Atomic Layer Deposition of Pd Nanoparticles on N-Doped Electrospun Carbon Nanofibers: Optimization of ORR Activity of Pd-Based Nanocatalysts by Tuning Their Nanoparticle Size and Loading

Optimization of size, loading and chemical composition of catalytic nanoparticles is a crucial step to achieve cost-effective and efficient (electro) catalysts. This report elaborates optimization of palladium (Pd) nanoparticle size and loading on the electrospun based N-doped carbon nanofibers (nCNF) towards oxygen reduction reaction (ORR) for the energy devices like fuel cell, metal air batteries. Electrospinning was utilized to produce one-dimensional (1D) polyacrylonitrile nanofibers followed by a two-step carbonization process obtaining well-defined conductive nCNF having diameters in the range of 200–350 nm. As-synthesized nCNF was decorated with discrete Pd nanoparticles ranging from 2.6±0.4 nm to 4.7±0.5 nm via thermal atomic layer deposition (ALD) technique. We found that nCNF deposited Pd nanoparticles having 3.9±0.6 nm size (Pd20/nCNF) showed the best ORR activity with the smallest Tafel slope of 58 mV dec⁻¹ along with four electrons involved in the ORR. In addition, high value at half wave potential (E_1/2=806 mV vs. RHE) and exchange current densities (i₀=6.998 mA cm⁻²) at Pd20/nCNF makes it efficient catalyst among other Pd decorated nCNF. Moreover, we found that electrocatalyst with lower loading/density of Pd nanoparticles showed enhanced ORR activity.