entropy is negative 113 joules per mole per kelvin, indicating that the activated complex is more ordered than the reactant state. The activation Gibbs free energies are 79.8, 80.9, 82.0, and 83.2 kilojoules per mole at 303, 313, 323, and 333 kelvin, respectively, confirming a finite energy barrier.

The table shows a spread of three point four kilojoules per mole across those four temperature columns. The proposed mechanism gives each component a distinct role: palladium catalyses methanol adsorption and dehydrogenation, manganese dioxide generates reactive hydroxyl species, and exfoliated boron provides conductivity and homogeneous palladium dispersion.

Methanol adsorbs on exposed palladium and undergoes successive dehydrogenation through methoxy, formaldehyde, formyl, and finally strongly adsorbed carbon-monoxide-like intermediates. Manganese dioxide adsorbs hydroxide ions to generate surface hydroxyl species, which migrate to adjacent palladium sites and react with adsorbed carbon-monoxide-like intermediates to produce carbon dioxide and water, regenerating the palladium surface.

The exfoliated-boron nanosheets provide a continuous conductive network and suppress aggregation of manganese-dioxide and palladium nanoparticles, while higher temperature accelerates methanol adsorption, hydroxide-ion transport, electron transfer, and oxidation of adsorbed intermediates.

In the upper sequence, the figure traces methanol from adsorption on palladium through methoxy, formaldehyde, formyl, and carbon monoxide-like intermediates, with electron loss marked across the steps. The lower schematic then shows hydroxide activation on manganese dioxide, spillover toward palladium, and electron movement through the exfoliated-boron network, while the rightmost panel depicts carbon dioxide and water formation as palladium sites are regenerated.

The Pd/MnO2/eB ternary composite was successfully synthesized and showed higher methanol-oxidation activity than Pd/C: 4.5 milliamperes per milligram of palladium versus 2.75. Temperature-dependent studies found significantly enhanced oxidation with increasing temperature and optimum performance at 323 kelvin, while Arrhenius and Eyring analyses gave an activation energy of 47.9 kilojoules per mole, activation enthalpy of 45.5, and activation entropy of negative 113 joules per mole per kelvin.

The combined evidence attributes the enhanced performance to palladium for methanol activation, manganese dioxide for supplying reactive hydroxyl species that oxidize poisoning intermediates, and exfoliated boron for rapid electron transport. The Pd/MnO2/eB composite outperformed Pd/C, reaching 4.5 mA per milligram of palladium versus 2.75, with the best temperature-dependent response at 323 kelvin.

Its proposed advantage comes from dividing the chemistry among palladium, manganese dioxide, and exfoliated boron.