formation of nickel hydrides by hydrogen evolution in alkaline media: effect of temperature

Journal of Electroanalytical Chemistry 457 (1998) 205–219

Formation of nickel hydrides by hydrogen evolution in alkaline media:effect of temperature

Michele Bernardini, Nicola Comisso, Giuliano Mengoli *, Livio Sinico

Istituto di Polarografia, CNR IPELP, Corso Stati Uniti 4, 35127 Pado6a, Italy

Received 22 April 1998; received in revised form 21 July 1998

Abstract

The effect of temperature on the hydrogenation of sintered Ni by the hydrogen evolution reaction (her) in 0.6 M K2CO3+H2Oelectrolyte was investigated in the range 25–95°C. At 25°C, hydrogenation leads to the formation of three different species,provisionally identified as b-Ni-, hexagonal-Ni- and a-Ni-hydride phases, based on their respective redox potentials andthermostability. Their penetration into the metal ranged from a few tens to a few hundred monolayers. Above room temperature,the overall efficiency and penetration of Ni hydrogenation increased significantly. Furthermore, there is definite evidence that, atT]65°C, the b-Ni-hydride phase evolves into new Ni–hydrogen species of higher thermal stability, regarding which there is noprevious knowledge. The results of electrolytic Ni deuteration by electrolysis of K2CO3+D2O conformed to those obtained inlight water. © 1998 Elsevier Science S.A. All rights reserved.

Keywords: Electrolytic reduction; Hydrogen (Deuterium) evolution reaction (her); Nickel hydrides; Temperature effects

1. Introduction

It was found recently that the formation of hydro-genated species of Ni by hydrogen evolution in alkalinemedia is inhibited strongly at solid Ni cathodes [1].Instead, Ni hydriding becomes feasible, although withlimited metal penetration, when:� solid Ni has undergone some pre-treatment, either

chemical etching or anodic oxidation [2], causingdeep modifications in the original surface texture;

� solid Ni is replaced by sintered Ni or Ni powder-fiber composites.

Even Ni samples behaving as good hydrogen absorbersunder mild cathodic polarization are often deactivatedirreversibly when submitted to forced and/or prolongedpolarization in the hydrogen evolution reaction.

These findings accord with data from Angely et al.[3,4] on hydrogen adsorption, both at cold worked(solid) Ni or vacuum-deposited Ni films subjected to

various annealing procedures. Only a small surfacefraction of solid Ni was available for hydrogen adsorp-tion (at Eeq, UH:0.03), whereas the number of adsorp-tion sites, corresponding to surface Ni atoms of lowco-ordination number, was very high for samples withdefective textures and small surface crystallites.

Therefore, if the activity (i.e. thermodynamic poten-tial) of surface hydrogen drives its penetration into themetal, the different behaviour towards hydrogen ab-sorption of ‘virgin’ solid Ni and ‘activated’ Ni is ex-plained, as well as the deactivation occurring underforced hydrogen evolution reaction (her), which proba-bly induces surface reconstruction.

However, some of our other findings do not agreewith current literature data on the H2–Ni system, forinstance, the voltammetric behaviour of hydrogenatedNi samples. When swept positive to the her, every typeof Ni (‘activated’ solid Ni, sintered Ni and Ni-fibercomposite) showed a more or less well-defined three-peak oxidative pattern which we ascribe to the ‘extrac-tion’ of inserted hydrogen: only the peak at the most* Corresponding author. Fax: +39 49 8295853.

0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved.PII S0022-0728(98)00307-6

M. Bernardini et al. / Journal of Electroanalytical Chemistry 457 (1998) 205–219206

Fig. 1. Kelac 47 Ni. Linear sweep voltammetries at 25°C (curve a) and 65°C (curve b), after 5 min cathodic polarization. Sweep rate=5 mV s−1.Reference electrode Ag � AgCl.

negative potential could be associated with a knownphase (b) of Ni hydride [1,5]. Some preliminary data [6]also indicate that electrolytic hydrogen insertion into Niis enhanced at temperatures above 25°C.

The present work, through the study of temperatureeffects, was aimed at obtaining more information onthe Ni–hydrogen species forming after hydrogen evolu-tion in alkaline media. Sintered Ni was chosen for thisinvestigation, since it provides quantitative data on Nihydrogenation with reproducibility much better thanother Ni types [1]. The electrolyte was again 0.6 MK2CO3 (pH 11.3), to ensure both good conductivityand a less aggressive attack on the cell glass thanalkaline hydroxides, especially above roomtemperature.

2. Experimental

2.1. Materials

The electrolyte was reagent grade K2CO3 supplied byFluka; H2O was of Millipore grade; D2O was 99.8%isotopic purity supplied by Acros; N2, H2, D2, and O2

were 99.9% purity gases from SIO.

