Electrocatalytic Production of Hydrogen Using Iron Sulfur Cluster

In response to the energy crisis, rising fossil fuel costs and global climate warming, this study focuses on the electrocatalytic reduction of proton into hydrogen using an iron sulfur cluster in the presence of pentafluorothiophenol. The direct reduction of pentafluorothiophenol at vitreous carbon electrode occurs at Ep-1.3 V vs Ag/AgCl in Tetrabutylammonium tetrafluoroborate [Bu4N][BF4]-DMF solution. Interestingly, in the presence of Iron Sulfur Cluster [Fe4S4(SPh)4][Bu4N]2, the reduction potential shifts significantly to -0.98 V vs Ag/AgCl. Based on gas chromatography analysis, the formation of H2 has been confirmed with a current efficiency of ca. 63% after two hours, while the chemical yield at the carbon electrode was about 46%. On the other hand, no H2 gas was detected without catalyst. Importantly, the increment of the concentration of acid (up to 18 equivalents) led to a positive shifting in the reduction potential until a value of 0.18 V. These results reflect the exquisite electrocatalytic efficiency of the protein-like iron sulfur cluster in Hydrogen Evolution Reaction (HER).


Introduction
The exquisite target of using hydrogen as sustainable energy has attracted significant attention because of many advantages including its innocent impact on the environment with energy conversion free of CO 2 emission. However, the ultimate problem resulting from its production -mostly from fossil fuels such as coal and oil-remains in the release of CO 2 which endanger the environment and global health (Aulice, 2004& Maria, 2007 Therefore, researchers are interested in developing many other methods for hydrogen production including water splitting and proton reduction (Artero, 2014;Bhugun, 1996;Kumar, 2010;Bak, 2002& Kelly, 2008. Transition metal complexes are of great interest for electrocatalytic reduction of proton into hydrogen at low potential (Perutz, 2012;Kellett, 1985;Koper, 2013;David, 2015& Han, 2014. Alenezi (2017) has studied the electrochemical catalysis of proton reduction into hydrogen using mesotetrakis-(pentafluorophenyl)porphyrin iron(III) chloride in the presence o triethylamine hydrochloride f Et 3 NHCl as source of proton. The reduction potential E p was marked with a significant shift from -1.6 V to -1.3 V vs Ag/AgCl in the presence of the catalyst. A current efficiency of ca. 58% after 3.8 h was recorded, with a yield of 8 µmoles and a turnover number of 8 while the chemical yield was about 80% . On the other hand, {Fe 4 S 4 }-clusters have been known for over 40 years. However, the studies of their electrocatalytic properties have been extremely limited. These entities, commonly known as iron sulfur clusters, play important roles in biological systems; they are involved in electron transfer as well as catalytic, structural, and sensory roles (Pickett, 2003& Goh, 1996. In this study, we focus on the electrochemical catalysis of proton reduction into hydrogen using the synthetic iron sulfur complex [Fe 4 S 4 (SPh) 4 ] 2at the vitreous carbon electrode in [Bu 4 N][BF 4 ]-DMF solution.

Methed
Pentafluorothiophenol was purchased from Aldrich and used as received. N,N/-dimethylformamide (DMF) was purified by distillation over calcium hydride. The iron sulfur cluster [Fe 4 S 4 (SPh) 4 ][Bu 4 N] 2 was prepared according to the literature method (Pamphilis, 1974). Electrochemical experiments were conducted to evaluate the electrocatalytic production of hydrogen by iron sulfur clusters following a reported procedure (Alenezi, 2017).
Cyclic voltammetry experiments were carried out using an Autolab PGSTAT 30 potentiostat. A conventional three-electrode arrangement was employed, consisting of a vitreous carbon working electrode (0.07 cm 2 ), a platinum wire as the auxiliary electrode (2cm 2 ), and Ag + /AgCl as a reference electrode.
gas. 5 ml of this solution was placed in the working electrode compartment. About 9-10 ml of the gas phase took place at the working electrode part. 1 mM of the catalyst [Fe 4 S 4 (SPh) 4 ] 2was added and stirred under argon in the electrochemical cell.
The electrolysis was carried out at -0.85 V vs Ag + /AgCl and the current were recorded during the course of electrolysis vs time; the electrolysis was stopped when the current decayed after two hours. Gas chromatography was carried out using a Perkin-Elmer Clarius 500 instrument, fitted with a 5Å molecular sieve column (800/100 mesh, 6' x 1/8") and thermal conductivity detector. The operating conditions were: 80 °C oven temperature, 0.5 ml injected volume, and 10 min retention time. The external standard calibration was performed on the same day

