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Scanning electrochemical microscopy (SECM) : study of the adsorption and desorption of hydrogen on platinum electrodes in Na2SO4 solution (pH = 7)
ELSEVIER
Journal of Electroanalytical
Chemistry
418 (1996)
99-107
Scanning electrochemical microscopy ( SECM) : study of the adsorption and desorption of hydrogen on platinum electrodes in Na,SO, solution (pH=7) Yi-Fu Yang, Guy Denuault * Depurtment
oj’Chemi.vtry,
University
Received
24 April
oj’.Southompton. 1996; revised
Southampton 17 May
SO17 IBJ,
l/K
1996
Abstract Hydrogen adsorption and desorption at polycrystalline platinum electrodes in a neutral Na*SO, solution were studied with a scanning electrochemical microscope. Experiments were carried out with the tip-substrate voltammetry mode, where the faradaic current flowing to the tip is recorded while cycling the potential of the substrate, and with the tip-substrate chronoamperometry mode, where the tip faradaic current is recorded as a function of time following the application of a potential step to the substrate. The tip current was made pH sensitive by holding the tip potential in a region where a pH-dependent reaction occurs. Proton reduction was used to monitor pH decrease, whereas platinum oxide formation was selected to detect pH increase. The results showed that a transient pH decrease as high as 2.3 pH units exists during hydrogen desorption and that a great pH increase occurs during hydrogen adsorption. The mechanisms of hydrogen adsorption and desorption were analysed by comparing tip current vs. substrate potential curves, which reflect the exchange of Ht between the adsorbed layer and the solution, with substrate current vs. substrate potential curves, which reflect the exchange of electrons between the adsorbed layer and the electrode. New conclusions have been drawn. Keywv~ds:
SECM;
Scanning
electrochemical
microscope;
Hydrogen
adsorption-desorption;
1. Introduction Hydrogen adsorption and desorption on platinum electrodes are important reactions in electrochemistry. During the last decade, a large number of studies has focused on the relationship between the characteristics of hydrogen adsorption-desorption and the surface structure of platinum electrodes. In most cases results were obtained from cyclic voltammetry on single crystal [l-7] and polycrystalline platinum electrodes [8,91. The influence of the pH of the electrolyte solution on the characteristics of hydrogen adsorption-desorption has also been emphasized by many authors [5,8,10- 121. It is agreed that in acidic media, hydrogen adsorption-desorp tion reactions can be written as [ 13- 151 M+H++e-*M-H,,
* Corresponding
(1)
author.
0022.0728/96/%15.00 Copyright PII SOO22-0728(96)04768-7
0 1996 Elsevier
Science
Platinum
electrode
where M represents electrode. In neutral reaction is [ 161 M+H,O+e
the surface atom of the platinum or alkaline solutions, the proposed
- + M-H,,,
+ OH-
(2)
The reactions in Eqs. (1) and (2) imply that electron transfer and proton transfer proceed simultaneously. A
survey of the literature shows that most of the data concerning the adsorption and desorption of hydrogen on platinum have been obtained by voltammetric methods; these data therefore relate only to the transfer of electrons at the electrode surface. However, since the solution pH influences the reactions, it is worth considering the overall mechanismof adsorption-desorption as being made up of two distinct processes:electron transfer between the adsorbed layer and the electrode, and Ht transfer between the adsorbedlayer and the solution. The latter is expected to induce transient proton concentrations in the vicinity of the electrode surface. Few articles report investigations of these two aspects of hydrogen adsorption-desorption. Ogasawara and Ito
S.A. All rights reserved
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[17] studied the electron transfer during hydrogen adsorption on different platinum single-crystal faces by voltammetry and detected the state of the adsorbedproton with in situ infrared reflection adsorption spectroscopy.KBtz et al. [18] presentedresults on the detection of proton fluxes at the solidlliquid interface during oxide formation/reduction on a polycrystalline gold electrode. They used probe beam deflection and achieved a good sensitivity; however, the technique was reported to be much less sensitive for platinum electrodes, where over 40 scanswere claimed to be necessaryin order to obtain similar results [19]. Proton concentration changes near electrodes have also been probed with a rotating ring disc electrode. A bismuth ring electrode was operated in the potentiometric mode to detect the changes in pH produced by reactions taking place on the disc electrode [20]. This technique proved to be very sensitive to the fluxes of H+ and OH- and to the variations of surface coverage down to a fraction of a monolayer. Scanning electrochemical microscopy (SECM) is a technique developed in recent years [21]. It has the ability to image the topography and reactivity of surfaces with a spatial resolution better than a micrometre. Tip-substrate voltammetry is one of the main operational modes of SECM where the tip current Ztip is recorded as a function of the substrate potential Esubwhile subjecting the substrate to a potential ramp and keeping the tip within a few micrometersfrom the substratesurface. It hasproved to be a powerful method which provides direct information about ion fluxes related to reactions on the substrate surface. This method was used to probe the ingressand egressof protons from a polyaniline film [22,23]. In a recent article we presentedthe results of an SECM study of pH changes near platinum electrode surfaces in pH4 sodium sulphate solutions [24]. The basic principles of the technique were developed and the merits of the approach were assessedboth qualitatively and quantitatively. Experiments consistedin acquiring a seriesof tipsubstrate voltammograms where the faradaic tip current was made pH sensitive by holding the tip potential in a region where a pH-dependent reaction occurred. Three reactions were considered: hydrogen evolution, Pt oxide formation and oxygen evolution. Results provided direct evidence of a pH decreaseclose to the substrate during hydrogen desorption, oxide formation and oxygen evolution. Conversely, a pH increasewas observed during oxide reduction, hydrogen adsorption and hydrogen evolution. A quantitative assessment of pH changeswas proposed; variations as large as one pH unit were observed. The present article reports the results of a similar study carried out in pH7 sodium sulphatesolutions; whereasthe previous paper aimed at establishing the principles of the SECM pH detection, the work presentedhere focuses on the hydrogen adsorption-desorption reactions. Tip-substrate voltammetry and tip-substrate chronoamperometry were used to monitor the tip responsewhile promoting the
Chemistry
418 (1996)
99-107
adsorption and desorption of hydrogen from the substrate. Different tip responseswere recorded depending on the amount of protons generatedor consumed: the steady-state tip current for proton reduction was used as a measureof the local proton concentration during hydrogen desorption (increase of the proton concentration within the tip-substrate gap) whereasthe tip current for tip oxide formation was used to follow hydrogen adsorption on the substrate (decrease of the proton concentration within the gap). Reaction mechanismsfor hydrogen adsorption and desorp tion on platinum electrodes were analysed by comparing tip and substrateresponses.The tip responsevs. substrate potential curve reflects the variations in H+ concentrations with substratepotential, while the substratevoltammogram informs about the changes in electron transfer with substrate potential.
2. Experimental Although several experimental conditions used in the present study are similar to those reported in Ref. [24], the basic details are listed below. The SECM apparatus has been described previously [25]. A home-madepotentiostat and a home-madebipotentiostat were usedto control the potentials of the electrodes. Potential waveforms were generated by a Hi-Tek PPRl waveform generator. All the electrochemical measurements were carried out with the cell placed inside an aluminium Faraday cage. The SECM tip, a 25 p,m diameter Pt microdisc sealedin glass, was polished on emery paper then on alumina lapping film (3M) to form a truncated cone; the ratio of the radius of glassto that of platinum was about 4. The SECM substrate was a 0.5 mm diameter Pt disc also sealed in glass. Before use, both tip and substrate were polished with alumina powder (0.05 km) on a moistenedpolishing cloth (Buehler) and rinsed with ultrapure water. Both electrodes were then cleaned as follows: in the first step the top of the electrode was heated at a temperaturejust below the melting point of glass for 2min, then the temperature was slowly lowered till the electrode was fully cooled to an ambient temperature. In the second step the electrode was immersed in 1 moldmm3 H,SO, solution and its potential was cycled between + 0.8 V and - 0.68 V (vs. HglHg ,SO,Isat. K ,SO,) until a reproducible characteristic voltammogram was obtained. Instead of a saturated calomel electrode, which would introduce interference from chloride ions, a home-made Hg ]Hg, SO, reference electrode with saturatedK 2SO, solution was used. All potentials are referred to this electrode. The auxiliary electrode was a pure platinum wire. Electrolytes were prepared with Aristar grade Na,SO, (BDH) and ultrapure water freshly made from a Millipore purification system (resistivity greater than 18Ma cm). This pure water was alsoused for rinsing the electrode and
Yi-Fu Yung. G. Denuuult/Journul
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the cell. The pH of the solutions was adjusted by addition of carbonate-free NaOH solution, Analar (BDH). pH was measured with a 3010 pH meter (Jenway Ltd) which was regularly calibrated with standard solutions. All solutions were thoroughly deoxygenated with pure N, before experiments were carried out. The influence of CO, on the pH of the solutions was also tested. The results showed that the pIi of pure fresh water opened to the air decreased gradually due to the dissolution of CO, from the air. In less than 30 min the pH had decreased by about 1. However, the pH of an Na,SO, solution initially at pH 7 did not change during deoxygenation with N, gas. When deoxygenated and non-deoxygenated Na,SO, solutions were opened to the air, the pH only decreased by 0.2 over 2 h. The pH of the solutions remained constant when a blanket of N, gas was kept above the solutions. These results indicated that under our experimental conditions, the influence of the CO, could be ignored. In order to get reproducible results, a strict experimental procedure was followed. Both tip and substrate electrodes were cleaned as described previously. The substrate was inserted vertically facing upward from the bottom of the cell. A sheet of parafilm was used to cover the cell and a hole was made in this to let the tip penetrate into the cell. The tip was mounted on the microstage and its position was adjusted manually very close to the substrate surface. The cell was purged continuously with nitrogen gas, the deoxygenated solution was introduced into the cell; finally, reference and auxiliary electrodes were put in place via two branch tubes on either side of the cell. Another two glass branch tubes were used to maintain a blanket of N, gas above the solution in order to avoid disturbances from 0, and CO, during the experiment. One of the cell walls was made with optical glass to facilitate observation of the tip-substrate distance with a stereomicroscope (Gallenkamp). The quality of the substrate and of the tip was assessed by recording voltammograms between +0.7 and - 1.2 V at a scan rate of 50mV s- ’ until they became stable (typically after several cycles). The tip-substrate distance was then adjusted with the piezocontroller down to about 1 pm. The bipotentiostat was used to control the potentials of the tip and of the substrate independently. Tip-substrate voltammograms were acquired by recording the tip current at a fixed tip potential while cycling the potential of the substrate; various tip responses were obtained by holding the tip potential at different values. Tip-substrate chronoamperometry was carried out by holding the tip potential, stepping the substrate potential and recording the tip current as a function of time. All the tip-substrate voltammograms shown in this article were recorded on the same day, with the same solution, same tip, same substrate and same tip-substrate distance (i.e. the tip was not moved between the various recordings). Similarly all the tip chronoamperograms were acquired the same day, with the
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same solution, same tip and same substrate. Experiments were repeated and the results discussed in Section 3 were found to be reproducible.
