Note: Descriptions are shown in the official language in which they were submitted.
TITLE OF THE INVENTION
Porous Ni electrodes and a method of fabrication thereof
FIELD OF THE INVENTION
[0001] The present invention relates to Ni electrodes. More specifically, the
present disclosure is concerned with
porous Ni electrodes and a method of fabrication thereof.
BACKGROUND OF THE INVENTION
[0002] Electrochemical water splitting is a promising approach to provide
clean and storable chemical fuels (H2).
When connected to renewable energy sources whose production is intermittent,
water electrolyzers can play a
fundamental role in the development of a sustainable energy network. Several
approaches to water splitting
catalytic processes, such as microbial, photo and photo-electro for example,
still present sluggish oxygen
evolution reaction (OER) kinetics that limits the overall efficiency of the
process. Among materials exhibiting good
activity and stability for the OER, oxide compounds are the most active,
notably binary noble metal oxides (Ru,
Ir) and those having complex structures (perovskite, spinel, layered) [1-5].
In strongly alkaline media (pH 13),
Ni metallic alloys are materials of sustained activity [6].
[0003] In combination with improving the intrinsic catalytic properties of OER
catalysts, micro-structuring of the
electrode surface is used to increase the number and surface density of
reactive sites having good electronic
connectivity to the underlying substrate and easy access to the electrolyte,
and nano-engineering of the electrode
surface is used facilitate the escape of gas bubbles, in view of applications
and device operation in practical
electrolysis conditions (j 100 mA cm-2). Indeed, the release of 02 bubbles at
large current density is known to
alter the reaction efficiency due to overpotentials associated with greater
bubble resistance [7]. The mechanisms
responsible for this increased inefficiency include 02 bubble formation
leading to a net decrease of the available
underlying catalytic Ni sites; 02 bubbles coalescing near the Ni surface which
may also cause large ohmic losses
due to the formation of non-conductive gas layers; and pH modification
(increase) which may lead to possible
instability of the catalyst's corrosion processes. In this context, it is of
utmost importance to facilitate the release
of gas bubbles from the surface of electrodes participating in gas evolving
reactions like oxygen evolution.
[0004] The size, size distribution, adsorption, and residency time of gas
bubbles on the electrodes can be varied
Date Recue/Date Received 2020-05-08
through ultra-gravity and ultrasonic treatment [8, 9, 10, 11], leading to
decreased overpotentials and increased
current density. However, these methods are difficult to implement in
industrial production and not cost-effective
for commercial systems. More recently, it was reported that passive control of
the bubble behavior can be
accomplished through nano-engineering of the electrode surface to impart
intrinsically active materials with
carefully tailored porosity that facilitate the detachment of oxygen bubbles
from the surface and, in turn, improved
the extrinsic (overall) performances of electrodes. These electrodes are
termed "superaerophobic" as gas bubbles
trapped at their surfaces typically exhibit very large contact angles [12]. In
the literature, several oxides and
hydroxides containing various amounts of Ni, Co, Fe and Zn superaerophobic
electrodes with nano-engineered
surface have shown improved OER characteristic [13-17]. This improvement of
the extrinsic properties of
electrodes for gas evolving reactions through nano-engineering of the
electrode surface is not restricted to the
OER and was also observed for other reactions, such as hydrogen evolution [18-
20]. Indeed, the ability to fabricate
materials and electrodes with optimized porosity has reignited interest in
research areas involving Li batteries,
capacitors, sensors, and catalysis [21-24]. However, in most of these studies,
the materials investigated and the
methods used to impart the necessary nano-engineered characteristics to the
electrode surface may not be
relevant to industrial applications and commercial devices.
[0005] There is still a need in the art for Ni electrodes and a method of
fabrication thereof
SUMMARY OF THE INVENTION
[0006] More specifically, in accordance with the present invention, there is
provided a method of fabrication of
Ni electrodes by hydrogen bubbles dynamic templated electrodeposition of Ni on
a substrate, the method
comprising one of: i) selecting a current, and selecting an electrodeposition
time at the selected current according
to a deposit target thickness on the substrate; and ii) selecting an
electrodeposition time, and selecting a current
during the selected electrodeposition time according to the deposit target
thickness on the substrate.
[0007] Hydrogen bubbles dynamic templated Ni film, comprising micrometer-sized
pores at a surface thereof,
and pore walls having a cauliflower-like secondary structure.
[0008] Hydrogen bubbles dynamic templated Ni electrode having a ratio between
anodic (Qa) and cathodic (Qc)
coulombic charge of redox transition of a mean value of 1.00 0.13., and Qa
values in a range between 62 4
mC cm-2 and 539 57 mC cm-2.
Date Recue/Date Received 2020-05-08
[0009] Dynamic hydrogen bubble templated Ni films, comprising a microporous
primary structure and a highly
porous cauliflower-like secondary structure.
