Laser structured nickel-iron electrodes for oxygen evolution in alkaline water electrolysis

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Laser structured nickel-iron electrodes for oxygen evolution in alkaline water electrolysis Matthias Koj a,*, Thomas Gimpel b, Wolfgang Schade b,c, Thomas Turek a,b a

Institute of Chemical and Electrochemical Process Engineering, Clausthal University of Technology, Leibnizstr. 17, 38678 Clausthal-Zellerfeld, Germany b Research Center Energy Storage Technologies, Clausthal University of Technology, EnergieCampus, Am Stollen 19A, 38640 Goslar, Germany c Fraunhofer Heinrich Hertz Institute, EnergieCampus, Am Stollen 19H, 38640 Goslar, Germany

article info

abstract

Article history:

In the present work, the ultra-short pulse laser ablation method is applied to create novel

Received 31 July 2018

surface alloys on NiFe electrodes for the oxygen evolution reaction (OER) in alkaline water

Received in revised form

electrolysis. The nickel-to-iron ratio in the alloy can be controlled with the ultra-short

19 December 2018

pulse laser ablation method by varying the thickness of electrochemically deposited iron

Accepted 3 January 2019

layers onto the nickel mesh substrate. Besides the application of the additional catalyst,

Available online 5 February 2019

the laser treatment enhances the surface area and a defined micro- and submicrometer structure is created in a single step. The laser structured nickel-iron electrodes show a

Keywords:

significantly lower overpotential of 249 mV than an electrochemically deposited Ni-NiFe

Alkaline water electrolysis

alloy with 292 mV at 10 mA cm
2, 298 K and 32.5 wt% KOH for the OER, although some

Oxygen evolution reaction

loss of iron over time could not be prevented.

Anode

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Ultra-short pulse laser ablation method Structured mesh electrodes Nickel-iron based catalysts

Introduction Water electrolysis for hydrogen production is one of the most promising energy storage technologies in the context of an increasing share of renewable energies and the associated fluctuating and intermittent amounts of energy [1e3]. The water electrolysis is a well-established technology and may offer an important contribution to stabilize the electricity grid of the future with the possibility of dynamic operation [1]. Commercially available technologies for the electrochemical

conversion of water to hydrogen and oxygen are the alkaline water electrolysis (AEL) and the proton exchange membrane (PEM) water electrolysis. In contrast to PEM water electrolysis, the alkaline environment in AEL makes it possible to utilize non-noble metals, which presents a major advantage concerning e.g. material accessibility and investment costs [3,4]. However, a drawback of state-of-the-art AEL electrolyzers is still the limited current density [5,6]. Even with a zero-gap assembly and highly conductive electrolyte (20e30 wt% KOH) at temperatures of 313e363 K, current densities are

* Corresponding author. E-mail address: [email protected] (M. Koj). https://doi.org/10.1016/j.ijhydene.2019.01.030 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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typically not higher than 200e400 mA cm
2 [5]. However, a recent study shows that both low temperature water electrolysis technologies may achieve similar performance under optimized conditions [7]. In the last decades great progress has been made in the fields of catalysts, separators, cell designs, optimization of operating parameters and process modelling but there are still some challenges to overcome [7e12]. In particular the oxygen evolution reaction is the bottleneck in the water electrolysis process due to its slow kinetics, insufficient stability of some catalysts and - in case of the AEL - also additional ohmic losses by bubble detachment [2,13]. Electrodes for the AEL are made of meshes and expanded metals or metal foams as substrates, which are coated with different active catalysts such as nickel-, iron-, cobalt-based oxides, spinel-type oxides or perovskites [2,4,13,14]. Especially the combination of nickel or nickel hydroxide and a second active catalyst (Fe, Co, Cr, Mn) enhances the electrode performance, where iron shows the highest catalytic activity towards the OER [2,4,15,16]. DiazMorales et al. [15] studied Ni-based double hydroxides theoretically by DFT-based analysis and also investigated the catalytic effect of introduced Fe, Cr, Mn, Co, Cu and Zn during the OER experimentally. The results suggest that NiFe double hydroxide reveals the highest activity and shows a good stability in 0.1 M KOH. However, activity, stability, morphology and composition of NiFe alloys are strongly dependent on the preparation conditions and methods. In this context Ullal and Hegde [17] developed novel nanocrystalline NiFe alloys with different compositions, which were electrochemically deposited. The authors found that varying the deposition current density influences the alloy composition, the surface morphology, the phase structure and therefore the activity of the catalyst. Furthermore, these authors pointed out that the electrochemically prepared NiFe alloys exhibit a good corrosion resistance in the utilized 6 M KOH. Another method to vary and adjust the composition of NiFe alloy is reported by
rez-Alonso et al. [18] by control of the Ni/Fe ratio within the Pe precursor salt solutions. Moreover, the authors show that the alloy composition as well as the nature of the substrate (e.g. nickel mesh, stainless steel mesh and nickel foam) influences the electrochemical activity, stability and adhesion to the substrate of the NiFe coatings during OER. From the literature several other methods are known for the application of the catalyst onto the substrate besides electrochemical deposition [15e22]. Examples are thermal decomposition [6] and chemical reduction [6,23] of the catalysts, the creation of a highly

