Study of the Hydrogen Evolution Reaction Using Ionic Liquid/Cobalt Porphyrin Systems as Electro and Photoelectrocatalysts

Abstract

In this work, the design and manufacture of graphite paste (Gr) electrodes is carried out, including N-octylpyridinium hexafluorophosphate (OPyPF₆) ionic liquid (IL) as binder and modification with Co-octaethylporphyrin (Co), in order to study the hydrogen evolution reaction (HER) in the absence and presence of light. The system is characterized by XRD and FESEM-EDX (Field Emission Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy), confirming the presence of all the components of the system in the electrode surface. The studies carried out in this investigation confirm that a photoelectrocatalytic system towards HER is obtained. The system is stable, efficient and easy to prepare. Through cyclic voltammetry and electrochemical impedance spectroscopy, was determined that these electrodes improve their electrochemical and electrical properties upon the addition of OPyPF₆. These effects improve even more when the systems are modified with Co porphyrin. It is also observed that when the systems are irradiated at 395 nm, the redox process is favored in energy terms, as well as in its electrical properties. Through gas chromatography, it was determined that the graphite paste electrode in the presence of ionic liquid and porphyrin (Gr/IL/Co) presents a high turnover number (TON) value (6342 and 6827 in presence of light) in comparison to similar systems reported.

1. Introduction

Hydrogen evolution reaction (HER) corresponds to the generation of hydrogen gas from water electrolysis. This reaction has been widely studied because of its potential use as fuel in the production of clean and sustainable energy [1,2]. Electrodes manufactured with carbonaceous materials [3,4] have been designed to electrocatalyze this reaction, for instance in the case of carbon paste electrodes [5,6]. These electrodes can include other components in order to improve their electrochemical and electrical properties.

These components can include ionic liquids (IL), which are attractive materials since they can be used as binders on paste electrodes, increasing current values compared to the use of other conventional binders (mineral oil or paraffin) [7]. They have also shown important advantages according to their physicochemical properties that allow their use in green chemistry [8]. The ionic liquids based on N-octylpyridinium (OPyPF₆) show favorable responses towards electrochemical reactions of interest when studied in paste electrodes [9,10,11].

Porphyrins are azo-macrocyclic complexes composed of a system of pyrrole rings with aromatic character that can be coordinated with a transition metal [12]. Metalloporphyrins can be used to modify electrodes, since they have an interesting redox activity and stand out for their high electrocatalytic performance towards electrochemical reactions such as HER [13,14,15]. Moreover, when irradiated at determined wavelengths, they can become photocatalysts [16,17] due to the electronic transition from HOMO to LUMO. Exited species present a very labile electron while an electronic vacancy (h⁺) is produced. In this way, both reduction and oxidation processes are photocatalyzed [18]. In previous works, it was determined that Co (II) and Cu (II) octaethylporphyrins deposited on glassy carbon are capable of catalyzing HER [14], becoming even more efficient when they are in the presence of light [15].

Based on the aforementioned information, graphite paste electrodes bound with ionic liquid OPyPF₆ and their modification with Co (II) octaethylporphyrins are manufactured in order to study the electro and photoelectrocatalytic effect of these systems towards HER. Characterization of the material obtained was also performed using XRD and FESEM-EDX. Finally, the quantification of produced H₂ was performed through gas chromatography.

2. Results and Discussion

2.1. Crystallographic Characterization

The different systems, graphite and the incorporation of Co porphyrin and IL are studied by XRD. Figure 1a shows the signals corresponding to the crystal phases of graphite powder with a signal at 2θ = 26.383° associated to the plane C(002), typical of graphite materials. This peak indicates the presence of a hexagonal structure typical of carbonaceous materials [19]. In Figure 1b, the signals of IL are shown, accounting for a high crystallinity since the diffraction peaks are narrow and intense compared to the baseline. For the Co porphyrin sample in Figure 1c, there is low crystallinity reflected on the noise and the low intensity of the diffraction peaks. In Figure 1d, corresponding to the graphite paste/ionic liquid (Gr/IL) system, typical graphite signals were observed, with higher intensity than the signals of the ionic liquid observed between 20 and 30°. On the other hand, the system Gr/IL/Co (Figure 1e) shows a diffractogram like the one of Gr/IL. Given the low crystallinity of the Co porphyrin, it was not possible to observe the diffraction peaks. Finally, in Figure 1f the comparison of the diagrams of Gr, Gr/IL and GR/IL/Co is presented. The inserted figure corresponds to an approximation between 24 and 28° of the signal associated to plane C(002). Here, it can be noted that the intensity of the signals was different for each case, diminishing when IL and Co were present. For the Gr/IL and Gr/IL/Co systems, the signal moves to the right, centering at 26.400° and 26.429° respectively. This is due to the tension on the material in which the distances within the unit cell vary in size. In this case, as the movement of the signal occurred to higher angles, the tension comprised of the unit cell, therefore decreasing the values in the lattice parameter values [20].