[0010] Other objects, advantages and features of the present invention will
become more apparent upon reading
of the following non-restrictive description of specific embodiments thereof,
given by way of example only with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] In the appended drawings:
[0012] FIGs. 1A-H show the effect of electrodeposition time on the
morphological features of NiDHBT, with
deposition conditions are of -2 A cm-2 in 0.1 M NiC12=6H20 + 2 M NH4CI : cross-
section (A-D) and corresponding
top-view images (E-H);
[0013] FIGs. 2A-H are contact angle images for a 5 pL air bubble (A-D) and a 5
pL water droplet (E-H) on Ni
plate (A, E); NiDHBT with Electrodeposition times (Td) = 50s (B, F); NiDHBT
with Electrodeposition times (Td) = 250s
(C, G); and NiDHBT with Electrodeposition times (Td) = 450s (D, H);
[0014] FIG. 3 shows cyclic voltammograms (5 mV s-1) in 1 M KOH for Ni
electrodes obtained by the dynamic
hydrogen bubble template electrodeposition method; the electrodeposition time
is shown for each electrode;
[0015] FIGs. 4A-B show (A) chronopotentiometric curves at +250 mA cm-2 in 1 M
KOH for NiDHBT electrodes
prepared at different electrodeposition times; (B) corresponding potential
values recorded at t = 900 s; the error
bars were obtained from three independent measurements performed on a set of
three electrodes prepared in
the same conditions (three replicates, see FIG. 20); the open symbols (0) are
for NiDHBT electrodes measured in
1 M KOH spiked with 10 ppm of Fe impurities;
[0016] FIGs. 5A-C show (A) the effect of the presence of FeCl2 (10 ppm) on the
CVs of NiDHBT film; (B),
chronopotentiometric curves recorded at +250 mA cm-2 in 1 M KOH spiked with
FeCl2 (10 ppm); (C), the variation
of the iR-corrected overpotential vs the logarithm of the steady-state current
density, j; the electrolyte was 1 M
KOH spiked with 10 ppm FeC12; the Tafel slopes are 31 and 29 mV/dec for Ni
plate and NiDHBT film, respectively;
[0017] FIGs. 6 show raw data without normalization for the geometric surface
area (0.4 cm2) of the substrate in
Date Recue/Date Received 2020-05-08
experiments of optimization of Ni dynamic templated electrodeposition (DBTH)
on pressed Ni Foam;
[0018] FIGs. 7A- 7C show the comparison with Ni dynamic templated
electrodeposition (DBTH on a Ni plate;
[0019] FIG.8A-B show (A) the front side, facing the counter electrode; and (B
) the back side, not facing the
counter electrode;
[0020] FIGs. 9 A-C are SEM photos of A- Ni Foam (A); (B) Ni Foam + DHBT
(600s); and (C) Ni DHBT(450s) on
Ni plate;
[0021] FIGs. 10A-B show Ni dynamic templated electrodeposition (DBTH on Ni
Foam, with and without Fe;
[0022] FIGs. 11A--C show short-term chronoamperometric curves of Ni dynamic
templated electrodeposition
(DBTH) on Ni foam electrodes at 10 and 250 mA cm-2 in 1 M KOH at 22 C with and
without 10 ppm FeCl2,
[0023] FIG. 12 show SEM micrographs of Ni dynamic templated electrodeposited
(DBTH) on Ni VECO
samples;
[0024] FIGs. 13 show Ni VECO electrodes with and without Ni DHBT tested in 1 M
KOK with 10 ppm FeCl2,
[0025] FIGs. 14 show Ni VECO electrode with and without Ni DHBT: FIG. 14A
shows OER activity; FIG. 14B
shows uncompensated resistance; and FIG. 14C shows overpotential at 10 and 250
mA cm-2;
[0026] FIG. 15 shows effect of deposition times on the mass of Ni coatings on
a 1 cm2 Ni plate substrate;
[0027] FIGs. 16 show a fractal analysis of NiDHBT film: FIG. 16A shows the
original SEM cross-section image of
a NiDHBT film (Electrodeposition times (Td) = 450 s) at x500 magnification;
FIG. 16B shows the contour image
extracted from FIG. 16A; FIG. 16C shows In plot of box count N vs box size r;
FIG. 16D shows the derivative plot
of In(N) vs In(r);
Date Recue/Date Received 2020-05-08
[0028] FIG. 17 shows the effect of deposition times on the coulombic charge,
Qa, of the redox transition
observed at ca t41 V, obtained from CV profiles recorded at 50 mV s-1 in 1 M
KOH; the y-axis on the right-hand
side displays the ratio between Qa and the mass of the deposits;
[0029] FIGs. 18 show SEM micrographs of Ni foam (1mm thick) and NiDHBT film;
[0030] FIG. 19 shows normalized current density vs electrode potential curves
obtained following normalization
of the CVs shown in FIG. 2 by the corresponding Qa values; the unit of the y-
axis is s-1 and the area under the
Ni(OH)2/Ni(00H) redox transition has unit of V s-1; upon division by the scan
rate (5 mV s-1), the area under each
Ni(OH)2/Ni(00H) redox transition is dimensionless and has a value of 1;
[0031] FIG. 20 shows chronopotentiometric curves at +250 mA cm-2 in 1 M KOH
for different Ni plates and
NiDHBT electrodes : FIG. 20A shows Ni plates, FIG. 202B shows NiDHBT with
electrodeposition times (Td) =
250 s, and FIG. 200 shows NiDHBT with electrodeposition times (Td) = 450 s;
error bars shown in FIG. 3B were
obtained from these measurements;
[0032] FIG. 21 shows variation of the iRs-corrected electrode potential
reached after 15 minutes of electrolysis
at +250 mA cm-2 with respect to the coulombic charge of the Ni(OH)2/Ni0OH
transition Qa;
[0033] FIGs. 22 show SEM images for NiDHBT films with electrodeposition times
(Td)ep = 450 s prior to (FIGs. A,
B and C) and after (FIGs. D, E and F) polarization at 250 mA cm-2 for 15 min
in 1M KOH; indicating no
morphological change due to strong 02 gas evolution; and
[0034] FIG. 23 shows CVs of NiDHBT films with electrodeposition times (Td) =
450 s recorded before and after
the data of FIG. 5C were taken; the electrolyte being1M KOH spiked with 10 ppm
FeCl2 and CV profiles recorded
at 5 mV s-1; the charge under the redox peaks centered at ca 1.39 V is hardly
changed, although the shape of
the oxidation and reduction peaks are slightly modified.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0035] The present invention is illustrated in further details by the
following non-limiting examples.
