This article is licensed under CC-BY 4.0 http://pubs.acs.org/journal/acsodf Article Low-Cost, High-Yield Zinc Oxide-Based Nanostars for Alkaline Overall Water Splitting Gisella Maria Di Mari, Maria Chiara Spadaro, Francesco Salutari, Jordi Arbiol, Luca Bruno, Giacometta Mineo, Elena Bruno, Vincenzina Strano, and Salvo Mirabella* Cite This: ACS Omega 2023, 8, 37023−37031 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The investigation of high-efficiency and sustainable electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media is critical for renewable energy technologies. Here, we report a low-cost and high-yield method to obtain ZnOHF-ZnO-based 2D nanostars (NSs) by means of chemical bath deposition (CBD). The obtained NSs, cast onto graphene paper substrates, were used as active materials for the development of a full water splitting cell. For the HER, NSs were decorated with an ultralow amount of Pt nanoparticles (11.2 μg cm−2), demonstrating an overpotential of 181 mV at a current density of 10 mA cm−2. The intrinsic activity of Pt was optimized, thanks to the ZnO supporting nanostructures, as outlined by the mass activity of Pt (0.9 mA mgPt−1) and its turnover frequency (0.27 s−1 for a Pt loading of 11.2 μg cm−2). For the OER, bare NSs showed a remarkable result of 355 mV at 10 mA cm−2 in alkaline media. Pt-decorated and bare NSs were used as the cathode and anode, respectively, for alkaline electrochemical water splitting, assessing a stable overpotential of 1.7 V at a current density of 10 mA cm−2. The reported data pave the way toward large-scale production of low-cost electrocatalysts for green hydrogen production. ■ INTRODUCTION The transition toward net-zero emission within 2050 requires H+[H 2O] + Hads + e V H 2[ + OH ] (4) an unprecedented transformation of the energy production At the state-of-the-art, noble electrocatalysts such as Pt- strategy.1 Hydrogen will certainly have a leading role in this group metals and Ru/Ir-based compounds are used as efficient transformation, as it has a huge energy content in terms of electrocatalysts for the HER and OER, respectively, with very energy density, 120 MJ kg−1 (33.3 kW h kg−1), well higher low overpotentials. Nonetheless, these materials are highly than that of gasoline (44.4 MJ k kg−1).1 The chance to use expensive and scarce,7 also classified as Critical Raw Materials hydrogen as a next future fuel is connected to the possibility of (CRMs) since 2011 by the European Union.8,9 A solution to producing it in huge quantities by means of sustainable overcome the extensive noble metal utilization for water methods.2,3 One of the most efficient and sustainable splitting is to develop proper substrates where ultralow approaches is electrochemical water splitting, by which water amounts of the noble metal are efficiently utilized. In turn, is converted into hydrogen and oxygen molecules through the this leads to a significant increase of the noble metal intrinsic hydrogen evolution reaction (HER) which occurs at the activity.10−12 cathode and the oxygen evolution reaction (OER), which Acidic water electrolysis still lacks in the development of occurs at the anode.4,5 The HER process, in acid or alkaline highly active and reasonably stable OER catalysts made of media ([ ]), occurs through a reaction which involves 2 earth-abundant and low-cost materials.13 Hence, it is electrons:6 convenient to perform the electrochemical water splitting in an alkaline medium, where the range of effective HER and 2H+[2H 2O] + 2e V H 2[ + 2OH ] (1) OER electrocatalysts is enlarged.14−19 Metal oxides composed of low-cost and earth-abundant Traditionally, HER mechanism is assumed to proceed by the elements are promising candidates as catalysts for water initial formation (Volmer step) of a hydrogen intermediate splitting purposes. Compared with other types of metal (denoted as Hads) which is formed via protons or water charge compounds, one of the advantages of metal oxides lies in the induced discharge, depending on the pH of the step:6 structural and compositional heterogeneity, which offers an H+[H 2O] + e V Hads[ + OH ] (2) Received: June 20, 2023 The Volmer step is then followed by a chemical Accepted: September 8, 2023 recombination, named the Tafel step:6 