3. Results and discussion 3.1. Cyclic voltammograms
on platinum electrodes
While cycling the potential of a platinum electrode between hydrogen evolution and oxygen evolution, many factors will determine the peak potentials for hydrogen adsorption and desorption. Fig. 1 shows two cyclic voltammograms recorded with a Pt electrode in 0.5 moldmp3 Na,SO, (pH7) at two different scan rates. With a low potential scan rate, 50mV s-’ (Fig. l(a)), the adsorption of hydrogen occurs at -0.975V (strong) and -1.115V (weak); the corresponding desorption occurs at - 0.61 V and - 0.69 V respectively. Keeping all other conditions the same, the voltammogram changes greatly (Fig. l(b)) when the scan rate is increased to 200mV s- ‘. For the hydrogen adsorption reaction, the peak potentials for strong and weak adsorptions are respectively - 1.Ol V and - 1.16 V, which correspond to negative shifts of 35mV and 45mV. This result indicates that the rate of the weak adsorption is slower than that of the strong one. For the hydrogen desorption reaction, however, the situation becomes more complicated at higher potential scan rates (Fig. l(b)). The
20 10 -3.
o-
-
-10 -20 -30 -40
I -1.2
I
I -0.8
I
I1 -0.4
I 0
I
I 0.4
I 0.8
E/V Fig. 1. Cyclic voltammograms solution with N, gas flowing rate: (a) 50mV s- ‘; (b) Hg/Hg,SO,Isat. K*SO,.
at a Pt electrode in 0.5 moldmm3 Na,SO, on the top of the solution. Potential scan 200mV s-‘; potentials in volts vs.
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418 (1996199-107
the other is in the potential region between -0.75 and - l.OV with a peak at - 0.94 V. The latter peak corresponds to the desorption of hydrogen in an increased pH environment caused by the adsorption of hydrogen on the substrate as the potential is scanned negatively, whereas the former results from the effect of solution stirring by the N, gas. These observations can be explained in terms of the gas flow induced convection which increases the rate of mass transport of H+ from the bulk solution to the vicinity of the electrode where the high concentration of OH- resulting from the adsorption of hydrogen can be neutralized by the reaction H++ OH-+
-1.2
-0.8
-0.4
0
0.4
0.8
EIV Fig. 2. Cyclic voltammograms at a Pt electrode in 0.5moldm-2 Na,SO, solution. Potential scan rate: 1OOmV s- ‘. (a) Without N, gas flowing on the top of the solution; (b) with N, gas flowing weakly on the top of the solution; (c) with N, gas flowing strongly on the top of the solution. Potentials in volts vs. HgIHg,SO,jsat. K,SO,.
desorption of hydrogen is separated into two parts; the first one is in the potential region between - 0.48 and - 0.79 V with peaks at -0.60 and -0.67V. This part is the same as that in Fig. l(a). The second one is in the potential region between - 0.85 and - 1.02 V with a single peak at - 0.95 V. This peak could be explained by assuming that hydrogen desorption took place in a high pH (> 7) environment as a result of the adsorption of hydrogen when the potential was scanned negatively. This is an indication that the adsorption and desorption of hydrogen are sensitive to the pH of the solution in the vicinity of the electrode. Fig. 2 shows the stirring influence induced by the N, blanket flowing over the solution. In Fig. 2(a), the N, gas flow was stopped. Hydrogen adsorbed at potentials - 1.O V (strong) and - 1.15 V ( weak), and desorbed in the region between - 1.07 V and -0.66V marked by a single broad peak at -0.95 V. When a slow N, gas flow passes over the solution (Fig. 2(b)), the solution is gently stirred. The adsorption of hydrogen occurs at - l.OV (strong) and - 1.14V (weak). However, the desorption of hydrogen is separated into two peaks. One is in the potential region between - 0.48 and - 0.75 V with a peak at - 0.595 V,
H,O
(3)
The extent of the recombination depends on the strength of the stirring. With a weak gas flow the local solution is not completely replenished with H+; some adsorbed hydrogen desorbs in a higher pH (> 7) environment, some desorbs in a neutral environment. This explanation can be further supported by the voltammogram recorded with a faster N, gas flow, Fig. 2(c). With stronger stirring, the desorption peak in the potential region between - 0.75 and - 1.OV disappeared but the peak height in the region between - 0.48 and - 0.75 V increased and separated into two peaks at - 0.59V and - 0.69V respectively. These two peaks correspond to the desorption of strongly and weakly adsorbed hydrogen. This result indicates that in such conditions all adsorbed hydrogen desorbs in a neutral environment. With the stronger gas flow, the adsorption of hydrogen occurs at potentials of -0.985 (strong) and - 1.135 V (weak), which corresponds to a positive potential shift of 15mV for both the strong and the weak adsorptions. From the results shown in Figs. 1 and 2, it is clear that the change of the solution pH has influenced both hydrogen adsorption and desorption. The peak potential shift for hydrogen adsorption is smaller than that for desorption when using stronger stirring; this is possibly caused by the fact that the potential difference between hydrogen adsorption and platinum oxide reduction (which also consumes Hf and hence increases pH) is smaller than that between hydrogen desorption and adsorption; the adsorption of hydrogen thus always occurs in an increased pH environment. The influence of convection and potential scan rate presented here are in agreement with the findings of Pletcher and Sotiropoulos [8] who carried out a series of experiments to investigate the effects of mass transport (using rotatin g d’isc and microdisc electrodes) and scan rate on the voltammetric response of Pt electrodes in unbuffered neutral Na,SO, solution. 3.2. H ’ desorption
Fig. 3 illustrates tip-substrate voltammograms recorded with two different tip potentials. In Fig. 3(a) Etip was set at - 1.2 V; at this potential, proton reduction is the tip reac-
Yi-Fu
Yang,
G. Dmuuult/Journal
ofElectrcxmcrlyticul
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-0.8
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418
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proton reduction, see Fig. 3(b). The basic characteristic of the peak in Fig. 3(b) is the same as that in Fig. 3(a) except for the zero baseline due to the absence of water reduction; the peak height in Fig. 3(b) (measured as 6.25nA) can be used to calculate the transient pH change. As the tip is very close to the substrate (1 km), the tip-substrate ensemble behaves like a leaky thin layer cell with the tip and the substrate as the two electroactive walls. For a neutral solution, the situation is different from that described in Ref. [24] at pH4, and the calculation of pH changes resulting from the desorption of proton from the substrate surface is also different. Using the analogy with a four-electrode thin layer cell, where one wall acts as the generator and the other as the collector, the steady-state tip current is given by 1261
0.8
1V
Fig. 3. Tip-substrate voltammograms in 0Sn1oldm~~ Na,SO, solution. Potential scan rate for Esub SOmVs-‘, dtipmsub = 1 Fm. (a) Etip = - 1.2V; (b) Etip = - l.OV. Potentials io volts vs. Hg/Hg,SO,lsat. KZS04.
