Catalysis Today 423 (2023) 113916 Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod PS-b-P4VP block copolymer micelles as a soft template to grow openly porous nickel films for alkaline hydrogen evolution Roberto Fagotto Clavijo a, Marta Riba-Moliner b, Arántzazu González-Campo b, Jordi Sort a, c, Eva Pellicer a, *, Konrad Eiler a, * a Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain b Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain c Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Spain A R T I C L E I N F O A B S T R A C T Keywords: Highly porous Ni films have been potentiostatically synthesised by micelle-assisted electrodeposition using Electrodeposition custom-made PS-b-P4VP block copolymer micelles as a soft template. Two PS-b-P4VP block copolymers with PS/ Porous materials P4VP block ratios of 1:1 and 1:4 were used for the micelle-assisted electrodeposition, resulting in Ni films with Electrocatalysis large pores of diameters varying from 25 to 600 nm (1:1), and from 10 to 230 nm (1:4). As a result of the Soft templating interconnected porosity, and hence the drastic increase of the surface-to-volume ratio, the electrocatalytic per­ Micelles formance at hydrogen evolution reaction (HER) in alkaline media is significantly improved in comparison to a dense Ni film, and—more importantly—even in comparison to a highly mesoporous Ni film with monodisperse 10 nm wide pores. Most remarkably, it is discovered that the openly porous Ni electrocatalysts not only lead to a simple increase in HER current density, but also to a lower overpotential and a better long-term performance. While the bulk of the films is metallic, Ni(OH)2 is formed on the surfaces of all Ni films during HER. This effect leads to an initial decrease of the catalytic activity, but provides excellent stability in alkaline media. The pre­ sented synthesis process for pure Ni may be readily adopted to any other electroplatable metals and alloys. 1. Introduction electrochemically active materials for low temperature hydrogen fuel cells, its cost is extremely high due to its low abundance. Alternative Current global challenges, such as the climate crisis, demand for materials are being investigated to make catalysts more accessible and technological advances towards more efficient and sustainable energy affordable [5]. Recent studies have shown that by partially substituting management. In order to decrease the use of fossil fuels, changes in platinum by an electrocatalytically active transition metal such as infrastructure, energy storage, and energy production are required [1]. nickel, the cost-effectiveness may increase [6]. Furthermore, the nano­ In a future low-carbon economy, hydrogen energy will play a major role structuring of electrocatalysts, such as by application of porous mate­ due to its high versatility and ability to provide energy for the trans­ rials, adds up to the efficiency by increase of the surface-to-volume ratio portation, storage, and domestic energy applications in today’s society (S/V) [7]. Micelle-assisted electrodeposition has proven to render [2]. high-quality mesoporous materials in a rather simple synthesis process. One of the most promising and emission-free means of energy stor­ Micelles are formed by block copolymers (BCP) in an aqueous electro­ age and production is through eletrolyser and fuel cell technologies [3]. lyte above critical micelle concentration. The micelles codeposit Making use of the hydrogen evolution reaction (HER) as an electro­ together with the discharged metal ions, leaving spherical pores in the catalytic process, green hydrogen can be produced emission-free when electroplated metal or alloy once the polymer is dissolved [8]. the required electric current is obtained by renewable energies [4]. Recently, highly mesoporous Ni-Pt thin films with a pore size of 10 Nanostructured materials have shown tremendous impact on nm have shown an improvement of HER activity with respect to fully fostering the development of electrolysers and fuel cells towards com­ dense films, however, the electrocatalytic activity did not increase as mercialisation. While platinum-based catalysts are the most much as could be anticipated from the high porosity [9]. One possible * Corresponding authors. E-mail addresses: eva.pellicer@uab.cat (E. Pellicer), konrad.eiler@uab.cat (K. Eiler). https://doi.org/10.1016/j.cattod.2022.09.022 Received 30 May 2022; Received in revised form 20 September 2022; Accepted 