porous surface by leaching of aluminum [24] or zinc (e.g. Raney nickel) or the deposition of active powders by electrochemical fixation [6,25,26]. However, some of these application methods lead to adhesion and conductivity problems between the catalyst and the substrate on the one hand. On the other hand, for instance in case of the electrodeposition method from aqueous solvents, deposition potentials limit the selection of catalysts to specific materials and alloy compositions [27]. A stable and large electrochemically active surface is also obtained by means of processing the surface with a femtosecond laser [28,29]. A defined micro and submicrometer structure is created influencing the physical properties (hydrophobicity/hydrophilicity), which leads to an improvement of the bubble detachment behavior [28,29]. Although hyperdoping materials with desired elements from a gaseous or a solid source is known for instance on silicon [30,31], the enhancement of the electrochemical activity on metals has been only investigated with respect to the large surface area or employing a platinum catalyst, which was deposited after the laser process [32]. In the present work, the ultra-short pulse laser ablation method is for the first time applied to create entirely new alloy components of NiFe. The alloy composition, i.e. the nickel-toiron ratio, is controlled by the laser ablation method through varying the amount or rather the layer thickness of the electrochemically deposited iron on the nickel substrate. The resulting composition and distribution of nickel and iron within the surface structure is analyzed by EDS and SEM. Selected samples are electrochemically characterized under technically relevant conditions (32.5 wt% KOH and 353 K) and current densities of up to 800 mA cm
2 and compared with an electrochemically deposited NiFe coating.

Experimental Preparation of laser structured NiLFe samples In the two-step manufacturing process, iron is first deposited electrochemically onto a nickel mesh and in the second step the coated electrode surface is treated on one mesh side with the ultra-short pulse laser ablation method as shown in Fig. 1. As substrate and current collector a nickel mesh (Ni 99.2 wt%, aperture width 0.5  0.5 mm, wire thickness of 0.14 mm, sintered and calendered to a mesh thickness of 0.2 mm,

Fig. 1 e Fabrication process of laser structured nickel-iron electrodes.

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Haver&Boecker, Germany) is used for all prepared samples. For pretreatment the nickel meshes are washed with deionized water (0.5 mS cm
1) and isopropanol each for 120 s in an ultrasonic bath. To improve the adhesion of the deposited iron layer, the meshes are immersed for 120 s in 7 M HNO3 and washed again with deionized water. The electrochemical iron deposition is conducted in an aqueous solution containing 1.62 mol L
1 iron (II)sulfate heptahydrate (FeSO4 7H2O, MERCK, p.a.) and 0.18 mol L
1 ammonium sulfate ((NH4)2 SO4, MERCK, p.a.). The cathodic deposition on an area of 25 cm2 takes place under continuous stirring, N2 flushing, at a temperature of (328 ± 2) K, a constant current density of 120 mA cm
2 and in a 500 mL glass cell [33]. As counter electrode a graphite compound plate (BMA-5, Eisenhuth, Germany) is used. To produce different alloy compositions by laser ablation, nickel meshes are loaded with different amounts of iron by varying the deposition time. Note that iron is deposited on the entire wire and therefore on the back side, front side and in between spaces of the mesh. The resulting iron loads and corresponding iron layer thicknesses, deposition times and nomenclatures of the prepared six sample are summarized in Table 2. The nomenclature of NiLFe indicates that an alloy composition of nickel and iron is produced by the laser ablation method. In a second step, femtosecond laser structuring is performed with a Ti:Sa based regenerative amplifier system, set up similarly as published in Ref. [32] at a repetition rate of 10 kHz with a Gaussian laser spot and a measured laser spot diameter of 80 mm, where the intensity profile drops to 86.5% of the maximum value (intensity 1/e2). The fluency is set to 5.2 J cm
2. The samples are scanned meandering for three times at a linear scan velocity of 10 mm s
1 with a line pitch of 40 mm under continuous N2 process gas flow at pressures of approximately 800 mbar. For application as electrode, samples are laser treated on the front side (cf. Fig. 1), which is defined as the side facing the counter electrode during the iron deposition.

Preparation of the deposited Ni-NiFe sample To evaluate the intrinsic activity and the influence of surface enhancement by laser ablation, an electrochemically cathodic deposited nickel-iron coating onto a nickel mesh (Ni-NiFe) is prepared with the same alloy composition as the laser structured nickel-iron (NiLFe) samples. The nomenclature of NiNiFe indicates that an alloy composition of nickel and iron (NiFe) is deposited onto a Ni mesh. The alloy composition of Ni-NiFe is controlled by the ratio of nickel and iron within the precursor salt solutions [16,18,19]. Therefore a ratio of 70 wt% nickel and 30 wt% of iron is chosen. For preparation of the NiNiFe sample the same pretreatment method is used as described above. The aqueous deposition solution contains 1.1 mol L
1 nickel (II)sulfate hexahydrate (NiSO4$6H2O, MERCK, p.a.), 0.51 mol L
1 iron (II)sulfate MERCK, p.a.) and heptahydrate (FeSO4$7H2O, 0.16 mol L
1 L(þ)-ascorbic acid (C6H8O6, VWR Chemicals, p.a.). To adjust a pH of about 3, boric acid (H3BO3, Fluka, p.a.) is added, typically with concentrations of 0.25 mol L
1. As counter electrode a nickel plate (thickness 0.3 mm, 99.2 wt% Metall Jobst, Germany) is used. The deposition takes place under the same conditions as described earlier except for the

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current density which is adjusted to 32 mA cm
2 for 120 min to insure a relatively smooth and homogeneous surface.