Published: September 29, 2023 2Hads V H 2 (3) or by the transfer of a second electron in the Heyrovsky6 © 2023 The Authors. Published by American Chemical Society https://doi.org/10.1021/acsomega.3c03958 37023 ACS Omega 2023, 8, 37023−37031 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 1. (a) Schematic of electrode preparation; (b) high-magnification SEM image of NSs-Pt; (c) NS SEM images at low, (d) medium, (e) and high magnification; and (f) NS cross section SEM image. electronic and crystal structure flexibility with various desirable amount of Pt, an overpotential of 181 mV at 10 mA cm−2 for physical/chemical properties.20,21 Among the others, zinc HER was recorded, using 11.2 μg cm−2 of Pt. Also, an OER oxide (ZnO), an inorganic semiconductor largely abundant overpotential of 355 mV at 10 mA cm−2 in alkaline media was in nature and with good electrochemical activity, can be easily recorded for the bare NSs. Therefore, an alkaline full cell nanostructured in a multitude of morphologies by employing electrolyzer was set up fully based on ZnO NS (bare and Pt- different methods.22 In particular, solution routes for ZnO decorated as the anode and cathode, respectively), showing a nanostructure synthesis have a plethora of advantages, such as current density of 10 mA cm−2 with a potential of 1.7 V for cost containment, simple laboratory setup, low-temperature 24h. These data open the way to ZnO-based full electrolyzers processes, and fast kinetics growth.23−27 Nonetheless, ZnO has for water splitting. poor charge transfer aptitude and a high electron−hole recombination rate, which limits its direct application in electrocatalysis. A successful strategy to overcome such limitation is coupling ZnO nanostructures with noble metals, ■ MATERIALS AND METHODS Synthesis of ZnOHF-ZnO NSs and of Pt Nano- narrow-band semiconductors, and other materials, creating particles. ZnOHF-ZnO NS powders were synthesized heterostructures with increased electron and hole mobility.28,29 through CBD using a bain-marie configuration. The starting Sun et al. reported a Ag-MoS2−ZnO, prepared by a double- aqueous solution of 25 mM zinc nitrate hexahydrate, 25 mM step hydrothermal method. They tested the electrochemical hexamethylenetetramine, and 16 mM ammonium fluoride was activity for HER in alkaline media (0.1 M KOH) finding an kept at 90 °C for 10 min to obtain the NSs.24,33 The obtained overpotential of about 530 mV at 10 mA cm−2 for the bare nanostructures were washed 4 times in Milli-Q water and dried ZnO and about 300 mV at 10 mA cm−2 for Ag-ZnO and in an oven at 100 °C for 16 h. The powders were then MoS2−ZnO composites.30 Saini et al. reported a ZnO-MXene dispersed in an aqueous solution of Milli-Q water. nanocomposite, obtained by a simple hydrothermal synthesis, In order to obtain a dispersion of Pt nanoparticles (NPs), we with a variable ZnO content for electrocatalysis applications. employed a green chemical reduction method under ambient The glassy carbon electrode with the bare ZnO as active conditions.34 Thirty microliters of 33 mM ascorbic acid material presents an overpotential higher than 700 mV at 10 aqueous solution, used as reducing agent, were added in 30 mL mA cm−2 in acidic media.31 of 0.2 mM H2PtCl6 (Sigma-Aldrich, St. Louis, MO, USA, ≥ Menesez et al. reported a cobalt−zinc oxide catalyst for the 99.9%) aqueous solution. The slightly yellow solution was then OER, obtained by annealing treatments of the, respectively, stirred at room temperature for 5 min and used without further hydroxide carbonate precursors.32 However, bare ZnO did not treatments. show any activity in alkaline media, as reported by the authors. Electrode Preparation. With the purpose of removing Thus, while undecorated ZnO is not promising for water impurities from the substrates, graphene paper electrodes (GP, splitting application, there is a growing attention on ZnO 1 × 1.8 cm2, 240 μm thick, Sigma-Aldrich, St. Louis, MO, nanostructures as proper and efficient support for active USA) were washed with Milli-Q water and dried in N2. NSs materials in electrocatalysts. were deposited through drop casting by using 35 μL of a 2 mg Here, we report a ZnO-based nanostructure, called nanostar mL−1 NSs dispersion. Samples were then dried on