The Ni samples were: a massive sintered ‘Kelac 47’Ni plate, 0.05 cm thick (Wiggin Alloys, UK), with�25% porosity; Ni powder sintered on to a Ni-platedsheet, 0.04 cm thick (SAFT, France), having �75%porosity; solid Ni foil, 0.025 cm thick, 99.5% pure(Goodfellow, UK).

2.2. Apparatus and procedures

The electrolytic cell was the same glass cylinder usedin the previous investigation [1]. It was double-jacketed,so that thermostatic control of temperature in the elec-trolyte was accomplished by a mantle of silicon oilcirculated by a Haake F3 thermostat. The gases, eitherproduced by electrolysis or the customary N2 made toflow across the electrolyte, exited through a joint on thecell neck to a refrigerated column refluxing the vapour.The typical working electrode was a flag made of thechosen Ni material, pressure-bonded to a Ni wire:electrical connexion was by means of a pyrex tubethrough the lid. The counter-electrode was a Pt coil:one end was connected to the outside through the lidand the other end was coiled round the Ni flag.

The reference electrode was Ag � AgCl � KCl(sat.).This electrode was placed in a (not thermostated) com-partment external to that of the cell: connection be-

M. Bernardini et al. / Journal of Electroanalytical Chemistry 457 (1998) 205–219 207

Fig. 2. Kelac 47 Ni. Linear sweep voltammetries at 95°C after 5 min cathodic polarization. Sweep rate 6 : (a) 1 mV s−1; (b) 2 mV s−1; (c) 5 mVs−1; (d) 10 mV s−1. Reference electrode Ag � AgCl.

tween the compartments was achieved through a Lug-gin probe. When the electrolyte was thermostatedabove room temperature, the reference electrode hadlower temperature increments with minor effects on thepotential. Standardization of Ag � AgCl towards RHEat the various electrolyte temperatures was obtained byreplacing the working (Ni) electrode with a platinizedPt sheet in H2 saturated 0.6 M K2CO3 solution. Thepotential attained by Ag � AgCl was: 0.878 V at 25°C;0.884 V at 65°C; 0.886 V at 95°C. This measurementwas used to convert voltammetric data achieved versusAg � AgCl into the RHE scale (Table 2).

Electrochemical instrumentation consisted of AMELapparatus or a PAR potentiostat [1]. Potentiometricmeasurements were made on a Keithley 175 A au-toranging multimeter. Capacitance data were obtainedon a Solatron 1250 frequency response analyzerequipped with an 1186 electrochemical interface.

3. Results

3.1. Voltammetry

To investigate the voltammetric behaviour of hydro-gen in Ni, the Ni samples were submitted first to 5 min

of cathodic polarization in the potential range −1.20to −1.05 V (vs Ag � AgCl) which about coincided withthe beginning of the her at the various temperatures:cathodic electrolysis for more than 5 min was generallynot adopted, as the amount of sorbed hydrogen eitherincreased slightly (Kelac 47) or did not increase at all(SAFT). Hydrogen extraction was then carried out byapplying a linear sweep potential scan from the her at−0.35 V to the Ni electrode.

Fig. 1 compares the oxidative patterns obtained froma Kelac 47 sample at 25°C (curve a) and 65°C (curve b)at the same potential sweep rate 6=5 mV s−1.

At 25°C, the peak just positive of the her onset (peak1) was followed by a second one at −0.6 V (peak 2)and a shoulder at �−0.48 V (peak 3). At 65°C peak1 was seen to decrease and split, originating a new one(peak 1%): at the same time peaks 2–3 were definedmore sharply than at 25°C, while the current increasedsignificantly.

A temperature increment from 25 to 65°C thereforecaused the greatest evolution in the voltammetric be-haviour of the hydrogenated sample, and the situationshown in Fig. 1 did not evolve substantially any furtherwhen the electrolyte temperature was taken to 95°C, asFig. 2 demonstrates. Four oxidation peaks are clearlyoutlined, but the relative peak currents (mainly as


Fig. 3. SAFT Ni. Linear sweep voltammetries after 5 min cathodic polarization. Continuous line 65°C; dotted line 95°C; sweep rate=5 mV s−1.Reference electrode Ag � AgCl.

regards to peaks 1–1%) do change significantly with thesweep rate 6. Thus, at 10 mV s−1, peak 1 currentlargely exceeds that of peak 1%; at 5 mV s−1, the twocurrents are comparable and the total current engagedby the whole oxidative pattern is about twice thatmeasured at lower temperatures (Fig. 1); at 1–2 mVs−1, the peak 1 fades with respect to peak 1%. In otherwords, peak current dependence on 6 highlights thetime instability of the species of peak 1 in favour of thatof peak 1%.