tion chosen for monitoring the concentration of H+ near the substrate surface. In a pH7 Na,SO, solution the concentration of Hf is, however, too low to give a measurable steady-state current for H+ reduction, and at - 1.2 V only the water molecule can be reduced according to the reaction 2H,O + 2e--+ H, + 20H-
Chemistry
(4 The dashed line in Fig. 3(a) indicates the magnitude of the tip current for water reduction at this potential. This was recorded by holding the substrate potential in the double layer region at about -0.2V and waiting until the tip current had become stable. Any situation leading to an increase of the concentration of protons within the tipsubstrate gap, e.g. a substrate reaction releasing protons, will promote the reduction of protons in addition to that of water and will lead to a significant increase of the tip current. Fig. 3(a) shows that when Esub is swept positively, ZtiP increases quickly from a potential of Esub = - 0.8 V, forms a single sharp peak at Esub = -0.55 V and then decreases quickly. Compared with the corresponding substrate behaviour (Fig. l(a)), it is apparent that this tip current peak is the result of hydrogen desorption on the substrate which causes a transient increase of H+ concentration. In fact, when Esub is between - 0.8 and - 0.35 V the tip response comes from two contributions, i.e. water reduction and proton reduction, but it is not easy to calculate the latter independently. When EtiP is held at - l.OV the current for water reduction approaches zero and the observed Zpeak-tip can be analysed in terms of the sole contribution from
where n is the number of electrons involved, F is the Faraday constant, D is the diffusion coefficient of H+, a is the radius of the microdisc electrode, c,,~ is the proton concentration on the substrate surface (not the coverage), ctip is the proton concentration on the tip surface and is the tip-substrate distance. As Erip is set in the dtip-sub steady-state current potential region for H+ reduction, ctip = 0. With n = 1, D = 7.7 X lo-‘cm* s- ’ [27], a = = 6.25nA, the tran12.5 km dtip-sub = 1 p,m and Ztipmpeak sient proton concentration at the surface of the substrate C subis calculated as 17.1 X 10m6M which correspondsto a pH of 4.77. It is now important to compare Zpeat-subin Fig. l(a) in Fig. 3(b); Isub reflects the transfer of wi* ‘peak-tip electrons between the adsorbed layer and the electrode, whereas Zrip reflects the transfer of protons between the adsorbed layer and the solution. It is thus possible to compare the transfer of electronic and ionic chargesduring the hydrogen desorption process.One can seethat Zpeakmtip begins at Esub= - 0.8 V, i.e. at a potential 150mV more positive than that of Zpeakwsub. This result indicates that the desorption of weakly adsorbedhydrogen proceeds in two separatesub-steps;the first one is the transfer of electrons from the adsorbedlayer to the electrode and the secondis the releaseof protons into the solution. At Esub= -0.6 V, Ztip bends to give a steeper current increase and forms a peak located at Esub= - 0.55 V, i.e. at a potential 60 mV more positive than the potential of Zpeakesub for the desorption of strongly adsorbed hydrogen (peak potential at Esub= - 0.61 V) on the substrate cyclic voltammogram. From diffusion theory, it is easy to calculate that, at drip-sub= 1 p.m, the time taken by H+ to diffuse from the substrate surface to the tip surface is about 1.3 X 10m4s, whereasa 60mV peak potential difference correspondsto a delay of 1.2 s between the substrate and tip responses (the potential scanrate is 50mV SK’). Hence, it is reason-
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6
5
4
P .
3 2
2
1
0 0
1
2
3
4
5
6
t/s
Fig. 4. Tip-substrate chronoamperograms step from -1.12 to -0.3V. E,,,=-l.IV. 4O*m; A 50p,m; l 6Opm.
at different dtlp-sub with dtiperub: n 30km;
Esub r
able to consider that the desorption of strongly adsorbed hydrogen also proceeds in two consecutive sub-steps, where the electron transfer occurs prior to the release of Hf. The tip current peaks in the region Esub = -0.3 to + 0.7 V (both in Fig. 3(a) and 3(b)) come from an increase of the H+ concentration resulting from the formation of oxide on the substrate. The results of an SECM study of oxide formation and reduction will be discussed in a separate publication [28]. In Fig. 3(a), when Esub is swept negatively the tip current decreases and forms two positive (with respect to the dashed line) peaks, one for Esub between -0.3 and - 0.8 V which corresponds to the reduction of oxide on the substrate, the other between - 0.9 and - 1.1 V which corresponds to the adsorption of hydrogen on the substrate. There are two peaks because water reduction depends on the pH near the electrode surface. During oxide reduction and hydrogen adsorption on the substrate, the concentration of OH- increases in the vicinity of the substrate surface and the reaction in Eq. (4) can be slowed down. In other words, water molecules are reduced at more negative potentials when the pH increases. In Fig. 3(b), where Etip is held at - l.OV the water reduction current is around zero and only the reduction of substrate oxide which results in a great pH increase can be detected. albeit with a much lower sensitivity. Fig. 4 illustrates tip-substrate chronoamperograms recorded as Esub was stepped from - I. 12 to - 0.3 V. The experiment proceeded as follows: first Esub was maintained at - 1.12 V for 2 min (this potential is just slightly more positive than that for water reduction) then Esub was
Chemistry
418
(1996)
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stepped to - 0.3 V causing the desorption of both weakly and strongly adsorbed hydrogen to occur. EtiP was held at - 1.1 V, which is in the potential region for H+ reduction when pH < 7. Tip current transients were recorded for several dtipmsub values. Fig. 4 shows the typical distortional effects of diffusion [29]; as the tip is moved closer to the substrate, the tip current peak becomes higher and the time needed for the appearance of the current peak is shorter. This is because at higher &P-sub protons need a longer time to diffuse from the substrate to the tip. This figure also points out that the transient change in H’ concentration only lasts for several seconds; this result can well explain the effect of potential scan rate on the shape of the substrate voltammogram shown in Fig. 1. The tip current peak heights measured at various tipsubstrate distances are listed in Table 1. One can see that the peak current, 72.51~4, is much at 4ipsub = 1 km higher than that in Fig. 3(b) (6.25nA). This suggests that the rate of proton release is controlled by the potential. At a tip substrate distance of 60 Km, however, the tip current peak is very low; nevertheless, this suggests that the substrate surface reaction has significantly affected the pH of the solution over 60 km. No attempt was made to transform the tip current value into a substrate surface proton concentration because Eq. (5) becomes less valid as the tip-substrate distance increases. When dtip-sub = 1 pm the diffusion time is about 130 p,s and we can assume that the proton concentration instantaneously reaches a steady-state linear profile from c sub to cfiP = 0; this is no longer true when dtipesub increases, e.g. it takes on average 13 ms for the wave of H+ to reach the tip when dtipYsub = 10 km. 3.3. H + adsorption
As mentioned previously, the proton concentration at pH7 is too low to give a measurablesteady-state current on the tip. Any further decreaseof the proton concentration - e.g. due to a substratereaction consumingprotons, such as hydrogen adsorption - cannot be probed directly with the reduction of Hf on the tip. However, some information about hydrogen adsorption can still be ob-
Table I Relationship d op-sub /pm I
5 10 20 30 40 50 60
between
dlipmrusb and Ipeaketip I peak. ,,p in.4
72.50 33.46 18.18 9.55 5.58 2.80 1.15 0.50
Table 2 Relationship between ELRP, IpEaketip and substrate hydrogen relative coverage 6. EL,, VS. Hg~Hg~SO~~~t. K2S0, 0 %a, /v jperk I tip/nA -0.8 1.0 0.165 - 0.9 1.37 0.226 - 1.05 2.23 ii.376 - I.1 2.8 0.461 - 1.2 6.07 1
-
-6
-7
-1.3
--J-.--L.-I1 -1.1
-0.9
-0.7 %a
-0.5
-0.3
-0. I
f V
Fig. 5. Tip-substrate voi~mmog~ams in 0.5moldm-3 Na$O, solution with different Erub regions. Potential scan rate for Esub SOmVs- ‘, Etip = - 1.1v, drjp-s”b = 1 ym. EsUbregions: (11 -0.2 to - 1.2V; (21 -0.2 to - I.lV; (3) -0.2 to - 1.05v; (4) -0.2 to -0.9v; t.51 -0.2 to -0.8V. Potentials in volts vs. HglHg,SO,lsat. K,SO,.
tained, but indirectly, from the response of the tip to hydrogen desorption on the substrate. The approach is illustrated in Fig. 5, which shows a set of tip-substrate voltammograms recorded within different Esub regions outside the influence of piatinum oxide formation and reduction on the substrate. In these ~x~rimeuts the upper reversal potential of Esuh was held at -0.2 V while the lower reversal potential ELRP was changed from
-1.2
-1.0
-0.8
-0.6
one voltammogram to the next. The variations of Ipeak.+ with ELRp are listed in Table 2. The adso~tio~ of hydrogen on platinum is loyally considered as a reversible reaction, and its equilibrium coverage is determined by the electrode potential [30]. Since the tip current is proportional to the concentration of protons released by the substrate, we can therefore expect that the tip current peak is proportional to the hydrogen subs~~te coverage at a given ELRP. Let us suppose that I peak-tip = 6.07nA (E,,, = - 1.2VI represents the full coverage, then the ratio of Ipeakmtipfor another ELKr value to 6.07nA gives a relative coverage 8. The calculated results are listed in Table 2. The voltammogram in Fig., l(a) suggests that - 0.8 V is the initial potential for hydrogen adsorption. Table 2, however, indicates that the proton coverage on the platinu~~ electrode surface at -0.8 V is already as high as 16.5%. This suggests that the adsorption of protons starts at a potential more positive than - 0.8 V, although this feature cannot be seen from a substrate voltammogram. In agreement with the results from tipsubstrate voltammet~, linear potential scan ex~rin~ents (from -0.2 to - 1.2V) also show that hydrogen adsorp-
-0.4
-0.2
0
0.2
0.4
0.6
f-Lb1 v Fig. -6. Tip-substrate vol~mmogmms in 0.5 mol dm -’ Na,SO, solution at diffe~nt Esubregions. Potential scan rate for f& drip-sub = 1p.m. Esub regions: (a) iO.5 to - 1.2V; (b) -0.2 to - 1.ZV. Potentials in volts vs. ~gl~g*S~~{sat. K,SO,.
