JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

Femtosecond Direct Laser Interference Patterning of Nickel Electrodes
for Improving Electrochemical Properties
Fabian Ränke1*, Stephan Moghtaderifard1, Lis Zschach1, Robert Baumann1, Bogdan Voisiat1, Marcos Soldera1, Leonard
Günther1, Sebastian Hilbert2, Christian Immanuel Bernäcker2, Thomas Weißgärber2, and Andrés Fabián Lasagni1,32

1Institut für Fertigungstechnik, Technische Universität Dresden, Germany
Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Germany
3
Fraunhofer-Institut für Werkstoff- und Strahltechnik IWS, Germany*
Corresponding author’s e-mail: Fabian.raenke@tu-dresden.de

The present work utilizes Direct Laser Interference Patterning (DLIP) to impart periodic microstructures onto the surface of nickel foils. A pulsed laser source emitting green radiation (514 nm) withpulse duration between 282 fs and 1 ps is utilized to produce line and cross-like pattern geometries
having a spatial period of 3.0 µm and a maximum structure depth of ~ 2.8 µm. Both the pulse duration and number of consecutive scans are varied in order to assess their influence on the structureformation. Afterwards, the treated Ni- electrodes are used for alkaline water electrolysis. The mostactive electrode exhibits an overpotential of -227 mV at a current density of -0.5 A cm-2 for the Hydrogen Evolution Reaction (HER). Thus, the technique here proposed permitted a 51 % reduction in
the overpotential of the HER compared to the untreated Ni-electrode, opening up possibilities forthe development of more efficient electrodes for hydrogen production.
DOI: 10.2961/jlmn.2024.02.2002
Keywords: direct laser interference patterning, nickel electrodes, alkaline electrolysis, hydrogen
evolution reaction
1. Introduction
Climate change represents an urgent and pressing challengethat demands immediate attention and concerted action.
One crucial aspect of mitigating carbon emissions and facilitating a sustainable future lies in the adoption of hydrogen as a key energy carrier and storage medium [1]. Hydrogen possesses immense potential for energy recovery
when utilized in fuel cells, as its combustion yields electrical energy, with water being the sole by-product. Amongthe various methods of large-scale hydrogen production,
alkaline water electrolysis stands out as a particularly
promising technique for meeting the growing demand forhydrogen in a sustainable manner, contributing to the transition to a low-carbon economy [2]. However, the efficiency and durability of alkaline water electrolysis systems rely
heavily on the quality and performance of their electrodes.
In case of the hydrogen evolution reaction (HER), it has
been shown that the reaction efficiency can be enhanced by(i) increasing the real surface area and/or (ii) improving the
intrinsic activity of the electrode materials, leading to a
reduction of the overpotential ȠHER [3–5]. In the hydrogen evolution reaction, the overpotential refers to the reaction rate and is directly proportional to the additional energy required to drive the reaction forward and is directly
related to the efficiency of the HER, as it affects the rate at
which hydrogen is produced [6,7]. Nickel and nickel alloys(e.g. Raney Ni, Ni-Mo) are common choices for this process due to their excellent electro catalytic activity andlong-term stability [8–10]. In particular, it has been shownthat the HER activity of these electrodes can benefit tremendously when the surface area is increased [9,11,12]. In
recent years, laser texturing of nickel electrodes has
emerged as an attractive manufacturing method to improvethe efficiency of the electrode towards HER [3,6,13,14].
Generally, laser texturing of surfaces has many advantages
over conventional methods, since they allow a precise control of the size, shape, and distribution of surface features[15,16]. In previous studies, complex surface architectures
on metallic electrodes were produced with repetitive or
quasi-periodic arrangements using laser-based methods. In
this way, they have been implemented to significantly enlarge their surface area and thus improving their electrochemical activity. For example, Direct Laser Writing(DLW) was used to structure nickel electrodes for applications in electro-catalysis and energy storage [4,6,17]. In
this context, Rauscher et al. employed a femtosecond
pulsed laser to fabricate self-organized conical microstructures on nickel electrodes, resulting in a 45% enhancementof the HER efficiency [6]. However, when features withlateral sizes below a few microns are needed, DLW faces
the limitations of a reduced throughput as well as resolutions restrictions imposed by the diffraction limit. One
promising laser texturing method capable to overcome
these challenges is Direct Laser Interference Patterning(DLIP) [18]. This method has been used to create a variety
of surface structures, such as microchannels, micropillars,
and microdimples, on various nickel alloys. The DLIP
method is based on overlapping two or more coherent laser
beams on the sample surface to produce an interference
pattern with a periodic distribution of the laser intensity[19,20]. Subsequently, material ablation or melting predominantly takes place in areas corresponding to the interference maxima positions where the melting/ablationthreshold fluence/s is/are surpassed. Conversely, areas subjected to destructive interference (interference minima positions) remain unaffected. In the context of alkaline water

JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

electrolysis, laser texturing of nickel electrodes by DLIP
has recently shown to significantly improve their performance due to the creation of microscale surface featuresthat enlarge the surface area, generating active centres forhydrogen evolution. For example, Baumann et al. utilized
DLIP with ps-pulses on open cell metal foams (steel 316L
(1.4404)) [21]. In the mentioned work, the electrochemical
performance of the textured foams revealed an increase of
the double-layer capacitance from 702 μF cm-2 (unstructured material) to 1330 μF cm-2 for the DLIP treated foam,
what was related to an enlargement of the electrochemical
surface by a factor of almost two. However, the impact of
implementing shorter pulses up in the femtosecond (fs)range on the efficiency of DLIP treatment for achieving
higher performances remains unclear until now. This work
explores the benefits of DLIP texturing of nickel electrodes
using pulse durations in the fs range in order to improvetheir performance in the hydrogen evolution reaction. The
structuring of the Ni-foils is performed with a laser source
emitting green radiation (514 nm) with pulse durations of
282 fs, 1 ps and 10 ps, and varying the laser fluence andnumber of scans. The topography of the treated surfaces ischaracterized using confocal and scanning electron microscopy methods. Finally, the performance of the treated samples if characterized in an alkaline water electrolysis.

Λ=λ2 ⋅ sin(θ )
2

.

(1)

2. Experimental Section
2.1 Materials
For all laser structuring experiments, commerciallyavailable nickel sheets (99.95% purity, Goodfellow, USA)with a thickness of 125 μm and dimensions of 300 mm х
300 mm were used. Prior to laser processing, the samples
were cleaned with ethanol (ethanol absolute ≥ 99.9 %,Supelco, Germany) to remove any possible contaminationand subsequently dried with compressed air.
2.2 Laser texturing process
The laser structuring through DLIP was performed byusing the optical configuration illustrated in Figure 1a. The
fs-DLIP setup is equipped with a femtosecond solid-statelaser (Carbide-40W-SH-TH, Light Conversion UAB, Lithuania) with a maximum pulse energy of 400 µJ. The laser
operated at a wavelength ʎ of 514 ± 3 nm with a tunable
pulse duration τ ranging from 282 fs to 10 ps and a repetition rate frep of 1 kHz. The beam path consisted of telescope
system that acts as a beam expander, and a recently developed optical head for DLIP (xDLIP head, SurFunction
GmbH, Germany) that utilizes a Diffractive Optical Element (DOE) to split the main laser beam into two subbeams, which are later shaped to elongated lines (see Figure 1b) and overlapped on the sample. This optical configuration had the advantage over previous setups that can beused with shorter pulse durations (in the fs range) as wellas an impressive depth of focus of ~ 10 mm [22]. The resulting focal spot had an elliptical shape with dimensionsof (dy × dx) 0.1 mm × 2.5 mm (Figure 1b).
Using this optical configuration, an intercepting angle Ɵ
between the overlapping beams of 9.8 ° is obtained. Considering this angle as well as the used laser wavelength ʎ,
the spatial period of the interference pattern can be calculated by using equation (1):Fig. 1 (a) Schematic drawing of the used experimental setup
showing the two-beam DLIP optical configuration combined witha xDLIP head.[22] The inset denotes the two overlapping subbeams producing a line-like interference pattern. (b) Process
strategy for structuring surfaces with elongated laser spots andcorresponding process parameters.

