Electro-Reactivity of Resorcinol on Pt(111) Single-Crystal Plane and Its Influence on the Kinetics of Underpotentially Deposited Hydrogen and Hydrogen Evolution Reaction Processes in 0.1 M NaOH Solution

Abstract

This article primarily presents cyclic voltammetry, Tafel polarization and ac. impedance spectroscopy electrochemical examinations of resorcinol (RC) electro-reactivity on the Pt(111) surface and its influence on the kinetics of UPD H (underpotentially deposited hydrogen) and the HER (hydrogen evolution reaction) in a 0.1 M NaOH supporting solution. The collected data provided evidence of the RC-ion’s surface adsorption and its further electroreduction in the presence of surface-adsorbed H radicals along with their primary beneficial role on the kinetics of the UPD H process. The above was elucidated through an evaluation of the associated charge-transfer resistance and capacitance parameters, and was carried out on the platinum (111) electrode plane, comparatively, for the RC-free and resorcinol-modified NaOH electrolyte. In addition, the recorded cathodic charge transients (obtained by injecting small amounts of RC-based 0.1 M NaOH solution to initially resorcinol-free electrolyte, carried out at the constant electrode potential characteristic to the UPD H potential zone) provided evidence that the RC species undergoes electrocatalytic reduction through the involvement of the Pt(111)-chemisorbed hydrogen radicals.

1. Introduction

The exceptional catalytic properties of platinum catalysts have led to wide-ranging research into the electrochemical performance of different organic molecules and well-ordered single-crystal Pt planes [1,2]. The current research can be traced back to some earlier works published from this laboratory on the electrochemical reactivity of small organic molecules, including guanidine [3], acetamidine [4], formamidoxime [5], urea [6,7] and single Pt crystals, as well as recently published articles on resorcinol (RC) [8,9] on polycrystalline Pt electrodes, examined in both H₂SO₄ and NaOH solutions.

The new work reported in the present article refers in particular to the process of the electrosorption of resorcinol ions onto the surface of a Pt(111) plane in a 0.1 M NaOH solution, which then, in the adsorbed state, become reduced by H radicals and influenced the kinetics of underpotentially deposited hydrogen (UPD H). The importance of this surface choice relates to the fact that under alkaline conditions the (111) is the only Pt plane that provides a wide potential separation between the H and OH species adsorption regions. The above not only allows for the quantification of the effect of RC adsorption on the extent of H or/and OH co-adsorption, but also for an examination of the Pt surface-based RC’s reactivity over the potential range of double-layer charging.

The UPD H appears at potentials positive to the H₂’s reversible potential, when the Gibbs energy of the hydrogen atoms’ binding with the metal surface (observed at e.g., Pd, Rh, Ir and Pt) is numerically greater than half of the Gibbs energy needed to bond to a hydrogen molecule [10,11,12,13] (see Equation (1) for the UPD of H on Pt in a neutral/alkaline medium with H atoms being extracted directly from water molecules instead of H₃O⁺ in acidic media).

$${\mathrm{H}}_2\mathrm{O}+\mathrm{P}\mathrm{t}+1{\mathrm{e}}^{-}\hspace{0.33em}\hspace{0.33em}\iff \hspace{0.33em}\hspace{0.33em}\mathrm{P}\mathrm{t}-{\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+\mathrm{O}{\mathrm{H}}^{-}$$

(1)

On the other hand, resorcinol (phenol’s hydroxy derivative) is extensively used in industry in the production of rubber and many organic chemicals. The presence of water-soluble phenolic species such as resorcinol could have a significant impact on the performance of fuel cells and water electrolyser systems. This is because RC molecules under suitable external conditions could become susceptible to catalytic (particularly significant for semi-noble and noble metals, and their alloys) surface adsorption and electrooxidation processes. Then, under such circumstances, the RC-modified catalyst’s reactivity (e.g., Pt for hydrogen oxidation or H₂ evolution reaction) might significantly change [14,15,16]. Also, phenolic species (including phenol and resorcinol) are known in the literature to be prone to selective catalytic hydrogenation in the presence and absence of external hydrogen [17,18,19,20].

2. Materials and Methods

A 0.1 M NaOH supporting electrolyte was made from sodium hydroxide monohydrate pellets (MERCK, Darmstadt, Germany, 99.99%) and ultra-pure water produced by Millipore Direct-Q3 UV water purification system (MERCK, Darmstadt, Germany) with 18.2 MΩ cm water resistivity. The resorcinol concentration (Sigma-Aldrich, St. Louis, MO, USA, >99.0%) in the working solution was in the order of 1.5 × 10⁻³ (and comparatively 1.5 × 10⁻⁵) M.

