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Electrodeposited NiFeCo and NiFeCoP alloy cathodes for hydrogen evolution reaction in alkaline medium
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 2 7 6 2 e1 2 7 7 1
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Electrodeposited NiFeCo and NiFeCoP alloy cathodes for hydrogen evolution reaction in alkaline medium V. Bachvarov a,*, E. Lefterova b, R. Rashkov a a
Institute of Physical Chemistry, Bulgarian Academy of Sciences, 11 Acad. G. Bonchev Str., 1113 Sofia, Bulgaria Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, 10 Acad. G. Bonchev Str., 1113 Sofia, Bulgaria
b
article info
abstract
Article history:
The present paper reports on the electrodeposition of crystalline NiFeCo and amorphous
Received 23 February 2016
NiFeCoP alloys at various conditions of the electrolysis process (different current densities)
Received in revised form
leading to differences in component composition. Alloys are characterized by SEM, XRF,
17 May 2016
EDX, XRD and XPS methods. The electrocatalytic activity of the materials for hydrogen
Accepted 18 May 2016
evolution reaction (HER) is studied in a 6 M KOH solution using polarization curves and
Available online 8 June 2016
electrochemical impedance spectroscopy (EIS). Presence of iron and cobalt in nickel alloys decreases significantly overvoltage of hydrogen evolution and the charge transfer resis-
Keywords:
tance is about 1000 times smaller compared to pure nickel. Alloy amorphization by in-
Hydrogen evolution reaction
clusion of phosphorus enhances additionally electroactivity of these materials for the HER.
Electrocatalytic activity
Results from both electrochemical methods indicate that the best electrocatalytic behavior
Electrodeposition
is demonstrated by amorphous alloys having high iron content. The latter is associated
Amorphous NiFeCoP alloys
with a more developed surface of these materials and possible synergy among Ni, Fe and
Impedance spectroscopy
Co. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen as a conventional fuel alternative is attracting an exceptional amount of interest. One of the hydrogen production methods is water solution electrolysis. Nickel proves to be a feasible pretender to replace noble metals as catalysts for HER, exhibiting very promising physical, mechanical and corrosion characteristics. A possible way to enhance nickel matrix catalytic properties is to shape electrodes so as to provide for a highly developed surface. Some authors have obtained nickel-based nanocomposite coatings with carbon [1], graphene [2], TiOx [3]
and molybdenum [4,5]. They have come to the conclusion that increasing surface roughness renders an enhanced catalytic has reported [6] that activity for HER of such electrodes. Jovic along with an enhanced electrochemically active area on Ni(Ebonex/Ir) electrode surface, such electrodes exhibit also a higher internal catalytic activity for the HER in comparison with Ni due to the presence of Ir active sites on the surface. Several authors have established that binary alloys of nickel with Co [7e11], Mo [12e15], Fe [11,14,16], W [14,17], Sn [18] have an improved electrocatalytic activity for the HER compared to pure nickel due to both a more developed surface and a synergic electrocatalytic effect. When left hand side
* Corresponding author. E-mail address: [email protected] (V. Bachvarov). http://dx.doi.org/10.1016/j.ijhydene.2016.05.164 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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elements of the transition group having vacant d-orbitals (hypo-d-electronmetals) are combined with metals of the right hand side of the same group having complete internally coupled d-orbitals (hyper-d-electronmetals), the intermetallic compounds obtained undergo electron interactions and binding energy change leading to an increased catalyst activity [19,20]. Studying the HER in triple systems such as Ni-Co-Mo [21] and NieSeMn [22], some authors have established that in alkaline water medium electrodeposited tri-component materials are more active than the respective two-component systems. According to Lu et al. [23], tri-component NiMoP exhibit catalytic properties comparable to those of platinum foils. The observed high electrocatalytic activity for the HER is attributed to the hydrogen absorption ability of such electrodes. One of the most promising hydrogen reduction electrocatalysts are based on the elements of the iron group e iron, nickel and cobalt [24,25]. Amorphous materials have attracted substantial research interest due to their exceptional properties for the HER [25]. Some authors have claimed that catalytic properties augmentation in NiP layers is caused by crystal size reduction and hydrogen absorption in the alloys thus altering electron structure and improving electrocatalytic activity of the obtained material [26e29]. According to De Giz et al. [30], amorphous NieFeeP alloys have great potential as an electrode material for the HER since electrocatalytic activity exhibited in alkaline water electrolysis is sufficiently high. However, the increased electrocatalytic activity requires the amorphization coupling with an adequate material composition. A decisive role in the course of the hydrogen evolution reaction is played by the association of Ni with Fe and Р. Studies of Kreysa and Ha˚kansson [31] have indicated that industrially manufactured multicomponent amorphous materials based on Fe, Co, Ni, Si and B show lower polarization for the HER than Pt and Ni. As established in our previous works, multicomponent iron group-based alloys containing P [32] island structures and W and P [33] thick films are better catalysts for the HER than pure nickel and the respective binary or ternary alloys. All of the above gives grounds to assume that nickel bonding with iron, cobalt and phosphorous will result in an improved alloy electron structure and reduced crystal size, i.e., enlarged catalytic surface. It is reasonable to expect that optimizing the effect of these two factors will substantially diminish polarization for the hydrogen evolution reaction. The purpose of the present work is to obtain and physically characterize of crystalline NiFeCo and amorphous NiFeCoP alloy materials, using various current densities. Further to that, via galvanostatic polarization dependences and electrochemical impedance spectroscopy to study electrocatalytic activity of such alloys in view of their possible practical application as cathode material for the HER.
Experimental
to prior polishing with a disk-pulley using a chromium paste. This procedure secured plate roughness of below 0.6 mm. The polished samples were cleaned ultrasonically in acetone and ethanol, and then rinsed with distilled water. Copper plates were degreased and etched in a 1:1 HCl solution. Two electrolytes, I and II, were used for obtaining NiFeCo and NiFeCoP alloys, respectively. Electrolyte I contained NiSO4.7H2O e 40 g L1; NiCl2.6H2O e 40 g L1; CoSO4.5H2O e 3.2 g L1; FeSO4.7H2O e 8 g L1; glycine ndash; 24 g L1; b-alanine e 24 g L1. Electrolyte II contained: NiSO4.7H2O e 40 g L1; NiCl2.6H2O- 40 g L1; CoSO4.5H2O e 5 g L1; FeSO4.7H2O e 30 g L1; glycine e 26 g/L; b-alaninee26 g L1; H3PO2 50% acid and NaH2PO2.H2O with a total phosphorous content of 4.4 g L1 . Nickel anodes were used for alloy electrodepositing at a current density of 3, 5 and 10 А dm2 and a temperature of 50 C. For the purposes of comparison, a layer of pure nickel was deposited from a sulphamate electrolyte containing 80 g L1 Ni2þ, 30 g L1 H3BO3 and 10 g L1 NiCl2. The current density was 4.5 А dm2.
Physical characterization Depending on the electrolysis conditions, the content of nickel, iron and cobalt in the NiFeCo deposits was determined by X-ray fluorescence analysis (XRF ndash; Fischerscope X-RAY XDAL) in 9 points (three points in the bottom, middle and top of the sample, respectively). Scanning electron microscopy (SEM) using Leo 1455VP and Leo Supra 55VP microscopes with energy dispersion X-ray (EDX, Oxford Inca 200 instrument, Software INCA-Vers.4) was applied to characterize surface morphology of NiFeCo and NiFeCoP deposits and perform elemental analysis of NiFeCoP layers. Diffractograms of the samples were recorded on Philips PW and Empyrean ndash; PANalytical using CuKa1 radiation. Phase identification, precise determination of quantitative ratios of phases present and crystal size were done using a software package High Score Plus with ICSD data base. The surface chemical composition and valence states of the elements of deposited films were determined by X-ray photoelectron spectroscopy (XPS). Measurements were carried out on AXIS Supraelectron-spectrometer (Kratos Analitycal Ltd.) using monochromatic Al Ka radiation with photon energy of 1486.6 eV. The high-resolution spectra were calibrated towards the C 1 s line at 284.8 eV. Casa XPS software package [34] was used for peak analysis. Shirley-type background, Gaussian/Lorentzian line shapes as well as respective asymmetric forms were applied in the fitting procedure.