Conversely, no decomposition within the time scaleof the voltammograms of Fig. 2 apparently occurs forthe species of peaks 2 and 3. For this reason, when theKelac 47 electrode was disconnected after 5 min ofcathodic polarization (at 95 °C), its open circuit poten-tial (oc), which was �0.9 V negative to Ag � AgCl 30 slater, increased with time rather slowly, so that, after�1 h at oc it was still �0.75 V negative to thereference electrode.

Fig. 3 compares the voltammetric patterns (6=5 mVs−1) obtained from a cathodized SAFT Ni sample at65°C (plain curve) and 95°C (dotted curve): four peaksare again outlined, but peak 1% is more evident at 95°Cthan at 65°C, meaning that the thermal stability ispredicted to be higher for peak 1% than for peak 1.

The oxidative patterns shown in Fig. 4 were all

obtained from the same SAFT Ni sample at 65°C, butstarting the voltammetric scan from the oc potentialobtained after increasing delay times from cathodicpolarization. Thus, the first voltammogram, recordedwith no delay, clearly accords with the analogue in Fig.3, but subsequent ones, with delay times from 2 to 30min, differ significantly from the first owing to ocdecomposition of the species related to the differentpeaks.

This oc instability of hydrogenated Ni species,greater when they form at SAFT than at Kelac 47sintered Ni, has already been tested at 25°C [1].

In conclusion, the voltammetric data indicate thatthe her at Ni above room temperature apparently leadsto four hydrogenated species whose formation,achieved here with two different sintered Ni samples, isprobably intrinsic to every Ni type.

3.2. Potentiostatic hydrogen extraction

Kelac 47 samples were submitted to the her for 5 minand then anodized in succession at −0.8, −0.7 and−0.5 V (vs Ag � AgCl) to achieve selective hydrogenextraction at peaks 1 (1%), 2 and 3, respectively: thedependence of the extraction current on time at eachpotential could thus be examined.


Fig. 4. SAFT Ni. Linear sweep voltammetries at 65°C at increasing times in open circuit after 5 min cathodic polarization. Delay time equal to:(a) 0 min; (b) 2 min; (c) 5 min; (d) 20 min; (e) 30 min; sweep rate 5 mV s−1. Reference electrode Ag � AgCl.

Fig. 5 shows the current transient recorded at 25°Cfor the three typical extraction potentials. At −0.8 V(curve a), the extraction current is not a simple timefunction, as it drops by �75% in a few hundred ms,then decreases much more slowly, requiring severalminutes for total hydrogen extraction. At −0.7 V(curve b) current behaviour is similar; conversely at−0.5 V (curve c) all the current transient appears to bea well defined function of time, as shown by the inset ofFig. 5, where the j versus t1/2 and log j versus t func-tions are plotted.

Fig. 6 shows that at 65°C the initial drop in currentat −0.8 and −0.7 V (curves a, b) is much smaller thanthat occurring at 25°C, and the fraction of currentdeclining slowly with time increases significantly. Ifthese current transients, neglecting the first 500 ms, areplotted as either j versus t1/2 or log j versus t functionslinearity is obeyed by more than 90% of the current: forinstance, the inset of Fig. 6 shows the linearization ofthe extraction current at −0.8 V. The transient at−0.5 V confirms the same time dependence as theanalogue at 25°C.

Fig. 7 shows current transients recorded at 95°C. At−0.8 V, the overall current increases significantlyabove the values measured at 25 and 65°C, while thedrop in current in the first few hundred ms is now

negligible compared with that of Figs. 5 and 6. How-ever, the subsequent current versus time decrease is notaccounted for by a simple j(t) relation.

At −0.7 V, the current transient fits the j versus t1/2

or log j versus t relationships (inset of Fig. 7) and thesame happens for the transient at −0.5 V.

These results (Figs. 5–7) are explained as follows.At −0.8 to −0.7 V and particularly at 25°C, the

consumed extraction current fraction in the initial fewhundred ms may be associated with oxidation of hydro-gen adsorbed on the Ni surface; this fraction is in factseen to decrease with temperature, as expected for theLangmuir-like adsorption isotherm [7]:

UH

1−UH

:K1 exp(−hF/RT) (1)

where UH is fractional surface hydrogen coverage.For times greater than a few hundred ms, the current

transients (except those at −0.8 V and 95 °C) areaccounted for by j(t), showing that extracted hydrogendoes not come from surface species. In fact, linearitybetween j and t1/2 is typical of diffusional control, aspredicted for hydrogen originating from a subsurfaceNi hydride phase characterized by slow hydrogen diffu-sion [8]. Instead, a satisfactory interpolation with a log jversus t relationship fits the model suggested by Con-


