Linear potential sweep (LPS) voltammetry was applied to study the kinetics of hydrogen evolution in solutions containing glycolic, malic, tartaric, and gluconic acids. The CE mechanism of hydrogen evolution was analyzed invoking the 2nd Fick's law equations supplemented by terms that account for chemical interactions between diffusing particles. Acids are considered as components that are capable of releasing hydrated protons taking part in the charge-transfer step. Current peaks observed on LPS voltammograms are in linear dependence on ( is the potential sweep rate). They obey well-known relationships obtained for simple redox processes, provided that the concentration of oxidant is treated as total concentration of proton donors. Determination of surface concentrations as current density functions makes it possible to transform LPS voltammograms into linear Tafel plots normalized with respect to the surface concentration of hydronium ions. Similar kinetic parameters ( and 10 μA cm−2) obtained at pH 3 for all OA solutions indicate that the nature of OA has no noticeable influence on the charge-transfer process.

1. Introduction

Organic acids containing OH groups are often used as ligands capable of forming quite stable coordination compounds with different metals including copper. Due to their environmental compatibility, most of them have found use in various industrial applications. Tartaric and citric acids are used in plating baths as additives producing compact, fine-grained copper coatings. Besides, electroless plating of dielectrics involving Cu(II)-tartrate complexes is also worthy of mention.

Hydrogen evolution often attends the electrodeposition of metals. This side reaction should be taken into account when the main process is studied. Often the reduction of hydronium ions can be complicated and involves some chemical steps. In the case of CE mechanism (chemical + electrochemical step), electroactive particles are formed in a chemical reaction that precedes the electron transfer step. Components that produce hydronium ions are often referred to as proton donors. Aforementioned organic acids, which are commonly added to improve the coatings, can also play such a role.

Fundamentals of CE processes, that have been substantially elaborated for present (see, e.g., [13]), can serve as a basis for studies of hydrogen evolution. However, some difficulties emerge, when a wide range of proton donors is considered; then, certain extensions and generalizations are desirable. The literature data on this problem show that the knowledge in this area is not sound. Therefore, the investigations of kinetics and mechanism of hydrogen evolution in such systems still remain an actual problem. In this connection, we referred to some concepts [4] that have been applied by us for reduction of metal complexes. This paper presents the main characteristics of the processes occurring on the copper electrode in solutions containing some organic acids that act as proton donors.

2. Experimental Details

Solutions under investigation contained 0.04 M analytical grade glycolic, malic, or tartaric acid (Reakhim, Russia), 0.02 or 0.05 M sodium gluconate (Sigma-Aldrich, 99% purity). 0.3 M Na2SO4 or K2SO4 (Reakhim, Russia, high purity) and 0.5 M Na2SO4 (Lach-Ner, Czech Republic, 99% purity) served as supporting electrolytes. Some experiments were carried out with addition of 0.01 M CuSO4 ( Mallinckrodt, USA). Specified values of pH were adjusted by addition of H2SO4 or KOH. Twice-distilled water was used to prepare solutions. A pure argon stream was passed through solutions for 0.5 h prior to measurements. All experiments were carried out at 20°C.

To prepare a working electrode, a platinum disc was coated with a 5–7 μm thick copper layer in the solution containing (g dm−3): CuSO45H2O–200, H2SO4 –50 at 10 mA cm−2. Electrode potentials were measured with respect to the reference electrode and were converted to the standard hydrogen scale.

Voltammograms were recorded under linear potential sweep (LPS) conditions using a potentiostat PI-50-1 (Belorussia). Potential sweep rates ranged from 10 to 200 mV s−1. Some supporting experiments were carried out using a conventional RDE technique.

3. A Quantitative Model

Let us assume that an aqueous solution of weak acid LH2 is prepared and an excess of indifferent (supporting) electrolyte is added. The main reversible processes occurring in such a system can be depicted as follows: (in the case of dibasic acid the stability of intermediate anion is also characterized by the relationship similar to (12)).