As a result, the overlapped sub-beams produce an interference pattern, consisting of periodically distributed lines
with a spatial period Λ of ~3.0 µm, which are oriented perpendicular to the long axis of the elliptical laser spot (Figure 1b). During the processing, the elongated laser spot wastranslated along the direction of the interference lines bymoving the sample at different process speeds leading tovarious pulse-to-pulse distances PtP at a fixed repetition
rate frep. The movement of the nickel foils in two-JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

dimensional directions was done with mechanical stages(Aerotech PRO155-05, USA). Considering the size of thelaser spot in the y-direction (~ 100 µm, Figure 1b) and theused pulse-to-pulse distance PtP, then it is possible to calculate the overlap OV (in %) by using equation (2):OV = ( 1 -PtP) ⋅ 100% .
dy(2)

For the fabrication of line-like surface patterns with different structure depths, the number of consecutive passes N(scans) was varied from 1 to 30 for a fixed pulse-to-pulse
distance PtP of 5.5 µm (OV = 95 %). This resulted in different cumulated fluences (from 0.2 to 22.0 J cm–2), which
describes the total laser energy used to irradiated a specificarea. For the treatment of large areas, the hatch distanceHD, which is the lateral distance between pulses (Figure1b), was set to 130 times the spatial period Λ, resulting in
390 µm. Additionally, to understand the influence of thermal effects on the resulting structure topography, the laser
pulse duration τ was varied, using 282 fs, 800 fs and 1 ps
pulses.
2.3 Topographical characterization
For evaluating the surface topography of the laser structured samples, confocal microscopy images (Sensofar SNeox, Spain) were recorded by employing 50x and 150x
magnification objectives. The surface profiles and average
structure depth values were obtained using the SensoMAP
Advanced Analysis Software (Sensofar, Spain). In addition,high resolution images of the treated Ni-samples were taken using Scanning Electron Microscopy (SEM) operating
at an acceleration voltage of 30 kV (Quattro ESEM, Thermo Fischer Scientific, Germany). The resulting images
were also evaluated with Gwyddion software by using thetwo-dimensional Fast Fourier transformation, permitting
the determination of structural characteristics for fabricatedsurface structures.
2.4 Electrochemical characterization
The electrochemical characterization of the electrodes
was conducted in a conventional three-electrode cell configuration at a temperature of 60 °C with 30 wt.-%. KOH.
For the counter electrode a platinum sheet (8 cm²) was utilized, and a saturated Reversible Hydrogen Electrode(RHE) was used as the reference electrode. The separationof the electrodes was achieved by a Haber-Luggin. A twochamber cell with a diaphragm (Zirfon Perl 500 UTP,
Mortsel) was used to separate anolyte and catholyte. The
temperature of the double-wall electrochemical cell wascontrolled by an external thermostat (ΔT = ± 2 K). The
electrolyte solution was rinsed with gaseous nitrogen before the experiments. All current densities (jgeo) are referredto the geometric surface area of the electrodes (Ageo =2 cm2).
The Electrochemically active Surface Area (ECSA) ofthe electrodes was estimated by determination of the double-layer capacitance Cdl, using Electrochemical ImpedanceSpectroscopy (EIS) [23]. The EIS measurements were performed in a frequency range between 0.05 Hz and 15 kHzwith an ac amplitude of 5 mV. The RandlesCPE fitting rou-tine of the software “Gamry Echem Analyst” (Gamry Instruments) was used for data analysis. In addition, the dimensionless roughness factor Rf is calculated as the ratio
between the double-layer capacitance Cdl of the laser structured electrodes and untreated Ni-electrode. The steadystate polarization curves were galvanostatically recorded in
the range of jgeo = -0.001 to -0.5 Acm-2; each step for 20 sin the direction of decreasing current density. Previously,
the electrodes were polarized at a current density of -0.5 A
cm-2 for 1 h, inducing the evolution of hydrogen gas.
3. Results and discussion
3.1 Influence of process parameter on the surface topography
Line-like DLIP textures with a spatial period Λ of
~3.0 µm were firstly fabricated on the Ni-foils. As processparameters, the pulse duration τ and number of consecutive
passes N were varied to evaluate their influence on the
structure morphology. As mentioned before, the repetition
rate frep was kept constant for all experiments. In thecourse of the investigations performed, the pulse energy
was gradually increased from 41.1 to 90.8 µJ. The totalenergy applied per unit of area, known as the cumulated
laser fluence Φcum, was calculated using Equations (4) and(5),