Table 1 e Composition of NiFeCo and NiFeCoP alloys. Current density
Electrochemical deposition NiFeCo and NiFeCoP alloys were electrodeposited on copper plates of 2 and 4 cm2 surface area. The plates were subjected
3 A dm2 5 A dm2 10 A dm2
From electrolyte I
From electrolyte II
Ni wt %
Co wt %
Fe wt %
Ni wt %
Co wt %
Fe wt %
P wt %
53 51 53
16 15 14
31 34 33
76 67 56
9 13 13
2 9 21
13 11 10
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Electrochemical characterization Steady-state polarization curves and EIS measurements were used to study the electrocatalytic activity of electrodes for the HER at room temperature. The working electrodes were cathodic polarized at 100 mA cm2 for 30 min in 6 М KOH to provide the steady-state conditions for the polarization measurements. Polarization dependences were obtained galvanostatically by a step-wise cathodic current decrease from 100 mA cm2 to 2.5 mA cm2. For each current value, the
potential was measured after 2e5 min. The procedure was repeated for any sample until reproducible results were obtained. Potential value was corrected with the ohmic drop by the electrolyte resistance obtained from EIS measurements. The values of overvoltage are calculated with respect to the reversible HER potential at the given conditions (Erev ¼ 1.114 V versus SCE in 6 M KOH). The impedance measurements were conducted on Gamry Instruments potentiostat/galvanostat using an electrochemical threeelectrode cell equipped with a Luggin capillary to the
Fig. 1 e SEM images of electrodeposited alloys: NiFeCo at 3 A dm¡2 e a), e); NiFeCoP e 3 A dm¡2 e b), f); 5 A dm¡2 e c), g); 10 A dm¡2 e d), h).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 2 7 6 2 e1 2 7 7 1
Fig. 2 e XRD spectra of NiFeCoP electrodeposited: (a) at 3 A dm¡2, (b) at 5 A dm¡2 and (c) at 10 A dm¡2 and NiFeCo e (d).
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significantly on current density. Coatings formed at current density higher than 3 A dm2 were cracked and adhered poorly to the substrate. Most probably this is due to high internal tensions associated with hydrogen inclusion occurring at increased current densities. For this reason only the results obtained for NiFeCo system electrodeposited at a current density of 3 A dm2 will be subject to further discussions below. Quantities of incorporated phosphorus in alloys from electrolyte II decreased with the increase in cathodic current density. The same dependence is observed with respect to deposited nickel. However, with the rise of current density, the amount of iron in the layer increased substantially. Similar dependence has been observed by Sridharan et al. [35] for FeNiP alloys. As for codeposited cobalt, its quantity climbed up negligibly with current density rise up to 5 A dm2, and remained unchanged with any further current density rise. Fig. 1 shows SEM images of electrodeposited alloys. A typical globular morphology of nickel alloys is observed at a magnification of 5000х. Coatings deposited in the presence of phosphorous (Fig. 1b, c, d) are cracked. At high magnification (100 000х), a spindle-shaped surface structure is observed in NiFeCo systems (Fig. 1е). An increase in current density applied to produce NiFeCoP alloys leads to particle size reduction (Fig. 1f, g, h).
X-ray diffraction analysis
Fig. 3 e XRD spectra of annealed NiFeCoP layers electrodeposited: (a) at 3 A dm¡2, (b) at 5 A dm¡2 and (c) at 10 A dm¡2. working electrode. In both electrochemical techniques a saturated calomel electrode (SCE) was used as a reference electrode, and Pt plate of 20 cm2 e as counter electrode. EIS 300 software at 1.25 V (vs. SCE) in a frequency range of 100 kHz to 10 mHz with ten point per decade was used for EIS measurements. Sine signal 10 mV peak to peak was applied. The Zview 3.2 software package was used for equivalent circuit modeling and to analyze and data fit.
Results and discussion Obtaining NiFeCo and NiFeCoP alloys NiFeCo and NiFeCoP alloys were obtained electrochemically from electrolytes I and II at various current densities. Alloy element composition (as identified with XRF and EDX) is presented in Table 1. The results indicated that coating composition of alloys obtained from electrolyte I did not depend
XRD analysis indicated that NiFeCoP coatings deposited at all three current densities are amorphous and only the peaks of the Cu substrate are observed on the XRD patterns (Fig. 2 a,b,c). The NiFeCo samples are crystalline (Fig. 2d). Two broaden peaks are registered. The first peak at 2Q ¼ 41.8 is related with hexagonal crystal structure hcp of CoxM1-x (M ¼ Ni and/or Fe). The second peak at 2Q ¼ 44.98 corresponds to (111) reflection of cubic structure of Ni3Fe (ICDD/ JPCPD PDF#65-3244) as well as of fcc structure of solid solutions of NieFeeCo alloys. The crystallite size of hexagonal and cubic phases is 6 nm and 4 nm, respectively. The absence of other diffraction peaks (only a weak peak at 75.6 responsible for both phases) means that the layers are textured with (100) and (111) preferential orientation of the hexagonal and cubic phases, respectively. It is in correlation with the specific morphology of this film, as seen from SEM. Since X-ray diffraction cannot serve for the identification of NiFeCoP layers due to layers' amorphous structure, phase composition of amorphous coatings was determined by an inert medium of nitrogen (30 L h1) annealing of coatings at 550 C for 3 h. Crystallization temperature was detected by Perkin Elmer Diamond TG/DTA analysis. XRD spectra of annealed samples are given in Fig. 3. The Ni3P and FeNi2P (ICDD/JPCPD PDF#65-1605 PDF#51-1367) phases are identified on the annealed NiFeCoP layers. Hentschel et al. [36] have also registered Ni3P phase following annealing of NiPх films at a temperature of 417 C. The authors stipulate that amorphous phase phosphorus is concentrated at the borders around Ni nanocrystals. As seen by the relationship in Fig. 3, an increase of the amount of iron, suppressed Ni3P formation, and a FeNi2P compound of the same structure crystallized at its place, with a portion of nickel being substituted by iron (Fig. 3c). This trend was monitored by the change in the
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Fig. 4 e XPS spectra of NiFeCoP alloys electrodeposited at 3, 5 and 10 A dm¡2: a) Ni2p3/2; b) Co2p3/2; c) P2p3/2; d) О1 s.
parameters of elementary cells. The analysis also indicated that with a comparable cobalt quantity in all three samples, presence of iron caused crystallization of different Co-rich phases ndash; fcc at higher Fe content or hcp at low Fe content.
XPS analysis XPS method was used to characterize the valence state of elements and possible compounds present in the amorphous
NiFeCoP alloys. For instance, the positions of P2p3/2 could be an indication whether a phosphide and/or a phosphate are formed. Binding energy (BE) of pure amorphous phosphorus P2p3/2 is 130.2 еV, while of that of metal phosphides was below 130 eV [34]. Binding energies over 132 eV are an indication of phosphate formation. Fig. 4 depicts the high resolution photoelectron spectra of Ni2p3/2 (Fig. 4a), Co2p3/2 (Fig. 4b) and P2p3/2 (Fig. 4c). Fe2p is not shown since Fe2p3/2 line overlaps with the Co&Ni LМM Auger lines at Al radiation and the very small quantity is registered on the surface. The spectra were
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Fig. 5 e Polarization curves of Ni e curve 1, NiFeCo e curve 2 and NiFeCoP electrodeposited: at 3 A dm¡2 e curve 3; at 5 A dm¡2 e curve 4; at 10 A dm¡2 e curve 5 in 6 M KOH.