Structural investigations The samples are studied by a scanning electron microscope (SEM), Zeiss (EVO 50, MA10). Additionally, element analysis is performed by energy dispersive X-ray spectroscopy (EDS) using a QUANTAX 800-System from Bruker Nano with an XFlash 6. The EDS spectra are acquired by integrating a representative area at 500x magnification in top view. The deviations at the EDS measurements are the average standard deviations given by the statistical analysis of the EDS counts in the corresponding energy channel and are automatically calculated within the EDS software for each measurement and averaged for different measurement spots. Furthermore, a sample with a medium iron load is cross-sectioned by an Armilling cross section polisher in order to resolve the resulting distribution of the elements inside the material. The ion beam penetrates from the non-structured backside towards the laser induced conical spikes. The alignment is adjusted and focused to have the optimum cross-section surface on the laser-structured side. Multipoint Brunauer-Emmett-Teller (BET) measurements are performed with Kr gas using a 3Flex (Micromeritics Instruments) surface characterization analyzer. The BET measurements are carried out with a laser structured nickel sample (NiL) and the Ni mesh instead of a NiLFe and Ni-NiFe sample. However, the results obtained with the nickel mesh provide an indication of possible surface enhancement factors by the laser ablation treatment. Laser scanning microscopy (LSM) measurements are done with a Keyence VK-X150K/X160K digital microscope and analyzed with the Multi-File-Analyzer software VK-H1XMD. For each measurement we use the 20x objective and perform volume and area measurements on rectangles of about 180e250 mm x 80e100 mm. Software filter and area are set in order to have the inherent shape of the mesh negligible and simultaneously to have sufficient statistics on the heights of the structure. The thicknesses of the samples are determined with a € fer, Germany). thickness dial gauge (type: 12.5mm/0.001, Ka The layer thickness is calculated by Eq. (1) assuming a uniformly deposited coating on front and back side of the mesh. On the one hand the assumption of an uniform iron deposition is based on the observation that iron is deposited on the front and back side of the nickel mesh, as shown in Fig. 6a. On the other hand the penetration depth of the laser process is around 25e35 mm, the obtained alloy composition for sample NiLFe (61/39) with a determined iron layer thickness of 30.9 mm is only explainable under the assumption of a uniform iron deposition, otherwise the laser beam wouldn't be able to reach the underlying nickel phase. Since only the front side of the mesh is lasered, the layer thickness is determined for one side and is therefore divided by two. The standard deviations for the thickness of the electrodes were determined from seven measurements. layer thickness ¼

thicknessmesh:coating
thicknessmesh:bare 2

(1)

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In order to investigate the stability of the NiFe coatings, the iron content of the electrolyte is measured before and after electrochemical characterization with graphite furnace atomic absorption spectroscopy (GFAAS) performed with a SIMAA 6000 spectrometer (PerkinElmer).

Electrochemical measurements The electrochemical characterization of the prepared electrodes is carried out in a heatable modified half-cell with a three-electrode setup, shown in Fig. 2. The half-cell consists of four compartments of Polymethyl methacrylate frames (Perspex®) for the working electrode (WE), the reference capillary (RC), the counter electrode (CE) and the reference electrode (RE). The compartments are sealed with silicone
zaud GmbH) gaskets (LEZ-SIL 60-TF, 0.8 mm thickness, AET Le and located in a vice to adjust a contact pressure of 0.04 kg cm
2. The compartments of WE, RC and CE are separated from each other by a separator (Zirfon™ perl UTP 500, Agfa). The working electrode with a circular geometric area of 3.14 cm2 lays directly on the separator to simulate zero-gap assembly. The electrode potential is measured on the opposite side of the separator with a PTFE capillary (di ¼ 0.5 mm, da ¼ 1.6 mm, BOHLENDER GmbH, Germany) which lays directly and concentric on the separator. This capillary is connected to the compartment with the reference electrode, a reversible hydrogen electrode (RHE) (HydroFlex®, gaskatel, Germany). As counter electrode a nickel mesh as described above with a geometric area of 30 cm2 is utilized. The half-cell is heated by an external thermostat (CC-K6s, Huber, Germany) (±2 K) and adjusted to the temperature of WE compartment. The measurement protocol depicted in Fig. 3 is utilized to examine the electrochemically active surface, the catalytic activity and the stability of the prepared electrode samples. A three-day measurement program is applied to investigate the influence of unsteady measurement methods and the temperature changes on the electrode performance. The half-cell compartments (WE 35 mL, RC 3.14 mL, CE 70 mL, and RE 50 mL) are filled with 32.5 wt% KOH, which is prepared from

Fig. 2 e Schematic drawing of half-cell.

Fig. 3 e Protocol for measuring the electrochemically active surface area, catalytic activity, stability of electrodes during OER.

potassium hydroxide pellets (KOH, p.a, iron content < 5 ppm, Honeywell). The electrodes are held without potential overnight and the electrolyte is left in the half-cell and replaced with fresh electrolyte at least 2 h before starting the daily measurement sequence. The compartments are flushed with nitrogen to avoid explosive gas mixtures and contamination of the electrolyte by CO2 in air. The WE compartment is purged with oxygen at least 30 min before starting the measurement sequence and the temperature is adjusted according to the measurement program. For all measurements a Gamry Reference 3000 potentiostat is utilized. The OER equilibrium potential for overpotential determination, is calculated with the Nernst equation taking the temperature dependence according to Bratsch [34] and the activity coefficients and water vapour pressures from Balej [35,36] into account. For the mentioned conditions, equilibrium potentials (Eeq) for the OER vs RHE of 1.212 V at 353 K and 1.237 V at 298 K are determined. In order to study and compare the behavior of the samples over time no further pretreatment is carried out before the measurement sequences. At the beginning of the sequence, the electrochemically active surface area is measured by cyclovoltammetry (CV) to determine the electrochemical capacitance Cdl [20]. CV measurements are conducted by cycling 50 mV around the open-circuit potential (OCP) in the non-Faradaic current region, where the current only charges the electrochemical double layer and no oxidation or reduction takes places. The electrode polarization is performed from the more positive to negative potential at scan rates of 0.05e0.8 V s
1. Determination of the ohmic resistance is conducted by electrochemical impedance spectroscopy (EIS) before the measurements at OCP in the frequency range from

