Concurrent electrochemical CO₂ reduction to HCOOH and methylene blue removal on metal electrodes

Abstract

Experimental studies were done for simultaneous CO₂ reduction and methylene blue (MB) dye removal using Sn and Zn as cathode and Co₃O₄ as anode electrocatalysts. The extent of CO₂ reduction and dye removal were observed in NaHCO₃ and KHCO₃ electrolytes separately. The electrodes (cathode and anode) were prepared by coating the respective electrocatalyst on the graphite plate surface. A mixture of 0.5 M salt and 10 mg l⁻¹ of MB was considered as a base solution to conduct the reduction and dye removal studies at different applied voltages of 2 to 3.8 V. The reacted samples were collected for different time intervals of 5, 10, 15, 20 and 25 min to obtain the product formation using ultra-fast liquid chromatography (UFLC) and dye analysis by an UV visible spectrophotometer. The results confirm the formation of HCOOH for both cases with simultaneous CO₂ and MB removal. A maximum faradaic efficiency of 57.9% (KHCO₃) was observed using Sn as an electrocatalyst for a reaction time of 15 min and 31.7% (KHCO₃) in 5 min using Zn as an electrocatalyst. With increasing applied voltages the faradaic efficiencies were gradually decreased whereas dye removal is increased. However, the present studies were able to give a new topic for both simultaneous dye removal and CO₂ reduction in order to reduce atmospheric CO₂ concentrations and water purification. The selected electrocatalyst combinations were able to be used effectively for reduction studies which gives a new application for both CO₂ reduction and dye removal.

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1. Introduction

The utilization of fossil fuels has been increasing due to the dependency of their source energy for mankind. The combustion process is used for the generation of energy from these fossil fuels during which the main final product is CO₂. Therefore, CO₂ concentrations are increasing in the atmosphere which causes a global warming effect.^1–3 For sustainable growth and energy conservation, there is a need to utilize this CO₂ into value added products. Different approaches like electrochemical,^4–7 chemical,^8,9 photo chemical,^10,11 bio-chemical,^12,13 radio-chemical⁹ and thermo-chemical¹⁴ for the conversion of CO₂ are in practice. However, the electrochemical method is beneficial due to higher conversion and product selectivity. In this method water can be used as a source for proton generation and a way to store electrical or solar energy in the form of fuel.^15–18 Different studies were reported on the reduction of CO₂ to products electrochemically (RCPE) using different electrocatalysts. Based on the electrocatalysts used different products like methanol, ethanol, propanol, formic acid, formaldehyde, ethylene, methane and acetic acid were formed.^19,20 It was showed that hydrogen gas was formed along with the CO₂ reduction due to proton formation at anode.^21,22 The significant research in RCPE was started by Hori in 1985 who found that copper is the best catalyst in reducing the CO₂ to hydrocarbons.²³ But multiple products like ethanol, acetic acid, propanol and formic acid on copper catalyst were forming during the reduction reaction which is complex.²⁴ Reduction of CO₂ to single product is most desirable and research is continuing in this area. Different works are reported for reducing CO₂ to HCOOH on Sn, Zn and Pb electrocatalysts using Pt as anode.^25–29 Lu et al., given a review for the reduction of CO₂ to HCOOH with a clear reaction mechanism along with advancement in electrocatalyst reactor design and electrodes.³⁰ Koleli et al., studied the RCPE on Pb and Sn electrocatalysts in potassium based electrolytes for HCOOH generation at different applied voltages.³¹ The effects of Tin oxide and Tin for the CO₂ reduction to HCOOH were reported for promising catalysts towards fuel synthesis.³² Reduction of CO₂ using Zn electrocatalyst was studied in sodium and potassium based electrolytes and it was reported that HCOOH was formed as the only product in all electrolytes at different applied voltages.³³ Lv et al., studied the effect of CO₂ reduction for HCOOH formation using Sn electrocatalyst in KHCO₃ electrolyte solution. It was reported that faradaic efficiency of forming product depends on the applied voltage and electrolyte concentration.³⁴ Pt is mainly used as anode catalyst for water oxidation reaction to generate protons in all the above studies.^35,36 However, our earlier studies were reported on Co₃O₄ as catalyst for water oxidation, which is low cost and better alternative to Pt electrocatalyst.^37,38

