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Microporous Film of Ternary Ni/Co/Fe Alloy for Superior Electrolytic
Hydrogen Production in Alkaline Medium
To cite this article: Ibrahem O. Baibars et al 2021 J. Electrochem. Soc. 168 054509
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Journalof TheElectrochemicalSociety,2021 168054509
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Microporous Film of Ternary Ni/Co/Fe Alloy for Superior
Electrolytic Hydrogen Production in Alkaline Medium
Ibrahem O. Baibars, Muhammad G. Abd El-Moghny,z Awad S. Mogoda, and Mohamed
S. El-Deabz
DepartmentofChemistry,FacultyofScience,CairoUniversity,Cairo,Egypt
Thisstudyaddressesthesuperbelectrocatalyticactivityofaternary-alloycatalystofNi/Co/Fetowardsthehydrogenevolution
reaction(HER)inalkalinemedium.Thedynamichydrogenbubblingtemplate(DHBT)methodwasappliedtoelectrodepositthe
catalystlayerwithuniquemicroporousfoam-likemorphology.TheelectrodepositedNi/Co/Femicroporousfilmwassubsequently
electropassivatedbycyclingthepotentialin1.0MKOH.Thethus-preparedcatalystsupportsaHERcurrentof10mAcm−2ata
significantlylowoverpotentialof−50mVin1.0MKOH(comparabletobenchmarkingcatalysts)togetherwithmarkedprolonged
stabilityfor10hofcontinuousoperationat100mAcm−2.Interestingly,theelectropassivationprocesswasperformedwithanaim
toenrichthesurfaceoftheNi/Co/Fefilmwithmetal/oxygenatedmetalinterfacesthatfacilitatetheretardingwaterdissociation
step.Theobtainedresults,herein,indicatethatthedesignedNi/Co/FefilmisapromisingcatalystforalkalineHERandshedsome
light on the impact of the electropassivation process on the electrocatalytic activity towards alkaline HER. The morphology,
composition,andcrystalstructureoftheproposedcatalystsweredisclosedemployingSEM,EDSmapping,HRTEM,ICP-OES,
XRD,andXPSsurfacecharacterizationtechniques.
©2021TheElectrochemicalSociety(“ECS”).PublishedonbehalfofECSbyIOPPublishingLimited.[DOI:10.1149/1945-7111/
abfa57]
ManuscriptsubmittedJanuary31,2021;revisedmanuscriptreceivedApril9,2021.PublishedMay7,2021.
Fossil fuels have long been used as primary sources of energy. impact on the understanding and progress of other important and
They have been extensively used by the whole world.1 Their evenmorecomplicatedprocessesinelectrochemistry.7Comparedto
consumption is very rapid compared to the quite slow formation HER in acidic medium, HER in alkaline medium suffers from a
of fossil fuels, which eventually leads to inevitable depletion, sluggish reaction rate.8 The reason behind this inferior kinetics in
especially with the increasing population and the growing demand alkalinemediumcanbeexplainedbyexaminingandcomparingthe
fromindustries.Inadditiontothehighconsumptionlevelandnon- mechanisms; kinetics is inherently dependent on the reaction
renewabilityproblems,asignificantdrawbackconcomitantwiththe pathway.
utilizationoffossilfuelsistheassociatedpollutantsandgreenhouse Generally, HER mechanism in alkaline medium proceeds
gases emitted during combustion.2 Consequently, eco-friendly through two elementary steps. The first step is Volmer step, Eq. 1,
renewable energy sources seem to be the only viable long-term whereawatermoleculeiscleavedintoanadsorbedhydrogenatom
solution to the energy crisis. According to these considerations, andahydroxideion.ThesecondstepmaybeHeyrovskystep,Eq.2,
researchers have long been working to develop reasonable alter- where the hydrogen molecule is produced via the interaction
natives to traditional fossil fuels.3 Although solar and wind energy between an adsorbed hydrogen atom and a water molecule.