Table 2 e Parameters obtained by the analysis of the polarization curves presented in Fig. 5. b [mV dec1]
Sample
h10 mA cm2 h100 mA cm2
I II III Range range range Ni NiFeCo-3A dm2 NiFeCoP-3A dm2 NiFeCoP-5A dm2 NiFeCoP-10A dm2
mV
mV
76 50
108 63
165 106
378 120
512 196
30
51
95
87
155
30
42
75
83
139
28
42
73
78
135
analyzed and decomposed to the respective components on the basis of NIST [34] and [37e43]. BE of the first components of Ni2p3/2 (852.7 eV), Co2p3/2 (777.9) corresponds to the metal phases (M0) and phosphides [44,45]. Peak and satellite structure analysis show presence of oxides of the second valence, in smaller quantity for nickel and higher e for cobalt. BEP2p3/ 2 ¼ 129.3 eV, determined from the first phosphorus doublet, is the evidence for MxP (M ¼ Ni, Co) formation. The large peak observed at higher than 132 eV in the P2p spectrum is attributed to presence of phosphates while the hint of a peak at about 130.3 eV is an indication for a small quantity of phosphorus bonded to neither the metal nor oxygen (pure phosphorous). The oxygen peak (Fig. 4d) is decomposed to 4 components indicating various oxygen bonds (O1 530 eV e МО (O2), O2 ¼ 531.5 eV e OH, MeOeP, O3 z532.5 eV C]O, H2O, O4 > 533 eV e P]O). The results from XPS analysis confirm XRD data of annealed samples, i.e., existence of MxP (M ¼ Ni, Co).
Polarization measurements A criterion for the catalytic activity of a material regarding hydrogen evolution reaction could be the overvoltage of this
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reaction at a given current density. Comparison of various electrode materials points to the fact, that at one and the same current density the lower overvoltage for the HER is associated with higher catalytic activity. Polarization curves (Fig. 5) show that at all studied current densities the highest overvoltage for the HER is exhibited by the nickel electrode (curve 1). The parameters obtained by the analysis of the polarization curves on Fig. 5 are presented in Table 2. Tafel slopes b for Ni in all ranges show the highest values. This indicates the worst electrocatalytic activity for the pure nickel. Nickel codeposition with Fe and Co (NiFeCo alloy) decreases overvoltage of the hydrogen reaction dramatically which corresponds to the increase of catalytic properties. For instance, at cathodic current density at 100 mA cm2 (log i ¼ 1), this reduction is by 316 mV (Fig. 5, curve 2) compared to pure nickel (curve1and Table 2). Similar dependence has been detected by Solmaz et al. [16]. They claim that hydrogen evolution activity grows with increasing of iron content in NiFe alloys. They attribute this fact to the synergy between Ni and Fe. Even small quantities of Fe in NiFeP alloys (1 wt. % Fe) reduce HER overvoltage by approximately 200 mV, compared to mild steel and Ni [30]. Alloying Ni with Со leads also to increase electrocatalytic activity for the HER compared to pure Ni. This is due to improved internal activity of the material, attributed to the synergy between catalytic properties of Ni (low hydrogen overvoltage) and Co (high hydrogen absorption) [7]. As shown in Fig. 5, NiFeCoP alloys exhibit the best catalytic activity for the HER (curves 3, 4, 5). Irrespective of the differences in component composition (Table 1), however, they demonstrate very close catalytic behavior. For instance, alloys obtained at 10 A dm2, contained 7 times more iron than the ones obtained at 3 A dm2, and the maximum overvoltage change at cathodic current density of 100 mA cm2 (log i ¼ 1) is only 20 mV (Fig. 5 curves 3, 5 and Table 2). Obviously, the better catalytic activity of these alloys compared to a NiFeCo alloy is due to phosphorus codeposition which on its part leads to amorphization of the latter (Fig. 2), i.e. to a more developed surface. The results presented above of XRD and XPS analysis confirm that the amorphous coatings contain MxP (M ¼ Ni, Co) and FeNi2P compounds. They could be another reason for the best electrocatalytic properties of NiFeCoP systems. Paseka et al. [26] have established that the high activity of MeeP(x) (Me ¼ Ni, Co and FeeNi) electrodes is caused by dissolved hydrogen in the amorphous layer in the course of the electrodeposition. The high activity for the HER of the layer NieP electrodes prepared at T 53 C was caused by the internal stress in the layer. The stress originated during the electrodeposition of the NieP layer by co-deposited and absorbed hydrogen [27]. A correlation between the activity of the NiPx electrodes, and the amount of absorbed hydrogen was found by Burchardt [28] as the most active electrodes also absorbed the largest amount of hydrogen. The activity increases with increasing absorption. The catalytic activity is related to variations in the energy of adsorption of hydrogen. Burchardt proposed that the activity is related to the amorphous phase, consisting of Ni and P. The absorption of hydrogen in the amorphous phase probably changes the electronic structure of the metal, thereby increasing the catalytic activity [28,29].
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Fig. 6 e Nyquist plots for the HER at Ni, NiFeCo and NiFeCoP alloys in 6 M KOH.
Electrochemical impedance spectroscopy measurements Impedance studies provide additional information about ongoing processes on catalytic layers. Fig. 6 presents Nyquist plots of EIS measurements for various layers recorded at the same potential of 1.25 V. A substantial difference in the impedance spectra of pure nickel (Fig. 6a and e) and of multicomponent layers (Fig. 6b) is observed. One semicircle corresponding to one time constant is registered on spectra of Ni layer. The EIS of multicomponent NiFeCo and NiFeCoP
electrodes are more complicated. Three overlapped depressed semicircles (Fig. 6c) can be distinguished and connected to three different processes with three different time-constants. The high frequencies ac response (Fig. 6d) is independent on potential change and hence it is connected to electrode surface porosity [12,24]. The rest two are related to Faraday's reaction of hydrogen evolution and adsorption, respectively [9,12,14,18,24,46]. Impedance modeling and fitting are applied using appropriate equivalent circuits. A three timeconstant equivalent scheme reflecting the above mentioned
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reduces particle size, as illustrated also by SEM images (Fig. 1), thus leading to a more developed surface. Results of the impedance measurements correlate very well with those of the polarization curves (Fig. 5). They show big differences in Rct values between Ni and multicomponent NiFeCo and NiFeCoP layers. NiFeCoP layers obtained at various current densities have close impedance spectra and close overvoltages, differing by not more than 5e15 mV at high current densities. The low Rct value together with high CPE, respective high Cdl (i.e. high surface) results in high catalytic activity of NiFeCoP layer for the HER.