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100 kHz to 1 Hz with an AC amplitude of 5 mV and estimated from the Nyquist plot. Between the individual measurements, a delay of 5e10 min is applied. To investigate the electrode performance and catalytic activity at the beginning of the measurement sequence, three consecutive linear sweep voltammetry (LSV) runs are performed in the potential range of 1.0e1.7 V vs RHE at a scan rate of 1 mV s
1. For comparison of the obtained results, one additional LSV is recorded at the end of the sequence. To assign the LSVs of the induvial samples to the respective measurements, the following nomenclature is used in this work: LSVx-Day-d, with x for experiment numbers 1e4 and d for individual day (I-III). The determination of Tafel slopes and current exchange densities from the LSV measurements was carried out in the low overpotential region (hlow.298K ¼ 200e300 mV for 298 K and for 353 K hlow.353K ¼ 180e240 mV) for current densities between
3 and
1.5 log (A cm
2) according to Kubisztal et al. [26]. After the initial LSV measurements, EIS analysis is carried out in the frequency range from 100 kHz to 0.1 Hz with an AC amplitude of 5 mV at potentials vs RHE of 1.475, 1.5 and 1.55 V at a constant current density of 10, 50, 100 mA cm
2. However, the results of EIS measurements are beyond the scope of this work and will be published elsewhere. Three consecutive steady state multiple step chronopotentiometry (mCP) measurements are recorded only for measurements at 353 K in a range of 10e100 mA cm
2 in steps of 10 mA cm
2 while in the higher current density region at 100e800 mA cm
2 steps of 100e200 mA cm
2 are employed. The current density is held for 30 s in each step. Due to the

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influence of bubble formation at higher current densities, every single current step is iR compensated at the specific current density with two different methods. The compensation of the ohmic drop of the first mCP-I-EIS is conducted by EIS (freq. 100 kHz to 1 Hz and AC amp. 5 mV) for iR compensation. The third mCP-III-CI is iR compensated by the current interrupt (CI) method, whereby also conclusions on stability of the electrode can be drawn. To investigate the influence of both iR compensation methods on electrode performance, the second mCP-II is performed without iR-correction of each current density step. At the end of the daily sequence, a galvanostatic measurement is carried out at 10 mA cm
2 for 60 min and at 100 mA cm
2 for 15 min.

Results and discussion Structural investigations In order to compare the sample morphology on the micro and submicrometer scale, SEM micrographs are shown in Fig. 4aed. The Ni mesh has a relatively smooth surface on all scales. In a lower magnification the surface of electrochemically deposited NiFe coating appears to be smooth with same irregularities. However, in contrast to the Ni mesh the surface seems to be slightly rougher at higher magnification. The surface morphology of the prepared iron coatings before the laser treatment depends on the deposited amount of iron. For lower iron coating loads (<2 mg cm
2) the surface morphology

Fig. 4 e SEM micrographs on micro and submicrometer scale of (a) smooth Ni mesh, (b) electrochemically deposited NiFe coating (Ni-NiFe(81/19)), (c) electrochemically deposited Fe coating (8.4 mg cm¡2) and (d) resulting laser structured NiFe surface (NiLFe (70/30)).

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is similar to the NiFe coating. For higher iron loads, the morphology contains inhomogeneities on the microstructure whereas around these inhomogeneities a relatively smooth surface on the submicrometer scale is observed. However, no direct correlation between the surface quality of the iron coated samples and the resulting surface structure after the laser treatment is observed. This can be probably explained by the fact that the ablation depth of the laser process is around 25e35 mm and therefore the influence from a low number of smaller inhomogeneities is negligible. Furthermore, all electrochemically prepared coatings exhibit a good adhesion to the nickel substrate, since no ablation or extensive damages are observed. In contrast to the Ni mesh and the NiFe surface morphology, the surface modification of the iron coatings by the laser ablation method reveals a strongly jagged microstructure. On this structure, a riff-like submicrometer structure develops as shown in the higher magnifications of Fig. 4d. Depending on the selected laser parameters, the microstructure of a nickel surface can be adjusted from ripple formation over randomly distributed cones to randomly distributed cone-like spikes as it is the case in the present work [28,37]. Therefore, the structure of the laser treated nickel-iron alloy strongly resembles the structure of the laser treated pure nickel surfaces. Moreover, no obvious differences in the surface morphology occur by variation of the alloy composition.

Surface enhancement BET measurements are carried out with a laser structured nickel sample (NiL) and the Ni mesh. The comparison of the normally utilized specific BET surface area (sBET) would lead to a misinterpretation of the obtained values due to the fact that the sample mass for NiL is reduced by around 40% compared to the Ni mesh due to ablation. For a better assessment of the surface enhancement, the specific BET surface area (sBET) is recalculated by Eqs. (2) and (3) to the area factor (Af.geo), with the sample mass (ms) yielding the absolute BET surface area (SBET.total) and the geometric surface area (SA.geo) of the measured sample. Furthermore, the comparison of the real surface area (SBET.total) with the geometric surface area (SA.geo) is more useful for a comparison with the electrochemical results. SBET:total ¼ sBET ,ms

(2)

Af:geo ¼

SBET:total SA:geo

(3)

As an additional method LSM is applied, which in contrast to BET measurements only identifies structures well above the 100 nm range, while cavities, deep pockets and pores are not captured. Furthermore, the electrochemically active surface area (EASA) is determined from the double layer capacitance (Cdl) obtained during CV measurements and should be proportional to the real surface area. However, deviations from the real EASA may occur due to non-conductive materials [38] or areas physically blocked by gas bubbles at the electrode surface [39]. The three different methods to estimate the surface enhancement of the samples are compared using the roughness factor Rf.Ni_mesh, which sets the obtained area factor (Af.geo) in case of BET/LSM and Cdl values of the laser structured sample in relation to the bare Ni mesh (cf. Eq. (4)). All previously described values are listed in Table 1. Rf:Ni

mesh:BET=LSM

¼

Af:geo NiL Af:geo Ni mesh

Rf:Ni

mesh:CV

¼

Cdl Ni
NiFe Cdl Ni mesh

(4)