sources seem to be the most promising among renewable sources, Alternatively, the second step may be a chemical desorption step
theysufferfromtheirintermittentnature.Thegeneratedelectricityis betweentwoadsorbedhydrogenatoms(Tafelstep),Eq.3,thesame
naturally transient (i.e., must be used once generated); therefore, as present in acidic medium. The overall reaction, Eq. 4, has a
energy storage and conversion systems must be attached to solar standard potential of −0.826V against the standard hydrogen
arraysandwindfarmstogettheadvantageofthetotalcapacityand electrode (SHE). By comparing HER in acidic and alkaline media,
to use later on-demands.4 Reversible/Regenerative hydrogen fuel one can attribute the sluggish kinetics in alkaline medium to the
cells (RHFCs)store andconvert energy upon charge anddischarge additional water dissociation process, which is not found in acidic
operatingaswaterelectrolyzersandFCs,respectively.RHFCsoffer medium. This additional dissociation process represents an extra
theadvantageofthehighestenergydensityperunitmassbytheuse energybarrier.Moreover,theadditionalhydroxideionproductmay
ofhydrogengasasafuel.However,RHFCssufferfromlowcharge/ hinderhydrogenproductionbyoccupyingsomeofthecatalystsites
discharge efficiency. This low efficiency is attributed to many (poisoningeffect).8,9
kineticfactorslinkedtotheredoxreactionsofhydrogenandoxygen
electrodes. Our focus in this study is on the electrolyzer part, H2O +e- H*+OH- (Volmerstep) [1]
especially the cathodic reaction (hydrogen evolution reaction
(HER)) in alkaline medium. Enhanced electrochemical kinetics of H*+H O +e- H +OH- (Heyrovskystep) [2]
2 2
HER will improve the efficiency of the electrolyzer and, therefore,
increase the effectiveness of RHFCs for more efficient energy
2H*H (Tafelstep) [3]
storageandconversionandmorebenefitfromrenewablesources.5,6 2
HERonvarioussurfacematerialsinacidicandalkalinemediais
2H O +2e- H +2OH- (overallreaction) [4]
one of the most investigated processes in electrochemistry. The 2 2
researchers always aimed to understand HER mechanism and
Regarding the process of correlating HER activity in alkaline
establish an electrocatalytic theory that links the properties of medium with catalyst properties, efficient catalysts with maximum
catalysts to their activity or the HER rate, so that the optimum
activity should have near-zero hydrogen adsorption energy, pro-
catalytic materials with the highest efficiency and the lowest
motedwaterdissociationprocess,andpromoteddesorptionofOH*
drawbacks can be proposed. However, a full grasp of the HER
fromthewaterdissociationsites,andhydrogengasproductionsites
mechanism and the fundamentals of the electrocatalytic theory has to prevent poisoning.10,11 Consequently, one should regard the
notbeenreachedyet.Itisbelievedthatacompleteunderstandofthe
balance between the three aspects to design promising catalysts.
nature of HER and the catalytic descriptors will have its profound
HER in alkaline medium includes various adsorption/desorption
processesformultiplespecies;therefore,itisdifficulttouseasingle
chemical entity that matches all the required energies for each
zE-mail:[email protected];[email protected] species to achieve the desired HER activity.12 Moreover, tailoring
Journalof TheElectrochemicalSociety,2021 168054509
and modification of the catalyst properties seem to be hard with the Rafailović et al.45 with slight modification. The deposition bath
useof only onechemicalcomponent. Consequently, heterostructures consistsof0.090MNiSO ,0.053MCoSO ,0.030MFeSO ,0.400M
4 4 4
are proposed as the promising, most active catalysts for HER in H BO ,and0.280MNH Cl.Theelectrodepositionwascarriedouton
3 3 4
alkaline medium. Tailoring the convenient heterostructures can high purity Cu rod (99.99%) by allowing the passage of the same
optimize the adsorption/desorption energies by the synergetic effect amount of coulombs, employing two different values of current, i.e.,
and by tuning the electronic structure.9 Among the most promising 125mA(for5min)and12.5mA(for50min),thustwotypesoffilms
heterostructuresforalkalineHERarethosebasedonoxygenatedmetal areprepared.Alldepositionsolutionswerefreshlypreparedandused
species(oxides,hydroxides,andoxyhydroxides)whichareknownfor for one time to avoid the probable oxidation of ferrous ions.
theirabilitytofacilitatewaterdissociationprocessandimproveOH* Electropassivationprocesswasachievedvia150repetitiveCVcycles
desorptionwhenhybridizedwithproperHadsorbingspecies.13–20The in 1.0M KOH between 1.023V and 1.623V against RHE at a scan
otherenhancementaspects(fastmasstransfer, increased activesites, rate of 200mVs−1. This was done for the film obtained at higher
and durability) can be achieved for heterostructures based on current(125mAfor5min)toproducethethirdfilm.
oxygenatedmetalspeciesthroughthewell-planneddesignofcatalysts.