To the best of our knowledge, no studies were done on effective reduction of CO₂ and MB dye removal simultaneously on Sn, Zn (cathode) and Co₃O₄ (anode). The electrocatalyst used here were prepared by using electrodeposition method.^33,37 The experiments were done in bicarbonate salts of sodium and potassium at different applied voltages of (2, 2.3, 2.5, 2.8, 3.3 and 3.8 V). The performances of electrocatalysts combination on RCPE and dye removal were studied using a 2 electrode cell. The faradaic efficiency and dye removal with respect to time at different applied voltages for both Sn, Zn was investigated and explained in detail. This study will be helpful in removing dye from colored wastewater and reduction of CO₂ to HCOOH simultaneously.

2. Experimental

2.1. Materials

Graphite plates (1.5 × 2.5) cm² was used for electrode preparation and procured from Sunrise Enterprises, Mumbai. NaHCO₃, KHCO₃, iso-propyl alcohol and methylene blue were obtained from Merck, India. Nafion (5 wt%) solution was purchased from DuPont, USA. All the chemicals were used without any further purification. Deionized water was used in all experiments.

2.2. Preparation of cathode and anode electrodes

The electrodes (cathode and anode) were prepared by coating the electrocatalysts on the surface of procured graphite plates using catalysts ink. The ink was prepared by adding 7.5 mg of electrocatalysts in a binder solution of 1 : 5 (nafion + IPA (iso propyl alcohol)) of 200 μl and sonicated for 30 min. The catalyst ink was coated on the surface of graphite plate to get the catalyst coating of 2 mg cm⁻² at 80 °C for 2 h to form the fully prepared electrode. The respective electrocatalysts (Co₃O₄, Zn and Sn) were coated and electrodes were prepared for RCPE.

2.3. CO₂ reduction and MB dye removal

All electrochemical studies were performed using a 2-electrode glass cell for CO₂ reduction and MB removal. The experimental setup used in the present study was shown in Fig. 1. For all experiments an electrolyte solution of CO₂ saturated for 1 h in 80 ml of 0.5 M (10 mg l⁻¹ MB) was used as reaction solution. The electrodes were connected to DC source, which were immersed in prepared bicarbonate based dye electrolyte CO₂ saturated solution. The extent of reaction was observed at different applied voltages of 2, 2.3, 2.5, 2.8, 3.3 and 3.8 V with reaction time of 0, 5, 10, 15, 20 and 25 min.

Fig. 1 Experimental setup for CO₂ reduction and MB removal.

2.4. Product and dye removal analysis

Ultra-fast liquid chromatography (UFLC), (Shimadzu LC-20AD, UV-detector of deuterium lamp (SPD-20A at 205 nm)) was used for CO₂ reduction product analysis by injecting a 20 μl of reacting sample into the C-18 column (10 × 4 mm). The mobile phase (5 mM tetrabutyl ammonium hydrogen sulfate) was used at 1 ml min⁻¹ flow rate. MB dye removal analysis was done by UV-Visible Spectrophotometer (Perkin Elmer, Model: Lambda 35). The faradaic efficiency was calculated based on the product formed utilized charge to the overall charge obtained by the whole reaction. MB dye removal (%) with time was calculated from the dye concentration before and after the reaction.

3. Results and discussion

3.1. RCPE and MB dye removal using Sn and Co₃O₄ as electrocatalysts

Cathode (Sn) and anode (Co₃O₄) were used to study the effect of CO₂ reduction and MB removal in potassium and sodium bicarbonate electrolytes. The reduction and removal with applied voltage and time were discussed clearly in the subsequent sections.