27 September 2022 Available online 30 September 2022 0920-5861/© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). R. Fagotto Clavijo et al. Catalysis Today 423 (2023) 113916 that is significantly higher than the previously mentioned 10 nm, by micelle-assisted electrodeposition. Most commercial micelle-forming BCP, including Pluronic P-123, F-127 or Brij-56, have relatively low molecular weight, facilitating the spontaneous micelle formation in aqueous media, but therefore also form relatively small micelles, leading to rather small pore sizes when used in micelle-assisted synthesis methods [24]. Larger BCPs with high molecular weight do not form micelles spontaneously; in this case the micelles need to be stabilised in Fig. 1. Chemical structure of PS-b-P4VP. a multi-step process as described by Nagarajan on the example of PS-b-PEO [25]. Table 1 Experimentally, only few studies have focussed on the synthesis of Amounts of initiator, styrene, and 4-vinylpyridine used in the synthesis of the porous metals using high molecular weight BCP micelles. Yamauchi PS-b-P4VP block copolymers. et al. prepared mesoporous Pt particles from a lyotropic liquid crystal (LLC) phase of PS3800-b-PEO4800, resulting in homogeneously porous Molar Initiator amount Styrene amount 4-vinylpyridine amount ratio (mmol) (mmol) (mmol) particles with a pore size of 15 nm [26]. Later, higher molecular weight PS63000-b-PEO26000 was used for electrodeposition of mesoporous Ni and 1:1 61 40 40 1:4 36 17 63 Cu films, and yielded a pore size of around 50 nm [27,28]. The same BCP was used by Artal et al. for the electrodeposition of Ni-Pt alloy films, achieving a uniform pore size of approx. 30 nm [29]. Usually, the sol­ explanation is that the pore size was not sufficiently large such as for an vent exchange method is used, where the BCP is at first dissolved in a aqueous electrolyte to effectively penetrate the pores, although the non-selective medium such as tetrahydrofurane (THF), before a selective surface energy did not increase due to the porosity. Secondly, the formed hydrogen gas bubbles may become trapped inside the pores, or block these, leading to a reduction of the available surface area. It was hypothesised that if the size of the hydrogen bubbles resulting from proton reduction was much larger than the actual size of the pores, the latter would not be efficiently utilised. Indeed, formed hydrogen bubbles are usually much larger than 10 nm before released from the surface [10]. Another critical point is the presence of an open or a closed porosity; if the porosity is closed then the electrocatalytic processes are limited to the surface without taking advantage of the sub-surface porous structure. On the other hand, an entirely open porosity would theoretically allow to make use of the entire structure in electrocatalysis. Furthermore, porous structures may significantly alter the wetting behaviour of surfaces, which needs to be monitored [11]. Nickel and its alloys, such as commercial Raney Ni, are generally promising candidates for HER in alkaline and neutral electrolytes [12–16], as well as for various other electrocatalytic reactions [17,18]. In contrast to acidic electrolysis, alkaline hydrogen evolution does not require the use of platinum group metals indeed [19]. Apart from metallic Ni-based electrocatalysts, nickel oxides, hydroxides and phos­ phides may be equally active [20–23]. Fig. 3. CV curves recorded from the electrolytes used for the synthesis of dense Considering the aforementioned points, the goal of this work is to Ni, Ni(P123), Ni(1:1), and Ni(1:4). Arrows indicate the onset of reduction as synthesise a HER electrocatalyst with an open porosity, and a pore size well as the scan direction. Fig. 2. TEM micrographs showing PS-b-P4VP micelles with block ratios 1:1 (a) and 1:4 (b). 2 R. Fagotto Clavijo et al. Catalysis Today 423 (2023) 113916 Fig. 4. SEM micrographs of porous Ni(1:1) (a, b) and Ni(1:4) (d, e) films and the corresponding pore size distributions for Ni(1:1) (c) and Ni(1:4) (f) obtained by micelle-assisted electrodeposition with PS-b-P4VP, followed by polymer removal. Fig. 5. SEM micrographs of dense Ni (a) and Ni(P123) (b) films. 