All electrochemical experiments were performed with a typical, three-compartment Pyrex glass electrochemical cell. The cell contained three electrodes, including the Pt(111) working electrode, aligned to the chosen crystallographic orientation by means of the back reflection von Laue X-ray diffraction method [21] with S_A ≅ 0.067 cm², Pd RHE: reversible hydrogen electrode as reference (1.0 mm diameter, Sigma-Aldrich, 99.99%), and a counter electrode (CE) made from a coiled Pt wire (1.0 mm diameter, 99.9998% purity, Johnson Matthey Inc., London, UK). Before conducting experiments, solutions were de-aerated with high-purity argon (6.0, Linde, Woking, UK), the flow of which was also maintained above the solution’s during the measurements.

Furthermore, the charge-transient experiments were carried out at the constant electrode potential mode by injecting 100 µL amounts (Hamilton 710 N micro-syringe model, Hamilton Company, Reno, NV, USA) of 1.2 M RC into the 0.1 M NaOH (previously de-aerated by bubbling with high-purity Ar) to create an initially resorcinol-free (also de-aerated) unmodified sodium hydroxide solution. The experiments were performed (six independent trials) at the electrode potential characteristic of the process of the UPD H.

Biologic SP-240 Electrochemical System was used to carry out all electrochemistry tests. Cyclic voltammetry (performed at 50 mV s⁻¹), Tafel polarization (carried-out at the scan-rate of 0.5 mV s⁻¹) and ac. impedance spectroscopy experiments were conducted in this work. For the impedance measurements, the generator provided an output signal of 5 mV, whereas the frequency range was swept between 1.0 × 10⁵ and 1.0 Hz (or 20 mHz). The instrument was controlled by EC-Lab^® V11.36 software for Windows. Three impedance measurements were carried out at each electrode potential with the duplicability of such obtained results being in the order of 10% or below. Data analysis was performed with ZView 4.0 software package for Windows, where the impedance spectra were fitted by means of LEVM 6—a complex, non-linear, least-squares immittance fitting program, written by J.R. Macdonald in Ref. [22].

3. Results and Discussion

3.1. Cyclic Voltammetry, Linear Sweep Voltammetry and Tafel Polarization Behaviour of Pt(111) in 0.1 M NaOH, in the Absence and Presence of Resorcinol

The cyclic voltammetric behaviour of the Pt(111) electrode in contact with 0.1 M NaOH supporting solution, in the absence and presence of resorcinol (at 1.5 × 10⁻³ M), is shown in Figure 1 below. Hence, in the absence of resorcinol, the CV profile of the single-crystal Pt(111) plane in an alkaline medium is characterized by the presence of two well-separated adsorption regions over the potential ranges 0.05–0.40 V and 0.60–0.90 V vs. RHE, isolated by a double-layer charging region. The former corresponds to the reversible adsorption of H atoms (underpotential deposition of hydrogen), whereas the latter, so-called “butterfly”, region refers to the reversible electrosorption of OH⁻ anions [6,23] (see Figure 1). Hence, the mean electric charge measured between 0.05 and 0.90 V, after subtracting the double-layer charge contribution, came to 283 µC cm⁻², being a sum of 106 and 177 µC cm⁻², corresponding to the first low potential zone, and to the high potential OH adsorption “butterfly” region, respectively. These charges translate to a maximum coverage of H by UPD (θ_H) of a ca. 0.44 hydrogen monolayer and to an OH species (θ_OH) of about 0.73 of an equivalent monolayer of H (compare with e.g., Ref. [24]).

Having introduced resorcinol at the concentration of 1.5 × 10⁻³ M into the supporting NaOH solution, the total voltammetric charge (blue curve in Figure 1) radically depleted to reach 154 µC cm⁻², which split to 56 and 98 µC cm⁻², correspondingly, for the UPD H (θ_H/RC ≈ 0.23) and OH (θ_OH/RC ≈ 0.40) adsorption regions. The above is strongly believed to be a result of simultaneous Pt surface co-adsorption and the interaction of large RC ions (formed after complete resorcinol ionization in sodium hydroxide [25]) with hydrogen atoms and also OH groups (the latter is also evidenced through a significant thickening of the double-layer charging zone). The proposed mechanism for the resorcinol oxidative Pt electrosorption and the RC ion-to-H attractive interaction, i.e., via partial or complete negative charge on the oxygen atom vs. partial positive charge on the hydrogen atom, is illustrated in Scheme 1 below. The above is also confirmed through the recorded ac. impedance behaviour described in detail in Section 3.2.