Fig. 5. Kelac 47 Ni. Anodic current transients recorded at −0.8 V (a), −0.7 V (b) and −0.5 V (c) vs Ag � AgCl after 5 min cathodic polarizationat 25°C. Inset: �, j vs t1/2 and �, log j vs t plots of curve c.

way et al. for hydrogen insertion/extraction at leachedRaney Ni in an alkaline media [9]. These authorsassume that hydrogen is electrochemically absorbedinto a bulk Ni hydride phase by a Langmuir-typetridimensional equation:

xH

1−xH

:K exp(−hF/RT)/cH+ (2)

where xH is the fraction of hydrogen in the Ni lattice.Both insertion of hydrogen into Ni and extraction ofsorbed hydrogen are thus seen to depend on xH, andthe respective currents are easily related with time andpotential variations of xH. In particular, the extractioncurrent

j= −FdxH

dt(3)

is integrated as

j=F [A− (A+B)xH0 ] exp− (A+B)t (4)

where xH0 is the site fraction of hydrogen at t=0 for a

given polarization potential, and parameters A and Brepresent forward and backward steps of the Volmerreaction.

Eq. (4) alone provides a linear dependence on time ofthe logarithm of the hydrogen extraction current from abulk Ni hydride phase.

The peculiar shape of the current transient at −0.8V and 95°C (curve a, Fig. 7) may be due to transitionof the species of peak 1 to a new phase (peak 1%)characterized by different diffusivity and/or stability.

3.3. Amounts of sorbed hydrogen

Both voltammetric patterns and extraction currenttransients indicate that the overall current increaseswith temperature. This behaviour actually reflects in-


Fig. 6. Anodic current transients obtained as in Fig. 5 at 65°C. Inset: �, j vs t1/2 and �, log j vs t plots of curve a.

creased sorption of hydrogen, which was measuredquantitatively by the coulometry of each peak at thevarious temperatures.

Since the amount of hydrogen absorbed by the sameNi sample increases with charge/discharge cycling (es-pecially Kelac 47 Ni [1]), to obviate this ‘training effect’[10,11] the coulometric data of Table 1 were obtainedas follows:1. the Ni sample, cathodized for 5 min at 25°C, was

then anodized exhaustively at the three peakpotentials;

2. the same insertion/extraction electrolyses were re-peated at 65 and then 95 °C;

3. the succession (1)–(2) was repeated five times, andthe five sets of coulometric data obtained at thesame temperature for each peak were then aver-aged.

Runs 1–3 were carried out with SAFT Ni and runs4–6 with Kelac 47 Ni: in both cases, hydrogen absorp-tion was definitely seen to increase with temperature.

The coulometric data of the subsequent runs of

Table 1 were obtained with the last Kelac 47 specimen,which was hydrogenated with gaseous H2. Thus, fol-lowing an already devised procedure [1], the samplewas interacted at oc with H2 dissolved in the electrolyteuntil the potential of the peak 1-(1%) system (�−0.90/−0.85 V vs Ag � AgCl) was reached. Exhaustive oxida-tion at the three peak potentials was then performed,while N2 bubbling removed H2 from the cell. In runs7–9 Ni hydrogenation was carried out by H2 bubblinginto the electrolyte, and in runs 10–12 by H2 dissolvedfrom the gas phase above the electrolyte.

A definite increase in hydrogen absorption with tem-perature was again indicated by runs 7–9. At 25 and65°C, sample hydrogenation (runs 10–11) was stillmore efficient when performed in a standing solutionwith H2 equilibrating from the gas � electrolyte inter-phase: Ni hydride decomposition is in fact enhanced instirred solutions [1]. The opposite behaviour at 95°C(run 12) is explained by considering that the partialpressure of H2 above the electrolyte is reduced signifi-cantly by H2O evaporation.


Fig. 7. Anodic current transient obtained as in Figs. 5 and 6 at 95°C. Inset: �, j vs t1/2 and �, log j vs t plots of curve b.

3.4. Decomposition at open circuit

The oc decomposition rates of the various hydro-genated species of Ni (peaks 1–3) at 25, 65 and 95°C,respectively, are compared in Fig. 8(a–c): these kinet-ics were obtained from a ‘trained’ Kelac 47 sam-ple. Each point in the plots was found by measuringthe coulometry of each peak in the succession −0.8,−0.7 and −0.5 V for increasing times in oc condi-tions after cathodic loading. The charges measured(Qt) were then correlated to the respective charge mea-sured at t=0 (Q0) (i.e. immediately after cathodicloading).