3 R. Fagotto Clavijo et al. Catalysis Today 423 (2023) 113916 2. Material and methods 2.1. Polymerisation of PS-b-P4VP diblock copolymers The PS-b-P4VP BCPs were synthesised by living anionic polymeri­ sation following the methodology described by Varshney et al. [34]. A solution of α-methylstyrene in dry THF (100 ml) was prepared in a three-necked round-bottom flask. Then, the initiator sec-butyllithium (1.4 M in cyclohexane) was added dropwise to the solution using a stainless steel syringe until the colour of the solution persisted. After this, the reaction was cooled down to − 78 ∘C and the required amount of the styrene was added slowly. A sudden change of the colour of the initiator was noticed (from red to light yellow). When the solution turned again to intense red, the polymerisation of the P4VP started by addition of 4-vinylpyridine. The polymerisation was terminated by adding methanol, and a milky solution was obtained. Afterwards, the block copolymer was recovered by precipitation in hexane and it was thoroughly dried under vacuum. Fig. 6. XRD patterns of Ni films deposited by micelle-free (dense film) and The block ratios of PS and P4VP in the block polymers (the ratios of micelle-assisted electrodeposition. m/n in Fig. 1) were controlled by the amounts of styrene and 4-vinylpyr­ idine used. PS-b-P4VP with a block ratio of 1:1 was synthesised using 40 mmol of styrene and 40 mmol of 4-vinylpyridine, while a block ratio of 1:4 was obtained using 17 mmol of styrene and 63 mmol of 4-vinylpyr­ idine (Tab. 1). The successful polymerisation was verified by 1H NMR (nuclear magnetic resonance) using a Bruker AVANCEII 300 and Bruker Avance DRX 300. The molecular weights (Mw) of the copolymers were determined by gel permeation chromatography (GPC). The chromatographer was calibrated using standardised samples of PS. The column used was a TSK-GEL G3000H with a pore size of 6 μm. THF was used as eluent, the flow rate was fixed at 1 μl/min, the volume of sample injected in each experiment was 20 μl and the lamp wavelength was set at 254 nm. Fig. 7. Contact angle photographs of a droplet of ultrapure water on Ni(1:1) (a), Ni(1:4) (b), Ni(P123) (c), and dense Ni (d) films immediately after depo­ 2.2. Electrolyte preparation sition of the droplet. For the electrodeposition process, a total of four different electrolytes were prepared. Unless otherwise stated, chemicals of analytical grade Table 2 were used. In addition to the electrolytes containing PS-b-P4VP micelles Determined water contact angles of Ni films. with molecular ratios of 1:1 and 1:4, respectively, an electrolyte con­ initial CA CA (t > 60 s) taining commercial Pluronic P-123, as well as a micelle-free electrolyte for the deposition of nonporous films were prepared. Ni(1:1) 100∘ 71∘ Ni(1:4) 74∘ — A method for the stabilisation of PS-b-P4VP micelles of the polymer Ni(P123) 86∘ 64∘ has previously been developed [32], however, this method had to be dense Ni 91∘ 80∘ merged with the preparation of the electrolyte for electrodeposition. The preparation of PS-b-P4VP micelles for the electrolyte was following the methodology described by Artal et al. [29]. 15 mg of the polymer were medium—i. e. an aqueous solution—is slowly added, leading to an first dissolved in 5 ml of THF at 50 ∘C for 24 h. By dropwise addition of arrangement of hydrophobic and hydrophilic polymer chains and hence, 2.5 ml of ethanol and 17.5 ml of ultrapure water, the polymer chains are the formation of micelles. THF may then be evaporated from the solu­ oriented in a way that the hydrophilic P4VP segments of the polymer are tion. Herein, a similar approach is used with non-commercial poly­ surrounded by water, while the hydrophobic PS segments remain in styrene-poly(4-vinylpyridine) diblock copolymers (PS-b-P4VP) THF, making up the core of the micelles. To protonate the micelles, the synthesised in-house in order to use micelle-assisted electrodeposition pH was then adjusted to 3.0 by addition of concentrated HCl. In the end, for the synthesis of porous Ni films with sub-micrometre pores. The 200 mM NiCl2, 200 mM H3BO3, and 25 mM NH4Cl were dissolved in the PS-b-P4VP BCP is a relatively versatile polymer which can be readily micelle-containing solution. The final pH was adjusted to 2.7. The pro­ synthesised in a variety of configurations, and allows for the stabilisa­ tonation of the micelles in acidic pH guarantees electrostatic repulsion tion of different geometries, including micelles, through different and prohibits agglomeration of micelles. When the Ni(II) species is methods [30,31]. In this way, it can serve as a template for the growth of added, Ni ions are expected to replace protons at the surface of the a large variety of nanostructured materials [32,33]. Here, the Ni films