Fig. 7 e Schematic representation of the electrical equivalent circuit diagram.
processes is given in Fig. 7(T3). The electrolyte resistance Rel is in series with the high-frequency response (Rp-CPEp), while the elements associated with the kinetics of hydrogen evolution (Rct-CPEdl) and adsorption (Rad-CPEps) are presented with a modified Amstrong's element [47,48], were Rct is charge transfer resistance, Rad is resistance due to adsorbed hydrogen, CPEdl and CPEps are constant phase elements (CPE) used instead of double layer capacitance Cdl and pseudo capacitance Cps respectively. CPE is defined by the formula: a
Z ¼ 1=½TðiuÞ
(1)
where T is a constant representing CPE capacitive part, u is angular frequency (in rad/sec), i2 ¼ 1 is an imaginary unit and a is an exponent. CPE is used to account for the deviation from the ideal capacity caused by the non-homogenous nature of the catalytic layer, roughness, impurities, etc. This deviation is reflected in the exponent a, and if a ¼ 1, Equation (1) is equivalent to the one for capacity determination. When CPE is included in parallel to resistance, gives depressed semicircle on Nyquist plot [48]. A single time-constant model (Fig. 7) (T1) ¼ Rel(Rct-CPEdl) is used to modeling Ni impedance. Results of the fitting procedure are given in Table 3. There is a big difference in the parameters determined for HER in pure Ni and multicomponent catalysts. CPE is by three orders of magnitude lower while Rct is by three orders of magnitude higher. Lower CPEdl value indicate also smaller active catalytic surface while the higher Rct means lower catalytic activity. The HER parameters follow the dependences RctNi >> RctNiFeCo > RctNiFeCoP. Fig. 6 plots the impedance comparison of NiFeCoP catalysts deposited at various current densities. Differences are small and follow the order of Rctð3Adm2 Þ >Rctð5Adm2 Þ Rctð10Adm2 Þ and CPEð3Adm2 Þ
Table 3 e Parameters obtained by fitting EIS experimental spectra. Sample
Rct [U cm2]
a
CPEdl [F sa1 cm2]
Ni 2348 ± 38 0.000227 ± 0.00003 2.19 ± 0.019 0.179 ± 0.004 NiFeCo-3A dm2 NiFeCoP-3A dm2 0.372 ± 0.007 0.074 ± 0.003 0.143 ± 0.005 NiFeCoP-5A dm2 0.301 ± 0.008 0.442 ± 0.023 NiFeCoP-10A dm2 0.316 ± 0.014
0.87 0.94 0.88 0.88 0.85
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± 0.002 ± 0.002 ± 0.01 ± 0.01 ± 0.02
Conclusions Crystalline NiFeCo and amorphous NiFeCoP alloy material were obtained electrochemically at various current densities. The catalytic properties for the HER of these systems in 6 М KOH were studied by means of galvanostatic polarization dependences and electrochemical impedance spectroscopy. The results indicated a dramatic increase in electrochemical activity for the HER (reduction in the overvoltage of hydrogen reaction by over 300 mV and decrease in Rct values by 3 orders of magnitude) in NiFeCo and NiFeCoP systems compared to pure nickel. The behavior observed could be attributed to the synergy among Ni, Fe and Co, resulting in enhanced internal catalytic activity of the material. On the other hand, phosphorus presence in NiFeCoP alloys reduced particle size causing alloy amorphization which led to a more developed surface, i.e. higher catalytic activity for the HER compared to crystalline NiFeCo alloy. Amorphous alloys deposited at current densities of 5 and 10 A dm2 exhibited the best catalytic behavior due to the presence of both MxP (M ¼ Ni, Co) and FeNi2P phases. The very good catalytic properties of amorphous NiFeCoP systems established call for further studies in view of their practical application as an electrode material in electrolysers with a polymer anion-conducting membrane.
Acknowledgments The authors would like to thank the National Science Fund of Bulgaria for the financial support of this work through contract DFNI E 02/9.
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[4] Abdel-Karima R, Halim J, El-Raghy S, Nabil M, Waheed A. Surface morphology and electrochemical characterization of electrodeposited NieMo nanocomposites as cathodes for hydrogen evolution. J Alloys Compd 2012;530:85e90. [5] Domınguez-Crespo M, Plata-Torres M, Torres-Huerta A, Arce-Estrada E, Hallen-Lopez J. Kinetic study of hydrogen evolution reaction on Ni30 Mo70, Co30Mo70, Co30Ni70 and Co10Ni20Mo70 alloy electrodes. Mater Charact 2005;55:83e91. B, Jovic V, Lac njevac U, Gajic -Krstajic L, Krstajic N. Ni[6] Jovic (Ebonex-supported Ir) composite coatings as electrocatalysts for alkaline water electrolysis. Int J Hydrogen Energy 2015;40:10480e90. [7] Herraiz-Cardona I, Ortega E, Anton J, Perez-Herranz V. Assessment of the roughness factor effect and the intrinsic catalytic activity for hydrogen evolution reaction on Nibased electrodeposits. Int J Hydrogen Energy 2011;36:9428e38. [8] Ali D, Karcı I, Kardas‚ G. Effect of C-felt supported Ni, Co and NiCo catalysts to produce hydrogen. Int J Hydrogen Energy 2012;37:9470e6. rez-Herranz V. Impedance [9] Herraiz-Cardona I, Ortega E, Pe study of hydrogen evolution on Ni/Zn and NieCo/Zn stainless steel based electrodeposits. Electrochim Acta 2011;56:1308e15. [10] Lupi C, Dell'Era A, Pasquali M. Nickel-cobalt electrodeposited alloys for hydrogen evolution in alkaline media. Int J Hydrogen Energy 2009;34:2101e6. [11] Suffredini H, Cerne J, Crnkovic F, Machado S, Avaca L. Recent developments in electrode materials for water electrolysis. Int J Hydrogen Energy 2000;25:415e23. [12] Martinez S, Metikos-Hukovic M, Valek L. Electrocatalytic properties of electrodeposited Nie15Mo cathodes for the HER in acid solutions. Synergistic electronic effect. J Mol Catal A Chem 2006;245:114e21. [13] Krstajic N, Jovic V, Gajic-Krstajic Lj, Jovic B, Antozzi A, Martelli G. Electrodeposition of NieMo alloy coatings and their characterization as cathodes for hydrogen evolution in sodium hydroxide solution. Int J Hydrogen Energy 2008;33:3676e87. [14] Navarro-Flores E, Chong Z, Omanovic S. Characterization of Ni, NiMo, NiW and NiFe electroactive coatings as electrocatalysts for hydrogen evolution in an acidic medium. J Mol Catal A Chem 2005;226:179e97. J, Vojnovic M, Krstajic N. Kinetic analysis of hydrogen [15] Jak sic evolution at Ni-Mo alloy electrodes. Electrochim Acta 2000;45:4151e8. [16] Solmaz R, Kardas‚ G. Electrochemical deposition and characterization of Ni-Fe coatings as electrocatalytic materials for alkaline water electrolysis. Electrochim Acta 2009;54:3726e34. [17] Fan C, Piron D, Sleb A, Paradis P. Study of electrodeposited nickelemolybdenum, nickeletungsten, cobaltemolybdenum and cobaltetungsten as hydrogen electrodes in alkaline water electrolysis. J Electrochem Soc 1994;141:382e7. njevac U, Krstajic N, Jovic V. Service life test of the [18] Jovic B, Lac NiSn coatings as cathodes for hydrogen evolution in industrial chlor-alkali electrolysis. Int J Hydrogen Energy 2014;39:8947e58. М. Interionic nature of synergism in catalysis and [19] Jak sic electrocatalysis. Solid State Ionics 2000;136e137:733e46. М. Hypo-hyper-d-electronic interactive nature of [20] Jak sic interionic synergism in catalysis for hydrogen reaction. Int J Hydrogen Energy 2001;26:559e78. [21] Fan C, Piron D, Paradis P. Hydrogen evolution on electrodeposited nickelecobaltemolybdenum in alkaline water electrolysis. Electrochim Acta 1994;39:2715e22.