3.1.1. Effect of current density at Sn electrocatalyst on applied voltage. In both the electrolyte solutions, the current density results on applied voltages were given in Fig. 2(a). The current densities are proportional to applied voltages in KHCO₃ and NaHCO₃ electrolyte solutions. Increasing applied voltages depicted higher rate of reaction which was confirmed by the measured current density. The reaction rate corresponds to either hydrogen removal or CO₂ reduction. Higher current densities were obtained in KHCO₃ than NaHCO₃ at higher applied voltages which shows that the reaction rate is high in potassium based electrolyte solution (Fig. 2(a)). The CO₂ reduction reaction and MB dye removal for different time intervals for applying voltages were discussed in the preceding section.

Fig. 2 (a) Voltage vs. current density, (b and c) faradaic efficiency vs. time and (d and e), removal (%) vs. time in KHCO₃ and NaHCO₃ electrolytes on Sn electrocatalyst.

3.1.2. RCPE and MB dye removal in KHCO₃ solution. The results for simultaneous CO₂ reduction and MB dye removal in 0.5 M KHCO₃ (10 mg l⁻¹ MB) solution is shown in Fig. 2(b and d). It was observed that HCOOH was the only product formed in RCPE at different applied voltages. Low applied voltages were able to get high faradaic efficiencies compared with high voltages which may be due to more hydrogen evolution at higher voltages.²³ Similarly, higher applied voltages were favorable for high MB removal than low voltages. MB removal (%) was increasing with time in Fig. 2(d and e) with change in faradaic efficiencies of the formed product HCOOH may be due to HCOOH oxidation at anode. Further hydrogen evolution was shown in Fig. 2(b and c).³⁴ The reaction at 2 V was obtained with HCOOH faradaic efficiencies in 28.2, 38.4, 57.9, 4.7 and 5.5% by MB removal (12.5, 13.33, 13.39, 13.5 and 14.1%) for reaction time of 5, 10, 15, 20 and 25 min. The optimized condition towards maximum CO₂ reduction was 57.9% of reaction after 15 min. Similar studies were done at same experimental conditions without MB dye which shows the formation of HCOOH as a product.³⁷ The simultaneous dye removal and CO₂ reduction at 2.3 V was shown in Fig. 2(b and d) with faradaic efficiencies (7.7, 4.6, 6.8, 2.2 and 1.4%) and MB dye removal (17.8, 18.08, 18.4, 18.8 and 19.2%), respectively. The faradaic efficiencies were decreased at this applied voltage compared with 2 V may be due to high H⁺ generation at anode which reduced to hydrogen at the cathode.³⁸ The MB dye removal was increased with applied voltages and maximum removal of 19.2% after 25 min reaction was observed. At 2.5 V, the faradaic efficiencies and MB removal were obtained to be (6.6, 0.7, 1.6, 2.9 and 1.7%), (20.2, 21.1, 21.2, 21.9 and 22.2%) respectively. Maximum faradaic efficiency of 6.6% after 5 min reaction was observed after that a drastic decrease in efficiency shows that HCOOH oxidation at Co₃O₄ anode. The change in faradaic efficiency with time may be due to the oxidation of forming product at anode.³⁴ The faradaic efficiencies and MB removal at applied voltage of 2.8 V was found to be (4.9, 3.4, 1.09, 0.7 and 0.3%), (24.2, 24.3, 25.5, 25.9 and 26.5%) respectively. The optimized reaction condition for RCPE and MB removal was 4.9% (5 min) and 26.5% (25 min). For a reaction at 3.3 V, faradaic efficiencies of (0.12, 0.33, 0.75, 0.59 and 0.2%) and MB removal (25.6, 26.8, 28.4, 29.02, and 29.4%) were obtained. Very low faradaic efficiencies were obtained compared to other applied voltages though current density is high which may be due to high hydrogen evolution at Co₃O₄ anode.²³ Faradaic efficiencies (0.72, 0.29, 0.5, 0.4, 0.17%), MB dye removal (29.7, 31.7, 32, 33 and 33.6%) was obtained at 3.8 V which shows that maximum dye removal was obtained at this voltage with low faradaic efficiencies. Overall, the reaction in KHCO₃ was able to reduce CO₂ to HCOOH and MB dye removal simultaneously. High applied voltages were able to remove maximum MB dye removal whereas low applied voltages were favorable for high faradaic efficiencies.