Species oxidized at −0.8 V disappear in less than20 min at 25°C and still more rapidly at 65°C,whereas decomposition slowed down at 95°C. Thisbehaviour, which agrees with previous voltammetricdata, is explained both by the thermal instability of

peak 1 species and by the temperature-induced forma-tion of peak 1% species.

The higher stability at higher temperatures of thepeak 2 species is shown clearly in Fig. 8, in full agree-ment with the absorption data of Table 1 which haddemonstrated already that the formation of peak 2species is favoured by temperature.

Moreover, peak 2 species do not decay, but tend toincrease as long as peak 1 (1%) species do not decom-pose completely. Fig. 8 also shows that peak 3 speciesare indefinitely stable within the time scale of theexperiment, in O2-free solution.

Lastly the various Ni hydrogenated species decay inboth succession and rate, which follows their redoxpotential scale (from the most negative upwards). Thisdecay probably occurs by means of the partial inter-conversion of one species into another of less negativeredox potential.


Table 1Hydrogen absorptiona on sintered Nib at different temperatures

LoadingQ−700/mC Q−300c /mC Qtot/mCQ−500/mCRun T/°C Q−800/mC

cm−2 cm−2cm−2 cm−2cm−2

25 6898 5297 234931 — 354 ElectrochemicalSAFT 1451—300930 Electrochemical789465 7392SAFT 2

95 96911 10094 279929 — 475SAFT Electrochemical3Electrochemical294—180928599625 5597Kelac 47 4

8893 183928 — 348 ElectrochemicalKelac 47 5 65 7793124910 200910 — 422 ElectrochemicalKelac 47 6 95 9898

34910 2192 44924 2791 126Kelac 47 7 25 H2 bubbling into the electrolyte37915849433991786912 24665 H2 bubbling into the electrolyteKelac 47 8

114944 86950 125957 29914 354Kelac 47 9 H2 bubbling into the electrolyte95H2 above the electrolyte8479297356915529625 347937Kelac 47 10

11929600 3729142 9499 H2 above the electrolyte1979Kelac 47 11 65 321935309912 167933 110932 H2 above the electrolyte700Kelac 47 12 95 114937

a Measured from the charge extracted at the different potentials.b SAFT or Kelac 47.c Residual charge at −0.3 V was measured only in runs 7–12.

3.5. Penetration of hydrogen in Ni

The subsurface penetration of each hydrogenated Nispecies may be estimated, for instance from the extrac-tion current transients of Fig. 5. If, as we believe, thecharge extracted during the first 500 ms (curve a) is dueto adsorbed hydrogen whereas the change later ex-tracted comes from subsurface Ni hydride, the ratio ofthe latter to the former immediately gives the numberof hydride layers. The following assumptions are alsomade:� at 25°C and with highly active Ni (e.g. sintered Ni),

full hydrogen coverage (U=1) is attained [4,12];� the oxidation of an adsorbed hydrogen monolayer

(H/Ni=1) requires �0.3 mC cm−2 (of true surface)[4].Therefore, from the charges consumed for respec-

tively t5500 and t\500 ms by current transient a ofFig. 5, the roughness factor (i.e. the ratio true surfaceto geometrical area) of the Kelac 47 sample is 13.8,while the subsurface hydrogen penetration of peak 1species is equivalent to ]10 monolayers.

The ratio of the extracted charge after the first 500ms to that extracted for t5500 ms is again 10 fortransient b of Fig. 5, but the actual hydrogen penetra-tion is certainly deeper than ten monolayers, since theH/Ni ratio of peak 2 species1 is, predictably, signifi-cantly B1.

Species oxidized at −0.5 V (transient c) are charac-terized by penetration which is probably more than oneorder of magnitude deeper, for not only is the extractedcharge appoximately four times larger than before, butthe H/Ni ratio is �1.

The coulometric data of the transients of Figs. 6 and7 with respect to those measured at 25°C indicate thathydride penetration at 65 and 95 °C is perhaps doublethat at 25°C.

As an alternative route to determine hydride penetra-tion in the runs of Table 1, the absorption data werenormalized to the true surface area of the samples: inthis case, the roughness factor of sintered Ni wasestimated by double layer capacitance measurements.

Capacitance measurements for samples SAFT andKelac 47 (with respect to those of a specular solid Nisample of the same geometrical area) pose some prob-lems. Sintered Ni samples, if untreated, are affected bysurface oxides and, when etched by acid, they absorbhydrogen [1], so that in both cases double-layer capaci-tance may be distorted by pseudocapacitivecontributions.

Relative capacitance measurements at 25°C in 0.6K2CO3 electrolyte give roughness factors of �20 forSAFT and �70 for Kelac 47—somewhat surprising inview of the far higher porosity of the former.