For the laser structured (NiL) electrodes we observe a considerable surface enhancement with Rf.Ni_mesh.BET of 9. Thus, the Rf.Ni_mesh.CV value of the EASA e.g. for NiLFe (84/16) is closer to the BET measurement than to the Rf.Ni_mesh.LSM value (cf. Table 1). As a consequence, one can conclude that at least a part of the cavities, pockets and pores contributes to the EASA. Furthermore, the comparison of NiL and NiLFe values for Rf.Ni_mesh.LSM from LSM indicates that, although the surface enhancement could vary in a certain range (cf. Table 1), overall all methods show that laser ablation increases the surface area significantly.

Examination of alloy composition The resulting composition of the NiLFe and Ni-NiFe samples after the laser treatment is analyzed by EDS. The resulting iron loads, iron layer thicknesses as well as alloy compositions after laser treatment are summarized in Table 2 together with the nomenclature. In Fig. 5, the deposited iron layer mass and iron layer thickness after laser structuring is shown as a function of normalized nickel-to-iron ratio. The iron load (right axis, dotted line) correlates well with the determined iron layer thickness (left axis, solid line). Furthermore, Fig. 5 shows that the normalized iron content

Table 1 e Surface enhancement described by roughness factor Rf.Ni_mesh and BET suface area for Ni mesh and NiL (top) as well as by laser scanning microscopy (LSM) and cyclovoltammetry (CV) for Ni mesh, N-NiFe, NiL and NiLFe (bottom). Sample

ms / g

SA.geo / cm2

sBET / cm2 g
1

SBET.total / cm
2

Af.geo (BET) / cm2 geocm
2

Rf.Ni_mesh (BET) / -

Ni mesh NiL

2.48 0.39

57 13

78 70

193 280

3.42 22.03

1 9.0

Sample Ni mesh Ni-NiFe NiLFe (84/16) NiL

Af.geo (LSM) / cm2 geocm
2

Rf.Ni_mesh (LSM) / -

Cdl (CV) / mF cm
2

Rf.Ni_mesh (CV) / -

1.19 1.46 4.14 5

1 1.2 3.5 4.2

194 ± 5 436 ± 15 1436 ± 69 e

1 2.2 7.4 e

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Table 2 e Sample properties of Ni-NiFe and NiLFe coatings. Electrode

NiLFe (99/01) NiLFe (94/06) NiLFe (84/16) NiLFe (70/30) NiLFe (61/39)a Ni-NiFe(81/19) a b

Deposition time / min

Deposited iron mass / mg cm
2

Iron layer thickness / mm

4 7 14 35 45 120

3.6 8.9 11.2 16.8 22.0 7b

2.2 ± 0.7 8.3 ± 1.0 16.6 ± 1.5 24.8 ± 4.2 30.9 ± 1.8 5.5 ± 2.1

Alloy composition after laser treatment (EDS) Ni (norm. wt%) 99 ± 94 ± 84 ± 70 ± 39 ± 81 ±

2.6 2.5 2.2 2.1 2.0 2.0

Fe (norm. wt%) 01 06 16 30 61 19

± 0.2 ± 0.2 ± 0.5 ± 0.5 ± 1.0 ± 0.47

Sample utilized for cross section investigation. Deposited mass of nickel and iron.

(red marks) increases for thicker iron layers while the nickel content (green marks) decreases accordingly. The dependence can be explained by the fact that under identical laser parameter settings the penetration depth of the laser process (25e35 mm) is nearly constant and thus different alloy compositions are created depending on the loads and the corresponding thickness of the deposited iron layer. The figure shows, that it is possible to produce a specific average alloy composition by controlling the layer thickness of the materials. This finding should be transferable to other materials, as well as to alloy compositions of ternary or even more complex mixtures. Besides the overall composition of the surface, the distribution of nickel and iron within the micro structure is revealed by EDS on cross sections of the laser structured NiLFe (61/39) sample as shown in Fig. 6aec. In Fig. 6a the whole wire is presented, with the laser structured side at the bottom and the iron layer on the top of the images. The upper part of the iron layer contains an alloy composition of 67.5 wt% nickel and 32.5 wt% iron after laser treatments, probably due to the ablation residues which stick to the backside of the mesh during the laser process. Inside the cone structures mainly iron is present with 95e98 wt% as shown in Fig. 6b. The

Fig. 5 e Deposited iron layer thickness (left axis) and deposited iron mass (right axis) versus resulting alloy composition after laser treatment. The normalized composition is taken from EDS analysis.

