One‑Pot Fabrication of Hierarchical Flower‑Like Pd‑Cu Alloy on Graphene for Enhanced Ethanol Oxidation

Abstract

Direct ethanol fuel cells (DEFCs) demand robust, low‑cost catalysts that resist CO poisoning. We report a one‑pot synthesis of flower‑like palladium‑copper (Pd‑Cu) alloy nanoparticles anchored on reduced graphene oxide (Pd‑Cu_(F)/RGO). Structural analysis by SEM, TEM, XRD, XPS, and EDS confirms the alloy composition (≈1:1.4 Pd:Cu) and the unique hierarchical morphology. Electrochemical tests in 0.5 M NaOH reveal a peak current of 2416 mA mg^–1 Pd—more than twice that of commercial Pd black (847 mA mg^–1 Pd)—and an electrochemical surface area (ECSA) of 151.9 m² g^–1 Pd. The catalyst also exhibits superior long‑term stability, retaining 86 % of its initial activity after 3000 s. These results highlight the synergistic benefits of alloying and flower‑like architecture for ethanol electrooxidation in alkaline media.

Background

DEFCs offer high energy density, low operating temperature, and environmental friendliness. Yet, catalyst durability remains a critical bottleneck, largely due to CO‑like poisoning. Palladium is attractive because of its moderate cost and relatively low CO tolerance. Recent studies demonstrate that nanostructuring—such as nanoflowers, nanowires, and hierarchical microspheres—can dramatically increase active surface area and improve mass transport.

Cu plays a dual role: it reduces catalyst cost and promotes hydroxyl adsorption, accelerating alcohol oxidation. Alloying Pd with Cu shifts the d‑band center, enhancing electrocatalytic activity. Graphene and its derivatives provide a conductive, high‑surface‑area scaffold that facilitates uniform metal dispersion.

Building on these insights, we designed a facile, one‑pot hydrothermal route to produce flower‑like Pd‑Cu alloy nanoparticles on reduced graphene oxide (RGO), aiming to combine alloy synergy with a high‑surface‑area support.

Methods

Materials

All reagents were analytical grade: Cu(NO₃)₂·3H₂O, PdCl₂, ethylene glycol, ethanol, graphite powder (S.P.), H₂SO₄, KMnO₄, KBH₄, H₂O₂, NH₃, NaOH, PVP (MW = 30 000), Nafion (5 wt %), and 10 wt % Pd black (HESEN).

Preparation of Graphene Oxide (GO)

GO was synthesized from graphite via a modified Hummers method.

Synthesis of Pd‑Cu_(F)/RGO

A mixed solvent of 40 mL EG and 40 mL ethanol was sonicated with 160 mg PVP for 30 min. PdCl₂ (0.01 M) and Cu(NO₃)₂·3H₂O (0.02 M) were added, followed by NH₃ to adjust pH = 10.0. Simultaneously, 30 mg GO was dispersed in 5 mL EG + 5 mL ethanol, sonicated for 60 min, and combined with the metal solution. 2 mL KBH₄ (0.15 mg mL^–1) was introduced, and the mixture was sealed in a 50 mL Teflon‑lined autoclave at 160 °C for 6 h. After cooling, the product was centrifuged, washed with water and ethanol, and dried at 40 °C under vacuum. The resulting Pd‑Cu_(F)/RGO was obtained.

Control catalysts—spherical Pd‑Cu_(P)/RGO (using Na₂CO₃ instead of NH₃), Pd/RGO, and Cu_(F)/RGO—were prepared under identical conditions.

Electrochemical Measurements

All experiments used a CHI750D workstation with a three‑electrode cell at 25 °C. A Pt plate served as counter electrode; a saturated calomel electrode (SCE) was the reference. Working electrodes were fabricated by depositing 2 mg of catalyst onto a 5 mm glassy carbon electrode (GCE) via a 10 µL suspension, followed by a 5 µL drop of 5 wt % Nafion. Electrolytes were 0.5 M NaOH (N₂ saturated) and 0.5 M NaOH + 0.5 M C₂H₅OH (also N₂ saturated).