50mV s- I, Etip = - 0.1 V,
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tion starts at -0.72V. For strong adsorption, the difference between the starting potential of the tip and substrate responses suggests that hydrogen adsorption occurs before the transfer of electrons. The coverage increase seen when E,,, is changed from - 1.1 to - 1.2 V is more than 50%; this is much higher than expected from the substrate voltammogram. The difference between the response of the tip to the changes in Hf concentration and that of the substrate to the transfer of electrons indicates that a more complicated hydrogen adsorption mechanism needs to be considered. Fig. 6 shows a set of tip-substrate voltammograms obtained using tip oxide formation as the sensitive reaction for the detection of transient proton concentrations in the vicinity of the substrate. Etip was held at - 0.1 V, i.e. at the foot of the wave for oxide formation. As discussed in Ref. [24], the tip oxide coverage is a function of the tip potential and of the pH of the solution. Changing either of these two factors induces the formation or reduction of tip oxide and leads to a faradaic tip current. In a tip-substrate voltammetry experiment, &, is held at a constant value and the oxide coverage on the tip is mainly determined by the pH of the solution. The relationship between the peak potential for oxide formation/reduction and pH has been reported as about - 60 mV per pH unit [3 1I, which means that a pH increase in the solution leads to a negative shift of the potential for oxide formation and results in a positive tip faradaic current. Conversely, a pH decrease causes a negative faradaic current on the tip. In Fig. 6(a), Esub was cycled in a region between +0.5 and - 1.2V. Considering the sweep towards negative potentials, we see two significantly overlapped positive tip current peaks between - 0.18 and - 0.8 V. These peaks result from a pH increasedue to the reduction of oxide on the substrate surface. In the region between -0.8 and - 1.18V another two small positive tip current peaks occur at - 0.99 and - 1.15V. These two peaks result from a pH increase due to the adsorption of hydrogen on the substrate. Recalling the peak potentials for hydrogen adsorption in Fig. l(a) (-0.975V for strong adsorption and - 1.115 V for weak adsorption), we find a 15mV offset between the tip response and the substrate responsefor strong adsorption and a 35 mV offset for weak adsorption. Considering the sweeptowards positive potentials, we see wo negative tip current peaks between -0.82 V and -0.42V, respectively located at -0.68V and -0.58V. These peaks are caused by a pH decreasein the solution due to the desorption of hydrogen from the substrate surface. As the substrateupper reversal potential is changed to -0.2V, oxide formation/reduction on the substrate no longer occurs; a tip-substrate voltammogram illustrating this situation is shown in Fig. 6(b). It is quite different from that in Fig. 6(a), in that the tip current peaks corresponding to the adsorption of hydrogen on the substrate are enhanced; two clearly distinguished peaks seen at
Chemistry
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E sub = - 0.93 V and - 1.12V are respectively 60mV (for
strong adsorption) and 30mV (for weak adsorption) more positive than the samepeaks in Fig. 6(a). Comparing Fig. 6(b) and Fig. 6(a), it is clear that the reduction of substrate oxide has a serious influence on the adsorption of hydrogen; this influence may occur either in the form of an increase of the pH of the solution near the substrateor in the form of a change in the state of the substratesurface. Fig. 6(b) also indicates that the change of H+ concentration starts from Esub = -0.76V; this result is consistent with that in Fig. 5. Comparing the potential of Ipeaketip with that of I peak-sub, it is clear that in the caseof strong adsorption the peak potential for Ipeak-iip( - 0.93 V) is about 45 mV more positive than that for Zpeakesub (-0.975V in Fig. l(a)). This further proves that for strong adsorption the hydrogen adsorption sub-step precedes the electron transfer. However, in the caseof weak adsorption the peak potential for Ipeakmrip (- 1.12V) is almost the sameas that of Zpeakmsub ( - 1.115V in Fig. l(a)). This result suggeststhat the adsorption of weak hydrogen proceedssimultaneouslywith the transfer of electrons. is scannedpositively, the magnitude of the men Esub tip current increases from Esub = -0.9 V and produces two negative tip current peaks at potentials of -0.735 and - 0.59 V. These two peaks correspondto the desorption of weakly and strongly adsorbed hydrogen. Compared with Fig. 6(a), it is clear that the desorption of strongly adsorbed hydrogen is not affected by the history of the surface (i.e. whether oxide formation/reduction preceded hydrogen adsorption); however, the desorption of weakly adsorbedhydrogen is strongly affected by the state of the substratesurface. When the platinum surfacehas not been subjected to oxide formation/reduction prior to hydrogen adsorption, the peak potential for the desorption of weakly adsorbed hydrogen is about 1OOmV more negative than that in Fig. 6(a), thus indicating that the weakly adsorbed atoms are more loosely bonded to the Pt surface.
4. Conclusion The study presented in this article has shown that SECM is a powerful technique for probing local transient Hf fluxes resulting from hydrogen adsorption and desorption on platinum electrode surfaces.It is particularly able to probe the simultaneoustransfer of electrons between the adsorbedlayer and the electrode surface and the transfer of protons between the adsorbed layer and the solution; in this study the use of the SECM provides direct evidence for the chronology between electron and proton exchange during adsorption and desorption. During hydrogen desorption from platinum electrodes in a neutral Na,SO, solution, a pH decreaseas high as about 2.3 was observed. The desorption of both weakly and strongly adsorbedhydrogen can be divided into two
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sub-steps, the first is electron transfer and the second is H+ release from the platinum surface. The latter sub-step proceeds at more positive potentials than the former. The rate of H+ release is always a function of potential. Furthermore, the release of weakly adsorbed Hf is also sensitive to the state of the electrode surface. Weakly adsorbed hydrogen is more loosely bonded to the Pt surface when the surface has not been taken through oxide formation/reduction prior to hydrogen adsorption. This observation may have important consequences in the design of efficient surface treatments usually aimed at improving the electrode response. The release of strongly adsorbed H+ is faster than that of weakly adsorbed HC and is much less sensitive to the history of the platinum surface.
As for hydrogen adsorption, a pH increase has been observed using tip oxide formation as the sensitive reaction. For strong adsorption, the starting potential and peak potential of the tip responseare more positive than that of the substrate response; this shows that the sub-step for hydrogen adsorption precedes that for electron transfer. For weak adsorption, however, electron transfer and hydrogen adsorption appear to be almost simultaneous.The behaviour of hydrogen adsorption is seriously affected by the reduction of oxide on the platinum substrate surface, possibly through two routes: one is by increasing the pH value in the solution near the substratesurface, the second is by changing the state of the electrode surface. Both hydrogen adsorption and desorption are sensitive to the pH of the solution. A pH increasein the solution can result in negative potential shifts for all adsorption and desorption peaks; conversely, a pH decrease can cause positive potential shifts for all these peaks. In addition, hydrogen desorption releasesH+ and causesa transient pH decrease, while hydrogen desorption consumesH+ and causes a transient pH increase. These pH changes are located in a region within about 60 km from the electrode surface and can last for several seconds. In a neutral unbuffered solution, the behaviour of hydrogen adsorption/desorption therefore dependsstrongly on the mutual interactions between the electrode surface and its environment.
Acknowledgements The authors would like to thank Professor D. Pletcher for fruitful discussions.Y.F.Y. acknowledgesthe financial support of the British Council in South China. G.D. acknowledges the financial support of the EPSRC through the award of an Advanced ResearchFellowship and grant for the RISC workstation (GR/K14704).