Date Recue/Date Received 2020-05-08
[0036] A method for fabricating porous Ni electrodes, and Ni electrodes
fabricated therewith are described. The
method generally comprises using electrodeposition. Oxygen evolution reaction
(OER) current densities are
controlled, in particular within typical practical electrolysis conditions of
j 100 mA cm-2, at reduced overpotential.
The Ni porous electrodes have high surface area values porous.
[0037] According to an embodiment of an aspect of the present disclosure, a
method for fabricating
polycrystalline Ni electrodes generally comprises hydrogen bubbles dynamic
templated electrodeposition (DH BT)
of Ni alloy onto a substrate. The method comprises controlling the
morphological features of the deposit to
facilitate the release of oxygen bubbles during the oxygen evolution reaction
(OER). During the cathodic Ni
deposition, the method comprises selecting a large cathodic potential so that
hydrogen bubbles are concomitantly
evolved, thereby controlling a nano-engineered electrode surface with an open
porosity that reaches the
underlying substrate. The method is scaled up and the deposits are adherent,
superaerophobic and mechanically
stable under vigorous oxygen evolving conditions, and characterized by
specific OER properties as illustrated
hereinbelow.
[0038] In experiments, galvanostatic deposition (2 A cm2) from an aqueous
solution of 0.1 M NiC12=6H20
(ACROS Organics, ACS Reagent) and 2 M NH4C1(Fisher Chemical, Trace Metal
Grade) was used to form fractal
Ni foams having honeycomb-like primary and cauliflower-like secondary
structures. These electrodes were
denoted as NiDNBT (Dynamic Hydrogen Bubble Template) since both Ni deposition
and H2 evolution occur
simultaneously. In all cases, commercial Ni plates (Alfa Aesar, Puratronic
99.9945% (metal basis)) were used as
substrates. The films were deposited on one face of 1 cm x 1 cm Ni substrates.
The electrodes were then sealed
in bent glass tubes so that the electrode surface was maintained in a vertical
position and the Ni substrate
uncovered face was not exposed to the electrolyte. In all cases, the exposed
surface area was 1 cm2. A saturated
calomel electrode (SCE) and Pt gauze (Alfa Aesar, 99.9%) were used as a
reference and counter electrodes,
respectively. For the sake of clarity, all electrode potential values were
converted to Reversible Hydrogen
Electrode (RHE) scale. The distance between the counter and the working
electrodes was fixed at about 5 mm.
Ni electrodeposition was carried out using a Solartron 1480 A
multipotentiostat for durations (electrodeposition
times (Td)) up to 550 seconds. The faradaic efficiency for the Ni
electroplating was about 27 8%, independently
of the deposition duration. Following electroplating, the porous Ni
electrodeposits were rinsed with water and dried
under an Ar stream.
[0039] The surface morphologies of the obtained porous Ni films were
characterized by scanning electron
Date Recue/Date Received 2020-05-08
microscopy (SEM) (JEOL, JSM-6300F) and thicknesses were measured by SEM cross-
section analysis. Energy
dispersive X-ray (EDX, VEGA3 TESCAN) measurements were performed to determine
the Fe content. Contact
angle measurements were performed as following. Images of water droplets and
(captive) air bubbles in contact
with the electrode surface were captured by a Panasonic CCD camera (model GP-
MF552). The volumes of the
water droplets and air bubbles were 5pL in both cases. Contact angles were
determined using image processing
program ImageJ software with the Dropsnake plugin.
[0040] Electrochemical characterization in Ar-saturated (Air Liquid, 99.999%)
1 M KOH (Fisher Chemical, ACS
Reagent grade) was conducted in a conventional three-electrode system, using a
Pt gauze and a saturated
calomel electrode as auxiliary and reference electrodes, respectively. The
working electrode and the counter
electrode were not separated by a membrane. The solution (70 ml) was agitated
by Ar bubbling. The distance
between the working and the counter electrodes was 5mm. Following a period of
10 minutes under open circuit
potential (OCP) conditions, cyclic voltammograms (CV) (50 mV s-1) with
different potential windows (0.5V to 1.4
V, 0.5 V to 1.6 V, and 0.5 V to 1.9 V) were performed until steady-state
potentiodynamic features were obtained.
The last CV was recorded at 5 mV s-1. Galvanostatic oxidation was carried out
at 10 mA cm-2 for 15 min and then
at 250 mA cm-2 for 15 min, followed by a last CV (0.5V to 1.9 V, 5 mV s-1).
This sequence was applied to every
Ni electrodes in order to ensure full conversion of nickel to 8-Ni(OH)2. The
ohmic drop was measured by
Electrochemical Impedance Spectroscopy (EIS) and an ohmic drop correction was
manually applied to all
potential values mentioned hereinbelow.