From various techniques of catalysts preparation, dynamic Characterization.—The morphology and elemental composition
hydrogen bubble template (DHBT) was used herein to fabricate of the as-prepared films were determined using a scanning electron
the proposed catalysts. DHBT is used to design a variety of macro microscope (SEM, JSM-IT100) coupled with an energy dispersive
andmicroporouscatalystfilmswithnanostructuredporeedges,high X-ray spectrometer (EDS) unit. A high-resolution transmission
specificsurfaceareas,andlargenumbersofactivesites.WhileHER electron microscope (HRTEM, JEM-2100) was also used to investi-
is considered a problem accompanying the process of catalysts gate the obtained structures. The prepared films were dissolved in
electrodepositionathighcurrentdensities,theDHBTtechniquegets Conc. HNO , then inductively coupled plasma-optical emission
3
the advantage of this evolution through the use of bubbles as a spectrometer (ICP-OES Optima 2000 DV PerkinElmer) was used to
dynamic template for the growth of particles around them to design confirm the elemental composition. In addition, X-ray diffraction
3D porous catalysts.21 During the use of the DHBT technique, (XRD,BURKER,D8DISCOVER)usingCutarget(λ=1.54Å)was
relatively concentrated solutions of metal salts are used together employed to identify the crystallographic structure. Furthermore,
with chemical species such as H SO , HCl, HNO , NH Cl, and X-ray photoelectron spectroscopy (XPS) using Al Kα radiation was
2 4 3 4
(NH ) SO which act as hydrogen production sources.22 Nickel and employedtoprobethesurfacecompositionandoxidationstates.
42 4
copperporoussurfacesdepositedusingDHBTmethod23–25werethe
firstthoroughlystudiedsurfacesafterwhichmanyothersurfaceshave Results and Discussion
beenstudiedsuchasmonometallicsurfaces:Co,26Ag,27,28Bi,29Pb,30
Pd,31 and Ru,32 bimetallic surfaces: Cu-Au,33 Cu-Pd,34 Cu-Ag,35 Characterization of the designed Ni/Co/Fe films.—The mor-
Cu-Sn,36 Ni-Sn,37 Ni-Co,38 Fe-Ni,39 Ni-Cu,40 Ag-Pd,41 Pd-Ni,42 phologyofthepreparedfilmsisconsideredacrucialfactorfromthe
Au-Pt,43andCu-Pt,44andternarysurfaces:Ni-Co-Fe.45 catalyticactivitypointofviewduetoitseffectonthespecificsurface
area and the accessible active sites. Figure 1 displays SEM micro-
Herein,thealkalineHERactivityisprobedforaternaryalloyof
graphs of the designed films at different magnifications. The film
theirongroupmetals(Ni,Co,andFe)whichisdesignedintheform
of a microporous film via DHBT technique. Electropassivation by depositedatlowercurrent(12.5mAfor50min),seeFigs.1a,1a′,and
1a″, showed substantially larger particle size and lower degree of
repetitivecyclingofthepotentialin1.0MKOHwasappliedtothe
designed film with an aim of developing metal/oxygenated metal porositycomparedtothefilmdepositedathighercurrent(125mAfor
5min),asshowninFigs.1b,1b′,and1b″;therefore,hereinafter,we
interfaces in order to maximize the alkaline HER activity. To the
best of our knowledge, this is the first time for electropassivation will refer to them by the compact film and the porous film,
respectively. Both films have cauliflower-like structures proposing
process to be applied to HER catalysts in order to introduce such
similarityinthegrowthstyle,eventhoughthereisavastdifferencein
interfaces andimprovealkaline HERkinetics.
