Role of Electrode Thickness in NiFe Nanogranular Films for Oxygen Evolution Reaction

Abstract

Nanostructured materials may provide a route to overcome the electrode-limiting performance in water splitting, the oxygen evolution reaction (OER), within the framework of low-cost catalysts search. However, for alloyed NiFe nanostructures, the relationship among the OER efficiency and the electrode physical characteristics (morphology, porosity, size, thickness, or mass loading) is largely unknown. This work introduces a new type of alloyed NiFe (90/10% at) nanogranular electrodes obtained by supersonic cluster beam deposition and investigates the dependence of their catalytic activity toward the OER on the film morphological and stoichiometric properties. The synthesized alloyed NiFe nanoparticles with 0.3–3.8 nm size assemble from the gas phase to form ultrathin film electrodes with thickness in the 15–88 nm range, corresponding to 5–30 μg/cm2 mass loading. The fitting of the optical spectroscopic data by an effective medium approximation model suggests that, independent of the thickness, the films have a 20% porosity and are completely hydroxydated. The resulting catalytic efficiency is independent on the film thickness, while the turnover frequency decreases with increasing electrode loading. These data suggest that an excess of catalyst mass with respect to the OER active sites is deposited in the case of thicker electrodes and sets the 15 nm film as an upper loading limit to maximize the electrocatalyst efficiency. This study represents a crucial step toward thickness optimization of NiFe electrodes to fabricate low-cost OER catalysts.

Unexpected Resilience of NiFe Catalysts for the Alkaline Oxygen Evolution Reaction

“…The most significant spectrum is that of Ni 2p, shown in Figure d, with a comparison between the before- and after-CV conditions. The initial peak corresponded to a mix of metal (centered at 853 eV), oxide, and hydroxide phases (in the 854–863 eV range) . After the prolonged electrochemical activity, the spectrum showed a broad peak centered at 855 eV, whose area is 60 times reduced with respect to that of the fresh electrode, suggesting the occurrence of an exfoliation process.…”

“…In this work, we investigated NiFe nanoalloys (90%/10% alloy fraction) deposited by SCBD in the form of nanogranular films onto indium tin oxide (ITO) conductive substrates (Figure 1a,b) using the same protocol utilized for analogous samples of our previous work. 36 The resulting electrode consisted of a porous film, with a 35 ± 3 nm thickness and a 12 ± 1 μg/cm 2 mass loading, characterized by a 20% porosity, composed of very small NPs (0.3−2.8 nm), which agglomerated in 5−20 nm clusters, as shown in the SEM image in Figure 1c, determining a 4.8 ± 0.1 nm roughness (for more details, see ref 36). A minor fraction of the film surface (15%) was covered by 1−3 μm clusters.…”

“…A minor fraction of the film surface (15%) was covered by 1–3 μm clusters. As a result of the exposure to ambient air, only 37% and 17% of Ni and Fe surface sites were metallic, respectively, with a predominant oxide abundance (38% for Ni and 83% for Fe) coexisting with 25% of Ni hydroxide phases (see ref ).…”

“…The electrode consists of a NiFe film deposited by SCBD onto an indium–tin oxide (ITO)-coated glass substrate, similarly to what is reported in ref . In the SCBD technique, carried out at 10 –6 mbar of base pressure, a He gas injection synchronized with a discharge generates a plasma plume, which ablates some atomic clusters from a metal rod, which then condenses into NPs that are ballistically deposited on the substrate.…”

“…In this work, electrodes composed of an ITO substrate covered by a 35 ± 3 nm thick film of NiFe alloy (90%/10% weight fraction) have been considered, corresponding to 12 ± 1 μg/cm 2 mass loading. The morphological and chemical characterization of freshly deposited NiFe films is reported in ref .…”