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References [II
I. Clavilier, D. Armand, S.G. Sun and M. Petit, J. Electroanal. Chem., 205 (1986) 267. J. Electroanal. Chem., 199 [21B. Love, K. Seto and J. Lipkowski, (1986) 219. [311. Clavilier and D. Armand, J. Electroanal. Chem., 199 (1986) 187. N.S. MarinkoviC and R. Ad%, J. Electroanal. [41 N.M. MarkoviC, Chem., 241 (1988) 309. K. El Achi and J. Clavilier, J. [51 A. Rodes, M.A. Zamakhchari. Electroanal. Chem., 305 (1991) 115. [‘51H. Kita, S. Ye and Y. Gao, J. Electroanal. Chem., 334 (1992) 351. J.M. Feliu and A. Aldaz, J. Phys. [71 R. G6mez, A. Femhdez-Vaga, Chem., 97 (1993) 4769. [81D. Pletcher and S. Sotiropoulos, J. Chem. Sot. Faraday Trans., 90 (1994) 3663. Acta, 37 (1992) 2629. [91 M.P. Sumino and S. Shibata, Electrochim. N.M. MarkoviC and R.R. AdiiC. J. Electroanal. [IO1N.S. MarinkoviC, Chem., 330 (1992) 433. [III M.A. Climent, M.J. Valls, J.M. Feliu, A. Aldaz and J. Clavilier. J. Electroanal. Chem., 326 (1992) 113. and J. Sobkowski, J. Electroanal. Chem., 261 (1989) [I21 K. Franaszczuk 223. in J.O’M. Bock& B.E. Conway, E. Yeager and [131 P.K. Subramanyan, R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, Vol. 4, 1981, p. 41 I. M.K. Hanafey and C.N. Reilley, J. Electroanal. [I41 F.E. Woodard, Chem., 167 (1984) 43. M.K. Hanafey and C.N. Reilley, J. Electroanal. [I51 F.E. Woodard, Chem., 167 (1984) 65. [I61 A. Lasia, Pol. J. Chem., 69 (1995) 639. [171 H. Ogasawara and M. Ito, Chem. Phys. Lett., 221 (1994) 213. [I81 R. Kiitz, C. Barber0 and 0. Haas, Ber. Bunsenges. Phys. Chem., 97 (1993) 427. F.R. Mclamon and E.J. Cairns, in R. Varma and J.R. [I91 J.D. Rudnicki, Selman (Eds.), Techniques for Characterization of Electrodes and Electrochemical Processes, Wiley, New York, 1991, p. 157. J. Chem. Sot. Faraday Trans. I, 79 [201 W.J. Albery and E.J.Calvo, (I 983) 2583. Fan and M.V. Mirkin, in A.J. Bard (Ed.), [211 A.J. Bard, F.-R.F. Electroanalytical Chemistry, Vol. 18, Marcel Dekker, New York, 1994, p. 244. [221M.H. Troise Frank and G. Denuault, J. Electroanal. Chem., 354 (1993) 331. [231M.H. Troise Frank and G. Denuault, J. Electroanal. Chem., 379 ( 1994) 399. J. Chem. Sot. Faraday Trans., 92 I241 Y.F. Yang and G. Denuault, ( 1996) in press. i.251G. Denuault, M.H. Troise Frank and L.M. Peter, Faraday Discuss., 94 (1992) 23. [%I A.T. Hubbard and F.C. Anson, in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 4, Marcel Dekker, New York, 1970, p. 156, eq. 87. [271R. Mills and V.M.M. Lopo, Self Diffusion in Electrolyte Solutions, a Critical Examination of Data Compiled from the Literature, Physical Science Data, Vol. 36, Elsevier, 1989. 1281 Y.F. Yang and G. Denuault, in preparation. and E.W. Kristensen, Anal. Chem., [291 R.C. Engstrom, R.M. Wightman 60 (1988) 652. Chemistry, Vol. 9, [301R. Woods, in A.J. Bard (Ed.), Electroanalytical Marcel Dekker, New York, 1976, p. 27. and M.R. Tarasevich, J. Electroanal. Chem., 101 [311 V.S. Bagotzky (1979) 1.
Journal of Electroanalytical
Chemistry
418 (1996)
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Scanning electrochemical microscopy ( SECM) : study of the adsorption and desorption of hydrogen on platinum electrodes in Na,SO, solution (pH=7) Yi-Fu Yang, Guy Denuault * Depurtment
oj’Chemi.vtry,
University
Received
24 April
oj’.Southompton. 1996; revised
Southampton 17 May
SO17 IBJ,
l/K
1996
Abstract Hydrogen adsorption and desorption at polycrystalline platinum electrodes in a neutral Na*SO, solution were studied with a scanning electrochemical microscope. Experiments were carried out with the tip-substrate voltammetry mode, where the faradaic current flowing to the tip is recorded while cycling the potential of the substrate, and with the tip-substrate chronoamperometry mode, where the tip faradaic current is recorded as a function of time following the application of a potential step to the substrate. The tip current was made pH sensitive by holding the tip potential in a region where a pH-dependent reaction occurs. Proton reduction was used to monitor pH decrease, whereas platinum oxide formation was selected to detect pH increase. The results showed that a transient pH decrease as high as 2.3 pH units exists during hydrogen desorption and that a great pH increase occurs during hydrogen adsorption. The mechanisms of hydrogen adsorption and desorption were analysed by comparing tip current vs. substrate potential curves, which reflect the exchange of Ht between the adsorbed layer and the solution, with substrate current vs. substrate potential curves, which reflect the exchange of electrons between the adsorbed layer and the electrode. New conclusions have been drawn. Keywv~ds:
SECM;
Scanning
electrochemical
microscope;
Hydrogen
adsorption-desorption;
1. Introduction Hydrogen adsorption and desorption on platinum electrodes are important reactions in electrochemistry. During the last decade, a large number of studies has focused on the relationship between the characteristics of hydrogen adsorption-desorption and the surface structure of platinum electrodes. In most cases results were obtained from cyclic voltammetry on single crystal [l-7] and polycrystalline platinum electrodes [8,91. The influence of the pH of the electrolyte solution on the characteristics of hydrogen adsorption-desorption has also been emphasized by many authors [5,8,10- 121. It is agreed that in acidic media, hydrogen adsorption-desorp tion reactions can be written as [ 13- 151 M+H++e-*M-H,,
* Corresponding
(1)
author.
0022.0728/96/%15.00 Copyright PII SOO22-0728(96)04768-7
0 1996 Elsevier
Science
Platinum
electrode
where M represents electrode. In neutral reaction is [ 161 M+H,O+e
the surface atom of the platinum or alkaline solutions, the proposed
- + M-H,,,
+ OH-
(2)
The reactions in Eqs. (1) and (2) imply that electron transfer and proton transfer proceed simultaneously. A
survey of the literature shows that most of the data concerning the adsorption and desorption of hydrogen on platinum have been obtained by voltammetric methods; these data therefore relate only to the transfer of electrons at the electrode surface. However, since the solution pH influences the reactions, it is worth considering the overall mechanismof adsorption-desorption as being made up of two distinct processes:electron transfer between the adsorbed layer and the electrode, and Ht transfer between the adsorbedlayer and the solution. The latter is expected to induce transient proton concentrations in the vicinity of the electrode surface. Few articles report investigations of these two aspects of hydrogen adsorption-desorption. Ogasawara and Ito
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[17] studied the electron transfer during hydrogen adsorption on different platinum single-crystal faces by voltammetry and detected the state of the adsorbedproton with in situ infrared reflection adsorption spectroscopy.KBtz et al. [18] presentedresults on the detection of proton fluxes at the solidlliquid interface during oxide formation/reduction on a polycrystalline gold electrode. They used probe beam deflection and achieved a good sensitivity; however, the technique was reported to be much less sensitive for platinum electrodes, where over 40 scanswere claimed to be necessaryin order to obtain similar results [19]. Proton concentration changes near electrodes have also been probed with a rotating ring disc electrode. A bismuth ring electrode was operated in the potentiometric mode to detect the changes in pH produced by reactions taking place on the disc electrode [20]. This technique proved to be very sensitive to the fluxes of H+ and OH- and to the variations of surface coverage down to a fraction of a monolayer. Scanning electrochemical microscopy (SECM) is a technique developed in recent years [21]. It has the ability to image the topography and reactivity of surfaces with a spatial resolution better than a micrometre. Tip-substrate voltammetry is one of the main operational modes of SECM where the tip current Ztip is recorded as a function of the substrate potential Esubwhile subjecting the substrate to a potential ramp and keeping the tip within a few micrometersfrom the substratesurface. It hasproved to be a powerful method which provides direct information about ion fluxes related to reactions on the substrate surface. This method was used to probe the ingressand egressof protons from a polyaniline film [22,23]. In a recent article we presentedthe results of an SECM study of pH changes near platinum electrode surfaces in pH4 sodium sulphate solutions [24]. The basic principles of the technique were developed and the merits of the approach were assessedboth qualitatively and quantitatively. Experiments consistedin acquiring a seriesof tipsubstrate voltammograms where the faradaic tip current was made pH sensitive by holding the tip potential in a region where a pH-dependent reaction occurred. Three reactions were considered: hydrogen evolution, Pt oxide formation and oxygen evolution. Results provided direct evidence of a pH decreaseclose to the substrate during hydrogen desorption, oxide formation and oxygen evolution. Conversely, a pH increasewas observed during oxide reduction, hydrogen adsorption and hydrogen evolution. A quantitative assessment of pH changeswas proposed; variations as large as one pH unit were observed. The present article reports the results of a similar study carried out in pH7 sodium sulphatesolutions; whereasthe previous paper aimed at establishing the principles of the SECM pH detection, the work presentedhere focuses on the hydrogen adsorption-desorption reactions. Tip-substrate voltammetry and tip-substrate chronoamperometry were used to monitor the tip responsewhile promoting the
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adsorption and desorption of hydrogen from the substrate. Different tip responseswere recorded depending on the amount of protons generatedor consumed: the steady-state tip current for proton reduction was used as a measureof the local proton concentration during hydrogen desorption (increase of the proton concentration within the tip-substrate gap) whereasthe tip current for tip oxide formation was used to follow hydrogen adsorption on the substrate (decrease of the proton concentration within the gap). Reaction mechanismsfor hydrogen adsorption and desorp tion on platinum electrodes were analysed by comparing tip and substrateresponses.The tip responsevs. substrate potential curve reflects the variations in H+ concentrations with substratepotential, while the substratevoltammogram informs about the changes in electron transfer with substrate potential.