distance from the iron-nickel interface to the top of the cone is approximately 30 mm, which corresponds to the measured layer thickness of iron on this sample. The content of iron decreases to the tip and to the outer flanks of the cone structure to 61e75 wt% and the proportion of nickel rises accordingly to 25e39 wt%. Although it cannot be seen in the false-color images in Fig. 6, the valleys between the cones are also covered with a significant amount of iron within the surface layer in the range of 1e5 wt%. The distribution of nickel and iron reflects the main effects of the ablation process. Besides the following explanation more fundamental effects are described elsewhere in detail [40e42]. Once small ripples are formed during the process within the very first laser pulses, the energy of the subsequent pulses is reflected at the flanks. Consequently, the ablation becomes favorable at the valleys of the structures and the cones evolve selforganized close to the initial surface level. Simultaneously, parts of the ablation plume is resolidified at the surface. As soon as the iron layer is permeated, both nickel and iron is present in the ablation plume and an alloy is deposited during the relaxation process of each pulse. Since the fraction of nickel increases as the penetration depth increases, the content of nickel in the surface layer increases from the core to the shell of a cone. Most of the material is resolidified at the tips of the structures because it is the last condensation seed structure and less excited than the valleys. Thus, the formation of pores and the broader distribution of nickel within the iron matrix as shown in Fig. 6c can be explained. The electrochemically deposited Ni-NiFe sample has a Ni/ Fe ratio of 81/19 wt%, which is below of the expected theoretical value of the deposition bath with a Ni/Fe ratio of 70/ 30 wt%. The deposition of nickel and iron is a so-called anomalous deposition, which means that the deposition rate of iron should be higher than that of nickel. However, the deposition rate of iron and the resulting Ni/Fe ratio depends on many factors such as current density, temperature, pH value and the used additives and could thus vary in a wide range [43]. Nevertheless, the main aim of the used method is to provide a NiFe coating with a smooth and uniform surface as well as a good adhesion of the coating to the substrate. According to Louie et al. the achievable current density and the overpotential during OER vary only slightly for compositions of 15e50 wt% iron [19]. Therefore, selected samples within this range are particularly suitable for comparing the different fabrication methods on the influence of surface structuring and intrinsic activity.

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Fig. 6 e Cross section of laser structured electrode NiLFe61/39 with EDS distribution of nickel and iron, (a) wire, (b) lasered surface structure, (c) single cone structure.

Electrochemical measurements The laser structured samples with the alloy compositions NiLFe (84/16) and NiLFe (70/30) as well as the Ni-NiFe(81/19) sample and an untreated Ni mesh are characterized electrochemically according to the three-day measurement program (cf. Fig. 3) at temperatures of 298 K and 353 K in 32.5 wt% KOH. The results of the LSVs on the first day (LSV2-Day-I-298K) and the second day (LSV2-Day-II-353K) are presented in Fig. 7. Further characteristic values such as current density, the exchange current density and the Tafel slope of the initial LSV1-Day-I-298K are given in Table 3. Among the measured samples, the Ni mesh has the lowest activity with an overvoltage of 394 mV and a Tafel slope of 70 mV dec
1 at 10 mA cm
2 and 298 K. The measured activity of the pure Ni mesh is higher than reported by Klaus et al. [44] even taking the higher electrolyte concentration of 32.5 wt% KOH in this work into account. These authors reported that even minor Fe impurities in the electrolyte have a significant influence on the performance of pure nickel as catalyst in the OER. Additional literature describing the influence of iron impurities on the activity of nickel during the OER can be found in Refs. [44e48]. In order to monitor the iron content of the electrolyte, the concentrations are determined before and after the daily measurement sequence (cf. Table 4). Nevertheless, the comparison of Ni mesh and Ni-NiFe(81/ 19) clearly shows the considerable differences and the improved performance through electrochemical co-deposition of nickel and iron. Overvoltage of Ni-NiFe at 10 mA cm
2 and Tafel slope are reduced to 292 mV and 49 mV dec
1, respectively. The results from the first day measurements of the Ni-

NiFe(81/19) electrode are in a good agreement with the work
rez-Alonso et al. [18] for a comparable Ni/Fe ratio under of Pe similar measurement conditions. For the laser structured NiLFe (84/16) sample, a reduction of the overvoltage by 37 mV to e249 mV and even lower Tafel slopes of 39 mV dec
1 are observed. Obviously, the alloy composition in the investigated

Fig. 7 e Linear sweep voltammetry curves of Ni mesh, codeposited Ni-NiFe, laser structured NiLFe with different compositions (LSV2-Day-I-298K) and NiLFe (LSV2-Day-II-353K). Potential sweep rate 1 mV s¡1 and 32.5 wt% KOH.

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Table 3 e Tafel slope (b), exchange current density (j0) and overpotentials (h) for OER at 298 K and 353 K in 32.5 wt% KOH from initial LSV1-Day-I-298K and LSV1-Day-II-353K of investigated samples. Samples Ni mesh Ni-NiFe(81/19) NiLFe (84/16) NiLFe (70/30)

b (298 K) / mV dec
1

j0 (298 K) / A cm
2

h(10/100 mA cm
2, 298 K) / mV

b (353 K) / mV dec
1

j0 (353 K) / A cm
2

h(10/100 mA cm
2, 353 K) / mV

70 49 39 38

4.7$10
8 1.2$10
8 3.8$10
9 2.1$10
9

394/541 292/354 249/297 255/298

83 46 36 37

6.4$10
6 3.3$10
7 5.5$10
8 7.5$10
8

264/363 203/252 190/228 190/226

Note that j0 and h refer to Eeq (298 K, 32.5 wt.-%) ¼ 1237 mV vs RHE and Eeq (353 K, 32.5 wt.-%) ¼ 1212 mV vs RHE.

