The hydrogen evolution reaction on NiPx alloys

The hydrogen evolution reaction on NiPx alloys

Trygve Burchardt*

Agder College, Grooseveien 36, 4890, Grimstad, Norway

Abstract

The hydrogen evolution reaction (HER) was studied on NiPx electrodes containing 15±27 at% P prepared by

electrodeposition. The amount of P in the alloy varied with deposition potential. The activity of the electrodes wasdependent on the P concentration, and the formation of a passive ®lm. The P in the deposited Ni alters the reactionmechanisms for the HER signi®cantly. This is probably caused by the absorption of hydrogen into the NiPx alloy.

A potential step, from the cathodic to the anodic region, gave information about the di�usion of absorbedhydrogen. A correlation between the activity of the electrodes, and the amount of absorbed hydrogen was found Ðthe most active electrodes also absorbed the largest amount of hydrogen. 7 2000 International Association forHydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.

1. Introduction

Hydrogen is one of the most promising energy car-riers and alternative to fossil fuels. Gaseous hydrogencan be easily produced by water electrolysis and

further used as a non-polluting fuel directly in internalcombustion engines or converted to electricity in fuelcells. The hydrogen evolution reaction has been studiedon various electrode materials in order to ®nd better

electrocatalysts and to understand better the kineticsand mechanisms of this process [1±8]. In alkaline waterelectrolysis, a mechanically and chemically stable,

active electrode is required. Nickel based electrodes areamongst the more active electrode materials for theHER in alkaline solutions [9]. A combination of a

large surface area with an enhanced catalytic activity,enables codeposits of Ni with other metals such as Co,Mo or Fe, to operate at overpotentials for the HER

close to 100 mV under what is typical for industrial

conditions [10]. A thin layer of NiPx or NiSx alloysformed on a Ni surface has also shown remarkablyhigh activity for the HER [11±14]. For NiPx alloys,

con¯icting results have been reported on their activityin alkaline solutions [14,15]. Paseka [14] reported highcatalytic activity for NiPx alloys containing around 6

at% P. On the other hand Gonzalez et al. [15] foundlow activity for Ni70P30 electrodes.

2. Experimental

The NiPx electrodes were prepared by electrodeposi-tion on a Ni-plate with an exposed area of 3 cm2. Prior

to deposition, the Ni plate was polished with 1000Mesh SiC, etched in concentrated HNO3 and washedin distilled water. The deposition was conducted in a

glass cell with volume of 0.3 l. A three-electrode set-upwas used, with a Ni counter electrode and a saturatedcalomel reference electrode. For all the experiments the

temperature was monitored with an ETS-D2 probe,which also served as input for the temperature control

International Journal of Hydrogen Energy 25 (2000) 627±634

0360-3199/00/$20.00 7 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.

PII: S0360-3199(99 )00089-0

* Tel.: +47-2295-8731; fax: +47-2295-8749.

E-mail address: [email protected] (T. Burch-

ardt).

system. The temperature could then be kept stable at252 18C. To attain an inert atmosphere, the cell was

continuously bubbled with N2. The electrolyte con-sisted of 0.1 M NiSO4, 0.3 M NaH2PO2, 0.15 MH3BO3 and 0.1 M NaCl, giving a solution with pH of4.6.

The HER was performed in a glass cell, using a 1 MKOH solution at 258C. The counter electrode was awire of Pt. A saturated calomel electrode was used as

reference electrode. In order to prevent oxygen in thecell the counter electrode was separated by a saltbridge. All data were corrected on an IR ohmic drop

between the reference and working electrode.

3. Results and discussion

3.1. Variations in the catalytic activity due to the

amount of P in the NiPx layer

Varying the deposition potential can e�ectively con-trol the amount of P in an electrodeposited NiPx layer.In these experiments, the P content in the NiPx alloy

was varied between 15 and 27 at%. This was achievedby changing the deposition potential in the range fromÿ645 mV to ÿ820 mV (sce). The electrodeposition

time was 1 h for all samples, producing NiPx layerswith thicknesses from 1 mm to 8 mm depending on thedeposition potential.

Fig. 1 shows the deposition potential and the cataly-tic activity as a function of the P content of the alloy.As can be seen from the ®gure, the P content in the

alloy decreases with decreasing deposition potential.The hydrogen evolution reaction on electrodeposited

amorphous NiPx electrodes was studied in a 1 MKOH solution. Potentiostatic experiments were per-

formed at ÿ1.3 V (sce). The ®rst 30 min of polaris-ation caused a pronounced increase in the currentdensity. After the samples had been activated the cur-

rent density stabilised. Fig. 1 shows the stable currentdensities obtained for the various samples after 1 h ofpolarisation.

The hydrogen evolution reaction is signi®cantlyincreased when the P content in the alloy is in therange around 17 at%, with a maximum current densityof 0.032 A/cm2 at 17.1 at% P. This value being over

Fig. 1. The e�ect of deposition potential and current density

for the hydrogen evolution reaction on the P concentration in

a NiPx alloy.