[22] Shan Z, Liu Y, Chen Z, Warrender G, Tian J. Amorphous NieSeMn alloy as hydrogen evolution reaction cathode in alkaline medium. Int J Hydrogen Energy 2008;33:28e33. [23] Lu G, Evans P, Zangari G. Electrocatalytic Properties of Nibased alloys toward hydrogen evolution reaction in acid media. J Electrochem Soc 2003;150:551e7. [24] Jafarian M, Azizi O, Gobal F, Mahjani M. Kinetics and electrocatalytic behavior of nanocrystalline CoNiFe alloy in hydrogen evolution reaction. Int J Hydrogen Energy 2007;32:1686e93. [25] Safizadeh F, Ghali E, Houlachi G. Electrocatalysis developments for hydrogen evolution reaction in alkaline solutions e a review. Int J Hydrogen Energy 2015;40:256e74. [26] Paseka I, Velicka J. Hydrogen evolution and hydrogen sorption on amorphous smooth MeeP(x) (Me ¼ Ni, Co and FeeNi) electrodes. Electrochim Acta 1997;42:237e42. [27] Paseka I. Hydrogen evolution reaction on NieP alloys: the internal stress and the activities of electrodes. Electrochim Acta 2008;53:4537e43. [28] Burchardt T. The hydrogen evolution reaction on NiPx alloys. Int J Hydrogen Energy 2000;25:627e34. [29] Burchardt T, Hansen V, Va˚land T. Microstructure and catalytic activity towards the hydrogen evolution reaction of electrodeposited NiPx alloys. Electrochim Acta 2001;46:2761e6. [30] De Giz M, Tremiliosi-Filho G, Gonzalez E, Srinivasan S, Appleby A. The Hydrogen evolution reaction on amorphous nickel and cobalt alloys. Int J Hydrogen Energy 1995;20:423e7. [31] Kreysa G, Ha˚kansson B. Electrocatalysis by amorphous metals of hydrogen and oxygen evolution in alkaline solution. J Electroanal Chem Interfacial Electrochem 1986;201:61e83. [32] Mitov M, Chorbadzhiyska E, Rashkov R, Hubenova Y. Novel nano structured electrocatalysts for hydrogen evolution reaction in neutral and weak acidic solutions. Int J Hydrogen Energy 2012;37:16522e6. [33] Bachvarov V, Arnaudova M, Rashkov R, Zielonka A. Electrochemical deposition of alloys based on NieFeeCo, containing W, P and their characterization for hydrogen evolution reaction. Bul Chem Com 2011;43:115e9. [34] Casa Softaware Ltd., CasaXPS version 2.3.15, http://www. casaxps.com/. [35] Sridharan K, Sheppard K. Electrochemical characterization of FeeNieP alloy electrodeposition. J Appl Electrochem 1997;27:1198e206. [36] Th Hentschel, Isheim D, Kirchheim R, Mu¨ller F, Kreye H. Nanocrystalline Nie3.6 at.% P and its transformation sequence studied by atom-probe field-ion microscopy. Acta Mater 2000;48:933e41. [37] NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, Version 4.1, Data compiled and evaluated by Alexander V. Naumkin, Anna Kraut-Vass, Stephen W. Gaarenstroom, and Cedric J. Powell, ©2012 copyright by the U.S. Secretary of Commerce on behalf of the United States of America, Distributed by the Measurement Services Division of the National Institute of Standards and Technology (NIST) Material Measurement Laboratory (MML). web version: http://srdata.nist.gov/xps/. [38] Kim J-G, Pugmire DL, Battaglia D, Langell MA. Analysis of the NiCo2O4 spinel surface with Auger and X-ray photoelectron spectroscopy. Appl Surf Sci 2000;165:70e84. [39] Briggs D, Gibson VA. Direct observation of multiplet splitting in 2P photoelectron peaks of cobalt complexes. Chem Phys Lett 1974;25:493e6. [40] Paunovic P, Stoevska Gogovska D, Popovski O, Stoyanova A, Slavcheva E, Lefterova E, et al. Preparation and characterization of Co-Ru/TiO2/MWCNTs electrocatalysts in PEM hydrogen electrolyzer. Int J Hydrogen Energy 2011;36:9405e14.
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[45] Blanchard P, Grosvenor A, Cavell R, Mar A. X-ray photoelectron and absorption spectroscopy of metal-rich phosphides M2P and M3P (M ¼ CreNi). Chem Mater 2008;20:7081e8. [46] Birry L, Lasia A. Studies of the hydrogen evolution reaction on Raney nickelemolybdenum electrodes. J Appl Electrochem 2004;34:735e49. [47] Armstrong R, Henderson M. Impedance plane of a reaction with an adsorbed intermediate. Electroanal Chem 1972;39:81e90. [48] Lasia A. Electrochemical impedance spectroscopy and its applications. New York Heidelberg Dordrecht London: Springer; 2014. ISBN 978-1-4614-8932-0 ISBN 978-1-46148933-7 (eBook).
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Electrodeposited NiFeCo and NiFeCoP alloy cathodes for hydrogen evolution reaction in alkaline medium V. Bachvarov a,*, E. Lefterova b, R. Rashkov a a
Institute of Physical Chemistry, Bulgarian Academy of Sciences, 11 Acad. G. Bonchev Str., 1113 Sofia, Bulgaria Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, 10 Acad. G. Bonchev Str., 1113 Sofia, Bulgaria
b
article info
abstract
Article history:
The present paper reports on the electrodeposition of crystalline NiFeCo and amorphous
Received 23 February 2016
NiFeCoP alloys at various conditions of the electrolysis process (different current densities)
Received in revised form
leading to differences in component composition. Alloys are characterized by SEM, XRF,
17 May 2016
EDX, XRD and XPS methods. The electrocatalytic activity of the materials for hydrogen
Accepted 18 May 2016
evolution reaction (HER) is studied in a 6 M KOH solution using polarization curves and
Available online 8 June 2016
electrochemical impedance spectroscopy (EIS). Presence of iron and cobalt in nickel alloys decreases significantly overvoltage of hydrogen evolution and the charge transfer resis-
Keywords:
tance is about 1000 times smaller compared to pure nickel. Alloy amorphization by in-
Hydrogen evolution reaction
clusion of phosphorus enhances additionally electroactivity of these materials for the HER.