micelles and ensure successful co-deposition of Ni(0) and micelles as synthesised using two different configurations of the BCP are investi­ well as the establishment of a continuous Ni(0) matrix in the film during gated for HER in alkaline media, and compared to mesoporous Ni film deposition. with a defined porosity of 10 nm and dense Ni film, likewise obtained For the electrolyte containing P-123 and the micelle-free electrolyte, from electrodeposition. an aqueous solution of the above quantities of NiCl2, H3BO3, and NH4Cl was prepared. For the P-123 containing electrolyte, 10 mg/ml of the polymer were dissolved in the electrolyte. pH values were adjusted to 2.7 using HCl likewise. 4 R. Fagotto Clavijo et al. Catalysis Today 423 (2023) 113916 Fig. 8. LSV curves for evaluation of HER recorded in 1 M KOH for a) Ni(1:1), b) Ni(1:4), c) Ni(P123), and d) dense Ni films. Currents are normalised to the geometric area of the electrode. For simplification, the Ni deposits obtained from their respective gas was performed before each deposition, and the deposition was car­ electrolytes will be denoted as ‘dense’ for the micelle-free deposited Ni ried out during magnetic stirring at 200 rpm from an electrolyte volume films, ‘Ni(P123)’ for the Ni films obtained from the P-123 containing of 25 ml. Finally, Ni(1:1) was deposited at − 1.2 V vs Ag∣AgCl for 60 s electrolyte, and ‘Ni(1:1)’ and ‘Ni(1:4)’ for the Ni films obtained by and Ni(1:4) at − 1.3 V vs Ag∣AgCl for 30 s. Both dense Ni and Ni(P123) micelle-assisted electrodeposition using the PS-b-P4VP polymer in block were deposited at − 1.1 V vs Ag∣AgCl for 60 s. Furthermore, for the ratios of 1:1 and 1:4, respectively. electrolytes containing PS-b-P4VP micelles, the electrodeposition of Ni (1:1) and Ni(1:4) was conducted at a temperature of 50 ∘C, while deposition from the other electrolytes was performed at room 2.3. Electrodeposition of Ni films temperature. After the synthesis of the porous Ni films, the micelles were redis­ All electrochemical processes were performed using an Autolab solved by immersion of the samples in THF in an ultrasonic bath for 1 h, 302 N potentiostat controlled by GPES software. Cyclic voltammetries exchanging the solvent every 20 min, and subsequently a short chemical (CV) were run to characterise the baths in order to identify the reduction attack of a few seconds in piranha solution (concentrated solution of potential ranges for each electrolyte prior to electrodeposition. A three- H2SO4 and H2O2 in relation 3:1) was carried out. electrode set-up was utilised, using a vitreous carbon rod with 3.14 mm2 working area as the working electrode (WE), a Pt wire as counter elec­ trode, and an Ag∣AgCl (3 M KCl) reference electrode. CVs were run from 2.4. Characterisation − 1.3 V to 1.3 V vs Ag∣AgCl under stagnant conditions at a scan rate of 50 mV/s. The PS-b-P4VP micelles were characterised by transmission electron The substrates for Ni deposition consisted of silicon wafer material microscopy (TEM) in order to visualise and confirm their formation and sputter-coated with a thin seed layer of Ti and a Cu layer of approx. size distribution. Two drops of the micelle solution (before addition of 200 nm thickness. Before electrodeposition, the substrates of about the Ni salt and additives) were deposited onto a copper grid for TEM. 1 cm2 working area were first cleaned in ethanol in an ultrasonic bath The micelles were imaged in a TEM JEOL 1400 with an accelerating and then activated by submersion in concentrated H2SO4(aq) for a few voltage of 120 kV. seconds. The electrodeposited Ni films were analysed by scanning electron The electrodeposition was carried out in the three electrode set-up microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy used for CV, with the metallised silicon substrates as the WE placed (EDX) on a Zeiss Merlin electron microscope with an InLens detector at vertically in the electrolyte. The deposition was carried out potentios­ an accelerating voltage of 1 kV. EDX was performed at 20 kV acceler­ tatically at different cathodic potentials, until an optimum deposition ating voltage in order to check the composition of the films. Pore size potential was determined for each electrolyte. De-aeration with nitrogen distributions were determined from the SEM micrographs using the 5 R. Fagotto Clavijo et al. Catalysis Today 423 (2023) 113916 photoelectron spectroscopy (XPS) was conducted using a PHI 5500 spectrometer. Survey spectra and detail spectra of Ni2p, C1s and O1s were collected before and after Ar ion sputtering for 60 s to analyse both the surface as well as the bulk state of the films. The XPS was performed on Ni films both before and after the HER studies. 