Fig. 8 e (a) Multiple step chronopotentiometry curves with iR compensation of single current steps by EIS (mCP-EIS) in the range of 10e800 mA cm¡2, (b) iR-corrected overpotential at 100 mA cm¡2. Conditions: 353 K and 32.5 wt% KOH.

range does not have a great influence on the electrochemical activity of the laser structured samples, which is also evident from the results at higher temperatures. This observation is in line with the findings of Louie et al. [19] that in a range of 15e50 wt% Fe in co-deposited NiFe coatings the electrochemical activity changes only slightly. Apparently, this finding also holds for laser treated NiFe coatings. In general, the activity of all samples at a temperature of 353 K is significantly increased, as shown e.g. for the NiLFe in Fig. 7. The exchange current density for the Ni-NiFe(81/19) and NiLFe samples is increased by one order of magnitude and the Tafel slope decreases slightly when compared with the results at 298 K. As a result, the overpotential at 10 mA cm
2 is significantly reduced to 203 mV for Ni-NiFe(81/19) and even to 190 mV for the NiLFe samples. Multiple step chronopotentiometry (mCP-EIS) curves (with iR compensation of single current steps by EIS) at 353 K for current densities up to 800 mA cm
2 are presented in Fig. 8a. These results are in line with the obtained results of the initial LSVs1-Day-II-353K measurements except for the results of the Ni

mesh, meaning that the NiFe samples exhibit no obvious runin behavior, in contrast to the Ni mesh depicted in Fig. 8b. This observed run-in behavior or decrease in OER activity of the Ni mesh could be attributed to a reduction of active sites or due to reduced OH
/O2 transport rates within the interlayer volume as Trotochaud et al. presumed for the aging behavior of Ni(OH)2 films [47]. NiLFe (84/16) and NiLFe (70/30) have nearly identical overpotentials of 277 mV at 800 mA cm
2, which corresponds to a voltage increase of 6.3 mV per 100 mA cm
2 in the range of 100e800 mA cm
2, whereas the unstructured Ni-NiFe(81/19) coating reveals an overpotential of 364 mV at 800 mA cm
2 and a voltage increase of 14 mV per 100 mA cm
2. Furthermore, it is possible to estimate the influence of the surface structuring onto the bubble introduced ohmic resistance by iR compensation of every single current step through mCP-EIS. The effect of bubble formation or bubble coverage on the electrode surface has just a small influence within the investigated current density range. The bubble resistance is 16e24 mU cm2 for NiLFe and Ni mesh, which corresponds to a

Table 4 e Iron content in 32.5 wt% KOH electrolyte after electrochemical measurements at the individual measurement days and iron content in the fresh electrolyte. Samples Ni mesh Ni-NiFe(81/19) NiLFe (70/30) NiLFe (84/16)

Day-I298K / mgFe L
1

Day-II353K / mgFe L
1

Day-III298K / mgFe L
1

0.139 ± 8.3 1.077 ± 114 4.414 ± 259 4.530 ± 115

0.105 ± 9.6 1.067 ± 70 19.679 ± 582 22.103 ± 1764

0.132 ± 9.3 0.304 ± 6 3.373 ± 103 2891 ± 8.3

fresh / mgFe L
1 0.166 0.253 0.145 0.214

± 3.6 ± 20 ± 6.4 ± 8.3

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bubble introduced overvoltage of 13e19 mV at 800 mA cm
2. This effect could be more dominant at even higher current densities or for other substrates such as foams or meshes with a smaller aperture diameter [49]. However, the results show that the highly structured surface of NiLFe in the zero-gap assembly behaves similarly to a smooth Ni mesh structure, which may be caused by the improved wettability after the laser treatment [32]. In contrast the bubble resistance for NiNiFe(81/19) is with 84 mU cm
2 significantly higher in the investigated current density range, which can be attributed to a inhibited bubble detachment on the corresponding surface morphology [50]. For comparison of activity and stability of the samples over the three-day measurement program, results of double layer capacity, Tafel slope and current density at 10 mA cm
2 for the first and third day at 298 K are depicted in Fig. 9. The double layer capacity (Cdl), which is determined at the beginning of each measurement by CV experiments, reveals almost identical values of around 1440 mF cm
2 for the NiLFe samples and 436 mF cm
2 for Ni-NiFe(81/19) at the first day. Similar results obtained with laser-structured nickel foils were published by Rauscher et al. [28] (Cdl ¼ 1866 ± 336 mF cm
2 in 29.9 wt% KOH at 333 K) which, however, are not completely comparable due

to different conditions and electrode properties. To compare the specific capacitance of the NiFe materials the roughness factor (Rf.Ni-NiFe.CV) is defined as ratio between the capacitances of Cdl.NiLFe and Cdl.Ni-NiFe, which amounts to a value of z3.3 at the beginning of the measurement program. Compared to Day-I298K, the Cdl value at Day-III298K increases by 24% in case of Ni-NiFe(81/19), by 16% for NiLFe (84/16), and by 8% for NiLFe (70/30), which leads to corresponding Rf.Ni-NiFe value of z3.0 for NiLFe (cf. Table 5). The increasing capacitance could result from changes of the surface morphology by aging [31], phase transformation of nickel/iron oxides to hydroxides or dissolution of iron hydroxide (Fe(OH)2 and iron oxy hydroxide (FeOOH) [51,52]. Furthermore oxide, hydroxide and oxy-hydroxide layers could influence the determination of Cdl by the CV method [38]. Investigations of the iron content in the electrolyte after measurements, summarized in Table 4, reveal that the NiFe alloy might not be stable under the given conditions. The examined electrolyte samples of the NiLFe coatings reveals a significant higher iron concentration in comparison to Ni-NiFe. Especially at elevated temperatures the iron concentration of NiFe coatings increases, meaning that the dissolution rate of iron is considerably higher. Nevertheless, it can be seen in Table 4 that the iron

Fig. 9 e Comparison of electrochemical properties during the measurement program at the first and third day; (a) CV of NiNiFe(81/19), (b) CV of NiLFe (70/30), (c) double layer capacitance Cdl determined from CV experiments, (d) Tafel slopes and (e) overpotentials at 10 mA cm¡2 obtained from LSV. Conditions: potential sweep rate 1 mV s¡1, 298 K and 32.5 wt% KOH.