Results and Discussion

Structural Characterization

SEM and TEM images (Fig. 1) show Pd‑Cu nanoparticles densely decorating both sides of the RGO sheet. The Pd‑Cu_(F)/RGO particles adopt a flower‑like architecture with an average size of 80 ± 5 nm, whereas the Pd‑Cu_(P)/RGO particles are spherical (≈10 ± 2 nm). Cu_(F)/RGO shares a similar morphology, confirming the role of Cu and ammonia in directing the flower growth.

EDX (Fig. 2a) and ICP‑OES reveal Pd:Cu weight ratios of 1:1.4 and 15.8 wt % Pd / 21.4 wt % Cu, respectively, indicating successful alloy formation. STEM‑EDS line scans (Fig. 2b) confirm homogeneous elemental distribution.

XRD patterns (Fig. 3) display shifted (111) and (200) peaks for Pd‑Cu_(F)/RGO relative to pure Pd, evidencing alloying. Cu_(F)/RGO shows CuO and Cu₂O peaks, confirming oxidation of Cu during synthesis. A broad peak near 25° across all samples confirms reduction of GO to RGO.

XPS analysis (Fig. 4) identifies Pd⁰ and PdO, as well as Cu⁰, CuO, Cu₂O, and Cu(OH)₂ species. The presence of oxidized Cu supports the proposed two‑step reduction/oxidation mechanism in ammonia solution.

Thermogravimetric Analysis

TGA (Fig. 6) shows weight loss of 6 % for Pd‑Cu_(F)/RGO between 250–500 °C, compared to 14 % for Pd‑Cu_(P)/RGO and 22 % for Pd/RGO, reflecting reduced residual oxygen groups due to efficient GO reduction.

Electrochemical Performance

Cyclic voltammetry in 0.5 M NaOH (Fig. 7a) indicates an ECSA of 151.9 m² g^–1 Pd for Pd‑Cu_(F)/RGO, surpassing Pd‑Cu_(P)/RGO (123.4 m² g^–1), Pd/RGO (102.7 m² g^–1), and Pd black (88.1 m² g^–1).

In 0.5 M NaOH + 0.5 M C₂H₅OH (Fig. 7b), Pd‑Cu_(F)/RGO achieves a peak current of 2416.25 mA mg^–1 Pd—more than 1.4 × that of Pd‑Cu_(P)/RGO (1779.09 mA mg^–1), 2.4 × Pd/RGO (997.70 mA mg^–1), and 2.9 × Pd black (847.4 mA mg^–1). The onset potential is also most negative, indicating easier ethanol oxidation.

Cu‑only (Cu_(F)/RGO) shows negligible activity, underscoring Pd as the active site and the synergistic effect of alloying. The enhanced activity is attributed to (i) the bifunctional role of Cu supplying OH^– for intermediate removal, and (ii) the increased surface area afforded by the flower morphology.

Durability

Chronoamperometry at –0.35 V for 3000 s (Fig. 8) reveals a 21 % decay for Pd‑Cu_(F)/RGO versus 35 % for Pd‑Cu_(P)/RGO, and 42 % for Pd/RGO. The residual current after 3000 s is 2124 mA mg^–1 Pd for Pd‑Cu_(F)/RGO, outperforming all controls, confirming superior long‑term stability.

Conclusions

We have demonstrated a one‑pot, hydrothermal method to produce hierarchical flower‑like Pd‑Cu alloy nanoparticles on RGO. The synergy between alloy composition and flower morphology delivers markedly higher ethanol oxidation activity and durability in alkaline media than conventional spherical catalysts or commercial Pd black. These findings validate the design strategy of combining alloying with engineered nanostructures on conductive supports, paving the way for high‑performance catalysts in DEFCs.