a hot plate (NS), obtained in high yield through an easy and green at 80 °C for 15 min. Pt NPs were deposited onto the NSs chemical bath deposition (CBD), showing promising perform- electrode through the NP dispersion drop addition onto NSs. ances for water splitting. After decoration with an ultralow Decorated samples are labeled as NSs-Pt. 37024 https://doi.org/10.1021/acsomega.3c03958 ACS Omega 2023, 8, 37023−37031 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 2. (a) HRTEM images showing the ZnO nanostar surface detail, together with (b) the orange box image magnification with the lattice spacings in yellow and (c) corresponding power spectrum (FFT) analysis. (d) STEM-HAADF micrograph showing the nanostar morphology and light blue box indicating the area where the EELS spectrum image map has been acquired. The resulting (e−i) STEM-EELS maps are shown evidencing the detected elements (scale bar 100 nm). Figure 1a presents a schematic of the electrode preparation. scan rate of 5 mV s−1 in the same potential region of CVs. We We measured the mass of NSs on GP substrates through a were able to perform electrochemical impedance spectroscopy Mettler Toledo MX5Microbalance and reported a value of (EIS) analyses by using a 10 mV sinusoidal voltage in the (0.20 ± 0.01) mg. frequency range 104 ÷ 10−1 Hz, fixing the chosen potential for Pt-Decorated NS Characterization. Scanning electron the measurement just after the onset potential (Eonset, the microscopy (SEM, Gemini field emission SEM Carl Zeiss minimum potential at which a reaction product is formed at SUPRA 25, Carl Zeiss Microscopy GmbH, Jena, Germany) the electrode). Mott−Schottky (M-S) analyses were performed allowed us to analyze the surface morphology of bare and Pt- on bare and decorated electrodes in the potential range of −1 decorated nanostars. SEM images were further investigated ÷ − 0.15 V vs SCE, at 1000 Hz frequency. through ImageJ software.35 Pt Benchmark Sample. A Pt electrode was prepared as a High-resolution transmission electron microscopy benchmark for our electrode performances. A sputter (HRTEM) and scanning transmission electron microscopy apparatus Emitech K550X was used for the Pt depositions, (STEM) analyses were performed with a field emission gun in which the graphene paper substrate is located in the FEI Tecnai F20 microscope. High-angle annular dark-field cathode, facing the Pt source (purity of 99.999%), at a distance (HAADF) STEM was associated with electron energy loss of 40 mm and using Ar flux with a pressure of 0.03 mbar, and spectroscopy (EELS) in a Tecnai microscope by using a the substrate at room temperature. The deposition current was GATAN QUANTUM energy filter to obtain compositional fixed at 50 mA, with a deposition time of 16 min. Both sides of maps. STEM-EELS maps were performed using the O K-edge the graphene paper were deposited. at 532 eV (blue), the Zn L edge at 1020 eV (green), Pt M edge Electrochemical Data Analysis. The measured potentials at 2122 eV (red), and F K-edge at 685 eV (orange). (η′) were adjusted with the voltage drop compensation as Rutherford backscattering spectrometry (RBS, 2.0 MeV He+ follows: beam at normal incidence) allows us to evaluate the Pt amount = ’ i Ru (5) on NSs. All RBS analyses were conducted with a 165° backscattering angle by using a 3.5 MV HVEE Singletron where i is the current and Ru [Ω] is the uncompensated accelerator system (High Voltage Engineering Europa, the resistance, extracted from the EIS data. We got current Netherlands). RBS spectra were then analyzed via the XRump densities after the normalization to the sample geometrical area software.36 coated with the active material [1 × 1 cm], and the measured All the electrochemical tests were carried out at room potentials vs SCE were converted to the reversible hydrogen temperature by using a VersaSTAT4 potentiostat (Princeton electrode (RHE) using the well-known equation:37 applied research, USA) and a three-electrode setup with a Pt E RHE = ESCE + 0.059· pH + 0.244 (6) wire as the counter electrode, a KCl saturated calomel electrode (SCE) as the reference electrode, and the previously The turnover frequency (TOF) is described as the amount ZnO-based discussed electrodes as the working electrode of product