2.2. Morphological Characterization

The FESEM images (Figure 2) show the surface of graphite pastes that include mineral oil (Gr), ionic liquid (Gr/IL) and both the ionic liquid and the Co porphyrin (Gr/IL/Co). All the surfaces are significantly different: in the case of the systems Gr and Gr/IL, the type of agglutination varies because of the change of binder when using IL instead of mineral oil. When adding Co porphyrin, generating the Gr/IL/Co system, long fiber-shaped structures can be observed, which are typical for this type of porphyrin [21].

Through FESEM coupled to an X-ray microanalyzer (FESEM-EDX), the chemical composition of the Gr/IL/Co system was studied at a magnification 6525× (Figure 3). The weight percentage values for each element present in the surface are summarized in

Table 1. Weight percentage of the elements present in the surface of the system Gr/IL/Co obtained by FESEM-EDX.

Element	Weight Percentage (%)
C	69.65
O	0.77
F	21.56
P	7.19
Co	0.84

. The main percentage belonged to carbon (near 70%), which is expected as the carbon paste contains graphite and IL in 7:3 proportion. Phosphorous and fluorine content was also detected, corresponding to the anion of IL (OPyPF6). Moreover, traces of Co were quantified in this analysis (0.84%). This value was low, because the porphyrin mainly presented carbon atoms in its macrostructure and a Co atom in the center. Additionally, the porphyrin solution quantity used to modify the surface was small (10 μL, 0.2 mM).

2.3. UV-Vis Spectroscopy

Figure 4 shows the absorption spectra of Co porphyrin. The UV-vis spectrum of porphyrins in solution (Figure 4a) shows the Soret band between 350 and 450 nm. This band accounts for an electronic transition to the second excited singlet (S₀ → S₂) [22] and it is more intense than the Q bands. The later appears between 500 and 700 nm as a result of an electronic transition π–π* to the first excited singlet (S₀ → S₁) [22].

The UV-vis spectrum in solid (Figure 4b) characterizes the porphyrins deposited on glass as an approximation to the modification on graphite paste. It is interesting to highlight that the Co porphyrin shows variations in the adsorption spectra when in solution and in solid. The unfolding of the Soret band associates to the existence of two types of porphyrin populations: monomers and aggregates. The bathochromic effect of the Soret and Q bands indicates that these aggregates would be J-type or side-by-side [23,24,25].

2.4. Electrochemical and Electric Characterization

Graphite paste electrodes using mineral oil as a binder and the incorporation of IL (instead of mineral oil) and Co porphyrin were studied towards HER through voltammetric profiles and electrochemical impedance spectroscopy. In Figure 5a, the system Gr/IL/Co shows the highest electrocatalytic activity in terms of potential, followed by Gr/IL, Gr/Co and finally by Gr. This demonstrates that both the IL and the Co porphyrin could catalyze HER due to an enhancement between the materials. The electrode that contains both components (Gr/IL/Co) shows a higher shift towards less negative potentials in comparison to the systems that contain only one component.

Figure 5b shows Nyquist plots for the same systems. When charge transfer resistance Rct is smaller, the systems conduct the electric current better than electrodes with higher Rct [26]. Therefore, low Rct values imply more conductive surfaces. In Figure 5b, the system that shows the lowest Rct was Gr/IL/Co, followed by Gr/IL, Gr/Co and Gr. This result was coherent with the voltammetric study, establishing for this case a direct relation between catalytic activity and conductive capacity of the electrodes.

The equivalent circuit inserted in Figure 5b presents a solution resistance (Rs), a charge transfer resistance (Rct), an inductor (L), because of the generation of intermediary species in the surface, [17,18] and a constant phase element (CPE). These parameters are summarized in Table S1 of supplementary material.

Considering that Gr/IL/Co is the system that presents the higher electrocatalytic activity and knowing that Co porphyrin has the ability to absorb light in the UV-vis range, the study of HER on this modified electrode was performed by cyclic voltammetry and electrochemical impedance spectroscopy in the absence and presence of light. The nine wavelengths presented in Table S2 of supplementary material were used. In previous works, it has been established that the photoelectrocatalytic activity of the metal octaethylporphyrin is independent to the intensity of the lamps, [15] being decisive of the value of the applied wavelength.