2. Experimental Although several experimental conditions used in the present study are similar to those reported in Ref. [24], the basic details are listed below. The SECM apparatus has been described previously [25]. A home-madepotentiostat and a home-madebipotentiostat were usedto control the potentials of the electrodes. Potential waveforms were generated by a Hi-Tek PPRl waveform generator. All the electrochemical measurements were carried out with the cell placed inside an aluminium Faraday cage. The SECM tip, a 25 p,m diameter Pt microdisc sealedin glass, was polished on emery paper then on alumina lapping film (3M) to form a truncated cone; the ratio of the radius of glassto that of platinum was about 4. The SECM substrate was a 0.5 mm diameter Pt disc also sealed in glass. Before use, both tip and substrate were polished with alumina powder (0.05 km) on a moistenedpolishing cloth (Buehler) and rinsed with ultrapure water. Both electrodes were then cleaned as follows: in the first step the top of the electrode was heated at a temperaturejust below the melting point of glass for 2min, then the temperature was slowly lowered till the electrode was fully cooled to an ambient temperature. In the second step the electrode was immersed in 1 moldmm3 H,SO, solution and its potential was cycled between + 0.8 V and - 0.68 V (vs. HglHg ,SO,Isat. K ,SO,) until a reproducible characteristic voltammogram was obtained. Instead of a saturated calomel electrode, which would introduce interference from chloride ions, a home-made Hg ]Hg, SO, reference electrode with saturatedK 2SO, solution was used. All potentials are referred to this electrode. The auxiliary electrode was a pure platinum wire. Electrolytes were prepared with Aristar grade Na,SO, (BDH) and ultrapure water freshly made from a Millipore purification system (resistivity greater than 18Ma cm). This pure water was alsoused for rinsing the electrode and
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the cell. The pH of the solutions was adjusted by addition of carbonate-free NaOH solution, Analar (BDH). pH was measured with a 3010 pH meter (Jenway Ltd) which was regularly calibrated with standard solutions. All solutions were thoroughly deoxygenated with pure N, before experiments were carried out. The influence of CO, on the pH of the solutions was also tested. The results showed that the pIi of pure fresh water opened to the air decreased gradually due to the dissolution of CO, from the air. In less than 30 min the pH had decreased by about 1. However, the pH of an Na,SO, solution initially at pH 7 did not change during deoxygenation with N, gas. When deoxygenated and non-deoxygenated Na,SO, solutions were opened to the air, the pH only decreased by 0.2 over 2 h. The pH of the solutions remained constant when a blanket of N, gas was kept above the solutions. These results indicated that under our experimental conditions, the influence of the CO, could be ignored. In order to get reproducible results, a strict experimental procedure was followed. Both tip and substrate electrodes were cleaned as described previously. The substrate was inserted vertically facing upward from the bottom of the cell. A sheet of parafilm was used to cover the cell and a hole was made in this to let the tip penetrate into the cell. The tip was mounted on the microstage and its position was adjusted manually very close to the substrate surface. The cell was purged continuously with nitrogen gas, the deoxygenated solution was introduced into the cell; finally, reference and auxiliary electrodes were put in place via two branch tubes on either side of the cell. Another two glass branch tubes were used to maintain a blanket of N, gas above the solution in order to avoid disturbances from 0, and CO, during the experiment. One of the cell walls was made with optical glass to facilitate observation of the tip-substrate distance with a stereomicroscope (Gallenkamp). The quality of the substrate and of the tip was assessed by recording voltammograms between +0.7 and - 1.2 V at a scan rate of 50mV s- ’ until they became stable (typically after several cycles). The tip-substrate distance was then adjusted with the piezocontroller down to about 1 pm. The bipotentiostat was used to control the potentials of the tip and of the substrate independently. Tip-substrate voltammograms were acquired by recording the tip current at a fixed tip potential while cycling the potential of the substrate; various tip responses were obtained by holding the tip potential at different values. Tip-substrate chronoamperometry was carried out by holding the tip potential, stepping the substrate potential and recording the tip current as a function of time. All the tip-substrate voltammograms shown in this article were recorded on the same day, with the same solution, same tip, same substrate and same tip-substrate distance (i.e. the tip was not moved between the various recordings). Similarly all the tip chronoamperograms were acquired the same day, with the
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same solution, same tip and same substrate. Experiments were repeated and the results discussed in Section 3 were found to be reproducible.
3. Results and discussion 3.1. Cyclic voltammograms
on platinum electrodes
While cycling the potential of a platinum electrode between hydrogen evolution and oxygen evolution, many factors will determine the peak potentials for hydrogen adsorption and desorption. Fig. 1 shows two cyclic voltammograms recorded with a Pt electrode in 0.5 moldmp3 Na,SO, (pH7) at two different scan rates. With a low potential scan rate, 50mV s-’ (Fig. l(a)), the adsorption of hydrogen occurs at -0.975V (strong) and -1.115V (weak); the corresponding desorption occurs at - 0.61 V and - 0.69 V respectively. Keeping all other conditions the same, the voltammogram changes greatly (Fig. l(b)) when the scan rate is increased to 200mV s- ‘. For the hydrogen adsorption reaction, the peak potentials for strong and weak adsorptions are respectively - 1.Ol V and - 1.16 V, which correspond to negative shifts of 35mV and 45mV. This result indicates that the rate of the weak adsorption is slower than that of the strong one. For the hydrogen desorption reaction, however, the situation becomes more complicated at higher potential scan rates (Fig. l(b)). The
20 10 -3.
o-
-
-10 -20 -30 -40
I -1.2
I
I -0.8
I
I1 -0.4
I 0
I
I 0.4
I 0.8
E/V Fig. 1. Cyclic voltammograms solution with N, gas flowing rate: (a) 50mV s- ‘; (b) Hg/Hg,SO,Isat. K*SO,.
at a Pt electrode in 0.5 moldmm3 Na,SO, on the top of the solution. Potential scan 200mV s-‘; potentials in volts vs.
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the other is in the potential region between -0.75 and - l.OV with a peak at - 0.94 V. The latter peak corresponds to the desorption of hydrogen in an increased pH environment caused by the adsorption of hydrogen on the substrate as the potential is scanned negatively, whereas the former results from the effect of solution stirring by the N, gas. These observations can be explained in terms of the gas flow induced convection which increases the rate of mass transport of H+ from the bulk solution to the vicinity of the electrode where the high concentration of OH- resulting from the adsorption of hydrogen can be neutralized by the reaction H++ OH-+
-1.2
-0.8
-0.4
0
0.4
0.8
EIV Fig. 2. Cyclic voltammograms at a Pt electrode in 0.5moldm-2 Na,SO, solution. Potential scan rate: 1OOmV s- ‘. (a) Without N, gas flowing on the top of the solution; (b) with N, gas flowing weakly on the top of the solution; (c) with N, gas flowing strongly on the top of the solution. Potentials in volts vs. HgIHg,SO,jsat. K,SO,.
desorption of hydrogen is separated into two parts; the first one is in the potential region between - 0.48 and - 0.79 V with peaks at -0.60 and -0.67V. This part is the same as that in Fig. l(a). The second one is in the potential region between - 0.85 and - 1.02 V with a single peak at - 0.95 V. This peak could be explained by assuming that hydrogen desorption took place in a high pH (> 7) environment as a result of the adsorption of hydrogen when the potential was scanned negatively. This is an indication that the adsorption and desorption of hydrogen are sensitive to the pH of the solution in the vicinity of the electrode. Fig. 2 shows the stirring influence induced by the N, blanket flowing over the solution. In Fig. 2(a), the N, gas flow was stopped. Hydrogen adsorbed at potentials - 1.O V (strong) and - 1.15 V ( weak), and desorbed in the region between - 1.07 V and -0.66V marked by a single broad peak at -0.95 V. When a slow N, gas flow passes over the solution (Fig. 2(b)), the solution is gently stirred. The adsorption of hydrogen occurs at - l.OV (strong) and - 1.14V (weak). However, the desorption of hydrogen is separated into two peaks. One is in the potential region between - 0.48 and - 0.75 V with a peak at - 0.595 V,
H,O
(3)
The extent of the recombination depends on the strength of the stirring. With a weak gas flow the local solution is not completely replenished with H+; some adsorbed hydrogen desorbs in a higher pH (> 7) environment, some desorbs in a neutral environment. This explanation can be further supported by the voltammogram recorded with a faster N, gas flow, Fig. 2(c). With stronger stirring, the desorption peak in the potential region between - 0.75 and - 1.OV disappeared but the peak height in the region between - 0.48 and - 0.75 V increased and separated into two peaks at - 0.59V and - 0.69V respectively. These two peaks correspond to the desorption of strongly and weakly adsorbed hydrogen. This result indicates that in such conditions all adsorbed hydrogen desorbs in a neutral environment. With the stronger gas flow, the adsorption of hydrogen occurs at potentials of -0.985 (strong) and - 1.135 V (weak), which corresponds to a positive potential shift of 15mV for both the strong and the weak adsorptions. From the results shown in Figs. 1 and 2, it is clear that the change of the solution pH has influenced both hydrogen adsorption and desorption. The peak potential shift for hydrogen adsorption is smaller than that for desorption when using stronger stirring; this is possibly caused by the fact that the potential difference between hydrogen adsorption and platinum oxide reduction (which also consumes Hf and hence increases pH) is smaller than that between hydrogen desorption and adsorption; the adsorption of hydrogen thus always occurs in an increased pH environment. The influence of convection and potential scan rate presented here are in agreement with the findings of Pletcher and Sotiropoulos [8] who carried out a series of experiments to investigate the effects of mass transport (using rotatin g d’isc and microdisc electrodes) and scan rate on the voltammetric response of Pt electrodes in unbuffered neutral Na,SO, solution. 3.2. H ’ desorption
Fig. 3 illustrates tip-substrate voltammograms recorded with two different tip potentials. In Fig. 3(a) Etip was set at - 1.2 V; at this potential, proton reduction is the tip reac-
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proton reduction, see Fig. 3(b). The basic characteristic of the peak in Fig. 3(b) is the same as that in Fig. 3(a) except for the zero baseline due to the absence of water reduction; the peak height in Fig. 3(b) (measured as 6.25nA) can be used to calculate the transient pH change. As the tip is very close to the substrate (1 km), the tip-substrate ensemble behaves like a leaky thin layer cell with the tip and the substrate as the two electroactive walls. For a neutral solution, the situation is different from that described in Ref. [24] at pH4, and the calculation of pH changes resulting from the desorption of proton from the substrate surface is also different. Using the analogy with a four-electrode thin layer cell, where one wall acts as the generator and the other as the collector, the steady-state tip current is given by 1261
0.8
1V
Fig. 3. Tip-substrate voltammograms in 0Sn1oldm~~ Na,SO, solution. Potential scan rate for Esub SOmVs-‘, dtipmsub = 1 Fm. (a) Etip = - 1.2V; (b) Etip = - l.OV. Potentials io volts vs. Hg/Hg,SO,lsat. KZS04.