[0041] In a number of cases, CVs and polarization curves were recorded in 1 M
KOH electrolyte spiked with Fe,
and the concentration of Fe was varied between 0 and 10 ppm through the
addition of FeC12=6H20 (Alfa Aesar,
98%).
[0042] The morphological features of as-deposited NiDHBT films are shown in
FIGs. I. All electrodeposition
parameters remained the same (-2 A cm-2in 0.1 M NiC12.6H20+ 2 M NH4C1) except
for the electrodeposition times
(Td). As seen in FIGs. 1A to 1D, increasing the electrodeposition times (Td)
led to a gradual increase of the Ni
film thicknesses, from about 35 pm for (Td) = 50 s up to 220 pm for (Td) = 450
s. The deposited mass of Ni
increased linearly with the electrodeposition times (Td), up to 100 mg cm-2
for (Td) = 450s (see FIG. 15). The
porosity of the films, calculated from the deposited mass and the measured
thickness, varies between 30 and
50%. The mechanical stability of films deposited for longer duration ((Td) =
550 s) is found to decrease, with
some parts detaching from the substrate upon rinsing, which causes the
deposited mass to level off. The cross-
Date Recue/Date Received 2020-05-08
section SEM micrographs of FIGs. 1A to 1D also show numerous voids along the
observed dendritic structure of
the films, most of these voids extending from the film surface to the
underlying Ni plate substrate.
[0043] In top-view SEM micrographs (FIGs. lE to 1H), micrometer-sized pores
are observed at the surface of
the films, with pore diameter in a range between about 10 and about 30 pm.
Lower pore density and larger pore
diameters are obtained for increased deposition times. In all cases, the pore
walls exhibit a highly porous
cauliflower-like secondary structure, with much smaller pore diameters,
typically less than 500 nm. The structure
seen in FIGs. 1 was observed over the entire 1 cm2 geometric surface area of
the deposits. Similar Ni structures
may be formed on substrates with larger geometric surface areas.
[0044] Contact angle measurements on captive air bubbles at the surface of
NiDHBT films were performed and
results are displayed in FIG. 2. The contact angle of air bubbles is seen to
increase from about 139 for Ni plate
to about 160 for NiDHBT films, independently of the DHBT deposition times.
Water contact angle measurements
were also performed as a measure of the hydrophilicity of NiDHBT films, as an
assessment of wetting capacity of
the porous structure of NiDHBT films and of the contact of the porous
structure of NiDHBT films with surface-active
sites. To do so, sessile drop experiments were performed (5 pL of deionized
H20) (see FIG. 2). NiDHBT films
presented superhydrophilic properties, with contact angles well below 25 ,
sign of the strong affinity of NiDHBT films
toward water molecules, to be contrasted with angle values of 30 and 42
recently reported [43]. In contrast,
much larger contact angles (69 ) are obtained herein on Ni plate.
[0045] According to the Wenzel's model, the apparent contact angle on a rough
surface, er, is given by the
following relation:
[0046] cos Or = r cos 0 with cos 0 = a13¨a12 (3)
a23
[0047] where au, au, and a23 are the interfacial tensions of the solid-liquid,
the solid-gas, and the liquid-gas
interface, respectively, r is the ratio of the true area of the solid surface
to the apparent area, and 0 is the Young
contact angle as defined for an ideal surface of the same material. Because r
is by definition greater than or equal
to 1, it is determined from relation 3 above that roughness enhances the
wetting/non-wetting intrinsic properties
of a material, the extent of which is defined by the value of r.
Date Recue/Date Received 2020-05-08
[0048] An alternative way to characterize porous solid surfaces is provided by
the following relation (4) [44, 45]:
L
[0049] cos Of = )D-2 cos 0 (4)
[0050] where L and / are the upper and lower limit lengths of fractal
behavior, respectively, and D is the fractal
dimension of the solid surface, with 2 D 3. A fractal analysis based on the
SEM cross-section image of the
thicker NiDHBT film (Td) = 450 s) was conducted. The SEM cross-section image
of a NiDHBT sample (Td) = 450s)
was taken at x500 magnification as shown in FIG. 16A. The original image was
firstly converted to 8-bit grayscale
and then was segmented into features of interest and background by setting the
threshold interval in-between
105 and 255. The boundary of the structure was extracted by a Sobel edge
detector in image software ("find
edge"). Then, the 2D contour image was skeletonized to one pixel wide. The
final processed image is shown in
Figure 16B.The 2D contour fractal dimension was analyzed by a box counting
tool. The box size was set between
1 to 1024 pixels which corresponds to a scale from 0.5 pm to 554 pm in the
original image. The count of boxes
containing pixels at different box sizes is presented in an In-In-plot (FIG.
16C) of count N versus box size r. Over
a certain local range of length scales the box count shows linear relationship
with box size, indicating that porous
metal materials have obvious fractal characteristics. To determine the largest
and the smallest size limits of the
fractal behavior of the surface as well as the exact 2D fractal dimension, the
derivatives of In(N) in function of In(r)
were extracted from the In-In plot and is shown in FIG. 16D. The derivative
shows a plateau with a value of
1.79 0.05 in the interval of 6.49pm to 69.19 pm. Thus, the 2D fractal
dimension D2 is estimated to be 1.79 0.05
and the upper and lower limit lengths of fractal behavior are 69.19 pm and
6.49 pm respectively. The fractal
dimension D of the surface is obtained as D = 2D + 1=2.79.