Therefore, from the determined roughness factorsand the coulometric data of Table 1 the penetrationdepth of b-Ni hydride can be estimated from 10 to 20monolayers for SAFT Ni, and from a few to �10monolayers for Kelac 47 Ni. In the latter case someartefact in the capacitance datum probably leads to anunderestimate of the penetration.

As a matter of fact, after interaction of H2 (dissolvedin the electrolyte) with specular solid Ni samples (as-suming roughness=1) we have determined by cou-lometry a b-Ni-hydride thickness ranging between 50and 100 monolayers [1]. Since b-Ni hydride penetrationinto Ni is mainly limited by the elastic strain of the Nilattice, which should be rather independent of themacroscopic morphology of the sample, we think that

1 For Ni-hydrogen compounds different from b-Ni-hydride theH/Ni ratio is �1 [21,23].


Fig. 8. Oc decomposition kinetics under N2 flow of species oxidized at respectively: peak 1(1%); peak 2; peak 3. Temperature: (A) 25°C; (B) 65°C;(C) 95°C.

some tens of monolayers is the typical penetrationdepth of this phase under the experimental conditionsdescribed.

The penetration depth of less hydrogenated phases(peaks 2–3) was predictably more than one order ofmagnitude greater.


Fig. 9. Kelac 47 Ni. CV patterns (5 mV s−1) at 25°C (curve a) and 95°C (curve b) in 0.6 M K2CO3+D2O electrolyte. Reference electrodeAg � AgCl.

3.6. Experiments in hea6y water

Fig. 9 compares the CV patterns (5 mV s−1) ob-tained at 25 and 95°C, respectively, in 0.6 M K2CO3+D2O electrolyte on a Kelac 47 electrode.

Starting the CV scan from −0.4 V, the her ispreceded by underpotential hydrogen deposition [1],with the formation of the less hydrogenated Ni species,which correlates with the most positive oxidative peakseen in the return scan. The anodic current, couplingwith the onset of the her, ascribed to oxidation ofhighly hydrogenated Ni, is quasi-negligible at 25 °C(curve a) but increases strongly at 95°C (curve b), wherea multipeak oxidative pattern is clear.

However, at 25°C, with longer cathodic polarizationand a slower potential sweep rate than in Fig. 9, thecathodic insertion of deuterium in Ni is also seen tolead to a three-peak oxidative pattern, as Fig. 10 shows.

The great similarity with results in light water isfurther shown by Fig. 11, in which the linear potentialsweep scans of the Ni sample cathodized in D2O at95°C (curve a) and 65°C (curve b) account for thetemperature-induced evolution of peak 1 into the newpeak 1%.

Table 2 compares the potentials of anodic peaksobtained after cathodic loading of Kelac 47 Ni in H2O-and D2O-based electrolytes; as regards to peak 1, al-though the differences in peak potentials are very small,they do have thermodynamic significance (see below).

Despite the frame of quite similar voltammetric be-haviour, some differences were indeed found betweenthe electrolytic insertion of hydrogen and deuterium.Thus, at 25°C, the cathodic insertion of hydrogen iseasier than that of deuterium, especially for peak 1species: we were never able to insert deuterium intoKelac 47 Ni by simple exposure to D2 gas dissolved inthe electrolyte at 25 °C (whereas at 95°C we succeeded).

Instead, the insertion of deuterium, even more thanthat of hydrogen, takes advantage of temperature, sothat Ni deuterides at 95°C have increased stability withrespect to Ni hydrides. This behaviour is easily ex-plained both by the voltammogram (Fig. 11, dottedcurve) recorded at 95°C at oc 1.5 h after cathodicloading and the oc potential decay of cathodized Kelac47 Ni at various temperatures, as Fig. 12 shows.

Fig. 12 also shows the effect of O2 on the potentialdecay rate at 95°C. O2 induces much faster decomposi-tion of peaks 1–3 than that occurring in a N2 atmo-sphere: in fact, when O2 is subsequently removed from


Fig. 10. Kelac 47 Ni. Linear sweep voltammetries obtained at 25°C in 0.6 M K2CO3+D2O electrolyte after 5 min cathodic polarization. Sweeprate 6 : (a) 0.5 mV s−1; (b) 1 mV s−1; (c) 2 mV s−1. Reference electrode Ag � AgCl.

the electrolyte (by N2), the electrode potential does notmove from the value attained in O2. Such behaviour istypical of transition metal hydrides which are decom-posed easily by O2 with water formation.

4. Discussion

The proposed attribution to hydrogen in Ni of all theanodic processes described so far may be criticized, atleast with regards to peaks 2–3, since they cover (Table2) a potential range in which Ni oxidation is expected[13].