formed per unit time per active site: (WE) using a 1 M KOH (ph 14, Sigma-Aldrich, St. Louis, MO, I USA) solution as a supporting electrolyte. Cyclic voltammetry TOF = 2nF (7) (CV) curves were acquired at a scan rate of 10 mV s−1 in the potential range −0.5 ÷ − 2 V vs SCE to stabilize the where I is the current measured at a fixed overpotential, 2 is electrodes. Once stabilized, the catalyst HER activities were the electron number involved in the HER process, F is the investigated by using linear sweep voltammetry (LSV) at a Faraday constant, and n is the number of active sites moles.38 37025 https://doi.org/10.1021/acsomega.3c03958 ACS Omega 2023, 8, 37023−37031 ACS Omega http://pubs.acs.org/journal/acsodf Article Once the Pt amount loaded onto the electrodes is known through RBS analyses, the number of active Pt moles can be estimated as follows: DosePt[at cm 2] nPt[mol cm 2] = NA[at mol 1] (8) where NA is Avogadro’s number and DosePt is the RBS dose, describing the Pt atoms amount per cm2. Lastly, the mass activity, which is a different way to report the intrinsic electrocatalyst activity, is described by the ratio between a certain current density value and the catalyst loading (obtained by multiplying nPt obtained through the RBS analysis for the Pt atomic weight): j [A cm 2] mass activity = catalyst loading [mg cm 2] (9) Figure 3. Polarization curves of bare NSs, NSs-Pt, and of Pt ■ RESULTS AND DISCUSSION NS morphology is shown in Figure 1c−e, reporting SEM electrodes. images of NSs on GP substrates at low, medium, and high Two well-distinct behaviors characterized the bare and Pt- magnification, respectively. The star arms, each about 5 μm decorated samples. long and composed of batches of parallel wires, draw 6 angles (i) Bare NSs presented very high overpotential, on the order of 60°, equally spaced on the plane. Figure 1f represents an NS of 400 mV vs RHE. Such high energies were required to cross section SEM image. From the morphological inves- overcome the H+ adsorption and successive H2 step tigations, it is possible to appreciate the high surface to volume production. Even if ZnO-based nanostructures are not really ratio of these peculiar nanostructures. This represents a huge active for HER, we could note that our NSs presented advantage, especially if NSs are used as supports for promising results for HER in alkaline media, showing electrocatalysts. overpotential values significantly reduced in comparison to Figure S1 shows NSs onto the GP SEM image at low literature data (100 mV lower than in refs).30,39 Such an magnification. advantage could be attributed to defects present on the NS After Pt decoration by drop casting, Pt NPs, 2 nm in size,10 surface,40,41 as evidenced by photoluminescence analyses in were uniformly spread onto the NS sample. The Pt NPs tend our previous work.33 to aggregate, forming bigger aggregates with dimensions of (ii) The presence of Pt NPs decorating NSs drastically hundreds of nm, as shown in Figure 1b, reporting the SEM reduced the overpotential from 485 to 180 mV, bringing up an image of NSs-Pt at high magnification. enhanced catalytic action of Pt toward HER. For comparison, a Transmission electron microscopy (TEM) has been Pt electrode was reported with an overpotential of 27 mV. exploited in order to gain deeper insight into the atomic As far as the Pt decorated NSs are concerned, a huge structure of the NSs and the corresponding elemental reduction of the overpotential was gained. Still, the effect of Pt distribution. High-resolution TEM (HRTEM) images evi- decoration must be properly quantified after the Pt loading denced that the NSs present a wurtzite hexagonal crystalline mass. However, given the high surface exposure, NSs structure belonging to the P63mc (186) space group. In Figure represented an efficient support for electrocatalysts. 