**1. Introduction**

The production of hydrogen by electrocatalytic or photoelectrocatalytic (PEC) water splitting driven by renewable energy, and its subsequent use in a fuel cell, could represent a zero-emission process in which the storage of H2 could mitigate the spatial and temporal discontinuities of renewable energy resources [1,2]. However, due to the high overpotential losses involved in the gas evolution reactions occurring at the electrodes, the electrolytic H2 production is still not competitive, at large scale, with the traditional process of H2 production from fossil fuels [3]. In fact, noble metals such as Pt, Ru, and Pd are ideal electrocatalysts for hydrogen evolution reaction (HER) in terms of overpotential. Nevertheless, high cost and scarcity make them impractical choices, and the quest for finding inexpensive electrocatalysts is indeed an active area of research [3].

Although many earth-abundant HER catalysts such as transition metal chalcogenides [4,5] or carbides [6,7] have exhibited high activities in acid solutions approaching that of Pt, all of them cannot operate satisfactorily in alkaline electrolytes [8]. Recently, nickel-based electrocatalysts, either as monofunctional or bifunctional materials, were proposed as an economical and efficient replacement to these expensive metal precursors, that exhibited very promising electrocatalytic activity and stability toward oxygen and hydrogen evolution reactions [9–12].

Due to its electronic properties, high conductivity, and thermal stability, Ni has been a very frequent choice for designing electrocatalytic materials. Ni and its related oxides and hydroxides have also been proposed as cathode materials which are effective for HER in non-acidic solutions [13]. Moreover, the NiO p-type semiconductor has emerged as the most frequently used material in this field for its low cost, good stability, and suitable band position alignment, for the transfer of photogenerated holes to counter electrode, that is a key efficiency-determining step of PEC water splitting [14–16]. In order to increase the catalytic activity of nickel based materials towards the HER, two basic approaches could be adopted, namely the use of multicomponent catalysts and the increase of the real surface area [17]. Ni-based alloys, together with other transition metals or rare earths, have been widely studied and exhibited better catalytic capability than single Ni catalyst, due to synergistic effects of the elements forming the alloy [18,19]. Among the Ni-based alloy electrodes studied, Ni–Cu alloy has shown potential as cathode for alkaline HER due to the improved electrocatalytic activity [20,21], high corrosion resistance [22] and good stability [23].

As already stated, the second way to increase the catalytic activity is the preparation of an electrode with a high surface area, that represents a crucial point to which attention should be paid to achieve high efficiency [24–26]. However, the synthesis of ordered nanoporous metals faces great challenges, since metals at the nanoscale tend to present low surface area in order to minimize the surface energy [27]. Moreover, the simple increase of the surface area does not always reflect an increase in the electrochemically-active surface area, because the morphology and the dimension of pores may affect the accessible surface area for the electrochemical reactions.

Nickel foam with a nanoporous structure can be obtained by several methods, such as alkaline leaching of the aluminum from Ni-Al Raney nickel [28], chemical vapor deposition [29,30], electrodeposition [31] and template synthesis [32,33]. However, these processes were found to be imperfect due to limitations in controlling pore sizes and relative density which are important for metallic foams [34].

Based on the above results, in this work, the electrochemical corrosion of Cu-Ni co-deposit was selected for the fabrication of porous Ni-based electrodes, which combine both high surface area and good intrinsic catalytic activity in the process of HER. The selective electrochemical corrosion of copper from NixCu1−<sup>x</sup> alloy was demonstrated for the first time by Sun and co-workers; by taking advantage from the formation of a passive nickel oxide film in sulfamate aqueous solutions, nanoporous nickel was prepared through selective electrochemical dissolution of the more noble copper, rather than the less noble Ni. The authors performed electrochemical etching under potentiostatic conditions; depending on the composition of the deposited alloy, different morphologies and dimensions of the pores were obtained [35].

Similarly, Chang and co-workers obtained nano-hollow tubes starting from Ni-Cu alloys prepared by electrodeposition, which showed a columnar structure and contained separated Cu- and Ni- rich multiple phases; the selective etching of the less reactive copper from the alloys was performed in solutions containing H3BO3 0.5 M [36]. A dendritic Ni-Cu alloy foam with high surface area was fabricated by electrodeposition during HER by Jeong and co-workers; then nanoporous dendritic Ni foam was successfully prepared by selective electrochemical dealloying of copper from Ni-Cu alloy using a sulfuric acid solution [37]. In order to control the morphological features of the porous structure, the dealloying of a co-deposited Ni-Cu was performed under pulsed electric field. By tuning voltage and duration of the bias applied, different final composition and degree of porosity were obtained [38].

To the best of our knowledge, few works have been devoted to the systematic study of the effects of the corrosion conditions of copper on the morphology and dimensions of the pores of nickel, as well as to the amount of residual copper. In this work, we studied the effect of the dealloying conditions, with the application of a voltage waveform, on morphology, real area and composition of the resulting samples. Starting from the same deposit containing 30% of copper and 70% of nickel, different values of corrosion and relaxation times were adopted: two classes of samples were prepared with two values of the ratio between the corrosion and relaxation times.

Moreover, also the electrocatalytic performance of the developed electrodes for HER and for photoelectrochemical tests were studied. To distinguish the effect of both surface roughness and intrinsic activity of the material, the real active surface area of the catalysts, in terms of roughness factor (Rf), was determined.