3. Results and discussion 3.1. Characterisation of PS-b-P4VP block copolymer The formation of PS-b-P4VP was successfully confirmed by 1H NMR, returning the following proton chemical shifts. [(C8H8)n(C7H7N)m] (CDCl3, 300 MHz), δH (ppm): 1.30–1.72 (br, 2 H), 1.72–2.30 (m, 4 H), 6.17–6.86 (m, 5 H), 6.86–7.23 (br, 2 H), 8.13–8.63 (m, 2 H). The molecular weights determined by GPC are 114700 g/mol for PS- b-P4VP with a block ratio of 1:1, and 42500 g/mol for the block ratio of 1:4. 3.1.1. PS-b-P4VP micelle formation The formation of micelles from PS-b-P4VP containing solutions was verified by TEM, before the addition of Ni salt and additives. For both configurations of PS-b-P4VP, micelles were found on the TEM grid. PS-b- P4VP 1:1 micelles were homogeneously distributed on the TEM grid with diameters varying from 20 to 600 nm, while the mean diameter of PS-b-P4VP 1:4 micelles is 36 nm (Fig. 2). 3.2. Electrolyte characterisation The electrolytes were characterised to determine the potential range where the deposition of nickel takes place, and to compare the effects that the different BCPs might produce on the electroreduction process. The micelle-free electrolyte shows the lowest overpotential for the reduction of the Ni(II) species at around − 0.7 V (Fig. 3, black curve), as well as the highest reduction current density, suggesting that the deposition rate is also highest in this case. For all micelle-containing electrolytes, the reduction potential is shifted towards more negative Fig. 9. HER polarisation curves of all Ni films at the 10th sweep (a) and cor­ potentials. Interestingly, the electrolytes containing PS-b-P4VP micelles, responding Tafel slopes (b). Currents are normalised to the geometric surface despite their higher molecular weight, show an earlier onset of Ni(II) area of the electrode. reduction than the P-123 containing electrolyte. This behaviour may hint at a better coordination of Ni(II) ions with PS-b-P4VP micelles in image manipulation software ImageJ. comparison to the P-123 micelles. Ni ions may substitute protons on the HER was studied in 1 M KOH solution at room temperature in the previously protonated micelles, and the more Ni ions are coordinated same set-up as used for electrodeposition. 200 linear sweep voltamme­ with the micelles in relation to the micellar mass, the more positive the tries were run from a starting potential of − 1.20 V down to − 1.55 V vs reduction potential should be, assuming that the interaction between Ag∣AgCl at a scan rate of 50 mV/s. The potentials measured against micelles and Ni ions is not disrupted due to the electric field. Ag∣AgCl were converted to reversible hydrogen electrode (RHE) scale In the anodic scan, two major oxidation peaks are detected for all according to the following equation (Eq. 1): electrolytes. A peak at 0 V vs Ag∣AgCl potential is present in all cases, which partially overlaps with another oxidation peak between 0.2 V and ERHE = EAg|AgCl + 0.059 pH + E0Ag|AgCl (1) 0.3 V vs Ag∣AgCl. The peak at 0 V is attributed to the oxidation of adsorbed hydrogen, while at more positive potentials, oxidation may be where ERHE is the potential vs RHE, EAg∣AgCl the potential measured attributed to the formation of Ni(OH)2 and further oxidation to NiOOH against Ag∣AgCl, and E0Ag|AgCl the standard electrode potential of the [36,37]. The double peak is also observed in the oxidation response of Ni Ag∣AgCl reference electrode. The pH of 1 M KOH was considered 14, and (P123), although slightly shifted towards more cathodic potentials (see E0Ag|AgCl in 3 M KCl at 25 ∘C was taken as 0.210 V. inset in Fig. 3). X-ray diffraction measurements were recorded at a 2θ range from 35 to 100∘, using CuKα radiation in order to analyse the composition and 3.3. Morphology of the Ni films crystallographic structure of the Ni deposits. XRD patterns were ac­ quired on a Malvern-PANalytical X′ pert Pro MRD diffractometer. Riet­ Highly porous Ni films were successfully obtained by micelle- veld refinement of the diffraction patterns was performed using the assisted electrodeposition using PS-b-P4VP with different molar ratios software MAUD developed by Lutterotti [35]. of the blocks PS:P4VP (concretely, 1:1 and 1:4) after removal of the Contact angle measurements