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Table 5 e Comparison of double layer capacitance Cdl of NiFe coatings obtained from CV experiments and resulting roughness factors Rf.Ni-NiFe defined as ratio of Cdl.NiLFe and Cdl.Ni-NiFe, 298 K and 32.5 wt% KOH. Method Ni-NiFe(81/19) NiLFe (84/16) NiLFe (70/30)

Cdl Day-I298K / mF cm
2

Rf.Ni-NiFe / -

Cdl Day-III298K / mF cm
2

Rf.Ni-NiFe /-

436 ± 15 1436 ± 70 1441 ± 87

1 3.3 3.3

542 ± 31 1672 ± 37 1555 ± 78

1 3.1 2.9

concentration of the electrolyte samples from NiFe coatings is decreasing from Day-I298K to Day-III298K, which is particularly evident in case of Ni-NiFe(81/19). This might be an indication for a stabilization of the surface composition. The higher iron dissolution rate of NiLFe could be explained by the fact that the non-structured iron layer on the back side of the samples is also exposed to the electrolyte. As already mentioned, inhomogeneities within the laser structure and thus exposed spots containing higher iron concentrations could also be responsible. To avoid higher dissolution rates of iron a protective voltage of around 1.4 V during the stand-by should be implemented in the measurement program, which was shown to prevent potential driven phase transitions of the coatings [53]. As a measure for the electrocatalytic properties, the corresponding Tafel slopes and overpotentials during the measurement protocol are shown in Fig. 9d and e. For the NiLFe samples, a relatively constant Tafel slope is observed (35e39 mV dec
1). In contrast, the Tafel slope of Ni-NiFe(81/19) continuously decreases with the number of experiments from 49 to 39 mV dec
1, which is approximately the level of the NiLFe samples. The obtained overpotentials are reduced at the third day by 29 mV in case of Ni-NiFe(81/19) and by 9 mV for NiLFe (84/16) at 10 mA cm
2. A possible explanation for the increasing activity of the samples might be that the improved

Fig. 10 e Linear sweep voltammetry curves (LSV2-Day-III298K) for electrochemically deposited Ni-NiFe and laser structured NiLFe electrodes and results for NiLFe electrodes recalculated with Rf.Ni-NiFe defined as the ratio of Cdl.NiLFe and Cdl.Ni-NiFe Conditions: potential sweep rate 1 mV s¡1, 298 K and 32.5 wt% KOH.

kinetics is a result of iron dissolution which exposes the more active nickel-iron phase. Consequently, less material is occupied by the retarding iron oxide or hydroxide. During the anodic polarization highly active nickel-iron oxy hydroxide is formed. Whereas pure iron oxy hydroxide is a poor catalyst for the OER [19] than the combination of nickel and iron oxy hydroxides [13,45,47,52]. Furthermore, incorporation of iron within the nickel-iron hydroxide structure and a further activation of nickel centers might be an explanation for the enhanced electrochemical activity [47,48]. As at the third day the NiLFe and Ni-NiFe(81/19) samples approach relatively similar Tafel slopes, one might assume that the kinetic properties, especially at high current densities, are mainly caused by the enhanced laser structured surface area of the electrodes. By recalculating the geometric current density of the NiLFe samples with the earlier determined roughness factors one obtains approximately the same LSVs of NiLFe and Ni-NiFe as depicted in Fig. 10. This analysis clearly shows that laser structuring increases the surface area, but does not alter the intrinsic activity of the catalyst. Similar observations are reported by Rauscher et al. for laser structured nickel electrodes during the hydrogen evolution reaction [28]. We have shown in this work that the electrochemical activity of nickel-iron electrodes changes over time depending on the selected measurement methods and conditions. The samples prepared by the laser ablation method exhibit a high initial electrochemical activity and are less affected by the three-day measurement program than the electrochemically deposited Ni-NiFe coating. However, at the end of the measurement program both methods result in materials with similar intrinsic activity, but in case of NiLFe with a threefold surface area enhancement through the laser treatment. The described measurement program was also developed to study the stability and degradation of the samples over time. On the one hand, an activation of the samples is observed which should be studied in more detail as it could offer a way for further improvement of the activity of NiFe coatings during OER. On the other hand, the observed dissolution of iron, especially at elevated temperatures is not tolerable during long-term operation in AEL and has to be further investigated. Therefore, our future research activities will focus on both, the development of stable catalyst compositions under industrially relevant conditions for AEL and on the improvement of the intrinsic activity (e.g. by employing more complex catalyst formulations such as Ni/Fe/Co) using the laser ablation method. In order to gain a deeper insight of laser structuring onto material properties and phase transformation processes during electrochemical measurements, the samples will be examined in the future work by XPS before and after the electrochemical characterization.

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Conclusions This work demonstrates that a two-step manufacturing process, i.e. electrochemically depositing iron onto a nickel surface followed by treating the surface by an ultra-short pulse laser ablation method, allows to produce a defined nickel-iron surface alloy composition with increased surface area. The alloy composition can be adjusted by control of the deposited layer thickness. A three-day measurement program is applied to study the electrochemical activity and stability of pure smooth Ni meshes as well as electrochemically deposited and laser structured NiFe coatings. Interestingly, especially measurements at elevated temperatures (353 K) improve the activity of the electrochemically deposited NiFe coating. In contrast, the laser structured NiFe samples are less affected by the measurement program. The results reveal that incorporation of iron into the nickel phase by electrochemical deposition or by a laser treatment significantly increases the catalytic activity. Moreover, structuring the surface by means of a fs-laser improves the electrode performance significantly. However, it is shown that the intrinsic activity is most probably not affected by laser structuring and that the enhanced electrochemical performance is a result of the increased surface area. This developed method should be transferrable to other transition metals or additives, which could further improve the electrochemical activity and might have a positive influence on the electrochemical stability of electrodes during alkaline water electrolysis.

Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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