Φ cum =d y ⋅ f rep

,

(4)

EP
⋅ N pulses ⋅ N ,
Aspot

(5)

N Pulses =vscan

where Aspot = π × dx/2 × dy/2 is the laser spot area, andNpulses the number of applied laser pulses irradiating thesame effective area, that can be calculated from Equation(4). Figure 2 shows SEM images of selected patterned Nisurfaces, where well-defined line-like textures can be observed. The used pulse durations were 1 ps for the SEM
image shown in Figure 2a, d, 800 fs for Figure 2b, e, andfinally 282 fs for Figure 2c, f. Moreover, the structures
shown in images (a, b, c) were produced with a single scan,
whereas the patterns in (d, e, f) were fabricated with 20
consecutive passes. In all samples, the overlap OV was
kept constant at 95 % (see experimental section). The calculated cumulated fluences are given in the labels for eachsub-figure.
For the samples structured with a single pass (N = 1)
and different pulse durations, the appearance of a homogenous line-like DLIP pattern decorated with a sub-structure
can be observed. Upon inspecting the samples structured
with a pulse duration of 1 ps (Figure 2a) and 800 fs (Figure2b) at a higher magnification (see insets), the presence of awavy texture at the positions corresponding to the interference maxima can be observed. These ripples are oriented
perpendicular to the polarization of the applied laser radiation (double arrow E in Figure 2a) and follow along theDLIP produced lines. The measured periodicity of the ripples ranged from 350 nm to 410 nm, which corresponds to
68% - 80% of the used laser wavelength (514 nm). These
characteristics suggest that the sub-structure can be regarded as Laser-Induced Periodic Surface Structures (LIPSS)JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

and further classified as Low-Spatial Frequency LIPSS(LSFL), according to previous studies [24–26].
In the case of the texture shown in Figure 2c, producedwith the shortest pulse duration of 282 fs, no visible ripples
were found but instead a random sub-texture was produced.
Also nanoparticles distributed over the whole area are visible. Taking into consideration matter reorganization theories, it is known that phase transitions as well as hydrodynamic effects of the transiently melted surface are necessary for LIPSS formation [27]. For instance, the existing
theories are driven by local gradients and require the surface to stay long enough in an excited state that enables theunderlying mechanisms to act. Hence, these effects dominate for long pulse durations and could explain why the

LIPSS are only observed for the longer pulse durations(800 fs and 1 ps). An alternative perspective involves examining the correlation between threshold fluence andpulse duration, as both factors significantly influence theformation of LIPSS. Fundamentally, threshold fluences
decrease with decreasing pulse durations. When maintaining a constant cumulative fluence while decreasing pulse
durations, the formation conditions (typically close to thematerial's ablation threshold) for LIPSS structures shift intoa range well above the established formation regime. Consequently, no self-organizing LIPSS textures are produced[27]. By increasing the number of consecutive passes (N)to 20, the cumulated laser fluence was increased from 0.7
to 14.5 J cm-2. As a consequence, the ablation at the maxi-

Fig. 2 SEM images of DLIP line-like structures produced on nickel-foils at a repetition rate frep = 1 kHz and overlap OV = 95%. The
pulse duration, number of applied scans (N) and cumulated fluence (Φcum) are displayed in the corresponding labels.

JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

ma positions becomes stronger, leading to deeper DLIP
structures for all pulse durations as illustrated in Figure 2d,2e and 2f. The appearance of self-organized periodic micropits in the regions of the interference maxima and parallelto the DLIP lines could be also observed for all pulse durations. Furthermore, the micro pits exhibit a spatial distanceof 1.5 ± 0.5 µm. From the SEM images (Figures 2d-f), it
can be concluded that the formation of the micro pits was
not influenced by the pulse duration and predominantly
depended on the applied cumulated laser fluence. Furthermore, it is known that these features result from successive
accumulation of the laser radiation (in particular when applying several laser pulses) as already explained by otherauthors [28]. In addition, the laser structuring experiments
performed at 1 ps with 20 consecutive passes resulted in asurface with significant redeposition of ablated material on
the ridges of the line-like features as illustrated in Figure 2d.
Upon decreasing the pulse duration to 800 fs, the formationof ablation residues was significantly reduced (Figure 2e).
For the sample treated with a pulse duration of 282 fs, theproduced topography is free of residues. This can be explained due to the very rapid formation of vapor and plasma phases, with negligible heat conduction as well as theabsence of a liquid phase for the femtosecond regime. It isalso noticeable that the LSFL ripples in the region of maxima interference visible for one scan, vanished as a resultof the laser process with high cumulative fluences (for 20
passes). Interestingly, for the surfaces treated with several
over scans, fringes located in the walls of the producedlines are also visible. These features could result from interference effects between the incoming and reflected radiation, and are known as Newton fringes [29].
By confocal microscopy analysis, the dependence ofthe structure depth as a function of the cumulated laser
fluence and the number of passes N was characterized. The
results are summarized in Figure 3. The measured structure
depths for the samples treated with a single scan (N = 1),
using a pulse overlap of 95% and pulse fluence values between 0.02 and 0.04 J cm-2 are displayed in Figure 3a. The

depth of the line-like DLIP textures can be observed toincrease in an almost linear manner with the cumulated
fluence. This behavior has been observed for all pulse durations. For the shortest pulse duration of 282 fs, the deepest
DLIP features were generated along the whole range of
cumulated fluence values. The maximum fluence of 0.73 J
cm-2 resulted in case of a pulse duration of 282 fs in a
structure depth of 0.37 ± 0.03 µm, while for 800 fs, a structure depth of 0.26 µm ± 0.06 µm was obtained. In case of
the 1 ps pulse duration, the structure depth was 0.17 ±0.06 µm, being this value approximately the half of thedepth reached 282 fs. It is known that when using shorter
laser pulses (in particular with pulse durations shorter than
1 ps), a lower amount of energy per unit of area is required
to initiate ablation at the material surface since the absorbed energy is mainly used for the ablative process withalmost no thermalization of the remaining material [30].
This effect can enhance therefore the ablation rate, as already reported for different metals [31].
Interestingly, the error bars associated with the depth
measurements for the Ni-surfaces structured with a pulse
duration of 282 fs were smaller than those resulting from1 ps and 800 fs pulse durations. This indicates a higher
overall texture homogeneity for the DLIP pattern producedin the low fs regime. A possible explanation could be due tothe absence of ablation residues that are generated eachtime that the material surface is irradiated. In case of thelonger pulses, these features might lead to inhomogeneities.
For the fabrication of DLIP line-like structures withhigh aspect ratios, the number of consecutive scans N was
varied between 1 and 30. The measured structure depths asa function of the number of passes as well as the cumulative fluence values for the same pulse durations (282 fs,
800 fs and 1 ps) are shown in Figure 3b. As it can be seen,
the depth increased continuously up to 20 consecutive
scans, independently of the used pulse duration. For N = 20
passes and a cumulative fluence of 14.5 J cm-2, maximal
structure depths of 2.2 ± 0.19 µm, 1.5 ± 0.22 µm and 1.4 ±0.25 µm where obtained for pulse durations of 282 fs,

Fig. 3 Structure depth as function of the cumulated fluence for a) a single scan and b) multiple scans, using pulse durations of 282 fs, 1 ps
and 10 ps.

JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

800 fs and 1 ps, respectively. Clearly, since the samples arescanned several times, deeper structures can be obtainedsince a higher amount of material is ablated at the interference maxima positions. When the number of passes wasincreased further to 30, the structure depth decreased for all
pulse durations. Although this behavior has been alreadyreported in DLIP experiments performed with ns pulses,
the reasons for this reduction in the structure depth is still
unclear. It is possible that when periodic structured surfaces
with a high aspect ratio (~ 0.6 to 0.8) are re-irradiated, this
topography negatively affects the incoming pulse distorting
the pattern. Further detailed investigations are necessary toanswer this question.
Based on the information presented in the previous sections, larger Ni-electrodes with dimensions of 360 mm x
100 mm were processed, which were necessary for the hydrogen evolution reaction experiments. In addition to linelike patterns, also cross-like geometries were produced(also referred as pillar structures in the literature), by firstly
producing line-like structures and subsequently rotating thesamples by 90 ° and re-irradiating them. The reason for
producing the cross-like geometries lies in a further enhancement of the surface area that can have a beneficial
effect on the electrodes performance. In addition, since thedeeper features were obtained for a pulse duration of 282 fs
and 20 consecutive scans, only these process parameters
were implemented. The electrodes produced exhibit a material throughput of 0.05 m²/min and 0.025 m²/min for theline and cross-like DLIP structures, respectively.
SEM images of the resulting textured samples together
with the two-dimensional Fast Fourier Transform (2D-FFT)of the line and cross-like DLIP features are shown in Figure 4. The SEM images (Figure 4a and 4b) displayed highly uniform periodic surface features for both texture types.
The corresponding FTT spectra for the line- and cross-likepattern is pictured in Figure 4c and d, respectively, showing
the periodic distribution of clearly defined intensity peaks
with an equidistant spacing, resembling the spectrum of
perfectly ordered 1D and 2D diffraction gratings, respectively [32,33]. By measuring the distance between two adjacent intensity peaks, the spatial periods of the DLIP structures can be calculated, yielding a spatial period Λ of
3.0 µm for both texture types.
3.2 Determination of electrochemically active surface
area
Previous studies have shown that the enlargement of thenickel electrode surface area is beneficial for the electrocatalytic HER activity [8,9,11]. The electrochemically active surface area (ECSA) plays a crucial role in determining the HER activity as it represents the interface area between the electrolyte and electrode, where the charge transfer reaction occurs. Electrochemically, the active surface
could be estimated by measuring the double-layer capacitance Cdl [34]. Additionally, the enlargement of the ESCA
compared to flat nickel electrodes can be quantified by

Fig. 4 SEM images of DLIP line-like (a) and cross-like (b) structures produced on nickel-foils at a pulse duration τ = 282 fs, repetition
rate frep = 1 kHz, overlap OV = 95 % and hatch distance HD = 139 μm. The cumulated fluence (Φcum) and the total number of appliedscans (N) are displayed in the corresponding labels. Two-dimensional Fast Fourier Transform for line-like (c) and cross-like (d) DLIP
structure.

JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

Table 1 Estimated double-layer capacitance Cdl at |𝜂𝜂𝐻𝐻𝐻𝐻𝐻𝐻|=100 mV after 1h at -0.5 A cm-2 in a 30wt.-%. KOH at 60 °C. Calculatedroughness parameter Rf based on the capacitance values obtained from EIS. Overpotential |𝜂𝜂𝐻𝐻𝐻𝐻𝐻𝐻| at a geometric current density of -0.5
A cm ². Tafel slope parameter |𝑏𝑏| and exchange current density j0. Charge transfer resistance Rct at |𝜂𝜂𝐻𝐻𝐻𝐻𝐻𝐻| = 100 mV after 1h of the HER
at -0.5 A cm-2.
Nielectrodes