Due to a sufficiently high difference between the standard potentials of and electrodes, partial processes of Cu(II) reduction and hydrogen evolution are well-separated, unless very stable Cu(II) complexes are formed. This can be seen from a typical example shown in Figure 1. Experimental voltammograms contain two well-defined maxima that are indicative of two different cathodic processes. The first current peak observed at a relatively low cathodic polarization results from Cu(II) reduction. A further increase in cathodic polarization gives rise to the second process with a distinctive current peak observed at . This maximum may be conditioned by different reasons, such as Cu2O reduction or hydrogen evolution [12]. EQCM investigations of Cu(II)-glycolic acid solutions have shown [12] that no extra changes in copper electrode mass are observed in this region. Moreover, similar current peaks are also observed in Cu(II)-free solutions. The voltammograms obtained in the latter case and brought to the level of partial Cu(II) reduction current (dotted lines in Figure 1) coincide sufficiently well with the data recorded in the presence of Cu(II). This effect, as well as large difference between and , gives grounds to suppose that the two partial processes may be analyzed independently. The limiting current of Cu(II) reduction, specified by the relationship could serve as the base line for hydrogen evolution. In this connection, it should be said, that the thickness, , of the Nernst-type diffusion layer (that develops under the natural convection conditions) depends on the potential sweep rate . Voltammetric investigations of redox system have shown [13] that the empirical condition const is obeyed in this case. According to the results of our further investigations, this relation is also valid for Cu(II) systems containing OA. The procedures used for estimation of δ values are described elsewhere [13].

Solution pH should be mentioned in the first place as a factor which determines the rate of hydrogen evolution. This can be clearly seen from the typical data shown in Figure 2. Naturally, current density significantly falls with increase in solution pH, that is, when the concentration of hydronium ions decreases. However, when pH is kept constant, the rate of the process under discussion increases with the total concentration of acid . This effect will be analyzed below in more detail. Finally, the supporting electrolyte plays a part in these processes: the height of the second current peak decreases gradually when sulphate is replaced by perchlorate (Figure 3). Revealed experimental phenomena imply that not only or but also should be treated as a proton donor and should be included into (8).

The composition of solutions was calculated using modified material balance equations (7) and (8): and the expression written for total sulphate concentration: Stability constants   and are listed in Table 1; a similar characteristic of hydrosulfate, = 30, was taken from a handbook [8, 9] as best-matched to the ionic strength of the solutions under discussion. To obtain the concentration of hydronium ions from pH measurements, the activity coefficient was used. It was obtained by empirical equations given in [14, 15]. The concentration of ions was obtained from the ion product of water. An example of the results obtained is given in Table 2. It can be seen that the amount of hydronium ions is not great in the solutions containing a large excess of sulphate. Thus, LH molecules and hydrosulphate anions are the species that should be related to predominating proton donors.

Most of the experimental data obtained are typical of every OA under investigation. Therefore, to minimize the size of this paper, we present below freely selected results obtained for either system. According to them, current peaks (that further are simply symbolized as ) increase with the potential sweep rate , and linear dependences between and are observed (Figures 4 and 5). The slopes of these lines, , increase when the acidity of solutions grows (pH falls) and the acid concentration is increased. The observed phenomena are consistent with the aforestated assumptions concerning the mechanism of electrode reactions. Since the position of current peaks depends on (Figures 1 and 2 and the data given below), the cathodic process should be treated as irreversible. Then, the simplified kinetic equation is valid for sufficiently high overvoltages: where is an exchange current density, is a cathodic charge-transfer coefficient, overvoltage , and is equilibrium potential. Normalized current density is defined by the relationship: where subscripts and denote the surface and bulk concentrations of hydronium ions. Once the latter two quantities are determined, linear normalized Tafel plots (NTP) can be obtained.

Procedures of data treatment might be as follows. Firstly, special integration of experimental voltammograms, transformed into functions, is performed (see (10)) and quantities, as functions, are obtained (Figure 6). The latter operation needs a certain diffusion coefficient to be used. Traditionally, it can be obtained from RDE data and tested for the condition according to which the limiting value (see plateau in Figure 6) cannot exceed the bulk concentration of proton donors.

The second step consists, in determination, of distribution of proton donors at the electrode surface under cathodic polarization conditions. For this purpose, (15)–(17) are used. The surface value of total concentration of proton donors, , is decreased from the bulk value (initial state) to zero (limiting current region) keeping and constant. It should be emphasized that the surface concentration of the electrically active substance (hydronium ions) does not fall to zero in contrast to common redox processes (see (8) or (16)). In the absence of proton donors, a neutral medium is created at the electrode surface , otherwise alkalization occurs. An example of the data obtained is shown in Figure 7.

Notice that the same data can be presented as functions, since the interrelation between and is easily obtained from Figure 6. This makes it possible to assign the value of at any potential of voltammograms and to transform them into normalized Tafel plots. Some results of the procedures performed are shown in Figure 8. The data obtained at different are very close and can be approximated by one average NTP. The kinetic parameters of charge-transfer process obtained by (18) are shown in Figure 8 and listed in Table 3. Though the equilibrium potential of electrode is uncertain in free solutions, the exchange of current density, named as “effective”, was determined by extrapolation of NTP to the theoretical quantity defined by the relationship: Low values are responsible for a high overvoltage of hydrogen evolution: current densities are low over a wide initial range of potentials. The rise of voltammograms is observed at rather negative potentials ranging up to .