[0051] The value of (L//)D-2 obtained is 6.5. However, using the water contact
angle of Ni plate as a reference,
Relation 4 above predicts that cosa = 2.3, which is obviously not possible.
This discrepancy may be caused by
air trapped beneath the water droplet. In these conditions, wetting follows
the Cassie-Baxter wetting regime and
Relation (4) can be re-written as follows (5) [46]:
[0052] cos 0f = (9D-2 f, cos 0 +f ¨1 (5)
1
[0053] with fs the fraction of the surface that is wetted by water.
Date Recue/Date Received 2020-05-08
[0054] In this case, assuming that f8=0.6 considering that the water droplet
is wetting 60% of the NiDHBT film
underneath, the contact angle measurements are in agreement with the fractal
analysis. In the Cassie-Baxter
wetting regime model (Relation 5), the NiDHBT films are treated like porous
materials and partial spontaneous
invasion of liquid inside the texture of the NiDHBT films occurs through
capillary action. Further decrease of Of may
be achieved by increasing (L//)2 and/or fs, by selecting the NiDHBT deposition
conditions.
[0055] The above discussion on the wetting property, based on the ex-situ
contact angle observations under the
air entrapment assumption used in Relation 5 as opposed to in-situ
observations on the contact angle
measurement in real gas evolution situations, reflects hydrophilic properties
of NiDHBT films, or efficiency of NiDHBT
films in releasing the bubbles.
[0056] The electrochemical properties of porous NiDHBT coatings were first
determined through CV
measurements. Following repetitive potential cycles, as will be detailed
hereinbelow, until the formation of a
hydrous Ni oxide deposit was achieved, steady-state CV profiles were obtained
as shown in FIG. 3. All NiDHBT
CVs exhibit a large oxidation, at about 1A1 V, and reduction peak, at about
1.28 V, whose intensities grow with
the film thicknesses. These peaks are discussed hereinbelow. For each NiDHBT
electrode, the ratio between the
anodic (Qa) and the cathodic (Qc) coulombic charge of this redox transition
remains similar, with a mean value of
about 1.00 0.13. Qa values increase continuously from 62 4 mC cm-2 for a
deposition time (Td) = 50s to 539
57 mC cm-2 for a deposition time (Td) = 450s (FIG. 17). These values
correspond to electrochemically active
surface enhancement factors of about 30 and 270, respectively, considering the
Qa value of a commercial Ni plate
as a reference, of 2.1 0.1 mC cm-2). Once normalized to the deposited mass,
m (FIG. 17), the ratio Qa/m is
remarkably constant. This is a clear indication that the material deposited at
the beginning of the deposition period
is not occluded by the material deposited at the end of the deposition period.
This is consistent with the presence
of numerous small (< 500 nm) and large (between about 10 and about 30 pm)
pores seen in FIGs. I. For
comparison, there is a factor of about 25 increase between the Qa values of Ni
foams and NiDHBT films (FIG. 18).
[0057] The good mechanical stability, highly porous structure and increased
capacity of the NiDHBT films to store
charge provides for material and/or substrate for low-cost pseudo
supercapacitor devices, as charge density
values in excess of 500 mC cm-2 observed for NiDHBT of 450 s are well above
charge density values reported
recently in the art for hierarchical porous Ni/NiO electrodes [48]. Higher
electrochemically active surface areas
were obtained for NiDHBT of 550 s (660 mC cm-2); with mechanical stability
issues, considering some part of the
deposits might detach from the substrate, causing a large dispersion in the
data (see the error bar in FIG. 17).
Date Recue/Date Received 2020-05-08
The mechanical stability of the thickest films may be improved by a subsequent
heat-treatment through sintering
of Ni grains, therefore allowing the preparation of adherent films with larger
electrochemically active surface areas.
[0058] On thinner NiDHBT films (Electrodeposition times (Td) = 50 s), the main
oxidation peak is centered at about
1.39 V. It corresponds to the well-known ix-Ni(OH)2/y-Ni0OH transition [50,
49]. There is also a shoulder at about
1.43V, which is attributed to p-Ni(OH)2/13-Ni0OH transition. While both
contributions are observed as the NiDHBT
film thickens (FIG. 19), the relative intensity of the 13-Ni(OH)2/13-Ni0OH
transition increases steadily from the
thinnest to the thicker films, as can be assessed from the relative intensity
at 1.39 and 1.43 V. The position and
the relative intensity of both transitions do not vary with the scan rate (not
shown).
[0059] All NiDHBT films exhibit an additional oxidation wave at about t56 V,
whose intensity increases with
thickness. This oxidation wave may be attributed to formation of Ni (IV)
species, potentially at the edges of y-
Ni(OH)2/y-Ni00H domains [52, 50]. At more positive potentials (E 1.60 V), 02
evolution occurred with high
current densities, which systematically increased upon increasing NiDHBT film
thickness. For NiDHBT films of
deposition times 50 s and 450 s, current density values of about 25 mA cm-2
were obtained at 1/2 V and 1.64 V,
respectively. Conversely, at 1.64 V, the OER current density increased by a
factor of five, from 5 mA cm-2 to 25
mA cm-2, upon increasing NiDHBT deposition times from 50s to 450s.