3.1.3. RCPE and MB dye removal in NaHCO₃ solution. The experimental results on RCPE and MB dye removal in 0.5 M NaHCO₃ (10 mg l⁻¹ MB) electrolyte solution for different applied voltages are shown in Fig. 2(c and e). It was observed that low applied voltages give high faradaic efficiency and high applied voltages favor maximum MB dye removal. At 2 V, faradaic efficiencies (27.5, 20.5, 40.1, 35.6 and 28.2%), MB removal (12.4, 12.5, 12.7, 13 and 13.2%) were obtained in the reaction time of 5, 10, 15, 20 and 25 min. The maximum faradaic efficiency of 40.1% (15 min) and dye removal 13.2% (25 min) were the optimized reaction conditions at this applied voltage. The reduction studies were done in NaHCO₃ electrolyte solution with same experimental conditions without dye.³⁷ For reaction at 2.3 V, the faradaic efficiencies and MB dye removal were obtained to be (11.2, 6.8, 4.2, 2.7 and 2.2%) and (18.8, 19.3, 18.1, 18.5 and 18.9%), respectively. The efficiencies were reduced when compared to 2 V reactions which depicts high hydrogen evolution.^37,38 Low faradaic efficiencies of 3.9, 1.5, 3.5, 1.08 and 1.4% were obtained at 2.5 V with removal of 18.7, 20.9, 21.3, 21.8 and 22.1% (Fig. 2(e)). Though efficiencies were reduced, but removal percentage is increasing which shows that higher applied voltages are favorable for dye removal. The experimental studies at 2.8 V shows that faradaic efficiencies are decreased by 2.76, 2.3, 2.1, 1.9 and 1.89% and MB removal of 24.6, 25.5, 26, 25.7 and 26.2% are observed. Maximum removal of 26.2% was obtained after 25 min reaction. At 3.3 V, faradaic efficiencies (2.79, 1.6, 0.5, 0.19 and 0.2%) and MB removal of around 27.9, 29, 29.1, 29.8 and 30.3% (Fig. 2(e)) was observed with maximum dye removal of 30.3% after 25 min reaction. The decrease in faradaic efficiency may be due to high proton generation at anode which forms hydrogen at the cathode than participating in CO₂ reduction.³⁴ The reaction at 3.8 V shows very low faradaic efficiencies (0.3, 0.58, 0.76, 0.3 and 0.19%) with high dye removal of 29.8, 30.9, 31.8, 32.1 and 32.7% with maximum removal of 32.7%. From the discussion, high applied voltage 3.8 V shows more dye removal and low voltage 2 V favors high HCOOH efficiencies.

3.2. RCPE and MB dye removal using Zn and Co₃O₄ as electrocatalysts

The experimental studies were performed to study the performance of electrocatalysts using (Co₃O₄/G) and (Zn/G) as the anode and cathode, respectively. The effect of electrolyte on RCPE and dye removal was clearly explained for different voltages with time intervals and discussed in subsequent sections.

3.2.1. Effect of current density. Results related to RCPE and MB dye removal using Zn electrocatalyst as cathode is shown in Fig. 3. Increase of applied voltages in KHCO₃ and NaHCO₃ electrolytes shows high current densities (Fig. 3(a)). Maximum current represents a high reaction rate, which comprises of CO₂ reduction or hydrogen evolution at anode. The reaction in KHCO₃ electrolyte shows maximum current density compared to NaHCO₃ solution.