Electrocatalytic activity
Results from both electrochemical methods indicate that the best electrocatalytic behavior
Electrodeposition
is demonstrated by amorphous alloys having high iron content. The latter is associated
Amorphous NiFeCoP alloys
with a more developed surface of these materials and possible synergy among Ni, Fe and
Impedance spectroscopy
Co. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen as a conventional fuel alternative is attracting an exceptional amount of interest. One of the hydrogen production methods is water solution electrolysis. Nickel proves to be a feasible pretender to replace noble metals as catalysts for HER, exhibiting very promising physical, mechanical and corrosion characteristics. A possible way to enhance nickel matrix catalytic properties is to shape electrodes so as to provide for a highly developed surface. Some authors have obtained nickel-based nanocomposite coatings with carbon [1], graphene [2], TiOx [3]
and molybdenum [4,5]. They have come to the conclusion that increasing surface roughness renders an enhanced catalytic has reported [6] that activity for HER of such electrodes. Jovic along with an enhanced electrochemically active area on Ni(Ebonex/Ir) electrode surface, such electrodes exhibit also a higher internal catalytic activity for the HER in comparison with Ni due to the presence of Ir active sites on the surface. Several authors have established that binary alloys of nickel with Co [7e11], Mo [12e15], Fe [11,14,16], W [14,17], Sn [18] have an improved electrocatalytic activity for the HER compared to pure nickel due to both a more developed surface and a synergic electrocatalytic effect. When left hand side
* Corresponding author. E-mail address: [email protected] (V. Bachvarov). http://dx.doi.org/10.1016/j.ijhydene.2016.05.164 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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elements of the transition group having vacant d-orbitals (hypo-d-electronmetals) are combined with metals of the right hand side of the same group having complete internally coupled d-orbitals (hyper-d-electronmetals), the intermetallic compounds obtained undergo electron interactions and binding energy change leading to an increased catalyst activity [19,20]. Studying the HER in triple systems such as Ni-Co-Mo [21] and NieSeMn [22], some authors have established that in alkaline water medium electrodeposited tri-component materials are more active than the respective two-component systems. According to Lu et al. [23], tri-component NiMoP exhibit catalytic properties comparable to those of platinum foils. The observed high electrocatalytic activity for the HER is attributed to the hydrogen absorption ability of such electrodes. One of the most promising hydrogen reduction electrocatalysts are based on the elements of the iron group e iron, nickel and cobalt [24,25]. Amorphous materials have attracted substantial research interest due to their exceptional properties for the HER [25]. Some authors have claimed that catalytic properties augmentation in NiP layers is caused by crystal size reduction and hydrogen absorption in the alloys thus altering electron structure and improving electrocatalytic activity of the obtained material [26e29]. According to De Giz et al. [30], amorphous NieFeeP alloys have great potential as an electrode material for the HER since electrocatalytic activity exhibited in alkaline water electrolysis is sufficiently high. However, the increased electrocatalytic activity requires the amorphization coupling with an adequate material composition. A decisive role in the course of the hydrogen evolution reaction is played by the association of Ni with Fe and Р. Studies of Kreysa and Ha˚kansson [31] have indicated that industrially manufactured multicomponent amorphous materials based on Fe, Co, Ni, Si and B show lower polarization for the HER than Pt and Ni. As established in our previous works, multicomponent iron group-based alloys containing P [32] island structures and W and P [33] thick films are better catalysts for the HER than pure nickel and the respective binary or ternary alloys. All of the above gives grounds to assume that nickel bonding with iron, cobalt and phosphorous will result in an improved alloy electron structure and reduced crystal size, i.e., enlarged catalytic surface. It is reasonable to expect that optimizing the effect of these two factors will substantially diminish polarization for the hydrogen evolution reaction. The purpose of the present work is to obtain and physically characterize of crystalline NiFeCo and amorphous NiFeCoP alloy materials, using various current densities. Further to that, via galvanostatic polarization dependences and electrochemical impedance spectroscopy to study electrocatalytic activity of such alloys in view of their possible practical application as cathode material for the HER.
Experimental
to prior polishing with a disk-pulley using a chromium paste. This procedure secured plate roughness of below 0.6 mm. The polished samples were cleaned ultrasonically in acetone and ethanol, and then rinsed with distilled water. Copper plates were degreased and etched in a 1:1 HCl solution. Two electrolytes, I and II, were used for obtaining NiFeCo and NiFeCoP alloys, respectively. Electrolyte I contained NiSO4.7H2O e 40 g L1; NiCl2.6H2O e 40 g L1; CoSO4.5H2O e 3.2 g L1; FeSO4.7H2O e 8 g L1; glycine ndash; 24 g L1; b-alanine e 24 g L1. Electrolyte II contained: NiSO4.7H2O e 40 g L1; NiCl2.6H2O- 40 g L1; CoSO4.5H2O e 5 g L1; FeSO4.7H2O e 30 g L1; glycine e 26 g/L; b-alaninee26 g L1; H3PO2 50% acid and NaH2PO2.H2O with a total phosphorous content of 4.4 g L1 . Nickel anodes were used for alloy electrodepositing at a current density of 3, 5 and 10 А dm2 and a temperature of 50 C. For the purposes of comparison, a layer of pure nickel was deposited from a sulphamate electrolyte containing 80 g L1 Ni2þ, 30 g L1 H3BO3 and 10 g L1 NiCl2. The current density was 4.5 А dm2.
Physical characterization Depending on the electrolysis conditions, the content of nickel, iron and cobalt in the NiFeCo deposits was determined by X-ray fluorescence analysis (XRF ndash; Fischerscope X-RAY XDAL) in 9 points (three points in the bottom, middle and top of the sample, respectively). Scanning electron microscopy (SEM) using Leo 1455VP and Leo Supra 55VP microscopes with energy dispersion X-ray (EDX, Oxford Inca 200 instrument, Software INCA-Vers.4) was applied to characterize surface morphology of NiFeCo and NiFeCoP deposits and perform elemental analysis of NiFeCoP layers. Diffractograms of the samples were recorded on Philips PW and Empyrean ndash; PANalytical using CuKa1 radiation. Phase identification, precise determination of quantitative ratios of phases present and crystal size were done using a software package High Score Plus with ICSD data base. The surface chemical composition and valence states of the elements of deposited films were determined by X-ray photoelectron spectroscopy (XPS). Measurements were carried out on AXIS Supraelectron-spectrometer (Kratos Analitycal Ltd.) using monochromatic Al Ka radiation with photon energy of 1486.6 eV. The high-resolution spectra were calibrated towards the C 1 s line at 284.8 eV. Casa XPS software package [34] was used for peak analysis. Shirley-type background, Gaussian/Lorentzian line shapes as well as respective asymmetric forms were applied in the fitting procedure.
Table 1 e Composition of NiFeCo and NiFeCoP alloys. Current density
Electrochemical deposition NiFeCo and NiFeCoP alloys were electrodeposited on copper plates of 2 and 4 cm2 surface area. The plates were subjected
3 A dm2 5 A dm2 10 A dm2
From electrolyte I
From electrolyte II
Ni wt %
Co wt %
Fe wt %
Ni wt %
Co wt %
Fe wt %
P wt %
53 51 53
16 15 14
31 34 33
76 67 56
9 13 13
2 9 21
13 11 10
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Electrochemical characterization Steady-state polarization curves and EIS measurements were used to study the electrocatalytic activity of electrodes for the HER at room temperature. The working electrodes were cathodic polarized at 100 mA cm2 for 30 min in 6 М KOH to provide the steady-state conditions for the polarization measurements. Polarization dependences were obtained galvanostatically by a step-wise cathodic current decrease from 100 mA cm2 to 2.5 mA cm2. For each current value, the
potential was measured after 2e5 min. The procedure was repeated for any sample until reproducible results were obtained. Potential value was corrected with the ohmic drop by the electrolyte resistance obtained from EIS measurements. The values of overvoltage are calculated with respect to the reversible HER potential at the given conditions (Erev ¼ 1.114 V versus SCE in 6 M KOH). The impedance measurements were conducted on Gamry Instruments potentiostat/galvanostat using an electrochemical threeelectrode cell equipped with a Luggin capillary to the
Fig. 1 e SEM images of electrodeposited alloys: NiFeCo at 3 A dm¡2 e a), e); NiFeCoP e 3 A dm¡2 e b), f); 5 A dm¡2 e c), g); 10 A dm¡2 e d), h).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 2 7 6 2 e1 2 7 7 1
Fig. 2 e XRD spectra of NiFeCoP electrodeposited: (a) at 3 A dm¡2, (b) at 5 A dm¡2 and (c) at 10 A dm¡2 and NiFeCo e (d).
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significantly on current density. Coatings formed at current density higher than 3 A dm2 were cracked and adhered poorly to the substrate. Most probably this is due to high internal tensions associated with hydrogen inclusion occurring at increased current densities. For this reason only the results obtained for NiFeCo system electrodeposited at a current density of 3 A dm2 will be subject to further discussions below. Quantities of incorporated phosphorus in alloys from electrolyte II decreased with the increase in cathodic current density. The same dependence is observed with respect to deposited nickel. However, with the rise of current density, the amount of iron in the layer increased substantially. Similar dependence has been observed by Sridharan et al. [35] for FeNiP alloys. As for codeposited cobalt, its quantity climbed up negligibly with current density rise up to 5 A dm2, and remained unchanged with any further current density rise. Fig. 1 shows SEM images of electrodeposited alloys. A typical globular morphology of nickel alloys is observed at a magnification of 5000х. Coatings deposited in the presence of phosphorous (Fig. 1b, c, d) are cracked. At high magnification (100 000х), a spindle-shaped surface structure is observed in NiFeCo systems (Fig. 1е). An increase in current density applied to produce NiFeCoP alloys leads to particle size reduction (Fig. 1f, g, h).