[0060] Galvanostatic experiments (250 mA cm-2) were performed on NiDHBT
electrodes in 1 M KOH. The
corresponding results are presented in FIG. 4k Stable potentials were obtained
for NiDHBT electrodes right from
the beginning of the tests. In contrast, a gradual increase of the potential
was observed for bare Ni plates during
the first 10 minutes of electrolysis. For longer electrolysis periods, the OER
potential of Ni plates stabilized at 2.05
V. The electrochemical behaviors presented in FIG. 4A were reproducibly
obtained for a minimum of three
different Ni electrodes (see FIG. 20). In FIG. 4B, the iR-corrected
overpotentials reached after 15 minutes of
electrolysis at +250 mA cm-2, 17250, being plotted with respect to the
deposition time. There is about 300 mV
difference between r1250 of Ni plate and best performing NiDHBT films. As
shown previously, Qa is directly
proportional to the deposition time (FIG. 17) and can be used as an indirect
measure of the electrochemically
active surface area. FIG. 21 shows that E250 values of NiDHBT films scales
linearly with Qa plotted on a semi-
logarithmic scale, which is expected if all the material making up the NiDHBT
films is involved in the OER. This
suggests that, even at high current density (250 mA cm-2) and for the thicker
films, the electrolyte has access to
the whole porous structure and that the 02 bubbles do not lead to a decrease
of the available Ni catalytic sites.
Date Recue/Date Received 2020-05-08
[0061] The observation of a redox transition at 1.56 V before the onset for
the OER in FIG. 3 may be interpreted
as a clear signature of Ni(OH)2 aged or cycled in a rigorously Fe-free
electrolyte [52, 51]. Considering that, in
contrast, known studies indicate that cycling or aging of Ni(OH)2 in Fe-
contaminated KOH solution, even at the
ppm level, leads to a huge improvement of the activity for the OER, potential
cycling of NiDHBT electrodes 1 M
KOH electrolyte spiked with 10 ppm of FeCl2 was performed. As seen in FIG. 5A,
the onset potential for the OER
is shifted negatively by at least 100 mV in presence of Fe impurities,
pointing toward a reduction of the energy
barriers of some of the intermediates in the OER process. This occurs even if
the charge under the redox peaks
centered at about 1.39 V is hardly changed although the shape of the oxidation
and reduction peaks are slightly
modified, suggesting the surface density of active sites was not changed. The
Fe content of these electrodes
remains low (0.6%, as determined by EDX analysis). Galvanostatic curves (j =
250 mA cm-2) recorded in 1 M KOH
spiked with 10 ppm FeCl2 are shown in FIG. 5B. These potential vs time curves
are as stable as they are in the
absence of Fe impurities. The two sets of SEM micrographs taken before and
after electrolysis are virtually
undistinguishable from one another (FIG. 22), indicating that the electrode
structure is morphologically stable even
under vigorous 02 evolution. This is consistent with the CVs of electrodes
taken at the beginning and the end of
the polarization period being almost superimposed on each other (FIG. 23).
[0062] FIG. 5C shows the steady-state iR-corrected potential vs log(j) curves
(Tafel plot) on both Ni plate and a
NiDHBT electrode (Td) = 450 s with 10 ppm FeCl2 in the electrolyte. The Tafel
slopes are 31 and 29 mV/dec for Ni
plate and NiDHBT, respectively, which indicates that the mechanisms
responsible for the OER are the same on
both electrodes. Even if the NiDHBT films have an electrochemically active
surface area (EASA) 270x larger than
a Ni plate, Fe impurities interact with the Ni sites at this extended surface
in the same way they are with Ni sites
distributed on a flat surface. Part of the reason for this behavior may be
related to the open structure of NiDHBT
films that is not hampering the diffusion of Fe impurities through the film
and their interaction with Ni sites. This
assumption is supported by the results of FIG. 17, showing that the coulombic
charge, Qa, of the redox transition
at about 1A1 V scales linearly with the deposition time (Td), and thus with
the mass of the film. The data of FIG.
5C also show that, in the "Tafel region", there is a factor of about 230x
difference of the apparent current density
between both substrates, very close from the 270-time increase of the EASA
determined previously. This means
that most of the extended surface area of NiDHBT films is modified by Fe
impurities and is active for the OER.
[0063] Activities for the OER is typically assessed in the art by the
potential required to oxidize water at a current
density of 10 mA cm-2, a metric relevant to solar fuel synthesis. As shown in
FIG. 5C, the overpotential at 10 mA
cm-2, rpo, of the NiDHBT film optimized herein is 250 mV, which is 70 mV lower
than best performing materials
Date Recue/Date Received 2020-05-08
reported in the art for most promising electrode materials.. In presence of 10
ppm FeCl2, 77250 values as small as
310 mV were reached in present experiments for the NiDHBT electrode with (Td)
= 450 s. In comparison, the recent
art reported an OER overpotential at 100 mA cm-2, 77100, of 312 mV in 1M KOH
for iron-doped nickel hydroxide
prepared at room temperature on Ni foam [64] which is already better than
results reported in previous works [65,
66]. However, from the data of FIG. 50, this is still 32 mV larger than the
overpotential recorded on NiDHBT at the
same current density. Elsewhere, FeCoNi deposited on Ni foam were shown to
deliver 75 mA cm-2 at an
overpotential of 320 mV in 1 M KOH [64],which is 44 mV larger than at the
present NiDHBT films (rim = 276 mV
from FIG. 5C).