LPS voltammograms possess characteristic maxima, the coordinates of which are convenient to use in the kinetic analysis. At present, relationships for and are available for different mechanisms, including the case of simple redox processes that are controlled by the charge transfer and diffusive mass transport [13]. The latter case is somewhat similar to that presented here, but one peculiarity needs to be considered. When a simple redox process occurs, both current density and electrode potential depend on the concentration of the same electroactive species. However, the case under consideration offers the following distinctive feature: the electrode potential is determined by [H+], whereas the current density is determined by the total flux of all proton donors and is dependent on . Keeping this in mind, we reasoned that the certain modification of common relationships, with in place of c, is permissible. Then, the current peak expression takes the following form:

To check the correctness of this equation, the slopes of linear “” plots were analyzed with the values of , and taken from the foregoing analysis. According to the results obtained, experimentally determined slopes are in a good agreement with the quantities that follow from (21) (Table 3).

It can be also seen from these data that the kinetic parameters of the charge-transfer step, obtained for different OA, are quite similar. Then, according to (21), the linear interrelation between and should be governed only by the equilibrium and mass transport parameters. In this regard, all experimental data, obtained for different OA solutions at various and pH, were generalized and collected in Figure 9. It can be seen that a satisfactory approximation can be performed by single general regression line. The results obtained show that the processes of hydrogen evolution, proceeding in the systems under consideration, possess very similar features, and the same theoretical model is acceptable for their description.

It is of interest to view the data concerning the position of current maxima in the scale of potentials. The model presented above supposes a linear dependence between and , the slope of which is determined by the relationship: This regularity is consistent with the most experimental data obtained for moderately acidic Cu(II)-free solutions. For instance, the average slope of lines, presented in the lower part of Figure 10, is ca 50 mV/decade; then, the value that follows from (22) is in agreement with the experimental data listed in Table 3. However, ceases to depend on in more acidic (pH 2.0) 0.02 M gluconic acid solutions, this is indicative of the reversible character of the charge-transfer process. When pH of such solutions was increased, a tendency for a certain increase in magnitudes was observed, but this was not the case for more concentrated (0.05 M) solutions.

Nonlinear dependences on log v are observed in the case of Cu(II)-containing solutions (Figure 10(a)). It is necessary to point out that the situation at the electrode surface is not exactly the same as that in identical Cu(II)-free solutions containing the same and pH. As what was stated above, hydrogen evolution starts in the region of limiting current, where the surface concentration of Cu(II) approaches zero but remain unchanged. However, a certain amount of OA anions is released from the reduced Cu(II)-OA complexes. This results in the shift of chemical equilibria and in the relevant increase in surface pH. Again, simultaneous deposition of copper can also modify the properties of original coatings that were prepared as working electrodes (see Section 2).

The kinetic parameters listed in Table 3 have been determined using not only experimental, but also the literature data. It is common knowledge that stability constants expressed in concentration terms depend on the ionic strength of solutions and on the nature of supporting electrolyte. Unfortunately, data given in different handbooks concern, as a rule, perchlorate or nitrate media and the certain problems arise when fitting such data for sulphate media. Therefore, the reliability of the parameters established here also depends on the reliability of the data selected from the literature. Nevertheless, it can be stated that the kinetic parameters of hydrogen evolution, occurring on the same copper substrate, are very similar in the case of all the systems under investigation. Organic acids seem to be of minor importance for the charge-transfer process and act in general as sufficiently labile proton donors.

5. Conclusions

The CE mechanism of hydrogen evolution occurring in organic acids (OAs) solutions was analyzed invoking the 2nd Fick’s law equations supplemented by terms that account for chemical interactions between diffusing particles. OA are considered as components that are capable of releasing hydrated protons (hydronium anions) taking part in the charge-transfer step. Simple equations containing no kinetic terms were obtained for total concentration of proton donors and acceptors, . When the dissociation of OA is sufficiently fast, the surface concentrations of species can be obtained from the material balance equations involving the stability constants of proton donors.

Linear potential sweep (LPS) voltammetry was applied to study the kinetics of hydrogen evolution in the solutions containing glycolic, malic, tartaric, and gluconic acids. Current peaks observed on LPS voltammograms are in a linear dependence on ( is the potential sweep rate). They obey well-known relationships obtained for simple redox processes, provided that the concentration of oxidant is treated as .

Determination of surface concentrations as current density functions makes it possible to transform LPS voltammograms into linear Tafel plots normalized with respect to the surface concentration of hydronium ions. Similar kinetic parameters ( and μA cm−2)) obtained at pH 3 for all OA solutions indicate that the nature of OA has no noticeable influence on the charge-transfer process.