Fig. 2. Tafel sweep on passive and active NiPx electrodes with 17 at% P in 1 M KOH at 258C with a sweep rate of 1 mV/s.

T. Burchardt / International Journal of Hydrogen Energy 25 (2000) 627±634628

200 times larger than the lowest current density

(0.00028 A/cm2 at 27.1 at% P).An etched pure Ni electrode was polarised at ÿ1.3 V

(sce) in 1 M KOH. A hydrogen evolution current den-

sity of 0.001 A/cm2 was obtained. As can be seen fromFig. 1 this indicates that the presence of a largeamount of P in the alloy causes an inhibition of thehydrogen evolution reaction. On the other hand, a

small amount of P catalyses the reaction. The catalytice�ect is reduced when the P concentration in the alloy

moves towards 0 at%. The current density will then

probably move towards the value for pure Ni depo-sition.An activation of the NiPx electrodes occurs during

polarisation. This can be seen as an increase in thehydrogen evolution reaction with time before it stabil-ises. This is probably due to the removal of passivelayers on the NiPx electrodes. The characterisation of

passive layers on NiPx in alkaline solutions has beenstudied by Wronkowska [16].

Fig. 3. Hydrogen evolution current density for NiPx electrode with 15 at% P at 1 M KOH at 258C. Potentiostatic experiment at

ÿ1.3 V (sce).

Fig. 4. Cyclic sweep on NiPx electrode with 17 at% P in 1 M KOH at 258C with a sweep rate of 1 mV/s.

T. Burchardt / International Journal of Hydrogen Energy 25 (2000) 627±634 629

Fig. 2 shows Tafel sweeps taken on a NiPx electrode

with 17 at% P. Two curves are presented, one taken

before cathodic polarising the sample, the other after

30 min of polarisation. The Tafel curves obtained

before polarisation represent the removal of the passive

Ni(OH)2 layer in addition to the hydrogen evolution.

After the passive layer has been removed, a Tafel

curve characteristic for the hydrogen evolution reac-

tion is obtained.

The stability of the NiPx electrodes was tested by

polarising the samples at ÿ1.3 mV (sce) in a 1 M

KOH solution over a period of 14 days. Fig. 3 shows

the polarisation of a NiPx electrode with 15 at% P.The polarisation of the sample does not seem to a�ect

the e�ciency of the electrode. A reduction in the cur-rent density can be seen in the beginning of the exper-iment, and after exposing the electrode to air and

reintroducing it into the cell.

3.2. The e�ect of absorption of hydrogen on thecatalytic activity

In the work by Paseka [14] it is indicated that a

Fig. 5. Anodic current density for potentiostatic experiments at ÿ0.98 V (sce) in a 1 M KOH solution at 258C, after the electrodes

have been polarised at ÿ1.3 V (sce). On electrodes with various P contents.

Fig. 6. Cottrell plot of the anodic current density for potentiostatic experiments at ÿ0.98 V (sce) in a 1 M KOH solution at 258C,after the electrodes have been polarised at ÿ1.3 V (sce).


large amount of hydrogen is absorbed in the NiPx

alloys. This has been related to the high catalytic ac-

tivity of the hydrogen evolution reaction. This may bedue to a change in the energy of adsorption caused bythe absorption of hydrogen in the alloy.

Fig. 4 shows the cyclic polarization curve for acharacteristic NiPx alloy. A limiting anodic currentjust above the reversible potential for the hydrogen

reaction is observed. This indicates that the oxidationof hydrogen is limited by the di�usion of hydrogenabsorbed in the NiPx layer.

A series of NiPx alloys were formed by galvanostaticdeposition. The current density was varied between 0.3and 5.6 mA/cm2. In order to obtain and equal amountof deposited NiPx the time was adjusted accordingly.

The absorption of hydrogen on the NiPx electrodeswas studied by performing potentiostatic steps. Theelectrodes were ®rst polarised at ÿ1.3 V (sce), giving a

cathodic current that enables hydrogen to be absorbedinto the layer. Then the potential was stepped toÿ0.98 V (sce) where the hydrogen was oxidised.

Fig. 5 shows the anodic current densities as a func-tion of time after a potential steep from the cathodicregion. Based on the results of Paseka [14] and the lim-

iting anodic current observed in Fig. 4 it is assumedthat the anodic current density is due to the oxidationof hydrogen absorbed in the alloy. As can be seen, thecurrent density decreases with time. This is also to be

expected if the reaction is controlled by the di�usionof sorbed hydrogen. The changes in a di�usion currentdensity with time can be predicted by solving Fick's

second law. Under the assumption of a very fast elec-tron transfer, the solution of Fick's law is given by theCottrell equation [17]:

Id � ÿ�nFD0:5o �Hab��

p0:5t0:5�1�

where Id is the di�usion current, Do the di�usion con-stant, t the time and �Hab� the bulk concentration of

absorbed hydrogen.From Eq. (1) it can be seen that a plot of the current

density vs 1/(t 0.5) would give a straight line with a

slope containing the di�usion constant and the bulk con-centration of absorbed hydrogen. In Fig. 6 the oxidationcurrent for the various NiPx electrodes is plotted vs 1/(t 0.5). As can be seen, straight lines are obtained, indi-

cating that the oxidation of absorbed hydrogen is lim-ited by di�usion for all the NiPx electrodes. Accordingto Eq. (1), the di�usion coe�cient for absorbed hydro-

gen can be predicted from the slope of the lines, if theamount of absorbed hydrogen is known.The amount of absorbed hydrogen can be deter-

mined by integrating the current density time curves inFig. 5. This integration shows that the most activeelectrode, i.e. that containing 15 at% P, also has theT

able

1

ResultsforgalvanostaticdepositedNiP

xalloys

Depositioncurrentdensity

(mA/

cm2)

Currentdensity

atÿ1

.3V

(sce)in

alkalinesolution/m

A/cm

2Amountabsorbed

hydrogen/10ÿ9

mol/cm

2Pcontentin

Ni±Palloy/

at%

0.3

3.1

4.54

22

2.1

21.4

20.2

19.1

4.4

32.2

18.1

16

5.6

56.9

51.8

15


largest amount of absorbed hydrogen, (see Table 1). Asthe table shows, the change in the amount of absorbed

hydrogen is approximately equal to the changes inthe catalytic activity. This strongly indicates that theabsorption of hydrogen into the alloy in¯uences the

reaction mechanism for the hydrogen evolutionreaction.

3.3. The e�ect of ®lm thickness on the catalytic activityof NiPx

In the galvanostatic experiments the deposition po-tential changes during the experiment. Therefore, the Pcontent in the alloy must vary according to Fig. 1. In

order to keep the composition constant during the

Fig. 7. Polarisation sweeps on NiPx electrodes with various amounts of deposited alloy, in a 1 M KOH solution at 258C with a

sweep rate of 1 mV/s.

Fig. 8. Anodic current density for potentiostatic experiments at ÿ0.98 V (sce) in a 1 M KOH solution at 258C, after the electrodes

have been polarised at ÿ1.3 V (sce). On electrodes with various ®lm thicknesses.


deposition some potentiostatic experiments were per-

formed.

The e�ect of ®lm thickness was studied by changing

the deposition time at a stable potential. Fig. 7 shows

polarisation curves for the HER in an alkaline sol-

ution, on NiPx layers deposited at ÿ875 mV (sce), at

various deposition times. As can be seen the activity

increases with deposition time. For comparison an

electrode without P is also shown in the ®gure.

All the polarisation curves on the NiPx electrodes at

low current densities have a slope of 060 mV/dec. The

slope changes at high current densities towards

0120 mV/dec. The slope of pure Ni is 0120 mV/dec.

at all potentials. This indicates a change in the reaction

mechanism when P is present in the alloy. Paseka [14]

has also reported a 60 mV/dec. slope for the HER on

NiPx. From the accepted reaction mechanisms for the

HER a slope of 60 mV/dec. can be predicted if the

Frumkin adsorption isotherm is used instead of the

Langmuir. Lateral interactions between adsorbed

atoms then takes place [18]. This indicates that the

energy of adsorption of hydrogen is changed due to

the NiPx alloy. Absorbed hydrogen apparently changes

the electronic structure of the metal and thereby in¯u-

ences the electrocatalytic properties of the electrodes.

Fig. 8 shows the anodic current density vs time

curves for the oxidation of hydrogen on potentiostati-

cally formed NiPx electrodes. The electrodes have pre-

viously been cathodically polarised at ÿ1.3 V (sce) in

order to produce hydrogen. The potential is steeped to

ÿ0.98 V (sce). With increasing amount of NiPx depos-

ited onto the electrodes, an increase in the anodic cur-

rent density can be seen. For the pure Ni sample only

a small amount of hydrogen is absorbed. The amount

of absorbed hydrogen is found by integrating the

curves.

In order to determine the amount of ®lm, the weight

gain due to deposition was determined. Fig. 9 shows

that a linear relationship is found between the amount

of absorbed hydrogen and the weight gain. This indi-

cates that the total amount of ®lm is available for the

absorption of hydrogen. Since the activity of the elec-

trodes also increases with the increasing amount of

deposited ®lm, (see Fig. 7), the increase is probably

due to the amount of absorbed hydrogen.

4. Conclusions

Electrodeposited NiPx electrodes with di�erent

amounts of P have been studied towards the hydrogen

evolution reaction. The P content of the alloy could be

altered by changing the rate of deposition. The results

show that the catalytic activity of the electrodes varies

with the P content in the alloy. The highest activity is

obtained within a narrow range around 17 at% P. It

was found that the catalytic activity is related to the

amount of absorbed hydrogen. The activity increases

with increasing absorption. The catalytic activity is re-

lated to variations in the energy of adsorption of

hydrogen. The absorption of hydrogen in the alloy

probably changes the electronic structure of the metal,

thereby increasing the catalytic activity.

Fig. 9. The amount of absorbed hydrogen as a function of the weight gain due to the deposition of the NiPx alloys.


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The hydrogen evolution reaction on NiPx alloys