particle size. The porous film was found to have a pore diameter
Experimental ranging from 3 to 30μm while the compact film has significantly
wider valleys. The remarkable open structure of the porous film
Chemicals.—Analytical grade chemicals (purchased from Merck besides its small particle size gives rise to high specific surface area
andSigmaAldrich)wereusedasreceivedwithoutfurtherpurification. andincreasedactivesitescomparedtothecompactfilm.Inspectionof
the morphology of the porous film after electropassivation, see
Electrodesandelectrochemicalprocesses.—Allelectrochemical Figs. 1c, 1c′, and 1c″, reveals no change in morphology or particle
measurements, i.e., chronopotentiometry (CP), cyclic voltammetry sizecomparedtothenon-passivatedporousfilm.Thisobservationis
(CV), linear sweep voltammetry (LSV), and electrochemical im- alsoaffirmed by HRTEM analysis, which indicates that both porous
pedance spectroscopy (EIS), were performed in a three-electrode filmshavenanoscalebranchesrangingbetween30–100nminsize,as
electrochemical glass cell. A high purity (99.99%) Cu rod (d = shown in Fig. 2. Consequently, it is believed that electropassivation
4.0mm) sealed in a glass tube leaving a planar surface with a doesnotaffectthemorphologyorthenumberofactivesitesaswould
geometricsurfaceareaof0.125cm2,agraphiterod,andasaturated beprovedusingcapacitancemeasurements,cf.Fig.9.
Ag/AgCl/KCl electrode were used as the working, counter, and Moreover,EDSmappinganalysisasdisplayedinFig.3wasused
referenceelectrodes,respectively.Beforeanymeasurement,theCu to determine the elemental composition of the as-prepared films. It
rod working electrode was treated by soaking in 10.0% HNO3 and revealedthatthethreefilmshavethesameatomicratiosofNi,Co,and
washed with distilled water, then mechanically polished by a 2500 Fe,whichare57:24:19,respectively.Additionally,ICP-OESanalysis
grit emery paper and rinsed with distilled water. In this study, all is also performed and confirmed the elemental atomic ratios. The
potentialsarereportedagainstRHEaccordingtoEq.5.Allprocesses similarity between the atomic ratios in the deposition bath and the
werecarriedoutatroomtemperature(25±1°C)usingaBio-Logic atomicratiosintheresultingfilmsindicatesthattheapplieddeposition
potentiostat (modelVSP-300). currentsareinthediffusion-limitedregionwherethecompositionof
deposits is controlled by the bath composition.31,45 Comparing the
E(RHE)=E(Ag/AgCl/KCl)-(-0.197) -(-0.059pH) EDSmappingcoloroftheelectropassivatedporousfilmwiththatof
=E(Ag/AgCl/KCl)+1.023 [5] thenon-passivatedporousfilm,onecannoticethattheelectropassiva-
tion step introduced an evident increment in the oxygen content.
Whereas the atomic ratio of oxygen to the metals was 4:96 for the
Preparation of the ternary catalysts.—Ni/Co/Fe films were non-passivated porous film, it was 6:94 for the electropassivated
prepared via DHBT technique using the method reported by porousone.Thepresenceofoxygeninthenon-passivatedporousfilm
Journalof TheElectrochemicalSociety,2021 168054509
Figure 1. SEM micrographs of (a, a′, and a″) the non-passivated compact film, (b, b′, and b″) the non-passivated porous film, and (c, c′, and c″) the
electropassivatedporousfilmatdifferentmagnifications.
Figure2. HRTEMimagesof(a)thenon-passivatedporousfilm,(b)theelectropassivatedporousfilm.
maybeattributedtotheprobableoxidationbyair.46,47EDSmapping from the BCC crystallographic planes. For all films, the (111) facet
analysis alsoshowedthe homogenousdistributionofNi, Co,andFe oftheFCCstructurewasthepredominantorientationfromwhichthe
throughoutalldesignedfilms. average crystallite size was calculated according to Scherrer’s
Furthermore,XRDanalysiswasconductedtoexaminethecrystal equation,52 Eq. 6. The average crystal size of the compact film was
structureofthedepositedfilms.Figure4displaystheXRDpatternsof found to be 10.8nm, while for the porous non-passivated and
the three prepared films. XRD patterns of the three films showed electropassivated films was 9.5nm. It is worthy of being mentioned
reflectionsfromfourdistinctivecrystallographicplanes:(111),(200), herethattheelectropassivationstepdidnotimpartanychangeinthe
(220),and(311)at2θ;44°,52°,76°,and92°,respectively,which crystalstructure,asseeninFig.4.