Fig. 3 (a) Voltage vs. current density, (b and c) faradaic efficiency vs. time and (d and e), removal (%) vs. time in KHCO₃ and NaHCO₃ electrolytes on Zn electrocatalyst.

3.2.2. RCPE and MB dye removal in KHCO₃ solution. The reaction in 0.5 M KHCO₃ (10 mg l⁻¹ MB) using Zn electrocatalyst for faradaic efficiencies of HCOOH formation and MB dye removal is shown in Fig. 3. The only product formed in the said experimental conditions is HCOOH and respective MB dye removal is reported for reaction time of 5, 10, 15, 20 and 25 min. Faradaic efficiencies (31.7, 12.1, 12.3, 11.5 and 18.1%) and MB removal (7.7, 7.9, 8.1, 8.3 and 8.3%) were obtained at 2 V (Fig. 3 (b–d)). Maximum faradaic efficiency of 31.7% for reaction time of 5 min with low dye removal of this applied voltage. Experimental studies were reported using Zn as electrocatalyst for the formation of HCOOH as the only product from CO₂ in the absence of MB dye.³³ For a reaction at 2.3 V, faradaic efficiencies 15.4, 9.4, 8.02, 3.8 and 2.5% (Fig. 3(b)) were decreased compared with above voltage was due to high H⁺ generation at anode than CO₂ reduction with removal of 9.6, 11.17, 11.2, 11.37 and 11.43%.^23,37 The experimental result corresponds to 2.5 V was discussed for faradaic efficiency and MB dye removal was obtained to be (2.7, 4.9, 3.2, 5.5 and 5.3%) and (6.8, 7.4, 10.6, 13.4 and 13.5%), respectively. The dye removal increased than above applied voltages with decrease in faradaic efficiency. For reaction at 2.8 V, faradaic efficiencies (3, 2.08, 1, 1.7 and 0.6%) and MB removal (4.8, 9.9, 13.1, 17.2 and 19.4%) were obtained and shown in Fig. 3(d). The reaction at 3.3 and 3.8 V shows very low faradaic efficiencies (2.02, 0.76, 0.5, 0.8 and 0.3%) and (1.4, 0.6, 0.62, 0.35, 0.4) with high MB dye removal (17.8, 21.4, 26.07, 30.3 and 33.79) and (13.3, 19.5, 26, 38.9 and 42.6%), respectively. The decrease in faradaic efficiencies was due to high hydrogen evolution reaction which competing with CO₂ reduction at the cathode.²³ From the discussion it was confirmed that CO₂ reduction is taking with high efficiencies at low applied voltages and for the case of MB dye removal high applied voltages are favorable.