Upon irradiating the surface of the modified electrode, a higher effect in terms of electrocatalytic activity (Figure 6a) could be observed, and in terms of its conductive properties (Figure 6b) when a wavelength of 395 nm was used. It is important to mention that this wavelength coincided with the Soret band of the adsorption spectrum of Co porphyrin in Figure 4. As mentioned in a previous work [27], the activity of the catalysts depended on the wavelength applied and not on the intensity of the light. The onset potentials and charge transfer resistance for all the systems in study are summarized in

Table 2. Onset potential (*E_O) and charge transfer resistance (Rct) values.

System	EO/V	Rct/Ω
Gr	−1.46	3950
Gr/Co	−1.34	1010
Gr/IL	−0.99	1510
Gr/IL/Co	−0.91	960
Gr/IL/Co 395 nm	−0.89	545

* E_O was determined with respect to the Ag/AgCl reference electrode, considering the potential where the reduction current increase begins.

.

The Gr/IL/Co modified electrode presents a high electrocatalytic activity, which improves when irradiating at 395 nm. This system was selected to determine the HER mechanism through Tafel slope (Figure 7). For similar materials, Tafel slopes close to 160 mV per decade in aqueous solution (pH ≈ 7) have been reported [15,28].

For the Gr/IL/Co system, the Tafel slope is 148 mV per decade. This value is consistent with the Volmer–Heyrovsky mechanism, [23,29,30] according to adsorption and desorption processes presented in Equations (1)–(3). The slow stage would be the Equation (2) [31].

M + H₃O⁺ + e⁻ → M-H_ads + H₂O,

(1)

M-H_ads + H₃O⁺ + e⁻ → M + H₂ + H₂O,

(2)

M-H_ads + M-H_ads → H₂ + 2M.

(3)

This system also has the advantage of being stable, showing relatively constant currents in time for a 2-h period when applying a controlled potential of −1.3 V (Figure S1 of supplementary material).

2.5. Chromatographic Quantification

The quantity of hydrogen produced using the system Gr/IL/Co was determined through electrolysis at a fixed potential of −1.3 V in a two-compartment sealed electrochemical cell. The working electrode (Gr/IL/Co) with an electroactive area of 1.83 cm² produces the reduction of protons to gaseous hydrogen in one of these compartments. Another compartment contains a platinum counter electrode where oxygen is produced simultaneously. The generation of both gases (hydrogen and oxygen) is corroborated by the formation of bubbles in both sides of the cell. The electrolysis process is carried out for 2 h, and every 1 h, an injection of 50 µL of gas from the working electrode compartment is obtained to be studied by gas chromatography. The hydrogen concentration (CH) is calculated by performing a calibration curve (Figure S2 of supplementary material), whose linear equation is A = 604.45CH − 21.19; R² = 0.991. The values of CH obtained by gas chromatography are in

Table 3. Hydrogen concentration (C_H), amount of catalyst (N) and turnover number (TON) obtained using the systems Gr/IL/Co and Gr/IL/Co 395 nm.

System	CH/μmol	CH/μmol h−1 cm−2	N/mol	TON
Gr/IL/Co	56.5	15.4	9.01 × 10−9	6342
Gr/IL/Co 395 nm	57.3	15.7	8.40 × 10−9	6827

.

Then, after 2 h of electrolysis, the systems Gr/IL/Co and Gr/IL/Co 395 nm produced 56.5 and 57.3 μmoles of H₂, corresponding to 15.4 and 15.7 μmol∙h⁻¹∙cm⁻² respectively using the electroactive area (1.83 cm²). The turnover number (TON) was calculated through the ratio between the H2 moles determined by gas chromatography and catalyst moles [15]. The amount of catalyst deposited on the electrode was determined considering the relation between the electric current, charge and time of Equation (4) and Faraday’s Law of electrolysis (Equation (5)), [32], applied to the voltagramm of systems Gr/IL/Co and Gr/IL/Co 395 nm employed on the chromatographic determination (Figure S3 of supplementary material). In Equation (4), Q is the charge, I is the magnitude of the faradaic current (1.91∙10⁻⁴ and 1.8∙10⁻⁴ A for the systems in the absence and in the presence of light respectively) and t is time. The time is obtained from the ratio between magnitude of E_O and scan rate (0.1 V∙s⁻¹), therefore Q = 1.74∙10⁻³ C for Gr/IL/Co and 1.62∙10⁻³ C for Gr/IL/Co 395 nm.