Cyclic Voltammetry of 1 mM [Fe 4 S 4 (SPh) 4 ] 2in DMF-0.2M [Bu 4 N][BF 4 ]
The cyclic voltammetry of [Fe 4 S 4 (SPh) 4 ]2-in non-aqueous electrolytes [Bu 4 N][BF 4 ]-DMF was studied. In this solvent, there is a diffusion-controlled primary reduction step, corresponding to the [Fe 4 S 4 (SPh) 4 ] 2-/3couple at -0.98 V vs Ag/AgCl. In addition, a second partly reversible reduction wave was observed at a more negative potential -1.72 V vs Ag/AgCl (figuer 1). Herein, the electrochemical study was carried out at the first wave reduction of [Fe 4 S 4 (SPh) 4 ] 2-. The primary reduction process is a diffusion-controlled reversible one-electron step. Thus: (i) the plot of the peak current I i p red vs ν 1/2 is linear, with an intercept close to zero ( Figure 3); (ii) the peak potential separation of the first wave, ΔE p = E p ox -E p red = m . ΔE = 70 mV (theoretical separation for a one-electron process is ca 59 mV at 298K). The diffusion coefficient for 0.5 mM [Fe 4 S 4 (SPh) 4 ] 2is estimated to be 1.42 x 10 -5 cm 2 s -1 , according to Randles -Sevcik Equation: The C 0 is the bulk concentration in mol/cm 3 , D is the diffusion coefficient in cm 2 s -1 , v is the potential scan rate in V s -1 , and I is the current density. The electrocatalysis was carried out at the potential of the first reduction wave of [Fe 4 S 4 (SPh) 4 ] 2-, in both presence and absence of pentafluorothiophenol.

Cyclic Voltammetry of [Fe 4 S 4 (SPh) 4 ] 2in the Presence of Pentafluorothiophenol
We examined the cyclic voltammetry of [Fe 4 S 4 (SPh) 4 ] 2in the presence of pentafluorothiophenol as a source of proton ( Figure 4). The primary reduction wave for [Fe 4 S 4 (SPh) 4 ] 2increase in height and became irreversible while increasing the concentration of pentafluorothiophenol, which is consistent with the electrocatalysis of proton reduction by the [Fe 4 S 4 (SPh) 4 ] 2complex. In addition the potential shift is proportional to the concentration of pentafluorothiophenol; at a concentration as high as 18 mM, the potential shifts in about 180 mV. At high concentrations of pentafluorothiophenyl, we observe a plateau ( Figure 5) for the peak current and a precipitate is formed. under Ar 1 eq 2 eq 3 eq 4 eq 5 eq 6 eq 9 eq 12 eq 15 eq 18 eq http://ijc.ccsenet.org  The kinetics of the electrocatalysis of proton reduction by [Fe 4 S 4 (SPh) 4 ] 2 were also investigated. The peak catalytic current was normalized by dividing the peak current for the catalyzed process by that of the uncatalyzed process (i cat / i o ) at 100 mVs -1 ; this simplify the subsequent analysis by eliminating the surface area, along with the catalyst concentration and its diffusion coefficient (DuBois, 2013). Figure 7 shows a plot of the i 1 cat /i o , where i 1 cat is the peak current measured at 100 mVs -1 for the primary catalysis in the presence of different concentrations and i o is the peak current measured for the first one-electron reduction step at the same scan rate in the absence of the proton source. It is important to mention that at the vitreous carbon electrode, the values of i cat /i 0 become independent of the acid concentration at ca 12 M in DMF-containing 0.1M [NBu 4 ] [BF 4 ].
Following the approach of Dubois and others (DuBois, 2013), the rate constant k C obs at the carbon electrode can be estimated from the i cat / i 0 data using the relationship shown in Equation 1.1, where F, R, and T are the Faraday constant, the gas constant, and the temperature respectively, and n is the number of electrons involved in the turnover with i obs , measured at the potential where the current approaches a plateau at -0.98 V vs Ag/AgCl. The value of k obs is 18.9 s -1 .