As a matter of fact, most of the current literature onNi in alkaline media [14–17] ascribes the voltammetricpeak(s) observed in the range �0.2–0.5 V (vs RHE) tosurface conversion of Ni into Ni(OH)2. Such oxidationwould require an oxide-free metal surface provided byprevious cathodic polarization: in this view, the tran-sient cathodic wave preceding the her (a typical in-stance is given by curve a in Fig. 9) represents Ni(OH)2

reduction to Ni.However, some important data from the literature

cast doubt on the above interpretation.Visscher and Barendrecht [2] prepared authentic

Ni(OH)2 electrodes and demonstrated that Ni(OH)2 has

no voltammetric peak in the potential range 0.0–0.4 V(vs RHE) in either KOH or K2CO3 electrolytes. Inother words, the redox system positive to the her (curvea, Fig. 9) is unlikely to be explained by the reactions:

Ni(OH)2+2e−�Ni+2OH− (5)

Ni+2OH− −2e−�Ni(OH)2 (6)

Thus, the systematic observation of redox systemssimilar to that shown in Fig. 9 at several Ni-basedalloys was interpreted by Vrcar and Conway [18] asunderpotential hydrogen deposition (cathodic reaction)coupled with extraction of absorbed hydrogen (anodicreaction): the irreversibility of the system (spaced from�0.0 to �0.5 V vs RHE) was explained as due to slowhydrogen diffusion within the metal.

In this situation, we mainly focused our former inves-tigation [1] on identifying which process, hydrogenextraction or Ni oxidation, was really occurring atpeaks 2–3.

We chose the former process in view of the followingobservations:� the formation of the anodic pattern (as in Figs. 1

and 2, etc.) was hindered generally after forced and/or prolonged Ni polarization into the her. Thisbehaviour may be explained (see Introduction) if all


Fig. 11. Kelac 47 Ni. Linear sweep voltammetries obtained in 0.6 M K2CO3+D2O electrolyte after 5 min cathodic polarization at 95°C (curvea) and 65°C (curve b). Dotted curve at 95°C: loaded electrode was left in oc for 90 min. Reference electrode Ag � AgCl.

the peaks concern hydrogen, whereas there is noreason why reaction (6) should be inhibited;

� peaks 2–3 may be seen after simple Ni treatmentwith mineral acids, which agrees with the recognizedhydrogen absorption into Ni by spontaneous corro-sion [19];

� peaks 1–3 were decayed in succession at oc, withpartial conversion of one peak into the more positiveone: such a pathway may be expected only for metalhydride species, each decomposing into another oflower hydrogen content;

� Ni reacted with H2 and, conversely, peaks 1–3 re-acted with O2 in ways typical of, respectively, consti-tution and oxidation of metal hydride species.The data of the present work strengthen our prime

attribution to hydrogen of all the voltammetric peaksoriginating by cathodic Ni polarization.

Thus, analysis of the anodic current transients atthree typical potentials shows that either most of thecurrent or the whole current (peak 3) flowed underdiffusional control: this fact fits the hypothesis of hy-drogen oxidation limited by H diffusion within theguest metal, whereas it disagrees with a Ni surfaceoxidation process (reaction 6)). Furthermore, whateverexperimental design was adopted (electrolytic or gasloading, H2O- or D2O-based electrolytes, Kelac 47 or

SAFT Ni) both the charge thereafter extracted from themetal (especially those of peaks 2–3) was definitelyfound to increase with temperature, and the oc stabilityof peaks 2–3 likewise increased: how can these findingsfit the hypothetical occurrence of reaction (6)?

In conclusion, a comprehensive overview of all theresults of our former investigation as well as of those ofthe present one point to hydrogen in Ni as the maincause of the whole anodic voltammetric pattern. Theprincipal argument in favour of reaction (6) concernsthermodynamics which however cannot always be ver-ified in an electrochemical system, owing either tosluggish kinetics or overlapping side reactions: (in ourcase, hydrogen adsorption/desorption processes).Therefore, reaction (6), if any, is likely to take place asbackground current after exhaustive hydrogen extrac-tion [19].

As noted above, diffusional control in hydrogen ex-traction (Section 3.5) and overall increase in extractedhydrogen with temperature (Table 1—contrary to theprediction of gas–solid adsorption isotherms) also indi-cate that we are dealing with subsurface hydrogen. Thishydrogen permeates Ni not by simple intergranulardiffusion but by constitution of several Ni–hydrogenspecies, each characterized by a typical redox potentialand specific decomposition kinetics.