Double-layer capacitance Cdl/ mF cm-2

Roughnessfactor Rf / -Overpotential|ȠHER| / mV

Tafel slope |b|/ mV dec-1

Exchange current density
jo,geo / mA cm-2

Charge transfer resistanceRct /Ω cm-2

Untreated

0.5

1.0

464

145

0.2

20.7

DLIP
(line-like)

1.7

3.4

295

114

1.3

4.3

DLIP
(crosslike)

4.9

9.8

227

98

2.3

1.4

introducing the dimensionless roughness parameter Rf of
the untreated and the line- and cross-like structures [35].
Note that the ECSA could be smaller compared to the realelectrode area if, for instance, parts of the surface are physically obstructed by adherent gas bubbles, due to the electrode/gas interface [36,37]. The Cdl values determined by
EIS and the corresponding calculated roughness factor Rf
are summarized in Table 1. For untreated Ni-electrodes a
Cdl value of 0.5 mF cm-2 was observed under these conditions. An increased Cdl value and, therefore, an enhancedroughness factor was obtained for the DLIP-structured linelike and cross-like textures (Table 1). In case of the DLIP
line-like treated electrode, a Rf value of 3.4 was determined,
whereas DLIP with cross-like textures provided the largest
roughness factor of 9.8 which indicated an increase of the
ECSA of almost ten times in comparison to untreated Nielectrode.
3.3 Electro-catalytic activity towards HER
An evaluation of the electro-catalytic activity towards
HER in an alkaline medium was conducted on the DLIPtreated electrodes with line and cross-like geometries. The
obtained results are shown in Figure 5. The overpotentialtime curves for the laser textured specimens and the untreated Ni-electrode were recorded at a geometric current
density of -0.5 A cm-2 for 1 h (Figure 5a). All curves exhib-

ited an increasing overpotential over time, whereas theslope of the curve tended to slow down after the first 20 to
25 minutes. For the initial stages of HER, the overpotential
gradually increased, which might be related to the reduction of native oxides or hydroxides and/or the formation ofa thin nickel hydride layer consisting of a few atomic layers.
After analyzing the curves depicted in Figure 5a, it could
be concluded that treating the electrodes with fs-DLIP
caused a translation of the overpotential-time curves towards lower overpotential values, relative to the untreatedNi-electrode. The resulting overpotentials after 1h of HER
are listed in Table. 1.
The untreated nickel surface exhibited an overpotentialof 464 mV, which aligns with results from previous studies[6,7]. In comparison, the DLIP (line-like) electrode displayed an overpotential of 295 mV, while the DLIP (crosslike) notably exhibited the lowest voltage overpotential
value of 227 mV. In contrast to the untreated Ni-electrode,
DLIP (cross-like) surface topography led to a significant
decrease in overpotential by approximately 51 % and, consequently, led to higher reaction speeds towards HER.
To underpin the results for the HER activity, the kinetic
behavior of HER was assessed by analyzing the steadystate polarization curves shown in Figure 5b. The corresponding Tafel parameters (b, j0) for the tested electrodes
are listed also in Table 1. Generally, the steady-state polari-Fig. 5 (a) Overpotential-time curve of the hydrogen evolution reaction on the line and cross-like patterned Ni-electrodes compared tothe untreated reference at a geometric current density of -0.5 A cm ² in 30 wt.% KOH at 60 °C for 1 h. (b) Steady-state polarization
curves for untreated and line and cross-like DLIP structured Ni-electrodes.