On the other hand, both the UPD of H and the OH reversible adsorption regions are much more pronounced (in reference to the resorcinol free cyclic voltammogram) in the voltammetric profile of the radically reduced RC-ion concentration at the level of 1.5 × 10⁻⁵ M (see a dark blue line in Figure 1).

Furthermore, in order to explain the presence of a non-negligible negative current flowing between 0.05 and 0.20 V in the presence of resorcinol (see Figure 1 again), charge-transient experiments were performed at the electrode potential of 0.05 V. As a result, a large, average cathodic charge-transient density of ca. −220 µC cm⁻² was recorded (Figure 2) after subtracting the O₂ reduction contribution (about −45 µC cm⁻²). Hence, no charge-transient behaviour (beyond that involving inevitable oxygen reduction, as small amounts of air are always being sucked in along with solution’s sample) was observed for injections of resorcinol-free 0.1 M NaOH solution samples. In conclusion, under the experimental conditions, the observed cathodic charge-transients recorded at the potential value of 50 mV could only correspond to the process of resorcinol ion electroreduction at the Pt(111) single-crystal plane, realized via the underpotentially deposited hydrogen radicals. The resorcinol ion reduction process proposed here to form a 1,3-cyclohexanediolate ion [17,18,19] with six UPD H atoms per single RC entity is illustrated in Scheme 2 below.

Interestingly, additional linear-sweep voltammetry experiments, extended to the potential range negative to the hydrogen reversible potential (0.00 to −0.30 V/RHE), showed that the Pt surface-adsorbed resorcinol species (or its electroreduction product/s) exhibited a significant positive influence on the HER rates, which can clearly be seen in Figure 3A. Hence, for the cathodic overpotential of 0.3 V, the recorded current densities were 1.3× and 1.5× greater for the RC-ion concentrations of 1.5 × 10⁻³ and 1.5 × 10⁻⁵ M, respectively, than those recorded in the absence of resorcinol in the solution (contrast to the behaviour at low overpotentials, where higher concentration of the RC-ion provided superior facilitation of the HER kinetics).

In addition, a significant enhancement in the the HER characteristics upon introduction of resorcinol into the supporting electrolyte could also unambiguously be observed over the kinetically controlled, low overpotential region of the cathodic polarization curves in Figure 3B,C. The recorded cathodic Tafel slopes (parameter b) approached 66, 78 and 57 mV dec⁻¹ for the unmodified NaOH and the RC concentrations of 1.5 × 10⁻⁵, and 1.5 × 10⁻³ M, respectively (Figure 3C). Also, the presence of resorcinol somewhat facilitated Tafel-estimated values of an exchange current-density (j₀) parameter, which came to 1.1 mA cm⁻² (0.1 M NaOH), 2.2 mA cm⁻² (0.1 M NaOH + 1.5×10⁻⁵ M RC) and 1.3 mA cm⁻² (0.1 M NaOH + 1.5×10⁻³ M RC). The latter observation is in line with the literature-based j₀ values recorded for platinum or Pt-based HER catalysts [26,27]. Finally, as the HER rate begins to be restricted by the mass transfer rate (for overpotentials beyond ca. 200 mV, see Figure 3B again), all polarization curves begin to converge.

3.2. Ac. Impedance Behaviour of Pt(111) in 0.1 M NaOH, in the Absence and Presence of Resorcinol

The Ac. impedance electrochemical behaviour of the Pt(111) electrode in unmodified 0.1 M NaOH solution and in the presence of 1.5 × 10⁻³ M RC, examined over the potential range characteristic to the processes of UPD of H and the RC-ion electrosorption, is shown in Figure 4A–C and

Table 1. Resistance and capacitance parameters for UPD of H, electrosorption and electroreduction of resorcinol ion on Pt(111) electrode surface in contact with 0.1 M NaOH and in the presence of RC-ion (1.5 × 10⁻³ and 1.5 × 10⁻⁵ M, and 293 K), obtained by fitting the equivalent circuits shown in Figure 4D,E to the acquired impedance data (values of dimensionless φ parameter for the CPE circuits fluctuated between 0.73 and 0.98; impedance reproducibility typically remained below 10%; χ² = 3 × 10⁻⁵ to 5 × 10⁻³).