[0064] Several reasons may explain the OER performances of the present NiDHBT
films. The increased
electrochemically active surface area of NiDHBT films, as compared to Ni
plates, is in part responsible for the
improved OER performance. As stated previously (FIG. 21), the
electrochemically active surface area of NiDHBT
films is fully accessible to the electrolyte and participates in the 02
evolution reaction. It is to be noted that this
measure of the active area was performed in a potential region where no gas
evolution is occurring. Owing to the
porous structure of NiDHBT films, it may have been expected that, at more
positive potential in the OER region, 02
bubbles would increase the electrolyte resistance and/or be responsible for
occlusion of some of the pores.
Surprisingly, this is not what was obtained, and the EIS data of the present
disclosure shows that the double layer
capacitance is constant in the potential region where the 29 mV /decade Tafel
slope is observed. This indicates
that occlusion of Ni active sites by 02 bubbles is not a limiting factor.
[0065] The low Tafel slope (29 mV/decade) appears as an important factor
contributing to the performance of
the NiDHBT films. On NiDHBT films, the 29 mV/decade Tafel slope is observed
over a range of current densities that
far exceed that of Ni plate. Indeed, the "low Tafel slope region" extends up
to 100 mA cm-2 on NiDHBT films while
it is limited to 5 mA cm-2 on Ni plate. This striking difference is partly
responsible for the increased performance
of the NiDHBT films and is to be related to their specific morphologies.
[0066] The morphology of the electrodes is here shown as impacting the
adhesion force of gas bubbles to the
surface and the detachment diameter of the same gas bubbles upon release.
Indeed, both the adhesion force
and the detachment diameter of gas bubbles are diminished through
nanostructuring of the electrode surface.
According to the Fritz correlation, there is a linear relationship between the
gas bubble detachment diameter from
a surface and its water contact angle. As mentioned hereinabove, the water
contact angle decreases from 60 to
less than 25 as a result of the fractal geometry of the NiDHBT electrode.
Enhanced air bubble contact angle, which
Date Recue/Date Received 2020-05-08
is a direct consequence of increased hydrophilicity, translates into smaller
bubble adhesive forces on the electrode
surface, and smaller residency time, along with smaller radius of the contact
plane between air bubble and the
electrode surface, and thus larger contact area between the electrolyte and
the electrode active sites. There are
thus signs of significant decrease of the adhesion force and detachment
diameter of gas bubbles resulting from
nanostructuring of the electrode, which may explain the morphological
stability of NiDHBT films under vigorous
oxygen evolving conditions.
[0067] There is now shown, in relation to FIGs. 6 to 14 that porous structure
of Ni can be replicated conformally
on a range of Ni materials by the present dynamic hydrogen bubble template
electrodeposition method (DHBT).
Nouveau inventif
[0068] FIGs. 6 to 11 show optimization of Ni DHBT deposition on pressed Ni
Foam.
[0069] FIGs. 6A-6C show raw data without normalization for the geometric
surface area (0.4 cm2) of the
substrate. Deposition of Ni DHBT was done on 0.4 cm2 Ni foams at deposition
current = -2A cm-2 with 9 different
deposition times, as well as two repeats. The deposited mass varies more or
less linearly with the deposition time.
CVs were measured and the charge under the cathodic peak, Qc, was measured. Qc
is shown to increase with
the deposition time.
[0070] FIGs. 7A- 7C show a comparison of Ni DHBT deposited on Ni plate and Ni
foam with data of the current
art (ACS Applied Energy Materials 2 (2019) 5734-5743). The deposition time was
varied.
[0071] FIGs. 8A-8B show what, on the front side facing the counter electrode
(FIG. 8A) and, on the back side
not facing the counter electrode (B).
[0072] FIGs. 9 --C show SEM micrographs of Ni DHBT deposited on Ni foam.
[0073] As evidenced from FIGs. 6-9, Ni DHBT were thus deposited on pressed Ni
foams., and the structure of
the film thus obtained is similar to Ni DHBT film prepared on flat Ni plate.
[0074] FIGs. 10- show, for Ni DHBT on Ni Foam, electrochemical
characterization in 1 M KOH at 22 C with and
Date Recue/Date Received 2020-05-08
without 10 ppm FeCl2 Several electrochemical tests were performed, the result
of a few of a number of them are
described hereinbelow.
[0075] FIGs. 10A-10B show Ni DHBT on foam, with and without Fe. The same
electrochemical protocol was
repeated four times, the results of the last test are shown. In the right-hand
panel, the squares and the dots are
for Ni DHBT deposited on a Ni plate previously reported (ACS Applied Energy
Materials 2 (2019) 5734-5743).
[0076] FIGs. 11A-11C show short-term chronoamperometric curves at 10 and 250
mA cm2 in 1 M KOH at 22
C with and without 10 ppm FeCl2. The data of Ni DHBT deposited on Ni plate and
Ni foam electrodes are shown.
[0077] FIGs. 10 and 1111, show further evidence of replication of porous
structure of Ni conformally on a range
of Ni materials, by the dynamic hydrogen bubble template electrodeposition
method (DHBT).Noiuveau inventif.
Two Ni DHBT deposits were fabricated on large area foam electrodes. One was
electrochemically tested without
Fe (sample 1) and the other one with Fe in solution (sample 2). After all the
electrochemical tests were performed,
sample 1 was put in contact with a KOH electrolyte containing FeCl2.