tion chosen for monitoring the concentration of H+ near the substrate surface. In a pH7 Na,SO, solution the concentration of Hf is, however, too low to give a measurable steady-state current for H+ reduction, and at - 1.2 V only the water molecule can be reduced according to the reaction 2H,O + 2e--+ H, + 20H-
Chemistry
(4 The dashed line in Fig. 3(a) indicates the magnitude of the tip current for water reduction at this potential. This was recorded by holding the substrate potential in the double layer region at about -0.2V and waiting until the tip current had become stable. Any situation leading to an increase of the concentration of protons within the tipsubstrate gap, e.g. a substrate reaction releasing protons, will promote the reduction of protons in addition to that of water and will lead to a significant increase of the tip current. Fig. 3(a) shows that when Esub is swept positively, ZtiP increases quickly from a potential of Esub = - 0.8 V, forms a single sharp peak at Esub = -0.55 V and then decreases quickly. Compared with the corresponding substrate behaviour (Fig. l(a)), it is apparent that this tip current peak is the result of hydrogen desorption on the substrate which causes a transient increase of H+ concentration. In fact, when Esub is between - 0.8 and - 0.35 V the tip response comes from two contributions, i.e. water reduction and proton reduction, but it is not easy to calculate the latter independently. When EtiP is held at - l.OV the current for water reduction approaches zero and the observed Zpeak-tip can be analysed in terms of the sole contribution from
where n is the number of electrons involved, F is the Faraday constant, D is the diffusion coefficient of H+, a is the radius of the microdisc electrode, c,,~ is the proton concentration on the substrate surface (not the coverage), ctip is the proton concentration on the tip surface and is the tip-substrate distance. As Erip is set in the dtip-sub steady-state current potential region for H+ reduction, ctip = 0. With n = 1, D = 7.7 X lo-‘cm* s- ’ [27], a = = 6.25nA, the tran12.5 km dtip-sub = 1 p,m and Ztipmpeak sient proton concentration at the surface of the substrate C subis calculated as 17.1 X 10m6M which correspondsto a pH of 4.77. It is now important to compare Zpeat-subin Fig. l(a) in Fig. 3(b); Isub reflects the transfer of wi* ‘peak-tip electrons between the adsorbed layer and the electrode, whereas Zrip reflects the transfer of protons between the adsorbed layer and the solution. It is thus possible to compare the transfer of electronic and ionic chargesduring the hydrogen desorption process.One can seethat Zpeakmtip begins at Esub= - 0.8 V, i.e. at a potential 150mV more positive than that of Zpeakwsub. This result indicates that the desorption of weakly adsorbedhydrogen proceeds in two separatesub-steps;the first one is the transfer of electrons from the adsorbedlayer to the electrode and the secondis the releaseof protons into the solution. At Esub= -0.6 V, Ztip bends to give a steeper current increase and forms a peak located at Esub= - 0.55 V, i.e. at a potential 60 mV more positive than the potential of Zpeakesub for the desorption of strongly adsorbed hydrogen (peak potential at Esub= - 0.61 V) on the substrate cyclic voltammogram. From diffusion theory, it is easy to calculate that, at drip-sub= 1 p.m, the time taken by H+ to diffuse from the substrate surface to the tip surface is about 1.3 X 10m4s, whereasa 60mV peak potential difference correspondsto a delay of 1.2 s between the substrate and tip responses (the potential scanrate is 50mV SK’). Hence, it is reason-
104
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G. Denuault
/Journul
of’Electroamlytica1
6
5
4
P .
3 2
2
1
0 0
1
2
3
4
5
6
t/s
Fig. 4. Tip-substrate chronoamperograms step from -1.12 to -0.3V. E,,,=-l.IV. 4O*m; A 50p,m; l 6Opm.
at different dtlp-sub with dtiperub: n 30km;
Esub r
able to consider that the desorption of strongly adsorbed hydrogen also proceeds in two consecutive sub-steps, where the electron transfer occurs prior to the release of Hf. The tip current peaks in the region Esub = -0.3 to + 0.7 V (both in Fig. 3(a) and 3(b)) come from an increase of the H+ concentration resulting from the formation of oxide on the substrate. The results of an SECM study of oxide formation and reduction will be discussed in a separate publication [28]. In Fig. 3(a), when Esub is swept negatively the tip current decreases and forms two positive (with respect to the dashed line) peaks, one for Esub between -0.3 and - 0.8 V which corresponds to the reduction of oxide on the substrate, the other between - 0.9 and - 1.1 V which corresponds to the adsorption of hydrogen on the substrate. There are two peaks because water reduction depends on the pH near the electrode surface. During oxide reduction and hydrogen adsorption on the substrate, the concentration of OH- increases in the vicinity of the substrate surface and the reaction in Eq. (4) can be slowed down. In other words, water molecules are reduced at more negative potentials when the pH increases. In Fig. 3(b), where Etip is held at - l.OV the water reduction current is around zero and only the reduction of substrate oxide which results in a great pH increase can be detected. albeit with a much lower sensitivity. Fig. 4 illustrates tip-substrate chronoamperograms recorded as Esub was stepped from - I. 12 to - 0.3 V. The experiment proceeded as follows: first Esub was maintained at - 1.12 V for 2 min (this potential is just slightly more positive than that for water reduction) then Esub was
Chemistry
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stepped to - 0.3 V causing the desorption of both weakly and strongly adsorbed hydrogen to occur. EtiP was held at - 1.1 V, which is in the potential region for H+ reduction when pH < 7. Tip current transients were recorded for several dtipmsub values. Fig. 4 shows the typical distortional effects of diffusion [29]; as the tip is moved closer to the substrate, the tip current peak becomes higher and the time needed for the appearance of the current peak is shorter. This is because at higher &P-sub protons need a longer time to diffuse from the substrate to the tip. This figure also points out that the transient change in H’ concentration only lasts for several seconds; this result can well explain the effect of potential scan rate on the shape of the substrate voltammogram shown in Fig. 1. The tip current peak heights measured at various tipsubstrate distances are listed in Table 1. One can see that the peak current, 72.51~4, is much at 4ipsub = 1 km higher than that in Fig. 3(b) (6.25nA). This suggests that the rate of proton release is controlled by the potential. At a tip substrate distance of 60 Km, however, the tip current peak is very low; nevertheless, this suggests that the substrate surface reaction has significantly affected the pH of the solution over 60 km. No attempt was made to transform the tip current value into a substrate surface proton concentration because Eq. (5) becomes less valid as the tip-substrate distance increases. When dtip-sub = 1 pm the diffusion time is about 130 p,s and we can assume that the proton concentration instantaneously reaches a steady-state linear profile from c sub to cfiP = 0; this is no longer true when dtipesub increases, e.g. it takes on average 13 ms for the wave of H+ to reach the tip when dtipYsub = 10 km. 3.3. H + adsorption
As mentioned previously, the proton concentration at pH7 is too low to give a measurablesteady-state current on the tip. Any further decreaseof the proton concentration - e.g. due to a substratereaction consumingprotons, such as hydrogen adsorption - cannot be probed directly with the reduction of Hf on the tip. However, some information about hydrogen adsorption can still be ob-
Table I Relationship d op-sub /pm I
5 10 20 30 40 50 60
between
dlipmrusb and Ipeaketip I peak. ,,p in.4
72.50 33.46 18.18 9.55 5.58 2.80 1.15 0.50
Table 2 Relationship between ELRP, IpEaketip and substrate hydrogen relative coverage 6. EL,, VS. Hg~Hg~SO~~~t. K2S0, 0 %a, /v jperk I tip/nA -0.8 1.0 0.165 - 0.9 1.37 0.226 - 1.05 2.23 ii.376 - I.1 2.8 0.461 - 1.2 6.07 1
-
-6
-7
-1.3
--J-.--L.-I1 -1.1
-0.9
-0.7 %a
-0.5
-0.3
-0. I
f V
Fig. 5. Tip-substrate voi~mmog~ams in 0.5moldm-3 Na$O, solution with different Erub regions. Potential scan rate for Esub SOmVs- ‘, Etip = - 1.1v, drjp-s”b = 1 ym. EsUbregions: (11 -0.2 to - 1.2V; (21 -0.2 to - I.lV; (3) -0.2 to - 1.05v; (4) -0.2 to -0.9v; t.51 -0.2 to -0.8V. Potentials in volts vs. HglHg,SO,lsat. K,SO,.