were carried out to achieve better un­ polymer (Fig. 4). Spherical pores, ranging from mesoporous to macro­ derstanding of the films wetting behaviour. The experiments were porous range, are homogeneously distributed across the surface of the conducted in an ISO 7 clean room on a KRÜSS DSA 100 smartdrop films. Ni(1:1) shows a higher spread in pore size with respect to Ni(1:4); surface analyser. The measurements were done using ultrapure water ranging from 25 nm to 600 nm for Ni(1:1), and from 10 nm to 230 nm droplets of 4 μl, taking three measurements on each sample. X-ray for Ni(1:4). Due to the short chemical attack with piranha solution for removal of the micelles, the Ni films are also slightly etched; this effect is 6 R. Fagotto Clavijo et al. Catalysis Today 423 (2023) 113916 Fig. 10. Ni2p core-level XPS surface spectra before (left) and after Ar ion sputtering for 60 s (right) for Ni(1:1) (a, b), Ni(1:4) (c, d), Ni(P123) (e, f), and dense Ni film (g, h). All spectra were collected before HER in alkaline media. presumably due to higher interactions of the polymer chains due to insufficient hydrophilicity, thus the assembly into larger micelles. However, even though a higher number of hydrophilic components seems to lead to a more well-defined pore size distribution, the randomly distributed pore size of Ni(1:1) appears more promising as an electro­ catalyst in terms of S/V. The Ni(P123) and dense Ni films synthesised for comparison show a very similar fine-grained structure. In addition, Ni(P123) very clearly shows a highly mesoporous surface with a narrowly distributed pore size of 10 nm (Fig. 5). However, an interconnection of the mesopores in Ni (P123) is not appreciable from the on-top SEM images. 3.3.1. Structural analysis The X-ray diffractograms show all diffraction peaks for face-centred cubic (fcc) Ni for all four configurations, demonstrating that the Ni de­ posits are polycrystalline (Fig. 6). A specific texture is not apparent. Apart from Si and Cu reflections originating from the substrate mate­ Fig. 11. Ni2p core-level XPS surface spectra for Ni(1:1) after 200 LSVs of HER rials, no other phases, such as Ni oxides or hydroxides, are detected by in alkaline media (a) and in initial state (b). XRD. An additional, narrow peak at 62∘ present in all spectra could not be identified and may be an artefact coming from the sample holder. more pronounced for Ni(1:1). At large magnification, sub-surface The main apparent difference between the different Ni films is that porosity and in some cases even the substrate layer become visible, the diffraction peaks of the dense film are narrower than those of the showing that the porosity is interconnected and therefore leads to a very films produced by micelle-assisted deposition. This observation is most high S/V (Fig. 4b and e). While the pore sizes appear rather randomly apparent at the Ni{200} reflection. The crystal sizes obtained by Riet­ distributed for Ni(1:1), Ni(1:4) shows a large number of finely distrib­ veld refinement are 75 nm for the dense Ni film and 52 nm for Ni(P123); uted mesopores in addition to randomly spread larger pores. In statis­ these values correspond rather well with the grain sizes observed by tical terms, this results in a much narrower pore size distribution for Ni SEM (cf. Fig. 5). When the high molecular weight PS-b-P4VP polymers (1:4) (note the difference in pore size range and intervals between were used in the synthesis, the crystal size was further reduced to 27 nm Fig. 4c and f). for Ni(1:1) and 25 nm for Ni(1:4). It is conjectured that this smaller grain From the resulting pore sizes and their distributions it can be inferred size is the result of an interference of the polymeric micelles with the that the PS-b-P4VP with 1:4 ratio, containing larger hydrophilic P4VP crystal growth during deposition. block chains, leads to better stabilisation and separation of the micelles. The PS-b-P4VP polymer with a lower content of hydrophilic groups (PS/ 3.3.2. Contact angle measurements P4VP ratio of 1:1) led to the formation of many larger micelles, For electrocatalysis in aqueous media such as the HER, a good 7 R. Fagotto Clavijo et al. Catalysis Today 423 (2023) 113916 wetting behaviour is a necessity because the entire surface needs to be in can be attributed to the presence of Ni(OH)2 [39,40]. Ni(P123) shows contact with the electrolyte in order to completely benefit from the high the lowest fraction of metallic Ni. In addition, the