JLMN-Journal of Laser Micro/Nanoengineering Vol. 19, No. 2, 2024

zation curve reflected the HER activity presented in the
overpotential-time curve. In this context, the untreated Nielectrode exhibited the highest Tafel slope b (145 mV dec1
) and the lowest exchange current density j0 (0.3 mA cm-²),
indicating the lowest HER performance. In contrary, DLIPtreated the electrode (cross-like) showed the highest HER
activity, coupled with the lowest Tafel slope parameter b(98 mV dec-1), as well as the highest exchange current density j0 (2.3 mA cm-²). Generally, the activity towards HER
is mainly influenced by the intrinsic activity and the electrochemically active surface area [35,38]. Therefore, it was
further investigated whether the increase in ECSA is solely
responsible for the reduction of the overpotential or if additional intrinsic effects influence the HER activity. In thiscontext, the roughness factor Rf and the charge transfer
resistances Rct from the electrochemical impedance spectroscopy were further analyzed to examine the influence on
the HER activity. The results are shown in Table 1. The
DLIP microstructured electrode (cross-like) presented thehighest roughness Rf value coupled with the lowest overpotential (|ηHER|= 227 mV), indicating that a larger electrochemical active surface area is accessible to the HER. This
statement coincides with the SEM images displaying theperiodic microstructure of the electrode. When comparing
the roughness parameters (Rf) in relation to the overpotential (|ηHER |), it was observed that the DLIP (line-like) electrode exhibited an increase in ECSA by a factor of 3.4 (Rf =3.4) and a reduction in overpotential by 36% compared tothe untreated reference. In contrast, the periodic cross-likestructure resulted in an almost tenfold increase (Rf = 9.8) in
ECSA and a reduction in overpotential by approximately
51 % (as mentioned before). The non-proportional behavior
regarding the surface enlargement coupled with the overpotential drop indicated that the intrinsic properties were influenced by the laser treatment.
The change in intrinsic activity of each electrode can bequalitatively assessed through the transfer charge resistances (Rct). In this context, low Rct values indicate high intrinsic activity. The results of Rct are also indicated in Table 1.
The untreated Ni-electrode exhibited an Rct of 20.7 Ω cm-2
implying a low intrinsic activity. Whereby, for the lasertreated electrodes with line-like and cross-like textures
lower Rct values of 4.3 Ω cm-2 and 1.4 Ω cm-2 were observed, revealing that the enhanced HER activity for bothelectrodes was the result of the enlarged electrochemically
active electrode surface area combined with a higher intrinsic activity.
4. Conclusion
In this work Direct Laser Interference Patterning(DLIP) was used to functionalize the nickel-based electrode surfaces with periodic microstructures. A pulsed laser
source (ʎ = 514 nm) with tunable pulse duration between
282 fs and 1 ps, combined with a two-beam DLIP setup,
was used to generate surface patterns with spatial periodsof 3.0 µm and structure depths of up to ~ 2.8 µm. Both thepulse duration and number of applied scans were varied toassess the influence on the structure formation for line-like
DLIP features. It was found that line-like patterns fabricated with a pulse duration 282 fs and N = 20 resulted in highly uniform periodic surface features with sharp welldefined textures and no traces of attached ablation residuals.

In case of longer pulses (800 fs and 1 ps), the maximal
structure depths decreased to ~ 1.5 µm (~ 42 % less compared to the patterns produced at 282 fs). In addition, afurther increase of the scan numbers (from 20 to 30) resulted in narrower structures, probably due to distortions of theinterference pattern during the texturing process. The determining factor for the achievable aspect ratio is predominantly the quantity of energy reaching the trench's bottom,
which consequently facilitates material removal. Finally,
Ni-electrodes with line and cross-like structures were fabricated and evaluated for hydrogen evolution reaction (HER).
The results indicated that the DLIP-treated electrodes benefited from enlarged electrochemical active surface area andenhanced intrinsic activity which led to a considerable decrease of the overpotential of up to 51 % compared to theuntreated Ni-electrode. These findings indicate that DLIP
combined with femtosecond laser source can be employed
to fabricate Ni electrodes at industrial scale for highly efficient production of hydrogen.
Acknowledgments
This project was funded by the European Regional Development Fund (ERDF) and co-financed under taxation on
the basis of the budget adopted by the members of the Saxon State Parliament. Provision of the laser beam source by
TOPAG Lasertechnik GmbH.
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(Received: August 23, 2023, Accepted: March 29, 2024)