In Equation (5), N is the amount of catalyst deposited on electrode in moles, F is the Faraday constant (96485 C∙mol-1), [32] n is the number of electrons involved in the reaction, which in the case of HER are 2. In this way, it is determined the catalyst quantity and the turnover number (Table 3). These values are higher than similar systems previously reported in literature (

Table 4. Comparison of onset potential (E_O) and catalyst turnover number (TON) in similar systems.

System	pH	EO/V Ag/AgCl	EO/V RHE	TON	Ref.
Commercial Pt/C	7	≈−0.66 *	≈−0.05	-	[33]
[Co(TPP)](COOH)4]	≈7	−0.92	−0.31 *	51.8	[34]
[Co(TPP)](SO43H)4]	≈7	−0.75	−0.14 *	104.1	[34]
[Co(TPP)](NH2)4]	≈7	−1.01	−0.40 *	46	[34]
[Co(bpyPY2Me)(CH3CN)(CF3SO3)](CF3SO3)]	≈4	−1.11	−0.50 *	4200	[35]
dinuclear cobalt(II)etriazenido complex	4.6	≈−1.1	≈−0.5 *	4367	[36]
dinuclear cobalt(II)etriazenido complex	7	≈−1.3	≈−0.7 *	4367	[36]
GC + 4AP + Co(II)OEP	≈7	−1.17	−0.56 *	6000	[13]
Gr/IL/Co	7	−0.91	−0.30 *	6342	This work
Gr/IL/Co 395 nm	7	−0.89	−0.28 *	6827	This work

* Transformed values between Ag/AgCl and RHE reference electrodes.

).

$$I=\left(\frac{dQ}{dt}\right),$$

(4)

$$Q=N·F·n.$$

(5)

The faradaic efficiency f(%) is determined through Equation (6), where C_H is the number of moles of hydrogen produced, n is the number of electrons transferred (2 for the HER), F is 96485 C∙mol⁻¹ and q is the charge after 2 h of electrolysis obtained from Figure S1. The values of q and f(%) are summarized in

Table 5. q and f(%) values for the systems Gr/IL/Co and Gr/IL/Co 395 nm.

System	q/C	f(%)
Gr/IL/Co	19.1	57
Gr/IL/Co 395 nm	21.7	51

. Here, both systems Gr/IL/Co and Gr/IL/Co 395 nm show similar values of f(%) (over 50%).

$$f(\%)=\frac{C_H·n·F}{q}\times 100.$$

(6)

3. Materials and Methods

3.1. Reagents and Solutions

Potassium chloride, graphite powder, hexane, pyridine, acetonitrile (HPLC grade) diethyl ether and dichloromethane (DCM; analytical grade) were obtained from Merck. Ultra-pure water was obtained from a Millipore-Q system (18.2 MΩcm). Argon (99.99% pure) was purchased from AGA, Chile. Cobalt (II) 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine, potassium dihydrogen phosphate, disodium hydrogen phosphate, 1-bromooctane, potassium hexafluorophosphate and mineral oil were all purchased from Sigma-Aldrich Chile.

3.2. Synthesis of the Ionic Liquid

The ionic liquid (IL) synthesis is performed through a method consisting of two stages [37]. The first stage consists of the quaternization of pyridine to obtain OPyBr and the second stage is the anionic interchange to obtain OPyPF₆. First, 10 mL of pyridine, 21.5 mL of 1-bromooctane and 20 mL of acetonitrile were refluxed in Ar atmosphere at 100 °C for 48 h. The solvent was evaporated, and the compound was dried at 90 °C for 15 h (yield = 93%). Then, 12.54 g of OPyBr generated in the first step of the reaction was mixed with 8.70 g of KPF₆, 20 mL of DMC and 20 mL of ultra-pure water in a glass flask, and the mixture was stirred at room temperature for 2 h. After washing and extracting the organic phase, the solvent was evaporated and the compound was recrystallized in 95% ethanol, to later be dried at 90 °C for 24 h (yield = 91%). The structure of OPyPF₆ was confirmed by spectra characterization ¹H NMR, ¹³C NMR and ¹⁹F NMR.

¹H NMR (400 MHz, CDCl₃) δ 8.63 (d, J = 6.1 Hz, 2H, H2 and H6), 8.43 (t, J = 7.7 Hz, 1H, H4), 7.97 (t, J = 7.0 Hz, 2H, H3 and H5), 4.51 (t, J = 7.6 Hz, 2H, 2 × H1′), 1.98−1.84 (m, 2H, 2 × H2′), 1.32−1.15 (m, 10H, 2 × H3′-H7′), 0.82 (t, J = 6.6 Hz, 3H, 3 × H8′).