dense Ni film shows a S/V. Since porous materials often exhibit hydrophobic surface proper­ rather large contribution of oxidised Ni species at 854 eV, attributed to ties, the wettability of the porous Ni films needs to be investigated. NiO (Fig. 10). Initially, the highest water contact angle was found for Ni(1:1) with a After sputtering, the Ni2p photoelectron spectra do not show any mean value of 100∘ (Fig. 7). However, a strong time dependence of the significant differences between the Ni films obtained from the different contact angle was observed on the porous films during the measure­ electrolytes. A difference in intensity is appreciated due to the variation ments, where the contact angle gradually decreased over time (Tab. 2). in porosity. Other than that, the electrodeposited films consist mostly of After one minute, the contact angles of the porous films are significantly metallic Ni, in addition to a minor fraction of oxidised Ni. lower than initially. In the case of Ni(1:4), this process was much faster After the HER measurements, the surfaces do not show any contri­ and the contact angle decreased rapidly in just a few seconds. This time bution of metallic Ni. Instead, Ni(OH)2 has formed on the surface, as dependence may also be an indicator that the water penetrates the shown exemplarily for Ni(1:1) (Fig. 11). Therefore, the metallic Ni films’ openly porous structure; in fact, a loss in volume could be confirmed for surfaces are oxidised to Ni(OH)2 during HER and the electrocatalytically Ni(1:1) through a reduction of the cross-section of a droplet of 17% active species change from contributions of metallic Ni and Ni(OH)2 to within a minute, along with the reduction of the contact angle from 93∘ Ni(OH)2 only. This passivation also explains the initial decrease in HER to 63∘. The observed difference in time dependence may be related to the activity, since, in direct comparison metallic Ni shows superior activity difference in pore size distribution. Under the assumption that large over Ni(OH)2 [21]. However, Ni(OH)2 should provide a sufficiently pores are filled immediately by water and do not contribute to the time stable surface for long-term performance in alkaline media. dependence, then there is a lower amount of small pores available in Ni Taking into account all of the obtained results, the openly porous Ni (1:1), while the large amount of small pores in Ni(1:4) facilitates a faster films prepared by micelle-assisted electrodeposition using PS-b-P4VP penetration of the water through the porous structure. show a clear improvement in hydrogen evolution activity over meso­ The dense Ni film yielded a mean contact angle of 91∘, in comparison porous Ni(P123) and dense Ni films. This improvement cannot be to the porous films this angle remained relatively stable and only referred to any chemical or crystallographic differences, so that it is decreased slowly until 80∘ at the end of the experiment. Ni(P123) shows mainly related to the significantly higher surface areas resulting from the best wettability after one minute, which may also be caused by the the openly porous microstructures of Ni(1:1) and Ni(1:4). In HER, the soap-like behaviour of possible remnants of the BCP. After all, the open porosity, combined with the good wetting behaviour of the films, contact angle measurements demonstrate a hydrophilic behaviour of the theoretically allows for complete exploitation of the porosity because porous Ni films with improved wettability with respect to the dense film. movement of electrolyte and gases is promoted. In an openly porous structure, the electrolyte can penetrate the structure while the formed 3.3.3. Hydrogen evolution reaction in alkaline media hydrogen can escape through different channels in absence of capillary The LSV curves recorded to assess the HER activity of the films show forces. that the openly porous Ni films prepared using PS-b-P4VP micelles Furthermore, the openly porous Ni films do not only display higher exhibit significantly higher HER activity than the dense film and Ni HER performance than the dense and mesoporous films, but also a lower (P123), initially being about 60% higher with respect to the dense Ni loss in electrocatalytic activity over time, leading to an activity at HER film (Fig. 8). In turn, Ni(P123) exhibits a 49% improvement with respect which is a factor 3 higher at end of test. This observation seems sur­ to the dense film. All Ni films experienced a loss in the