arecorrespondingtotheface-centeredcubic(FCC)crystalstructureof
theternaryNi/Co/Fealloyasdescribedelsewhere.48–51Itwasreported t= 0.9l [6]
that the crystal structure of Ni/Co/Fe alloys intrinsically depends on b cosq
the elemental atomic ratios in a way that the body-centered cubic
(BCC) structure vanishes with the decrease in Fe percentage.48 As where t is the average crystal size, λ is X-ray wavelength, θ is the
expectedfromtheratiosobtainedfromEDSandICP-OESmeasure- diffractionangleindegrees,andβisthefullwidthathalfmaximum
ments, the three designed Ni Co Fe films showed no reflections (FWHM) ofthe diffraction peakin radians.
57 24 19
Journalof TheElectrochemicalSociety,2021 168054509
Figure3. SEMmicrographsandEDSmappingcoloranalysisofthenon-passivatedcompact,non-passivatedporous,andelectropassivatedporousfilms.
Figure5. 2ndand150thCVsfortheelectropassivationoftheporousNi/Co/
Figure 4. XRD patterns of the non-passivated compact, non-passivated Fefilmin1.0MKOHatapotentialscanrateof200mVs−1.
porous,andelectropassivatedporousfilms.
cycling led to the increase of oxidation current. It was continued
Effectofelectropassivation.—Theas-preparedporousNi/Co/Fe until a stable constant current is obtained at the 150th cycle, as
film was electrochemically treated in 1.0M KOH, see Fig. 5, by displayed in Fig. 5, and no further metallic sites are accessible to
repetitive cycling of the applied potential between 1.023V and oxidation. This repetitive cycling was performed in order to
1.623V against RHE at a potential scan rate of 200mVs−1. A maximize the metal/oxygenated metal interfaces. The presence of
noticeable pair of redox peaks appeared in this potential window, metallicsitesandthepreventionoffurtheroxidationisevidentfrom
which is assigned to the interconversion between Ni(OH) and the very low percentage of oxygen compared to the sum of metals
2
NiOOH species as addressed elsewhere.53–56 However, Co may percents disclosed by EDX analysis as shown in Fig. 3. It is also
contributetotheoxidationcurrent,asproposedbyIrinaetal.forthe evident fromthe forthcoming XPSspectra,see Fig.7.
Ni-Co oxide film.57 Anyway, this electrochemical treatment, ac-
cordingtoPourbaixdiagramsofthealloymetals:Ni,Co,andFe,58 The alkaline HER activity of the designed Ni/Co/Fe films.—
will result in the formation of a passivating layer consisting of The electrocatalytic activity of the as-prepared films towards alka-
oxygenatedspecies(mixedoxides,hydroxides,andoxyhydroxides) line HER was investigated in 1.0M KOH using linear sweep
of the three metals covering the metallic sites to produce different voltammetry (LSV) at a potential scan rate of 10mVs−1.
metal/oxygenated metal interfaces. The repetition of potential Figure 6a displays the LSVs of the as-prepared films: compact,
Journalof TheElectrochemicalSociety,2021 168054509
Figure6. (a)LSVsforHERontheas-preparedfilmsin1.0MKOHatapotentialscanrateof10mVs−1,(b)Chronopotentiogramsoftheas-preparedfilmsin
1.0MKOHat10mAcm−2,and(c)chronopotentiogramoftheelectropassivatedporousfilmin1.0MKOHat100mAcm−2.
porous,andelectropassivatedporous.Althoughthecompactfilmhas approach the heterojunctions where the O atoms interact with the
approximately twice the loading of the porous film as reported by oxygenated metal sites while H atoms interact with the adjacent
ICP-OES analysis, the polarization curves revealed that the porous metallic sites in a synergistic way that promotes water dissociation.