2a the NS is imaged along its [0001] axis parallel to the The electrochemical surface area evaluation was carried out, electron beam. Electron energy loss spectroscopy in scanning and it is reported in Figure S3. Also, the LSV curves for both TEM mode (EELS-STEM) confirmed the chemical composi- NSs and NSs-Pt-normalized ECSA are reported in Figure S4. tion of the NSs, evidencing the presence of decorating NPs Tafel plot curves are reported in Figure S5. In order to (Figure 2b). Residual fluorine is justified with the coexistence properly evaluate the amount of Pt loading on the NSs sample, of the ZnOHF-ZnO mixed phase. ZnO-ZnOHF NSs X-ray RBS measurements were conducted on flat silicon substrates diffraction analysis is reported in Figure S11, in which ZnO (Si) covered with the same amounts of drops of the electrodes. and ZnOHF PDF cards are reported below. We assumed that after drop casting, the Pt mass on Si was the Electrochemical Characterization. In order to perform a same as that on NSs. The RBS spectrum in Figure 4a proved proper electrochemical evaluation of bare and Pt-decorated the effective presence of Pt and enabled us to quantify the NSs in alkaline conditions, electrochemical analyses were amount of platinum present on the substrate, as the Pt loading performed in 1 M KOH (Figure 3, light blue and bordeaux was associated to the area of the Pt peak in the spectrum.42 lines respectively). The Pt sputtered electrode (yellow line) A Pt dose of 3.4 × 1016 cm−2 was evaluated, which means was used as Pt benchmark performance. All curves are 11.2 μg cm−2. A remarkably low amount of Pt loaded onto the corrected for the voltage drop, obtained from the EIS spectra, electrodes was evidenced with very promising HER perform- as reported in Figure S2. The polarization curves clearly ances, pointing out the great efficacy of our NSs as support for showed that the presence of Pt NPs drastically lowers the electrocatalysts. activation energy barrier for the H2 production, and a strong Turnover frequency (TOF) represents a key parameter in variation in the overpotential at a constant current of 10 mA the HER, as it provides the electrode intrinsic catalytic activity. cm−2 was recorded as shown in Table 1. Figure 4b shows the TOF, extrapolated from eq 7, in the range 37026 https://doi.org/10.1021/acsomega.3c03958 ACS Omega 2023, 8, 37023−37031 ACS Omega http://pubs.acs.org/journal/acsodf Article Table 1. Overpotential Values of NSs, NSs-Pt, and Pt; TOF, Mass Activity, and Pt Loading Values of NSs-Pt samples overpotential (@10 mA cm−2) (mV) TOF (@50 mV) mass activity (@10 mA cm−2) Pt loading NSs 485 NSs-Pt 182 0.27 s−1 0.9 A mgPt−1 11.2 μg cm−2 Pt 27 Figure 4. (a) RBS spectrum of Pt decoration performed on Si; (b) NSs-Pt TOF plots, in the range of 0−100 mV of overpotential, and (c) mass activity, with the comparison with state-of-the-art Pt-based electrodes. Figure 5. (a) NSs and NSs-Pt Mott−Schottky plots and (b) their schematic of energy levels at the surface. of 0−100 mV of overpotential. Pt-loaded NSs presented very the overpotential, as a significant potential drop could occur high TOF values, comparable with many other Pt-based along the NSs. catalyzers.40 To evaluate the effect of Pt decoration on ZnO NS, Mott− Figure 4c presents the mass activity (from eq 9), measured Schottky (M−S) analyses were performed. The Mott− at 10 mA cm−2, as a function of the overpotential of our Schottky plot for NSs and NSs-Pt are shown in Figure 5a. A decorated electrode compared with other Pt-based catalysts typical M-S plot reports the inverse of the squared capacitance under alkaline conditions present in the literature.7 Our sample as a function of the potential applied to the sample. For an n- showed quite high mass activity at moderately low over- type semiconductor, as the applied potential decreases, the C−2 potential, in good agreement with other Pt-based catalysts. It goes to zero.43,44 The intercept with the x-axis represents the should be noted that further optimization of NSs could reduce flat band potential VFB that needs to be corrected by the open 37027 https://doi.org/10.1021/acsomega.3c03958 ACS Omega 2023, 8, 37023−37031 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 6. Overall water splitting (a) and stability (b) tests of the NSs-Pt || NSs cell. circuit potential (VOC): ΔVM−S = VFB − VOC.42 Zinc oxide is an A series of electrochemical tests were also performed to n-type semiconductor: once it is immersed in an electrolytic evaluate the bare NS performances as a catalyst for the OER. solution, the net effect arising from the alignment of the Fermi All measurements were conducted in alkaline media (1 M energy of the