Preparative Electrocatalysis
Preparative bulk electrolysis reduction of protons in the presence of 0.5 mM [Fe 4 S 4 (SPh) 4 ] 2-(2.5 µmoles) was carried out in a closed system at -0.85 V vs Ag/AgCl (0.2 mM-[NBu 4 ][BF 4 ]-CH 3 CN; 296 K) using 18 mM (18 equivalents, H + 22.5 µmoles) of pentafluorothiophenol as a source of protons. The decay in current was monitored during the course of the experiment, while the working electrode compartment was stirred at a constant rate during electrocatalysis. Figure 8 shows the relationship between the current and time. During proton reduction, the initial current rapidly decayed to a proportion of about 10% of the initial current before falling off toward the end of electrolysis; the gas phase of the cell was analyzed by gas chromatography. The gas chromatography (GC-TCD) test confirmed the formation of H 2 with a current efficiency of ca 63% after two hours, which needed to be 3.2 C, while the yield of H 2 was 10.4 µmoles. No H 2 was formed at -0.85 V vs Ag/AgCl in the presence of 18 mM of pentafluorothiophenol and absence of [Fe 4 S 4 (SPh) 4 ] 2 (control experiment) (Table 1). In a separate experiment under the same conditions, the yield of H 2 was studied by gas chromatography as a function of electrolysis time and the charge passed versus current efficiency. Clearly, the yield of H 2 increased steadily in the first phase of the electrolysis and dropped off in the latter stages as the acid was consumed. The chemical yield of dihydrogen at the end of electrolysis, based upon the total acid available, was 43%; the electrochemical results are shown in Table 2. At the end of the electrolysis, a charge of 3.6 coulombs was passed and 5 µmoles of dihydrogen was detected in the headspace of the catholyte by thermal conductivity gas. All results show in figure 9.   [BF 4 ] was studied in presence of 18 mM of pentafluorothiophenol by comparing the difference in cyclic voltammetry before the addition of acid and after the electrocatalysis course ( fig. 10). The peak current of the first reduction wave of [Fe 4 S 4 (SPh) 4 ] 2before the addition of the acid source was 2.36 µA, while after the electrocatalysis course it was 1.742 µA. This confirms that 73% of [Fe 4 S 4 (SPh) 4 ] 2was recovered after two hours.

Conclusion
We have shown here that [Fe 4 S 4 (SPh) 4 ] 2is a powerful catalyst for the reduction of proton into hydrogen at the carbon electrode in DMF-[Bu 4 N][BF 4 ] at 23 0 C. The cyclic voltammogram of [Fe 4 S 4 (SPh) 4 ] 2displayed two successive reversible reduction peaks; at -0.98V and -1.72V vs Ag/AgCl. The HER occurs at E p = -0.98V which is a significant shift comparing to E p = -1.3 V; the reduction potential of pentafluorothiophenol without catalyst. The addition of high concentration of pentafluorothiophenol led to a positive shifting in the potential; while adding 18 equivalents, the potential shifts about 450 mV. The current efficiency to produce dihydrogen was about 63%, while the chemical yield was 46% and the turnover number was 4 after ca. 2 hours.