filmattainsahigheralkalineHERcatalyticactivitycomparedtothe Hydrogen atoms are adsorbed on vacant metallic sites and subse-
compactfilm.ItisobviousthattheporousfilmdeliveredlargerHER quentlyrecombinetobedesorbedashydrogengaswhileOH-anions
currentsalongthewholepolarizationcurvecomparedtothecompact aredesorbedfromthenearbyoxygenatedmetalsitesasproposedby
one.Thisenhancedactivityisnotintrinsicorrelatedtothechemical severalauthors.10,13,15,59–61Thispreferenceinadsorptionisattributed
composition but is due to the increased active sites and higher to the difference in H and OH− adsorption energies on metallic and
specificsurfacearea.Infact,thecompactfilmwasdesignedinorder oxygenated metal sites.12,60 Moreover, surface defects and oxygen
to highlight the effect of the unique morphology attained by the vacancies that may emerge in the oxygenated species upon the
porous film on HER activity. On the other hand, the electropassi- electropassivation treatment could be an additional cause for the
vated porous film showed a substantially improved HER activity observedenhancementofwateradsorptionanddissociation.62–64
along the whole polarization curve compared to the non-passivated In addition, the stability and durability of the designed catalysts
porousone.Interestingly,theelectropassivatedporousfilmdelivered were probed via chronopotentiometry. Figure 6b displays chron-
currents of 10 and 100mAcm−2 at overpotentials of −50 mV and opotentiogramsoftheas-preparedfilmsataconstantcurrentdensity
−220mV,respectively.Thissuperioractivitysignificantlysurpasses of 10mAcm−2 in 1.0M KOH without IR correction. All of the
those of previously designed Ni/Co/Fe alloys and outperforms or prepared films showed remarkable stability for 2h of continuous
approachesthoseassignedtothemostactivereportedcatalysts,even operation especially the electropassivated porous film which per-
thosebasedonnoblemetals,seeTableI.Thissignificantincrement formedwithoutanydecayinthemeasuredpotential.Moreover,the
in catalytic activity is intrinsic and ascribed to the change in electropassivated film showed outstanding durability and marked
chemical composition imparted by the electropassivation step. stability for a prolonged continuous operation of profuse HER for
Inspired by the well-known trend of improving the alkaline HER 10h at a current density of 100mAcm−2. The measured over-
activitybytheintroductionofhybridstructuresofmetal/oxygenated potential,whichamountstoca.−260mV,iseffectivelyunchanged
metalinterfaces firstlyreportedby Subbaramanetal.,13weapplied atsuchahighHERrate,seeFig.6c.Thisobservationreferstothe
theelectropassivationsteptotheas-preparedporousNi/Co/Fefilmin toleranceofthemetal/oxygenatedmetalinterfacesundersuchsevere
ordertodevelopsuchhybridstructureswhichsucceededinenhancing experimentalconditionsandopenstheavenueforindustrialscaling-
the alkaline HER activity. This type of heterojunctions acts as upoftheelectropassivatedporousfilmforapplicationincommercial
bifunctional catalysts in a way that water molecules preferentially electrolyzers.
Journalof TheElectrochemicalSociety,2021 168054509
TableI. AcomparisonbetweenthecatalyticactivitiestowardsHERinalkalinemedium.