Table 2Peak potentiala of the species formed after the her at Ni in K2CO3+H2O and K2CO3+D2O

Epeak 2/V Epeak 3/VSystem T/°C Epeak 1/V Epeak 1%/V

vs Ag � AgCl vs Ag � AgCl vs RHEvs Ag � AgCl vs RHEvs RHE vs Ag � AgCl vs RHE

— −0.60 0.280.6 M −0.4825 0.40−0.87 0.01 —K2CO3+H2O

−0.69 0.190.6 M −0.5165 −0.88 0.00 −0.79 0.09 0.37K2CO3+H2O

0.17 0.350.6 M −0.54−0.720.1095 −0.88 0.01 −0.79K2CO3+H2O

0.40−0.470.280.6 M —25 −0.60−0.90 −0.01 —K2CO3+D2O

−0.69 0.190.6 M −0.5965 −0.92 −0.04 −0.83 0.05 0.29K2CO3+D2O

0.05 −0.70 0.190.6 M −0.6095 0.29−0.92 −0.03 −0.84K2CO3+D2O

a Measured after potential scan at 1 mV s−1 versus Ag � AgCl and RHE reference.

We will thus discuss the species associated with eachvoltammetric peak as single Ni–hydrogen compoundsor well-defined Ni-hydride phases.

Based on its oc decay (within a few tens of min at25°C) [5] and oxidation potential (Table 2) [11,20], thepeak 1 species has already been identified with theb-Ni-hydride phase [1]. This identification is supportedhere by:� increasing instability with temperature, which is

predicted by b-Ni-hydride formation thermodynam-ics (DH:−8 kJ mol−1) [5] and also confirmed bythe report of Baranowsky et al. [11] that b-Ni-hy-dride no longer forms electrolytically in H2SO4

when the temperature is ]60°C;� the observation that the formation of peak 1 species

is much more hindered in D2O- than in H2O-basedelectrolyte, which is again predicted by the thermo-dynamics of b-Ni-deuteride and b-Ni-hydride, re-spectively [5].With regard to these thermodynamics, whereby a Ni

specimen is converted fully into b-Ni-hydride underthousands of H2 atm [5], the enigma of having ob-tained this phase with 1 atm H2 dissolved in the elec-trolyte (Table 1) may be explained by viewingb-Ni-hydride formation as an exothermal reaction(which indeed it is) becoming hindered energetically asthe b-NiHx phase expands into the Ni lattice. In ourcase, for the expansion of only a few tens of monolay-ers (Table 1), still favourable thermodynamics must beassumed.

As regards the other peaks shown by the voltam-mograms, we correlate the one at the most positivepotential (peak 3) with the stable solid solutions ofhydrogen in Ni (a-Ni-hydride) which form, accordingto the literature [21], in low to moderate H2 pressures.

Here, Ni behaves like an endothermal hydrogen oc-cluder to achieve stoichiometric H/Ni atom ratiosranging from B10−4 to \10−2 increasing with bothtemperature and pressure [21].

Peak 3, poorly detailed in voltammograms at 25°C,becomes much better defined at 65 and 95°C, owing tothe increases in formation kinetics, loading ratio anddiffusivity of interstitial hydrogen, all induced pre-dictably by temperature. In addition, peak 3 speciesshow high oc stability at any temperature in oxygen-free solutions.

The formation of peak 2 species is also seen to befavoured strongly by temperature. The hydrogen con-tent is probably higher than that of a-Ni-hydride, asindicated by both the more negative redox potentialand the far lower stability at oc.

This species may be explained for hydrogen intersti-tial to Ni crystallizing in less common shapes (te-tradecahedral instead of cubodecahedral [3]) or phases(hexagonal within a fcc matrix [3,22]) which pro-bably coexist in sintered Ni. In the latter case, peak 2species may be identified with the hexagonal Ni hy-dride phase reported occasionally in the literature[21,23].

The formation of metastable species oxidizing atpeak 1% again appears to be endothermal, but it hasnothing to do with the species of peaks 2–3. Con-versely, its hydrogen content cannot be too far fromthat of the b-Ni-hydride phase, to which peak 1% ap-pears to be related due to the proximity of the respec-tive redox potentials. The narrow experimental rangein which peak 1% species can form as well as their lowoc stability, are probably the reasons why the exis-tence of such a hydrogen–Ni compound has neverbeen noted before.


Fig. 12. Kelac 47 Ni. Eoc vs time decay after 5 min cathodic polarization in 0.6 M K2CO3+D2O electrolyte at various temperatures under N2

flow:�, 95°C;, 65°C; �, 25°C. Rapidly decaying potential transient (dotted curve) was recorded in O2-saturated electrolyte. Reference electrodeAg � AgCl.

References

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formation of nickel hydrides by hydrogen evolution in alkaline media: effect of temperature

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