material and the redox potential of the electrolyte KOH) as the HER tests. Figure S7 reports the polarization is the balancing of carriers at the solid−liquid interface. In curve for NSs, obtained from the LSV. The LSV curve is particular, the semiconductor is depleted of electrons, leading normalized by the voltage drop, as for the HER curves, and the to an accumulation of holes beneath its surface and to an corresponding EIS spectra are reported in Figure S8. NSs upward semiconductor energy band bending. By modulating possess an overpotential of 355 mV at a current density of 10 the applied voltage, it is possible to vary the degree of bending mA cm−2. This time, without any decoration, NSs presented a of the energy levels.41 The flat-band potential condition VFB is remarkable activity toward the OER, which is a significant reached when the applied potential is enough to flat the achievement for a ZnO-based nanostructure. semiconductor bands independent of the position of the The corresponding Tafel plot is reported in Figure S9. solution redox potential. Pt NP decoration clearly affected the As for the HER, the ECSA-normalized NS polarization curve position of the semiconductor energy levels at the ZnO-Pt is reported in Figure S10. interface, as proved by the Mott−Schottky analyses. Figure 5b Motivated by these data, an overall water splitting summarizes the band bending in the case of bare NSs and NSs- measurement in alkaline media was performed by using the Pt. Pt nanoparticles attracted the ZnO electrons at the NSs-Pt as a cathode and the bare NSs as an anode; a scheme of interphase, an effect called “spillover”, preventing the the cell used is in the inset of Figure 6a. The cell required a electrode−electrolyte electron flow.45,46 By increasing the Pt potential of 1.72 V to reach a current density of 10 mA cm−2 amount, there was a slight increase in the band bending. and 1.98 V to reach 40 mA cm−2, almost comparable to the Generally, a more negative CB position accounted for stronger commercial Pt/C || RuO2 standard cell, which has a potential reduction power, so the electrochemical performances of 1.63 V at 10 mA cm−2. improved with the Pt loading increase. Figure S6a−c shows In addition, the cell stability was tested by performing a 24 the COMSOL simulations of the Pt-ZnO system (see the h-long chronopotentiometry (Figure 6b), revealing very high Supporting Information). stability over a prolonged time, even improving its perform- 37028 https://doi.org/10.1021/acsomega.3c03958 ACS Omega 2023, 8, 37023−37031 ACS Omega http://pubs.acs.org/journal/acsodf Article ance with time. These last results confirmed that ZnO-Pt || Francesco Salutari − Catalan Institute of Nanoscience and ZnO electrodes represent very efficient electrocatalysts. Nanotechnology (ICN2), 08193 Bellaterra (Barcelona), Figure S12 shows a Pt-decorated nanostar SEM image after Catalonia, Spain the stability tests, in which it is evident how nanostars and Pt Jordi Arbiol − Catalan Institute of Nanoscience and NP structures are maintained. Nanotechnology (ICN2), 08193 Bellaterra (Barcelona), Catalonia, Spain; ICREA, 08010 Barcelona, Catalonia, ■ CONCLUSIONS We developed a highly effective electrocatalyst support for Spain; orcid.org/0000-0002-0695-1726 Luca Bruno − Dipartimento di Fisica e Astronomia “Ettore water splitting by synthesizing low-cost and high-yield ZnOH− Majorana”, Università degli Studi di Catania, 95123 ZnO nanostars. A decoration with an ultralow amount of Pt Catania, Italy; CNR-IMM, Catania (University) Unit, (11.2 μg cm−2) allowed us to measure an HER overpotential of 95123 Catania, Italy; orcid.org/0000-0003-2710-1154 182 mV at 10 mA cm−2. The NS shape allowed us to maximize Giacometta Mineo − Dipartimento di Fisica e Astronomia Pt utilization, as confirmed by the high values of mass activity “Ettore Majorana”, Università degli Studi di Catania, 95123 (0.9 mA mgPt−1) and turnover frequency (0.27 s−1) for a Pt Catania, Italy; CNR-IMM, Catania (University) Unit, loading of 11.2 μg cm−2. As far as the OER is concerned, bare 95123 Catania, Italy; orcid.org/0000-0001-6713-8811 NSs showed a remarkable value of 355 mV at 10 mA cm−2 in Elena Bruno − Dipartimento di Fisica e Astronomia “Ettore alkaline media, well lower than