Catalyst i(mAcm−2) η(mV) Tafelslope(mVdec−1) Medium References
ElectropassivatedMicroporousNi Co Fe 10 −50 84 1.0MKOH ThisWork
57 24 19
Co Ni Fe 10 −240 100 1.0MNaOH 65
53 33 14
Ni-Co-S/CF 10 −140 96 1.0MKOH 66
NiCo/NiCoO nanostructures 10 −155 35 1.0MKOH 67
x
CoO /Co/NF 10 −90 44 1.0MKOH 68
3 4
Ni/NiOcore/shellnanosheets 10 −145 43 1.0MKOH 69
Ni(OH)/MoS /CC 10 −80 60 1.0MKOH 70
2 2
FeNi-N/CFC 10 −106 115 1.0MKOH 71
FeNi/NiFeO nanohybrids 10 −99 45 1.0MKOH 72
3 x
Fe/CoP/Tifoil 10 −78 75 1.0MKOH 73
Pt/Ni NNSs/Nimesh 10 −50 36.5 1.0MKOH 74
3
Pt Cu Ni /CNF-CF 5 −150 54 1.0MKOH 75
13 73 14
Pt/C/CC 10 −38 68 1.0MKOH 17
Natureandroleofthemetal/oxygenatedmetalinterfaces.—The towardsalkalineHERinthefollowingpoints:1)facilitatesthewater
chemical nature of the electropassivated porous Ni/Co/Fe film was dissociation step and reduces its energy barrier, 2) provides
ex situ examined directly after preparation via the XPS analysis. desorption sites for OH− anions, 3) prevents the poisoning of
Deconvolution of the XPS spectra of the constituent elements; Ni, metallic sites by OH− anions, and 4) prevents further oxidation of
Co, Fe, and O is displayed in Figs. 7a–7d, respectively. Figure 7a themetallicsitesduringelectropassivationandHERmaintainingthe
displays the deconvoluted Ni 2p spectrum which reveals that Ni presence of metal/oxygenated metal interfaces. It is worthy to be
3/2
elementisfoundasmetallicNi,Ni(OH) ,andNiOOHwhileNiOOH mentioned here that oxygenated metal sites themselves without
2
isthemaincomponentasexpectedfromtheelectropassivationstep. hybridizing are considered inactive towards HER due to their
The deconvoluted Co 2p spectrum, see Fig. 7b, disclosed the improper H adsorption energy.12 On the other hand, the metallic
3/2
presenceofCoelementasmetallicCo,Co(OH) ,andCoOOHwhile sitescontributetothewaterdissociationstepandprovidesitesforH
2
CoOOH is the main component. On the other hand, the Fe 2p adsorption and recombination to finally be desorbed in the form of
3/2
spectrum,seeFig.7c,isdeconvolutedintometallicFeandFeOOH hydrogen gas. As a result, the electropassivated porous film with a
while FeOOH is the main component. Along the same lines, the maximizednumberofmetal/oxygenatedmetalinterfacesoperatesin
deconvolutionofO1sspectrum,seeFig.7d,indicatesthepresence a synergistic mechanism and substantially outperforms the activity
of metal–O and metal–O–H species while the hydroxide species, ofthe non-passivated porousfilm.
found in hydroxides and oxyhydroxides, is the main component in
agreementwiththespectraofmetals.Itisnoteworthyherethatthe Acceleration of water dissociation.—Tafel plots are generally
presence of metal/oxygenated metal interfaces in the electropassi-
used to investigate the kinetics of electrochemical reactions like
vatedporousfilmisevidentfromtheXPSspectra.Nevertheless,the
HER. Tafel slopes can provide insights into the rate-determining
percentageofoxygenatedmetalspeciesappearedlargerthanthatof
step (RDS) in HER mechanism. The theoretically calculated Tafel
metallic species due to the small depth examined by the XPS slopesforVolmer,Heyrovsky,andTafelstepsasthe RDSinHER
analysis.62 are 120, 40, and 30mV dec−1, respectively.9 Figure 8a displays
It is worthy of mentioning here that the chemical nature of the
Tafel plots of the designed catalysts in 1.0M KOH at a potential
oxygenated species of the iron group metals; Ni, Co, and Fe was scanrateof0.1mVs−1.InordertodetermineTafelslopes,thelinear
in situ studied in the HER potential region by Subbaraman et al.10 regions ofTafel plotswere fittedtoTafel equation (Eq. 7):
The authors declared ambiguous chemistries that approach these
compositions Ni(OH) , Co(OH) , and FeOOH. In order to investi- ∣h∣ =a +blog∣i∣ [7]
2 2
gatethesechemistries,weexsituexaminedthe XPSspectra ofthe
electropassivated porous Ni/Co/Fe film after the stability test, after wherebisTafelslopewhichequals2.303RT andaisaconstantwhich
HERfor2hat10mAcm−2,seeFigs.7e–7h.ThedeconvolutedNi anF
2p3/2spectrum,seeFig.7e,disclosedthepresenceofNielementas equals 2.3a0n3FRT log∣io∣.
