**3. Materials and Methods**

Boric acid (H3BO3) and potassium hydroxide (KOH) were purchased from Sigma-Aldrich (Sigma-Aldrich Chemie, GmbH, Schnelldorf; Germany). Sodium sulfate anhydrous (Na2SO4), nickel sulfate hexahydrate (NiSO4\*6H2O), copper sulfate pentahydrate (CuSO4\*5H2O) and potassium nitrate (KNO3) were supplied by Carlo Erba (Carlo Erba Reagents, Cornaredo, Milano, Italy). All electrochemical experiments were performed at room temperature using an AUTOLAB PGSTAT302N (Metrohm, Herisau, Switzerland) potentiostat/galvanostat equipped with a frequency response analyzer controlled with the NOVA software. Two cylindrical hand-made three-electrode cells realized by Teflon were used, in which the working electrode was at the bottom of the cell and the electrical contact consisted of an aluminium disc. A platinized titanium grid, placed in front of the anode at 1 cm distance, and a saturated calomel electrode (SCE) constituted the counter and reference electrodes, respectively. The preparation of the Ni-Cu co-deposits was performed using niobium foils (thickness 0.25 mm, 99.8%, Sigma-Aldrich Chemie, GmbH, Schnelldorf; Germany) as working electrode, cut as discs. Prior to deposition, niobium was mechanically polished with diamond paste (sizes: 3, 1, 0.5 μm) and colloidal silica gel (particles size: 0.05 μm), then submitted to sonication in acetone for 15 minutes and rinsed with distilled water. The cell used for the electrodeposition experiment contained 40 ml of solution (inner diameter = 5 cm, height 4 cm); the exposed geometrical area was 13.5 cm2. The electrodeposition was performed using solution containing 0.5 M NiSO4, 0.005 M CuSO4 and 0.5 M H3BO4 (pH = 4); linear sweep voltammetry was firstly performed in the electrodeposition solution at 5 mV s<sup>−</sup>1, starting from the OCP up to <sup>−</sup>0.8 V, after that a constant potential of <sup>−</sup>0.8 V for a total time of 130 minutes was applied while the solution was stirred. The charge amount recorded during the electrodeposition experiments was about 6 C/cm2. Considering unit faradaic yield and average atomic weight of 60 g/mol, the amount of deposited metals was roughly estimated equal to 1.8 mg/cm2, by Faraday's Law.

In order to study the effect of the selective corrosion, starting from the same co-deposit, after the deposition the discs were cut in six slices and subjected to anodic dissolution under different corrosion conditions. The cell used for corrosion experiment contained 10 ml of solution (inner diameter = 1.5 cm, height 3 cm): the exposed geometrical area was 0.5 cm2.

The runs were performed in aqueous solution, containing 0.5 M H3BO4 and 0.5 M Na2SO4, under stirring conditions, using pulsed voltage modulated between Vcorr (E = 0.5 V) and Vrelax (E = OCP) for time durations of *tcorr* and *trelax*, respectively. Values of the ratio between *tcorr* and *trelax* (called φ in the rest of the text) equal to 0.2 and 0.02 were adopted. A total time of 30 minutes were required (typically the dissolution current drops to zero within this period). Prepared samples were denoted as Stcorr-trelax where *tcorr* and *trelax* are the corrosion and relaxation times (in seconds), respectively. After oxidation, the electrodes were rinsed in deionized water and dried in a nitrogen stream. The charges amount, recorded during the corrosion test, ranged from 1.2 to 1.6 C/cm2. Also in this case, applying Faraday's Law, the amount of copper removed was evaluated from 0.4 to 0.55 mg/cm2.

A scanning electron microscope (SEM) equipped with EDX detector (Zeiss, Oberkochen, Germany) was used to characterize the morphology and the chemical composition of the nanoporous nickel electrodes. Auger electron Spectroscopy (AES) was also used to investigate the distribution of the copper and nickel.

X-ray diffraction (XRD) patterns were recorded in the range of 20–80 (2θ) on a Panalytical Empyrean diffractometer equipped with a Cu Kα radiation and an X'Celerator linear detector. XRD patterns were collected at a grazing incidence of 2 on the films mounted on a flat sample stage. Data were processed by Empyrean X'pert High Score software and phase identification was performed by comparison with the Powder Diffraction Files (PDF-2 JCPDS International Centre for Diffraction Data, Swarthmore, PA, USA) database.

Linear sweep voltammetry (LSV) were performed to study the behavior of the electrodes in the potential range from E = OCP to E = −1.0 V in cathodic direction at a sweep rate of 5 mV/s. The characterization of the electrode/electrolyte interface was also carried out through electrochemical

impedance spectroscopy (EIS). The measurements were performed in a frequency range from 100 kHz to 0.1 Hz with excitation amplitude of 10 mV. The impedance spectra were then fitted to an equivalent electrical circuit by using the ZSimpWin 2.0 software (EChem software).

A subsequent annealing treatment was performed in order to convert Ni(OH)2 and NiOOH groups in NiO, which is the semiconductive form of nickel. Thermal treatment was carried out in air atmosphere for 30 min at 500 ◦C, after ramping 5 ◦C min<sup>−</sup>1.

Photoelectrochemical measurements were carried out in a hand-made photoelectrochemical cell (PEC), equipped with a quartz window. The investigated samples were adopted as working electrodes while a platinum wire constituted the counter electrode and a SCE the reference. 0.1 M KNO3 aqueous solution was used as the supporting electrolyte. The PEC-cell was irradiated with a 300 W Xe lamp (LOT-Quantum Design Europe) equipped with AM 0 optical filter. The incident power density of the light was measured by LP 471 UVU or LP 471 PAR quantum radiometric probes: the recorded value was 138–140 W/m2. Photocurrent density was calculated with respect the nominal surface area of the samples as the difference between the current recorded under illumination and dark conditions.

#### **4. Conclusions**

In this work, a dealloying method was used to obtain porous nickel electrodes as possible alternative cathodic materials in HER. A suitable combination of corrosion and relaxation times in the pulsed potential steps made it possible to obtain different morphologies and pore size distributions. Depending on the samples, roughness factors ranging from 22 and 106 were obtained, but the increased surface area was not always exploitable. In fact, the Tafel analysis revealed that the exchange current density calculated with respect to the real surface area, was higher in sample S1–5 rather than sample S0.01–0.5, even if a higher surface area was measured at this last sample. The higher j0*<sup>r</sup>* value suggests that the inner porous surface area of sample S0.01–0.5 is not totally exploitable during HER, due to gas bubbles shielding. This behavior can be explained by considering the more open structure of sample S1–5 with respect to S0.01–0.5, thus confirming the strong effect of the pores (size, shape and distribution) on the resulting electrocatalytic performance. The presence of thin oxide layer of NiO, as well as of residual copper were indicated as responsible for the photocatalytic activity of the samples. Once again, the different pore size and distribution become crucial in determining the final performance of the samples: thus, for example, at the highest cathodic potential gas can be generated which may limit the exploitability of the whole pore structure, especially when small pores are involved in the structure, as at sample S0.01–0.5.

**Author Contributions:** Conceptualization, S.P. and A.V.; Investigation, L.M., M.M., M.F.C. and E.S.; Writing—original draft, L.M., S.P., E.S. and A.V.; Writing—review & editing, M.M. and J.R.

**Funding:** This research was funded by Fondazione di Sardegna, grant number project F71I17000280002-2017 and project F71I17000170002.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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