tained, but indirectly, from the response of the tip to hydrogen desorption on the substrate. The approach is illustrated in Fig. 5, which shows a set of tip-substrate voltammograms recorded within different Esub regions outside the influence of piatinum oxide formation and reduction on the substrate. In these ~x~rimeuts the upper reversal potential of Esuh was held at -0.2 V while the lower reversal potential ELRP was changed from
-1.2
-1.0
-0.8
-0.6
one voltammogram to the next. The variations of Ipeak.+ with ELRp are listed in Table 2. The adso~tio~ of hydrogen on platinum is loyally considered as a reversible reaction, and its equilibrium coverage is determined by the electrode potential [30]. Since the tip current is proportional to the concentration of protons released by the substrate, we can therefore expect that the tip current peak is proportional to the hydrogen subs~~te coverage at a given ELRP. Let us suppose that I peak-tip = 6.07nA (E,,, = - 1.2VI represents the full coverage, then the ratio of Ipeakmtipfor another ELKr value to 6.07nA gives a relative coverage 8. The calculated results are listed in Table 2. The voltammogram in Fig., l(a) suggests that - 0.8 V is the initial potential for hydrogen adsorption. Table 2, however, indicates that the proton coverage on the platinu~~ electrode surface at -0.8 V is already as high as 16.5%. This suggests that the adsorption of protons starts at a potential more positive than - 0.8 V, although this feature cannot be seen from a substrate voltammogram. In agreement with the results from tipsubstrate voltammet~, linear potential scan ex~rin~ents (from -0.2 to - 1.2V) also show that hydrogen adsorp-
-0.4
-0.2
0
0.2
0.4
0.6
f-Lb1 v Fig. -6. Tip-substrate vol~mmogmms in 0.5 mol dm -’ Na,SO, solution at diffe~nt Esubregions. Potential scan rate for f& drip-sub = 1p.m. Esub regions: (a) iO.5 to - 1.2V; (b) -0.2 to - 1.ZV. Potentials in volts vs. ~gl~g*S~~{sat. K,SO,.
50mV s- I, Etip = - 0.1 V,
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oj’Electroandyticul
tion starts at -0.72V. For strong adsorption, the difference between the starting potential of the tip and substrate responses suggests that hydrogen adsorption occurs before the transfer of electrons. The coverage increase seen when E,,, is changed from - 1.1 to - 1.2 V is more than 50%; this is much higher than expected from the substrate voltammogram. The difference between the response of the tip to the changes in Hf concentration and that of the substrate to the transfer of electrons indicates that a more complicated hydrogen adsorption mechanism needs to be considered. Fig. 6 shows a set of tip-substrate voltammograms obtained using tip oxide formation as the sensitive reaction for the detection of transient proton concentrations in the vicinity of the substrate. Etip was held at - 0.1 V, i.e. at the foot of the wave for oxide formation. As discussed in Ref. [24], the tip oxide coverage is a function of the tip potential and of the pH of the solution. Changing either of these two factors induces the formation or reduction of tip oxide and leads to a faradaic tip current. In a tip-substrate voltammetry experiment, &, is held at a constant value and the oxide coverage on the tip is mainly determined by the pH of the solution. The relationship between the peak potential for oxide formation/reduction and pH has been reported as about - 60 mV per pH unit [3 1I, which means that a pH increase in the solution leads to a negative shift of the potential for oxide formation and results in a positive tip faradaic current. Conversely, a pH decrease causes a negative faradaic current on the tip. In Fig. 6(a), Esub was cycled in a region between +0.5 and - 1.2V. Considering the sweep towards negative potentials, we see two significantly overlapped positive tip current peaks between - 0.18 and - 0.8 V. These peaks result from a pH increasedue to the reduction of oxide on the substrate surface. In the region between -0.8 and - 1.18V another two small positive tip current peaks occur at - 0.99 and - 1.15V. These two peaks result from a pH increase due to the adsorption of hydrogen on the substrate. Recalling the peak potentials for hydrogen adsorption in Fig. l(a) (-0.975V for strong adsorption and - 1.115 V for weak adsorption), we find a 15mV offset between the tip response and the substrate responsefor strong adsorption and a 35 mV offset for weak adsorption. Considering the sweeptowards positive potentials, we see wo negative tip current peaks between -0.82 V and -0.42V, respectively located at -0.68V and -0.58V. These peaks are caused by a pH decreasein the solution due to the desorption of hydrogen from the substrate surface. As the substrateupper reversal potential is changed to -0.2V, oxide formation/reduction on the substrate no longer occurs; a tip-substrate voltammogram illustrating this situation is shown in Fig. 6(b). It is quite different from that in Fig. 6(a), in that the tip current peaks corresponding to the adsorption of hydrogen on the substrate are enhanced; two clearly distinguished peaks seen at
Chemistry
418 (1996)
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E sub = - 0.93 V and - 1.12V are respectively 60mV (for
strong adsorption) and 30mV (for weak adsorption) more positive than the samepeaks in Fig. 6(a). Comparing Fig. 6(b) and Fig. 6(a), it is clear that the reduction of substrate oxide has a serious influence on the adsorption of hydrogen; this influence may occur either in the form of an increase of the pH of the solution near the substrateor in the form of a change in the state of the substratesurface. Fig. 6(b) also indicates that the change of H+ concentration starts from Esub = -0.76V; this result is consistent with that in Fig. 5. Comparing the potential of Ipeaketip with that of I peak-sub, it is clear that in the caseof strong adsorption the peak potential for Ipeak-iip( - 0.93 V) is about 45 mV more positive than that for Zpeakesub (-0.975V in Fig. l(a)). This further proves that for strong adsorption the hydrogen adsorption sub-step precedes the electron transfer. However, in the caseof weak adsorption the peak potential for Ipeakmrip (- 1.12V) is almost the sameas that of Zpeakmsub ( - 1.115V in Fig. l(a)). This result suggeststhat the adsorption of weak hydrogen proceedssimultaneouslywith the transfer of electrons. is scannedpositively, the magnitude of the men Esub tip current increases from Esub = -0.9 V and produces two negative tip current peaks at potentials of -0.735 and - 0.59 V. These two peaks correspondto the desorption of weakly and strongly adsorbed hydrogen. Compared with Fig. 6(a), it is clear that the desorption of strongly adsorbed hydrogen is not affected by the history of the surface (i.e. whether oxide formation/reduction preceded hydrogen adsorption); however, the desorption of weakly adsorbedhydrogen is strongly affected by the state of the substratesurface. When the platinum surfacehas not been subjected to oxide formation/reduction prior to hydrogen adsorption, the peak potential for the desorption of weakly adsorbed hydrogen is about 1OOmV more negative than that in Fig. 6(a), thus indicating that the weakly adsorbed atoms are more loosely bonded to the Pt surface.
4. Conclusion The study presented in this article has shown that SECM is a powerful technique for probing local transient Hf fluxes resulting from hydrogen adsorption and desorption on platinum electrode surfaces.It is particularly able to probe the simultaneoustransfer of electrons between the adsorbedlayer and the electrode surface and the transfer of protons between the adsorbed layer and the solution; in this study the use of the SECM provides direct evidence for the chronology between electron and proton exchange during adsorption and desorption. During hydrogen desorption from platinum electrodes in a neutral Na,SO, solution, a pH decreaseas high as about 2.3 was observed. The desorption of both weakly and strongly adsorbedhydrogen can be divided into two
Yi-Fu Yang. G. Denuault/Journnl
of‘Electroundytica1
sub-steps, the first is electron transfer and the second is H+ release from the platinum surface. The latter sub-step proceeds at more positive potentials than the former. The rate of H+ release is always a function of potential. Furthermore, the release of weakly adsorbed Hf is also sensitive to the state of the electrode surface. Weakly adsorbed hydrogen is more loosely bonded to the Pt surface when the surface has not been taken through oxide formation/reduction prior to hydrogen adsorption. This observation may have important consequences in the design of efficient surface treatments usually aimed at improving the electrode response. The release of strongly adsorbed H+ is faster than that of weakly adsorbed HC and is much less sensitive to the history of the platinum surface.
As for hydrogen adsorption, a pH increase has been observed using tip oxide formation as the sensitive reaction. For strong adsorption, the starting potential and peak potential of the tip responseare more positive than that of the substrate response; this shows that the sub-step for hydrogen adsorption precedes that for electron transfer. For weak adsorption, however, electron transfer and hydrogen adsorption appear to be almost simultaneous.The behaviour of hydrogen adsorption is seriously affected by the reduction of oxide on the platinum substrate surface, possibly through two routes: one is by increasing the pH value in the solution near the substratesurface, the second is by changing the state of the electrode surface. Both hydrogen adsorption and desorption are sensitive to the pH of the solution. A pH increasein the solution can result in negative potential shifts for all adsorption and desorption peaks; conversely, a pH decrease can cause positive potential shifts for all these peaks. In addition, hydrogen desorption releasesH+ and causesa transient pH decrease, while hydrogen desorption consumesH+ and causes a transient pH increase. These pH changes are located in a region within about 60 km from the electrode surface and can last for several seconds. In a neutral unbuffered solution, the behaviour of hydrogen adsorption/desorption therefore dependsstrongly on the mutual interactions between the electrode surface and its environment.
Acknowledgements The authors would like to thank Professor D. Pletcher for fruitful discussions.Y.F.Y. acknowledgesthe financial support of the British Council in South China. G.D. acknowledges the financial support of the EPSRC through the award of an Advanced ResearchFellowship and grant for the RISC workstation (GR/K14704).
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