X-ray diffraction analysis
Fig. 3 e XRD spectra of annealed NiFeCoP layers electrodeposited: (a) at 3 A dm¡2, (b) at 5 A dm¡2 and (c) at 10 A dm¡2. working electrode. In both electrochemical techniques a saturated calomel electrode (SCE) was used as a reference electrode, and Pt plate of 20 cm2 e as counter electrode. EIS 300 software at 1.25 V (vs. SCE) in a frequency range of 100 kHz to 10 mHz with ten point per decade was used for EIS measurements. Sine signal 10 mV peak to peak was applied. The Zview 3.2 software package was used for equivalent circuit modeling and to analyze and data fit.
Results and discussion Obtaining NiFeCo and NiFeCoP alloys NiFeCo and NiFeCoP alloys were obtained electrochemically from electrolytes I and II at various current densities. Alloy element composition (as identified with XRF and EDX) is presented in Table 1. The results indicated that coating composition of alloys obtained from electrolyte I did not depend
XRD analysis indicated that NiFeCoP coatings deposited at all three current densities are amorphous and only the peaks of the Cu substrate are observed on the XRD patterns (Fig. 2 a,b,c). The NiFeCo samples are crystalline (Fig. 2d). Two broaden peaks are registered. The first peak at 2Q ¼ 41.8 is related with hexagonal crystal structure hcp of CoxM1-x (M ¼ Ni and/or Fe). The second peak at 2Q ¼ 44.98 corresponds to (111) reflection of cubic structure of Ni3Fe (ICDD/ JPCPD PDF#65-3244) as well as of fcc structure of solid solutions of NieFeeCo alloys. The crystallite size of hexagonal and cubic phases is 6 nm and 4 nm, respectively. The absence of other diffraction peaks (only a weak peak at 75.6 responsible for both phases) means that the layers are textured with (100) and (111) preferential orientation of the hexagonal and cubic phases, respectively. It is in correlation with the specific morphology of this film, as seen from SEM. Since X-ray diffraction cannot serve for the identification of NiFeCoP layers due to layers' amorphous structure, phase composition of amorphous coatings was determined by an inert medium of nitrogen (30 L h1) annealing of coatings at 550 C for 3 h. Crystallization temperature was detected by Perkin Elmer Diamond TG/DTA analysis. XRD spectra of annealed samples are given in Fig. 3. The Ni3P and FeNi2P (ICDD/JPCPD PDF#65-1605 PDF#51-1367) phases are identified on the annealed NiFeCoP layers. Hentschel et al. [36] have also registered Ni3P phase following annealing of NiPх films at a temperature of 417 C. The authors stipulate that amorphous phase phosphorus is concentrated at the borders around Ni nanocrystals. As seen by the relationship in Fig. 3, an increase of the amount of iron, suppressed Ni3P formation, and a FeNi2P compound of the same structure crystallized at its place, with a portion of nickel being substituted by iron (Fig. 3c). This trend was monitored by the change in the
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Fig. 4 e XPS spectra of NiFeCoP alloys electrodeposited at 3, 5 and 10 A dm¡2: a) Ni2p3/2; b) Co2p3/2; c) P2p3/2; d) О1 s.
parameters of elementary cells. The analysis also indicated that with a comparable cobalt quantity in all three samples, presence of iron caused crystallization of different Co-rich phases ndash; fcc at higher Fe content or hcp at low Fe content.
XPS analysis XPS method was used to characterize the valence state of elements and possible compounds present in the amorphous
NiFeCoP alloys. For instance, the positions of P2p3/2 could be an indication whether a phosphide and/or a phosphate are formed. Binding energy (BE) of pure amorphous phosphorus P2p3/2 is 130.2 еV, while of that of metal phosphides was below 130 eV [34]. Binding energies over 132 eV are an indication of phosphate formation. Fig. 4 depicts the high resolution photoelectron spectra of Ni2p3/2 (Fig. 4a), Co2p3/2 (Fig. 4b) and P2p3/2 (Fig. 4c). Fe2p is not shown since Fe2p3/2 line overlaps with the Co&Ni LМM Auger lines at Al radiation and the very small quantity is registered on the surface. The spectra were
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Fig. 5 e Polarization curves of Ni e curve 1, NiFeCo e curve 2 and NiFeCoP electrodeposited: at 3 A dm¡2 e curve 3; at 5 A dm¡2 e curve 4; at 10 A dm¡2 e curve 5 in 6 M KOH.
Table 2 e Parameters obtained by the analysis of the polarization curves presented in Fig. 5. b [mV dec1]
Sample
h10 mA cm2 h100 mA cm2
I II III Range range range Ni NiFeCo-3A dm2 NiFeCoP-3A dm2 NiFeCoP-5A dm2 NiFeCoP-10A dm2
mV
mV
76 50
108 63
165 106
378 120
512 196
30
51
95
87
155
30
42
75
83
139
28
42
73
78
135
analyzed and decomposed to the respective components on the basis of NIST [34] and [37e43]. BE of the first components of Ni2p3/2 (852.7 eV), Co2p3/2 (777.9) corresponds to the metal phases (M0) and phosphides [44,45]. Peak and satellite structure analysis show presence of oxides of the second valence, in smaller quantity for nickel and higher e for cobalt. BEP2p3/ 2 ¼ 129.3 eV, determined from the first phosphorus doublet, is the evidence for MxP (M ¼ Ni, Co) formation. The large peak observed at higher than 132 eV in the P2p spectrum is attributed to presence of phosphates while the hint of a peak at about 130.3 eV is an indication for a small quantity of phosphorus bonded to neither the metal nor oxygen (pure phosphorous). The oxygen peak (Fig. 4d) is decomposed to 4 components indicating various oxygen bonds (O1 530 eV e МО (O2), O2 ¼ 531.5 eV e OH, MeOeP, O3 z532.5 eV C]O, H2O, O4 > 533 eV e P]O). The results from XPS analysis confirm XRD data of annealed samples, i.e., existence of MxP (M ¼ Ni, Co).