[0078] Thus, as illustrated 6-11, the specific surface area of the Ni DHBT
coating on pressed Ni foam was
optimized by controlling the time of electro-deposition. The electrochemical
active surface area of different Ni
DHBT coatings was determined by the coulombic charge, Qa, of the redox
transition observed at ca 1.41 V,
obtained from CV profiles. It was shown that at deposition time of 600s the Ni
DHBT coating on pressed Ni foam
reaches an optimal specific surface area. Then, the optimized deposition
condition was applied on large pressed
Ni foam (5.75cm2). The morphology of the as prepared Ni DHBT coating is shown
conformable to the morphology
obtained on Ni plate.
[0079] Further catalyzing the Ni DHBT coating on pressed Ni foam with small
amount of more active materials
such as Fe2+ was also shown. Catalization of Ni DHBT was achieved through
spiking of the 1M KOH electrolyte
with a small amount of FeCl2. Adsorption of Fe cations at the electrode
surface decreases the OER onset potential
and enhances the OER kinetics.
[0080] FIGs. 12 to 14 relate to Ni VECO samples. SEM micrographs of Ni DHBT
deposited on Ni VECO
samples of FIGs. 14show that NI DHBT is deposited conformally on the
substrate.
Date Recue/Date Received 2020-05-08
[0081] FIGs. 13 show Ni VECO with and without Ni DHBT tested in 1 M KOK with
10 ppm FeCl2. As may be
seen in FIGs. 13, the Ni electrochemically active surface area is increased by
50 (FIGs. 13A-B), and there is a
significant increase of the current density for the oxygen evolution reaction
for the range of electrode potential
(FIG. 130).
[0082] FIGs. 14 show, Ni VECO with and without DHB decrease of the
overpotential for the oxygen evolution
reaction at 10 and 250 mA cm-2.
[0083] As shown in FIGs. 12 to 14, Ni DHBT is deposited on Ni Veco sample and
the Ni DHBT deposit is
conformal. Ni DHBT coating was applied on Ni VECO textured sample. Still the
morphology of the deposited Ni
DHBT coating is identical to the morphology obtained on Ni plate.
[0084] Robust and mechanically stable electrodes are thus fabricated starting
from a cost-effective and
sustainable material. NiDHBT films are wetted by the electrolyte (fs = 0.6),
resulting in an increased electrochemically
active surface area. They also exhibit a superaerophobic character resulting
in in increased air bubble contact
angle and reduced air bubble adhesive force, both factors further contributing
to maximize the surface area
contact between the active sites of the electrode and the electrolyte even in
conditions of strong 02 evolution.
This results in a decreased overpotential even in conditions of vigorous 02
evolution. On this matter, it is worth
remembering that NiDHBT films are prepared by electrodeposition in conditions
where hydrogen evolution occurs
concomitantly with Ni metal deposition. As mentioned hereinabove, the faradaic
efficiency for Ni deposition is
close to 30%, which means that a large fraction of the current is used to
generate hydrogen gas that escapes the
electrode in the form of gas bubbles. As a result, right from their formation,
NiDHBT films are templated in such a
way that gas bubbles can freely escape the growing film without causing any
damage to its structure. The
existence of several paths through which gas bubbles escape without causing
damage to the film is shown to
contribute in the stability of the NiDHBT films. From a broader viewpoint,
such gas bubble-architectured materials
provide active and stable catalysts for other gas evolving electrochemical
reactions.
[0085] Dynamic hydrogen bubble templating is used to fabricate NiDHBT films
with a fractal structure, which
exhibits improved OER properties compared to Ni plate. Fabricated NiDHBT films
are highly porous and have an
electrochemically accessible surface area which is an increased by a factor of
270 as compared to the underlying
Ni plate. They are mechanically robust and resist degradation under vigorous
oxygen evolution. In presence of
ppm FeCl2, OER overpotential at 250 mA cm-2 is only 310 mV, contributed by
both the porous nature of the
Date Recue/Date Received 2020-05-08
deposit and the superaerophobic characteristic of the fractal Ni films, which
leads to an increase of the contact
angle of a trapped air bubble and a decrease of the adhesion force of 02 gas
bubbles. Industrial applications of
these NiDHBT templates depends on the availability of suitable pieces of
equipment for dynamic hydrogen bubble
templating on substrates with larger geometrical surface area.
[0086] There is thus provided a method of dynamic hydrogen bubble templating
of Ni (NiDHBT) electrodes to
fabricate highly porous films with enhanced properties towards the oxygen
evolution reaction (OER) . Upon
controlling the electrodeposition conditions, Ni films with a microporous
primary structure and highly porous
cauliflower-like secondary structure are formed. These films are able to
develop an extended electrochemically
active surface area, up to 270-fold increase compared to Ni plate. They
exhibit stable overpotential I (71250 = 540
mV) at] = 250 mA cm-2geometric in 1M KOH electrolyte, which is 300 mV less
positive than at Ni plate. Fe
incorporation onto these NiDHBT structures can further lower OER
overpotentials to ipso = 310 mV. NiDHBT films are
remarkably stable over prolonged polarization and are characterized by a low
Tafel slope (29 mV/ decade) that
extends up to] =100 mA cm-2geometr,c, contributed by both superaerophobic
characteristics with a contact angle of
about 160 between the surface and an air bubble and superhydrophilic
characteristics with less than 25
between the surface and a water droplet.
[0087] The scope of the claims should not be limited by the embodiments set
forth in the examples but should
be given the broadest interpretation consistent with the description as a
whole.
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