Optimizing PdAu/VGCNF Anode Catalyst for Enhanced Glycerol Fuel Cell Performance

Abstract

This study introduces a novel anodic PdAu/VGCNF catalyst designed to improve glycerol electro‑oxidation in alkaline fuel cells. By systematically varying catalyst loading, electrolyte temperature, and NaOH concentration, the optimal operating window was identified using response‑surface methodology (RSM). Under the most favorable conditions—5.24 M NaOH, 60 °C, and 12 wt.% catalyst loading—the catalyst achieved a peak current density of 158.34 mA cm⁻², demonstrating its promise for practical fuel‑cell applications.

Background

Global energy demands are pushing the limits of fossil fuels, prompting a shift toward cleaner alternatives. Fuel cells, which convert chemical energy directly into electricity, offer a compelling solution but traditionally rely on hydrogen—an energy carrier that is difficult to store and transport. Glycerol, a low‑toxicity by‑product of biodiesel production, presents an attractive liquid fuel for alkaline fuel cells due to its high energy density and abundant supply.

Glycerol’s complex structure leads to multiple oxidation intermediates, making catalyst selection and reaction conditions critical. Palladium–gold (PdAu) bimetallic nanoparticles supported on vapor‑grown carbon nanofibres (VGCNF) combine high catalytic activity with excellent dispersion and stability. VGCNF’s high surface area (10–200 m² g⁻¹) and edge‑rich lattice provide a robust platform for metal–support interactions, mitigating nanoparticle agglomeration and enhancing electrocatalytic performance.

Previous work has shown that higher temperatures and NaOH concentrations generally accelerate glycerol oxidation, but the optimal combination of these variables remains unclear for PdAu/VGCNF. This study applies RSM to identify the precise conditions that maximize current density while minimizing catalyst loading.

Experimental

Materials and Chemicals

Gold(III) chloride trihydrate, palladium chloride, trisodium citrate, sodium borohydride, VGCNF, sodium hydroxide, glycerol, 2‑propanol, and 5 wt.% Nafion solution were purchased from Sigma‑Aldrich.

Instrumentation

X‑ray diffraction (Bruker D8 Advance), field‑emission scanning electron microscopy (Gemini SEM 500), energy‑dispersive X‑ray spectroscopy, and transmission electron microscopy (Philips CM12) were employed to assess crystal structure, morphology, and elemental composition.

Catalyst Synthesis

The PdAu alloy was prepared by co‑reduction of PdCl₂ and HAuCl₄·3H₂O (1:1 molar ratio) in the presence of trisodium citrate, followed by impregnation onto VGCNF. Sodium borohydride (5–15 × metal‑ion molar ratio) served as the reducing agent. Catalyst loadings ranged from 10 to 30 wt.%. The final product was dried at 80 °C for 10 h.

Cyclic Voltammetry Tests

Electrochemical measurements were conducted on a glassy‑carbon electrode (active area 3 mm²) using an Autolab PGSTAT101. Catalyst ink (5 mg catalyst in water/IPA/Nafion) was deposited and dried. CV scans were performed from –0.8 to 0.4 V at 50 mV s⁻¹ in 0.5 M glycerol/0.5 M NaOH, with electrolyte NaOH concentrations (0.5–6.0 M) and temperatures (25–80 °C) systematically varied. All solutions were de‑oxygenated with N₂.

Experimental Design

A central composite design (CCD) comprising 20 runs (factorial, axial, and center points) evaluated the effects of NaOH concentration, temperature, and catalyst loading on peak current density. A second‑order polynomial regression was fitted to the data to generate a predictive model.

Results and Discussion

Physical Characterization

XRD confirmed the formation of highly alloyed PdAu nanoparticles (fcc structure) with a crystallite size of 4.5 nm. SEM images revealed moderate agglomeration, while EDX mapping showed a Pd:Au ratio close to the feed (≈55:44). TEM images demonstrated uniform dispersion of PdAu particles on VGCNF, with a size distribution of 2.5–9.5 nm (average 4.5 ± 1.0 nm).

Optimization Study

The quadratic RSM model yielded an R² of 0.986 and a highly significant p‑value (<0.0001), indicating an excellent fit. ANOVA revealed that all three factors—NaOH concentration, temperature, and catalyst loading—were statistically significant. The model predicted a maximum peak current density of 158.34 mA cm⁻² at 5.24 M NaOH, 60 °C, and 12 wt.% loading.

Effect of Operating Parameters

Contour plots illustrated that peak current density increases with NaOH concentration and temperature up to an optimum, beyond which it declines due to catalyst surface coverage and mass‑transport limitations. Catalyst loading exhibited a similar trend: current density rose up to ~12–15 wt.% and then decreased as excessive metal blocked active sites.

Confirmation Tests

Two validation runs were performed under the predicted optimal conditions and at the lower‑cost extremes (0.5 M NaOH, 45 °C). The experimental current densities (158.34 mA cm⁻² and 143.94 mA cm⁻²) closely matched the model predictions, confirming the robustness of the RSM approach.

Conclusions

RSM with a central composite design effectively optimized the PdAu/VGCNF anode catalyst for glycerol fuel cells. The optimal conditions—5.24 M NaOH, 60 °C, 12 wt.% loading—yielded a peak current density of 158.34 mA cm⁻², representing a >40 % improvement over non‑optimized runs and a significant reduction in catalyst loading compared to literature benchmarks. These findings demonstrate that PdAu/VGCNF is a highly efficient, low‑cost anodic catalyst for alkaline glycerol fuel cells.