electrocatalytic prising since the material itself is essentially the same, and the only activity upon cycling. Comparing the first and 200th sweep at a potential structural difference is in the crystal size. Even though Ni(P123) shows of − 0.4 V vs RHE, shows that the dense Ni film exhibits a decrease in lower amounts of metallic Ni at the surface, which may be deleterious cathodic current density of over 20 mA/cm2 (around 75%), Ni(1:1) for the initial catalytic activity, all Ni films’ surfaces are covered by shows a decrease of over 25 mA/cm2 (around 40%), and Ni(1:4) has the hydroxides after HER, as observed by XPS. Possible differences are in the lowest loss in electrocatalytic activity, namely 14 mA/cm2 (around thickness of the hydroxide layer, since a relatively thin hydroxide layer 25%). Meanwhile, Ni(P123) showed a decrease of over 30 mA/cm2 on top of the metallic Ni should be advantageous for HER due to better (around 70%). According to these data, both Ni(1:1) and Ni(1:4) showed conductivity. The formation of the hydroxide may very well have been significant improvement in electrocatalytic stability compared to dense influenced by the crystal size of the Ni films [41]. Judging from the Ni and Ni(P123). Due to this difference in stability, after 200 sweeps the structure of PS-b-P4VP, it is also thinkable that nitrogen may have been catalytic activity of both Ni(1:1) and Ni(1:4) is over three times higher incorporated into the Ni films, which may have limited the growth of the than the activity of the dense Ni film. hydroxide. The direct comparison of electrocatalytic activity is observed by Since the wetting behaviour is seen to be time-dependent, it is comparing the 10th sweep for each electrocatalyst (Fig. 9a). In this view possible that the alkaline electrolyte still continues to wet, i. e. to it is not only clear that the Ni films produced using PS-b-P4VP micelles penetrate the porous structure during the tests, so that the surface area yield higher HER current densities, but also that the onset overpotential in contact with the electrolyte is slowly increasing. The fact that both Ni is shifted towards more positive potentials. In terms of kinetics, which (1:1) and Ni(1:4), despite their apparent difference in pore size distri­ are evaluated based on the determined Tafel slope b, there are rather bution, exhibit rather similar activity at HER, leads to the conclusion little differences. The lowest Tafel slope is found for Ni(P123) with that the crucial property of the porous films is in fact their inter­ 123 mV. Ni(1:1) and the dense Ni films have almost identical Tafel connected porosity; the actual size and distribution of pores seems to slopes with 140 mV and 142 mV, respectively. Ni(1:4) has the highest have only minor influence. The initial loss of electrocatalytic activity is Tafel slope with 172 mV and therefore slower kinetics, however, this attributed to the superficial oxidation of metallic Ni to Ni(OH)2, as film exhibits the lowest overpotential at low current densities (Fig. 9b). confirmed by XPS. The Tafel slopes are close to what has been reported for pure Ni, sug­ Chen et al. obtained a high-surface-area Ni electrode by synthesis of gesting that HER follows the Volmer-Heyrosvky mechanism, the latter Ni nanowires, reaching an improvement of factor 7 at HER in neutral being the rate-determining step [38]. electrolyte with respect to a planar Ni electrode [42]. The Tafel slopes determined here are comparable to those of different Raney 3.4. Chemical state of the electrodeposited films before and after HER nickel-aluminium-molybdenum alloys determined in 1 M KOH, although those exhibit lower overpotentials [43], and Pt-decorated Ni All electrodeposited Ni films show surfaces containing mostly electrodes [44]. The Tafel slopes determined here are also considerably metallic Ni at 853 eV as well as oxidised Ni(II) species at 857 eV which lower than those obtained by Qian et al. on electrodeposited porous Ni 8 R. Fagotto Clavijo et al. Catalysis Today 423 (2023) 113916 electrodes in 30 wt% KOH which, in agreement with our results, show Data availability an initial loss in activity during galvanostatic long-term electrolysis, but a rather high stability afterwards [45]. This passivation effect is com­ Data will be made available on request. mon and has been observed for different Ni-based alloys, irrespective of the synthesis method [12,46]. References 4. 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