¹³C NMR (101 MHz, CDCl₃) δ 145.64, 144.07 (2C), 128.59 (2C), 62.57, 31.60, 31.36, 28.90, 28.81, 25.96, 22.53, 14.02.

¹⁹F NMR (376 MHz, CDCl₃) δ −72.23 (d, ¹J_P-F = 712.2 Hz, PF₆).

3.3. Electrode Preparation

As reported before [27], powder graphite was mixed with the binder (mineral oil or IL) in a 7:3 proportion in mass in an agate mortar, adding diethyl ether. The mixture was homogenized until the complete evaporation of diethyl ether, obtaining a paste that was used to fill a Teflon hollow electrode, compacting completely. A solution of 0.2 M Co (II) octaethylporphyrin was prepared in DCM/hexane (2:8 proportion). Of porphyrin solution 10 μL was added to the paste electrode surface and let to evaporate at room temperature. After 5 min, the surface of the electrode was carefully washed with ultra-pure water.

3.4. Instrumentation

A Bruker B8 Advance diffractometer was used, operating with a CuK_α1,2; K_β X ray tube generated at 30 rmp, 40 kV and 40 mA. The scans were performed with a period of 1.0 and 3.0 s, for values of 2θ between 10 and 80° and 20 and 80° and increase of 0.02°. A scintillation detector was employed.

A scanning electron microscope with a field emission cathode (FE-SEM) coupled to the composition analysis by the energy-dispersive X-ray (EDX) model FEI Quanta^TM 250 FEG was also employed.

UV-Vis spectroscopy was performed in UV-2600 UV-VIS SPECTROPHOTOMETER SHIMADZU equipment, with a module to study samples in solution inside a quartz cell, and a second module to analyze solid samples over a glass plate. On the analyses performed on both modules, the porphyrins on DCM/hexane 2:8 was studied. To study the sample (in solid), 0.5 mL of 0.2 mM Co (II) porphyrin solution was deposited over the glass surface and the solvent was dried at room temperature. The spectra were obtained between 350 and 700 nm.

The activity of electrodes was studied in a three-compartment cell. It used an Ag/AgCl reference electrode in 3 M KCl, the working electrode and a platinum counter electrode that contained 0.1 M phosphate buffer (pH = 7), saturated with argon for 20 min.

Voltammetry, electrical impedance spectroscopy (EIS) and chronoamperometry analyses were performed by using a CH Instruments 750D potentiostat. In the characterization by EIS, 100,000 Hz to 1 Hz frequencies and 0.005 V amplitude were used. The measurements were performed in 0.1 M phosphate buffer (pH = 7) bubbled with argon for 20 min at a potential of −1.3 V.

Photoelectrocatalytic studies were carried out within a dark and closed box. It used an electrochemical cell equipped with a quartz window to ensure an adequate irradiation of the surfaces by using a monochromator with nine available lamps between 380 and 660 nm (Table S2 of supplementary material). The distance between the monochromator and the modified electrode was 10 cm.

The quantification of hydrogen gas produced by the manufactured electrode was carried out in a sealed electrolysis cell in which HER was performed at the working electrode compartment at a fixed potential of −1.3 V. The compartments were separated by a salt bridge filled with agar at 2.5% m/V in KCl 1 M. While the electrocatalysis occurred, the phosphate buffer solution at pH = 7 (in Ar saturation) was agitated to favor the hydrogen bubbles rise to the Head Space of the cell. Every 1 h 50 μL of gas was obtained and analyzed by gas chromatography in a DANI-Instruments^® Gas Chromatograph.

4. Conclusions

In this work, a graphite paste electrode modified with the addition of OPyPF6 as a binder and Co porphyrin was obtained, in order to study HER. The study through XRD shows the presence of the components of the system and indicates that both the IL and the Co porphyrin generated a compression effect on the graphite unit cell. By FESEM, it was observed that the surfaces of the systems were evidently different and by FESEM-EDX the presence of IL (PF6-) anion and Co coming from the porphyrin on the surface of the Gr/IL/Co electrode were corroborated. It was determined that the Gr/IL/Co system was the electrode that shows a higher electrocatalytical effect and lower charge transfer resistance towards this reaction in comparison to the other systems. These properties improved even more when the system was irradiated at 395 nm, accounting for its photoelectrocatalytic ability. It is determined that, when using this system, HER occurred through the Volmer–Heyrovsky mechanism. Finally, in accordance to the chromatographic studies, the electrode shows a high TON value (6342 in absence and 6827 in presence of light) in relation to the use of similar systems reported in literature and a faradaic efficiency over 50%.