Polarization measurements A criterion for the catalytic activity of a material regarding hydrogen evolution reaction could be the overvoltage of this
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reaction at a given current density. Comparison of various electrode materials points to the fact, that at one and the same current density the lower overvoltage for the HER is associated with higher catalytic activity. Polarization curves (Fig. 5) show that at all studied current densities the highest overvoltage for the HER is exhibited by the nickel electrode (curve 1). The parameters obtained by the analysis of the polarization curves on Fig. 5 are presented in Table 2. Tafel slopes b for Ni in all ranges show the highest values. This indicates the worst electrocatalytic activity for the pure nickel. Nickel codeposition with Fe and Co (NiFeCo alloy) decreases overvoltage of the hydrogen reaction dramatically which corresponds to the increase of catalytic properties. For instance, at cathodic current density at 100 mA cm2 (log i ¼ 1), this reduction is by 316 mV (Fig. 5, curve 2) compared to pure nickel (curve1and Table 2). Similar dependence has been detected by Solmaz et al. [16]. They claim that hydrogen evolution activity grows with increasing of iron content in NiFe alloys. They attribute this fact to the synergy between Ni and Fe. Even small quantities of Fe in NiFeP alloys (1 wt. % Fe) reduce HER overvoltage by approximately 200 mV, compared to mild steel and Ni [30]. Alloying Ni with Со leads also to increase electrocatalytic activity for the HER compared to pure Ni. This is due to improved internal activity of the material, attributed to the synergy between catalytic properties of Ni (low hydrogen overvoltage) and Co (high hydrogen absorption) [7]. As shown in Fig. 5, NiFeCoP alloys exhibit the best catalytic activity for the HER (curves 3, 4, 5). Irrespective of the differences in component composition (Table 1), however, they demonstrate very close catalytic behavior. For instance, alloys obtained at 10 A dm2, contained 7 times more iron than the ones obtained at 3 A dm2, and the maximum overvoltage change at cathodic current density of 100 mA cm2 (log i ¼ 1) is only 20 mV (Fig. 5 curves 3, 5 and Table 2). Obviously, the better catalytic activity of these alloys compared to a NiFeCo alloy is due to phosphorus codeposition which on its part leads to amorphization of the latter (Fig. 2), i.e. to a more developed surface. The results presented above of XRD and XPS analysis confirm that the amorphous coatings contain MxP (M ¼ Ni, Co) and FeNi2P compounds. They could be another reason for the best electrocatalytic properties of NiFeCoP systems. Paseka et al. [26] have established that the high activity of MeeP(x) (Me ¼ Ni, Co and FeeNi) electrodes is caused by dissolved hydrogen in the amorphous layer in the course of the electrodeposition. The high activity for the HER of the layer NieP electrodes prepared at T 53 C was caused by the internal stress in the layer. The stress originated during the electrodeposition of the NieP layer by co-deposited and absorbed hydrogen [27]. A correlation between the activity of the NiPx electrodes, and the amount of absorbed hydrogen was found by Burchardt [28] as the most active electrodes also absorbed the largest amount of hydrogen. The activity increases with increasing absorption. The catalytic activity is related to variations in the energy of adsorption of hydrogen. Burchardt proposed that the activity is related to the amorphous phase, consisting of Ni and P. The absorption of hydrogen in the amorphous phase probably changes the electronic structure of the metal, thereby increasing the catalytic activity [28,29].
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Fig. 6 e Nyquist plots for the HER at Ni, NiFeCo and NiFeCoP alloys in 6 M KOH.
Electrochemical impedance spectroscopy measurements Impedance studies provide additional information about ongoing processes on catalytic layers. Fig. 6 presents Nyquist plots of EIS measurements for various layers recorded at the same potential of 1.25 V. A substantial difference in the impedance spectra of pure nickel (Fig. 6a and e) and of multicomponent layers (Fig. 6b) is observed. One semicircle corresponding to one time constant is registered on spectra of Ni layer. The EIS of multicomponent NiFeCo and NiFeCoP
electrodes are more complicated. Three overlapped depressed semicircles (Fig. 6c) can be distinguished and connected to three different processes with three different time-constants. The high frequencies ac response (Fig. 6d) is independent on potential change and hence it is connected to electrode surface porosity [12,24]. The rest two are related to Faraday's reaction of hydrogen evolution and adsorption, respectively [9,12,14,18,24,46]. Impedance modeling and fitting are applied using appropriate equivalent circuits. A three timeconstant equivalent scheme reflecting the above mentioned
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 2 7 6 2 e1 2 7 7 1
reduces particle size, as illustrated also by SEM images (Fig. 1), thus leading to a more developed surface. Results of the impedance measurements correlate very well with those of the polarization curves (Fig. 5). They show big differences in Rct values between Ni and multicomponent NiFeCo and NiFeCoP layers. NiFeCoP layers obtained at various current densities have close impedance spectra and close overvoltages, differing by not more than 5e15 mV at high current densities. The low Rct value together with high CPE, respective high Cdl (i.e. high surface) results in high catalytic activity of NiFeCoP layer for the HER.
Fig. 7 e Schematic representation of the electrical equivalent circuit diagram.
processes is given in Fig. 7(T3). The electrolyte resistance Rel is in series with the high-frequency response (Rp-CPEp), while the elements associated with the kinetics of hydrogen evolution (Rct-CPEdl) and adsorption (Rad-CPEps) are presented with a modified Amstrong's element [47,48], were Rct is charge transfer resistance, Rad is resistance due to adsorbed hydrogen, CPEdl and CPEps are constant phase elements (CPE) used instead of double layer capacitance Cdl and pseudo capacitance Cps respectively. CPE is defined by the formula: a
Z ¼ 1=½TðiuÞ
(1)
where T is a constant representing CPE capacitive part, u is angular frequency (in rad/sec), i2 ¼ 1 is an imaginary unit and a is an exponent. CPE is used to account for the deviation from the ideal capacity caused by the non-homogenous nature of the catalytic layer, roughness, impurities, etc. This deviation is reflected in the exponent a, and if a ¼ 1, Equation (1) is equivalent to the one for capacity determination. When CPE is included in parallel to resistance, gives depressed semicircle on Nyquist plot [48]. A single time-constant model (Fig. 7) (T1) ¼ Rel(Rct-CPEdl) is used to modeling Ni impedance. Results of the fitting procedure are given in Table 3. There is a big difference in the parameters determined for HER in pure Ni and multicomponent catalysts. CPE is by three orders of magnitude lower while Rct is by three orders of magnitude higher. Lower CPEdl value indicate also smaller active catalytic surface while the higher Rct means lower catalytic activity. The HER parameters follow the dependences RctNi >> RctNiFeCo > RctNiFeCoP. Fig. 6 plots the impedance comparison of NiFeCoP catalysts deposited at various current densities. Differences are small and follow the order of Rctð3Adm2 Þ >Rctð5Adm2 Þ Rctð10Adm2 Þ and CPEð3Adm2 Þ
Table 3 e Parameters obtained by fitting EIS experimental spectra. Sample
Rct [U cm2]
a
CPEdl [F sa1 cm2]
Ni 2348 ± 38 0.000227 ± 0.00003 2.19 ± 0.019 0.179 ± 0.004 NiFeCo-3A dm2 NiFeCoP-3A dm2 0.372 ± 0.007 0.074 ± 0.003 0.143 ± 0.005 NiFeCoP-5A dm2 0.301 ± 0.008 0.442 ± 0.023 NiFeCoP-10A dm2 0.316 ± 0.014
0.87 0.94 0.88 0.88 0.85
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± 0.002 ± 0.002 ± 0.01 ± 0.01 ± 0.02
Conclusions Crystalline NiFeCo and amorphous NiFeCoP alloy material were obtained electrochemically at various current densities. The catalytic properties for the HER of these systems in 6 М KOH were studied by means of galvanostatic polarization dependences and electrochemical impedance spectroscopy. The results indicated a dramatic increase in electrochemical activity for the HER (reduction in the overvoltage of hydrogen reaction by over 300 mV and decrease in Rct values by 3 orders of magnitude) in NiFeCo and NiFeCoP systems compared to pure nickel. The behavior observed could be attributed to the synergy among Ni, Fe and Co, resulting in enhanced internal catalytic activity of the material. On the other hand, phosphorus presence in NiFeCoP alloys reduced particle size causing alloy amorphization which led to a more developed surface, i.e. higher catalytic activity for the HER compared to crystalline NiFeCo alloy. Amorphous alloys deposited at current densities of 5 and 10 A dm2 exhibited the best catalytic behavior due to the presence of both MxP (M ¼ Ni, Co) and FeNi2P phases. The very good catalytic properties of amorphous NiFeCoP systems established call for further studies in view of their practical application as an electrode material in electrolysers with a polymer anion-conducting membrane.
Acknowledgments The authors would like to thank the National Science Fund of Bulgaria for the financial support of this work through contract DFNI E 02/9.
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