E/mV	RH/Ω cm2	Cdl/µF cm−2	Rct/Ω cm2	CpH/µF cm−2
0.1 M NaOH
50	17.6 ± 0.2	45.7 ± 3.8	-	664 ± 7
100	31.2 ± 0.3	46.5 ± 2.7	-	662 ± 8
150	53.4 ± 0.5	43.7 ± 1.9	-	657 ± 6
200	80.2 ± 0.7	48.3 ± 1.0	-	610 ± 7
300	311.6 ± 3.7	47.4 ± 0.7	-	116 ± 1
500	-	59.3 ± 0.7	-	-
0.1 M NaOH + 1.5 × 10−3 M RC
50	3.3 ± 0.2	53.2 ± 5.3	1005 ± 13	533 ± 25
100	4.5 ± 0.1	53.6 ± 10.1	-	556 ± 11
150	7.0 ± 0.2	54.7 ± 8.8	-	425 ± 8
200	11.1 ± 0.3	45.9 ± 4.9	-	232 ± 5
			RF/Ω cm2	Cp/µF cm−2
300	-	57.9 ± 0.6	91,366 ± 3820	128 ± 15
400	-	90.7± 0.7	10,906 ± 784	99 ± 5
500	-	56.4 ± 0.9	8880 ± 488	122 ± 4
0.1 M NaOH + 1.5 × 10−5 M RC
			Rct/Ω cm2	CpH/µF cm−2
50	3.1 ± 0.2	57.8 ± 5.5	1716 ± 23	698 ± 33
100	7.0 ± 0.5	29.4 ± 2.9	-	672 ± 31
150	11.3 ± 0.7	35.6 ± 3.2	-	523 ± 29
200	20.3 ± 1.3	37.2 ± 2.4	-	458 ± 26
			RF/Ω cm2	Cp/µF cm−2
300	-	55.9 ± 0.5	12,240 ± 178	242 ± 5
400	-	60.7± 0.6	68,072 ± 4120	72 ± 3
500	-	53.4 ± 0.8	69,479 ± 1589	66 ± 2

. The process of the UPD of H generates a semicircle present over the high and intermediate frequency range in the Nyquist impedance plot along with a vertical line (Figure 4A), which characterizes purely capacitive behaviour at relatively low frequencies. The latter often deviates from a 90° angle, due to the so-called capacitance dispersion effect [3,28], related to surface roughness and heterogeneity.

Hence, the corresponding charge-transfer resistance, the R_H parameter (proportional to the inverse of the exchange rate for the process of UPD of H), considerably increased from 17.6 (at 50 mV) to reach 311.6 Ω cm² at 300 mV (where H adsorption became radically depleted). Then, the double-layer capacitance, C_dl, values fluctuated between 44 and 48 μF cm⁻² for the corresponding electrode potential range. On the other hand, the H adsorption pseudocapacitance, C_pH, parameter stayed relatively constant (above 600 μF cm⁻²) over the potential range of 50–200 mV, which then dramatically dropped to 116 μF cm⁻² at the mentioned electrode potential of 300 mV vs. RHE (see Table 1 for details and a comparison of these results with analogous data on the kinetics of the UPD of H published in refs. [6,23,29]).

Then, the introduction of resorcinol (at the concentration of 1.5 × 10⁻³ M) into the supporting electrolyte caused the R_H parameter to become radically reduced, i.e., to 3.3 Ω cm² at 50 mV (5.3 times) and to 11.1 Ω cm² at 200 mV (7.2 times), as compared to the RC-free electrolyte. Then, the recorded C_dl and C_pH capacitance values largely resembled those of the RC-unmodified sodium hydroxide solution. Interestingly, the occurrence of another partial semicircle over intermediate and low frequencies, at the potential of 50 mV (see Figure 4C,E, and the respective R_ct value of 1005 Ω cm² in Table 1) coincides with the voltammetric cathodic feature observed in Figure 1, over the potential range of 0.05–0.20 V. The latter is strongly believed to be associated with the Faradaic process of RC-ion reduction, realized via surface-chemisorbed UPD H atoms (see Scheme 2 again) and is in line with the results obtained through the above-discussed charge-transient experiments in Figure 2 above.