metallicNi,Ni(OH) ,andNiOOHasfoundbeforeHERonthefilm. As shown in Fig. 8a, Tafel slope of the non-passivated porous
2
However,aclearswitchinthemainpeakisobservedreferringtothe filmis137mVdec−1,avaluewhichisapproximatelyequaltothat
Ni(OH) asthemaincomponent.Figure7falsorevealedaswitchin assignedtometallicNisurfacestudiedbyDanilovicetal.59Onthe
2
themainpeakreferringtoCo(OH) asthemainCospecies.Onthe other hand, a lower value, 84mV dec−1, was found for the
2
otherhand,Fig.7gshowedthesameFe2p spectrumwithFeOOH electropassivated porous film. These values refer to Volmer step
3/2
asthemainFespeciesasfoundbeforeHERonthefilm.Alongthe as the RDS. This lowering in Tafel slope is attributed to the
samepattern,thedeconvolutedO1sspectrum,seeFig.7h,unveiled facilitated water dissociation and the ameliorated kinetics on the
a decrease in the percentage of metal-O species, which is corre- electropassivatedporousfilm.
sponding to oxyhydroxides, supporting the spectra of metals. The Furthermore,electrochemicalimpedancespectroscopy(EIS)was
obtaineddataareinagreementwiththoseaddressedbySubbaraman used to further emphasize the reported alkaline HER activity.
et al.10 referring to Ni(OH) , Co(OH) , and FeOOH as the Nyquistplotsofthepreparedfilmsin1.0MKOHatanoverpotential
2 2
predominant oxygenated metal species during HER. Therefore, we of−150mVareshowninFig.8b.Theresultingsemi-circlesreferto
believethatapassivatinglayercomposedofthesechemistriesactsas a reaction that is controlled by charge transfer. Nyquist plots
the oxygenated metal sites that prevent further oxidation of the revealed that the lowest charge transfer resistance is assigned to
metallicsitesduringHER.Theseoxygenatedmetalsitestogetherare theelectropassivatedporousfilm,with7Ωonly,whilethevaluesfor
resistivetoreductionandcansurviveintheHERregionmaintaining the non-passivated porous and the non-passivated compact films
the hybridmetal/oxygenated metal interfaces. were around 16 Ω and 90 Ω, respectively. These data are in
Consequently,wecanoutlinetherolesplayedbythepassivating agreement with the proposed acceleration of the water dissociation
layer (oxygenated metal sites) to enhance the catalytic activity step impartedby the electropassivationtreatment.
Journalof TheElectrochemicalSociety,2021 168054509
Figure7. XPSspectraofNi2p (a)and(e),Co2p (b)and(f),Fe2p (c)and(g),O1s(d)and(h).(a)–(d)XPSspectraoftheelectropassivatedporousfilm
directlyafterpreparation,(e)–(h3)/2XPSspectraofthe3/2electropassivatedp3o/2rousfilmafterHERfor2hat10mAcm−2.
Journalof TheElectrochemicalSociety,2021 168054509
Figure8. (a)Tafelplotsofthenon-passivatedcompact,non-passivatedporous,andelectropassivatedporousfilmsin1.0MKOHatapotentialscanrateof
0.1mVs−1.(b)Nyquistplotsofthenon-passivatedcompact,non-passivatedporous,andelectropassivatedporousfilmsin1.0MKOHatanoverpotentialof
−150mV.
Nature of enhancement imparted by electropassivation.—It is andattributedtochemicalcomposition,nottothespecificsurfacearea
well-knownthatdouble-layercapacitance(C )isdirectlyproportional or the number of active sites. CV measurements at different potential
dl
to the electrochemical surface area (ECSA).76,77 Consequently, C scanratesin1.0MKOHinapotentialregion,wherenofaradiccurrent
dl
measurements were conducted to confirm that the improved alkaline isobserved,werecarriedouttoestimateC ofthenon-passivatedand
dl
HER activity imparted by the electropassivation treatment is intrinsic the electropassivated porous films, as shown in Figs. 9a and 9b,
Figure9. CVsof(a)thenon-passivatedporousfilmand(b)theelectropassivatedporousfilmin1.0MKOHatdifferentpotentialscanrates(60:160mVs−1).
Thevariationofcapacitivecurrentdensitieswithpotentialscanratefor(a′)thenon-passivatedporousfilmand(b′)theelectropassivatedporousfilm.
Journalof TheElectrochemicalSociety,2021 168054509
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