3.2.3. RCPE and MB dye removal in NaHCO₃ solution. Fig. 2(c and e) shows the results for simultaneous reduction of CO₂ and MB dye removal in 0.5 M NaHCO₃ (10 mg l⁻¹ MB) electrolyte solution. The experimental studies were done at different applied voltages of 2, 2.3, 2.5, 2.8, 3.3 and 3.8 V with reaction time of 5, 10, 15, 20 and 25 min. For reaction at 2 V, with faradaic efficiencies (6.7, 25.4, 21.7, 7.9 and 9.99%) and MB dye removal (5.5, 7.19, 8.1, 10.3 and 11.4%) (Fig. 3(e)) were obtained. The maximum faradaic efficiency of 25.4% for the reaction of 10 min was obtained which is the best condition for HCOOH formation. The experimental results showed that with the increase in time the faradaic efficiencies were varying which was due oxidation of formed HCOOH anode.³⁴ At 2.3 V, the faradaic efficiencies and MB removal were obtained to be (11.7, 4.7, 6.9, 1.9 and 5.4%), (10.7, 12.51, 13.5, 16.58 and 17.63%), respectively. Faradaic efficiencies were reduced at this applied voltage because of hydrogen evolution that competing with CO₂ reduction at Co₃O₄ anode. For a reaction at 2.5 V, the results were 2.7, 1.4, 1.64, 1.56 and 3.7% (Fig. 3(c)) for faradaic efficiency of HCOOH and 11, 12.1, 13.8, 16.3 and 18.3% for MB removal. Similar studied were reported in the absence of MB for HCOOH formation.³³ The CO₂ reduction and MB removal at 2.8 and 3.3 V were obtained to be [(3.77, 3.29, 1.6, 0.9 and 0.93%) and (10.09, 13, 15.2, 17.9 and 20.4%)] and [(3.58, 1.07, 0.56, 0.65 and 0.38%) and (13.9, 18.6, 22.06, 25.8 and 28.6%)], respectively. The faradaic efficiency of HCOOH (1.5, 1.2, 0.85, 0.83 and 0.43%) and MB dye removal (11.3, 17.2, 20.5, 24.05, and 33.55%) were obtained at 3.8 V. The faradaic efficiencies were decreased compared to low applied voltages which may be due to high proton generation at anode.^23,33 The above result proves the ability of electrocatalysts combination towards RCPE and MB removal simultaneously. The optimized conditions for both electrocatalyst Sn and Zn were given in Table 1 and MB removal in Table 2 in presence of potassium and sodium based solutions.

Table 1 Optimized conditions for HCOOH faradaic efficiency in different electrolytes

Voltage	Maximum faradaic efficiency
	Sn				Zn
	KHCO3		NaHCO3		KHCO3		NaHCO3
(V)	(%)	(min)	(%)	(min)	(%)	(min)	(%)	(min)
2	57.9	15	40.1	15	31.7	5	25.4	10
2.3	7.7	5	11.2	5	15.4	5	11.7	5
2.5	6.6	5	3.9	5	5.5	20	3.7	25
2.8	4.9	5	2.76	5	3	5	3.77	5
3.3	0.75	15	2.79	5	2.02	5	3.5	5
3.8	0.72	5	0.76	15	1.4	5	1.5	5

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Table 2 Optimized conditions for MB removal in different electrolytes

Voltage	MB removal (time)
	Sn				Zn
	KHCO3		NaHCO3		KHCO3		NaHCO3
(V)	(%)	(min)	(%)	(min)	(%)	(min)	(%)	(min)
2	14.1	25	13.2	25	8.3	25	11.4	25
2.3	19.2		18.9		11.4		17.6
2.5	22.2		22.1		13.5		18.3
2.8	26.5		26.2		19.4		20.4
3.3	29.4		30.3		33.7		28.6
3.8	33.6		32.7		42.6		33.5

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4. Conclusion

These studies report the ability of the selected electrocatalyst for simultaneous CO₂ reduction to HCOOH and MB dye removal on Zn and Sn as cathode and Co₃O₄ as anode. The abilities of the electrocatalyst were studied in both KHCO₃ and NaHCO₃ solutions at different applied voltages. HCOOH was the only product formed at all applied voltages. The high applied voltage was favorable for MB dye removal whereas low voltages are for maximum faradaic efficiencies. Maximum MB removal for Sn and Zn electrocatalyst was obtained to be [33.6% (KHCO₃), 32.7% (NaHCO₃)] and [33.7% (KHCO₃), 33.5% (NaHCO₃)]. For Sn as electrocatalyst, maximum faradaic efficiencies of 57.9% (KHCO₃) and 40.6% (NaHCO₃) was obtained for reaction time of 15 min. Using Zn as electrocatalyst, faradaic efficiencies of 31.7% (KHCO₃) in 5 min and 25.4% (NaHCO₃) in 10 min was obtained. Finally, the output of this investigation will explore the possibilities of improving a process for the simultaneous MB removal from wastewater and CO₂ reduction.

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