other ZnO-based nanostruc- Majorana”, Università degli Studi di Catania, 95123 tures. Finally, our NSs showed promising performances as an Catania, Italy; CNR-IMM, Catania (University) Unit, alkaline, all ZnO-based electrolyzer, showing an overpotential 95123 Catania, Italy of 1.7 V at 10 mA cm−2 and 1.98 for 40 mA cm−2, by using Vincenzina Strano − CNR-IMM, Catania (University) Unit, bare and Pt-decorated NSs (NSs-Pt || NSs) as the cathode and 95123 Catania, Italy; orcid.org/0000-0003-2053-4141 anode, respectively. Also, this cell was demonstrated to be very Complete contact information is available at: stable in time, with the potential at a current density of 10 mA https://pubs.acs.org/10.1021/acsomega.3c03958 cm−2 slightly decreasing with time. NSs demonstrated to be efficient electrocatalyst supports for HER, allowing to reach Author Contributions very high Pt mass activity. These cells based on abundant The manuscript was written through contributions of all materials pave the way toward a new era of electrocatalysts for authors. All authors have given approval to the final version of water splitting applications. the manuscript. ■ * ASSOCIATED CONTENT sı Supporting Information Funding This work has been partially funded by European Union (NextGeneration EU), through the MUR-PNRR project The Supporting Information is available free of charge at SAMOTHRACE (ECS00000022), by programma ricerca di https://pubs.acs.org/doi/10.1021/acsomega.3c03958. Ateneo UNICT 2020-22 linea 2 PIA.CE.RI ″NaTI4Smart Additional experimental details, materials, and methods Sviluppo di NAnomateriali e Tecnologie Innovative per Smart and images; NS low-magnification SEM image; Nyquist detection″, and by PRIN 2017 “CLEAN-Valorizing Sustainable plot of NSs, Pt-NSs, and Pt; ΔI plot; HER LSV Plastics through a CLEver use of NANoparticles” normalized by ECSA of NSs and Pt-NSs; Tafel plots of 20174FSRZS_003. ICN2 acknowledges funding from General- NSs, NSs-PT, and Pt; COMSOL simulations; polar- itat de Catalunya 2021SGR00457. This study was supported ization curve of NSs for OER; NS Nyquist plot; NS by MCIN with funding from European Union NextGener- Tafel plot; OER NS LSV normalized by ECSA; NS XRD ationEU (PRTR-C17.I1) and Generalitat de Catalunya. The patterns; and Pt-NS SEM image after the 24 h stability authors thank support from the project NANOGEN test (PDF) (PID2020-116093RB-C43), funded by MCIN/AEI/ 10.13039/501100011033/and by “ERDF A way of making ■ AUTHOR INFORMATION Corresponding Author Europe”, by the “European Union”. ICN2 is supported by the Severo Ochoa program from Spanish MCIN/AEI (Grant No.: CEX2021-001214-S) and is funded by the CERCA Pro- Salvo Mirabella − Dipartimento di Fisica e Astronomia gramme/Generalitat de Catalunya. Part of the present work “Ettore Majorana”, Università degli Studi di Catania, 95123 has been performed in the framework of Universitat Autònoma Catania, Italy; CNR-IMM, Catania (University) Unit, de Barcelona Materials Science PhD program. This work has 95123 Catania, Italy; orcid.org/0000-0002-9559-4862; received funding from the European Union’s Horizon 2020 Email: salvo.mirabella@dfa.unict.it Research and Innovation Programme under grant agreement Authors no. 654360 NFFA-Europe. Gisella Maria Di Mari − Dipartimento di Fisica e Astronomia Notes “Ettore Majorana”, Università degli Studi di Catania, 95123 The authors declare no competing financial interest. Catania, Italy; CNR-IMM, Catania (University) Unit, 95123 Catania, Italy Maria Chiara Spadaro − Dipartimento SIMAU, Università Politecnica delle Marche, 60121 Ancona, Italy; Catalan ■ ACKNOWLEDGMENTS The authors wish to thank G. Pantè, C. Percolla, and S. Tatì Institute of Nanoscience and Nanotechnology (ICN2), 08193 (CNR-IMM Catania, Italy) for the technical support and V. Bellaterra (Barcelona), Catalonia, Spain; orcid.org/ Iacono for the useful discussion and the experimental Pt 0000-0002-6540-0377 electrode preparation. 37029 https://doi.org/10.1021/acsomega.3c03958 ACS Omega 2023, 8, 37023−37031 ACS Omega http://pubs.acs.org/journal/acsodf Article ■ REFERENCES (1) Pathway-To-Critical-and-Formidable-Goal-of-Net-Zero-Emissions- (21) Bruno, L.; Scuderi, M.; Priolo, F.; Mirabella, S. 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