*2.1. Materials*

The microbial homobiopolyester PHB, P226F grade, was obtained from Biomer (Krailling Germany). This grade is certified as both compostable and food contact. It has a density of 1.25 g/cm<sup>3</sup> and a melt flow rate (MFR) of 10 g/10 min when tested at 180 ◦C using a 5 kg load.

Polycaprolactone (PCL) (Mw: Mn 80 kDa), 2,2,2-trifluoroethanol (TFE) with 99% purity and D-limonene with 98% purity were purchased from Sigma-Aldrich S.A. (Madrid, Spain). Hexadecyltrimethylammonium bromide (CTAB) with 99% purity and palladium (Pd) nano-powder, <25 nm particle size measured by transmission electronic microscopy (TEM) and ≥99.5% trace metals basis, were also purchased from Sigma-Aldrich S.A. Chloroform (≥99%) and 1-butanol (99.5%) were purchased from AppliChem. All products were used as received without further purification. CTAB was selected as surfactant for PHB, because it is currently permitted for food contact applications by FDA and EFSA.

The conventional cellulose fiber-based packaging substrate was prepared using commercial bleached Kraft eucalyptus pulp as raw material, which was kindly provided by Ence-Celulosas y Energia S.A. (Madrid, Spain). Briefly, the pulp was disintegrated in a pulp disintegrator for 1 h at 3000 rpm to achieve a consistency of 1.5%. Paper sheets of 700 × 16 mm<sup>2</sup> with a final grammage of 75 g/m<sup>2</sup> were fabricated in an isotropic Rapid-Köthen sheet former and conditioned at 23 ◦C and 50% of relative humidity (RH) according to ISO standard 187. The grammage and thickness were evaluated following ISO standards 536 and 534, respectively. Further details can be found in previous research [22,27].

#### *2.2. Preparation of the Films*

Before electrospinning, the PHB solution was prepared by dissolving 10 wt % in TFE under magnetic stirring conditions at 50 ◦C. The PHB/PdNP suspension was prepared by adding CTAB (0.25 wt. % in the fibers) surfactant and PdNP (1 wt. % in the fibers) to the PHB solution.

PCL was prepared by dissolving 10 wt % in butanol:chloroform (25:75) under magnetic stirring conditions at room temperature. To prepare the solution of PCL/PdNP, PdNP were added (1 wt % in the fibers) in the previous solution and dissolved while magnetically stirring.

The electrospinning device used was a high throughput Fluidnatek® LE-500, used in lab mode with temperature and relative humidity control pilot plant equipment from Bioinicia S.L. (Valencia, Spain), a variable 0–60 kV dual polarizer high-voltage power supply, and a scanning injector, to obtain a homogeneous deposition of fibers. To obtain the electrospun PHB layers, the biopolymer solution was transferred to a 30 mL plastic syringe and coupled by a Teflon tube to a stainless-steel needle (∅ = 0.9 mm) that was connected to the power supply. PHB and PCL solutions were electrospun at 25 ◦C and 30% RH on a flat metallic collector, for 2 and 1 h under a steady flow-rate of 6 mL/h and 2 mL/h, respectively, using a motorized injector, scanning vertically toward a metallic grid collector. A distance between the injector and collector was both optimal at 15 cm, and the applied voltage was 16 kV and 12 kV.

The electrospun PHB and PCL coatings were subjected to an annealing post-processing step below the polymers' melting points, at temperatures of 160 ◦C and 50 ◦C, respectively using a hydraulic press 4122-model from Carver, Inc. (Indiana, IN, USA). This post-processing thermal treatment was applied for 5 s without pressure, to ensure the coalescence of the fibers mat into a continuous film. The conditions were selected based on the research conducted in previous works [21,28,29].

#### *2.3. Characterization of the Films*