However, in contrast to the RC-free electrolyte, in the presence of resorcinol, another partial semicircle (seen over high and intermediate frequencies) with a low frequency capacitive line, at an inclination to the Z′ axis significantly lower than 90°, can be observed in the Nyquist impedance spectra. The latter was identified over the potential range of 300–500 mV/RHE, characteristic of the double-layer charging zone (see as examples a Nyquist impedance spectrum and a Bode phase-angle plot recorded at 500 mV in insets to Figure 4A, and Figure 4B, respectively). This behaviour is strongly believed to be associated with the Faradaic process of the RC-ion electrosorption on the Pt(111) surface, where its kinetics are several orders in magnitude slower than those of the process of UPD of H (see the respective charge-transfer resistance, R_F, C_p and C_dl capacitance parameter values, recorded in Table 1 and Scheme 1 above). Then, while in the adsorbed state, the RC ions and chemisorbed H atoms (now with significantly limited surface coverage though) undergo attractive interaction (see Scheme 1 again), which as a result considerably facilitates the kinetics of the process of underpotential deposition of H (again, compare the recorded values of the R_H parameter for the RC-free and resorcinol-modified NaOH solution in Table 1). Furthermore, as for platinum single-crystals’ electrodes, the UPD H kinetics play “a prerequisite role” in the Faradaic process of bulk hydrogen evolution [30,31], the surface-electrosorbed resorcinol entity (or its electroreduction product/s) might potentially be considered as an efficient, HER (hydrogen evolution reaction) catalytic Pt surface modifier.

Furthermore, comparative ac. impedance experiments carried-out at a radically lower RC-ion concentration in the working solution (1.5 × 10⁻⁵ M) provided kinetic UPD H data that are generally in-line with those obtained for a higher resorcinol concentration, although the corresponding charge transfer resistance, R_ct parameter value (recorded at 50 mV vs. RHE, Table 1) is about 1.7 times greater. The above implies that a higher RC-ion concentration in the electrolyte significantly facilitates its Pt surface reduction process. Moreover, contrary to the behaviour observed at higher RC concentration, at the resorcinol concentration of 1.5 × 10⁻⁵ M, the rate of RC electrosorption on the Pt surface seems to be inversely proportional to the electrode potential. The latter, we believe, might be related to RC-to-OH species surface competitive interactions, which could become significant at high anodic potentials. However, this effect requires further study and detailed elucidation.

Interestingly, in recent work from this laboratory [8], resorcinol has been found to have a detrimental effect on the kinetics of the process of UPD of H on polycrystalline Pt electrode in 0.5 M H₂SO₄, already at the concentration of 1.0 × 10⁻⁵ M RC (with θ_H/RC ≈ 0.29). The latter most likely results from the fact that under alkaline conditions (in contrast to the behaviour in acidic solution) surface electrosorption of resorcinol species is significantly limited, due to the existing repulsive interactions, exhibited between the adsorbed RC anions (Pt–RC^–-to-^–RC–Pt).

4. Conclusions

The employment of cyclic voltammetry, cathodic Tafel polarization, ac. impedance spectroscopy along with a chronoamperometric charge-transient electrochemical techniques to study the influence of resorcinol (a valuable industrial chemical and possible contaminant of electrolyser/fuel cell systems) on the kinetics of the UPD of H and HER processes, studied on a Pt(111) single-crystal electrode under alkaline conditions, provided confirmation of significant positive role of platinum-electrosorbed resorcinol ions (which simultaneously undergo a Faradaic reduction process, most likely to form 1,3-cyclohexanediol derivative in the presence of UPD H atoms) on the rates of underpotential deposition of hydrogen, which is a precursor of the hydrogen evolution reaction with platinum series catalysts. Furthermore, a constructive role (e.g., through facilitation of final H₂ surface desorption step) of the RC species adsorbed onto the Pt(111) surface (and/or its electroreduction derivatives) on the kinetics of bulk hydrogen evolution process was also illustrated through additional linear-sweep voltammetry and Tafel polarization experiments, conducted over the potential range of ca. 0.00 to −0.30 V vs. RHE.

However, further laboratory work (perhaps with the involvement of additional, in-situ infrared experiments) would be required in order to provide detailed understanding of the process of RC-ion adsorption on the surface of Pt(111) plane and to provide more insights into competitive RC-ion-to-UPD of H adsorption, especially on how it might affect the kinetics and mechanisms of processes of UPD and OPD of H, and HER Faradaic reaction, also in relation to the behaviour recently reported in Ref. [8] for 0.5 M H₂SO₄.