Conversion of carbon dioxide gas to hydrocarbon fuels by electrolysis in molten salts

Using fossil fuels in power generation has not been considered a serious issue until recent justification of the depletion of fossil resources in addition to the rising atmospheric temperature as a result of increasing CO2 emission from fossil fuel consumption. Technologies that can absorb CO2 from the emissions and, more substantially, convert CO2 economically into useful products, e.g. materials or fuels, with lower carbon intensity are urgently needed and more desirable than simply storing the gas underground. Performing such conversion at high temperatures (200-600 oC) can offer both thermodynamic and kinetic advantages. Molten salts are ideal media for high temperature reactions but their use for the conversion of CO2 and H2O to beneficial products has not yet been examined properly. Thus, this research aims to investigate the feasibility of producing hydrocarbons by the electrochemical reduction of CO2 and H2O (steam) in molten salts at the atmospheric pressure. Two different mechanisms were suggested in literature for hydrocarbon formation after the co-electrolysis of CO2 and H2O to CO or C (carbon) and H2 in molten salts. The first one is the partial oxidation of CH4 that is produced feasibly in molten salts (e.g. C+2H2=CH4). Due to the sufficient availability of oxide ions (O2-) in molten salts, where the partially reduced species of oxygen (O2-, O22-) are obtained, the produced CH4 can be transferred to C2 and other hydrocarbons by the catalytic oxidative coupling in liquid phase. However, there is a possibility of CH4 reaction with O2 to CO2 and H2O. The second mechanism is the direct reaction of carbon with atomic hydrogen adsorbed primarily on the cathode producing different hydrocarbons. However, some other studies detected CH4 attached with slight amounts of long-chain hydrocarbons in the cathodic gas product during the direct electrolysis of molten carbonates mixed with hydroxides. The electrolyser used in this work resembled that for CO2 reduction to CO in lithium containing molten carbonates at 900 oC using a cell with partitioned cathode and anode compartments. However, in addition to the ternary molten carbonates (Li2CO3-Na2CO3-K2CO3) of (43.5:31.5:25 mol%), this work also studied the molten chloride salts of KCl-LiCl of (41:59 mol%) and molten hydroxides of LiOH-NaOH (27:73 mol%) and KOH-NaOH (50:50 mol%). The electrolyser was employed at different temperatures (220-600 oC) depending on the molten salt applied. Various cathodic gases were produced during the electrolysis as confirmed by gas chromatography. At the specified temperatures in this work, olefin hydrocarbon species between (C2-C5) rather than paraffins were found (as the reaction of CO with H2 is feasible) by a total production rate of 0.06 mmol/h of the whole product associated with H2 and CO in molten carbonate electrolysis at 1.5 V and 425 oC under a feed gas of 15.6 molar ratio of CO2/H2O. The priority of olefin formation can be confirmed also by the mechanism of partial oxidation of CH4. The summation of current efficiencies for different cathodic products was close to 100%. The CH4 gas was the predominant hydrocarbon fuel produced during the electrolysis in molten hydroxide in general. No significant indication of hydrocarbon formation was found in the molten chlorides from CO2 reduction or conversion even at 1.3 bar of CO2. The effect of the molten salt temperature, applied electrolysis voltage and the CO2/H2O ratio of the feed gas were also examined during the electrolysis in molten carbonates and hydroxides. By increasing the electrolysis temperature from 425 oC to 500 oC, the number of carbon atoms in the hydrocarbon species produced in the cathodic gas rose to 7 (C7H16) with a production rate of 1.5 μmol/cm2.h at a CO2/H2O ratio of 9.2 increasing the average molecular weight of the product and thus the calorific value. However, the hydrocarbon fuel content in the cathodic gas product in general was found to be higher in the case of high inlet gas CO2 content (CO2/H2O=15.6) by 18% at 425 oC and 41% 500 oC which can be considered as the optimum condition for hydrocarbon formation in this research. Due to the prospective carbon formation, the electrolysis to produce hydrocarbon in molten carbonates was more feasible at 1.5 V than that performed at 2 V. In molten hydroxide case, the CH4 production rate increased when the applied voltage was increased from 2.0 to 3.0 V despite the reduced current efficiencies. Because the electrolytic conversion can be very fast and achieved without using any catalyst, such as the precious metals used in other CO2 reduction routes in water, the results reported in this thesis are promising and encouraging for further fundamental investigation and technological development

The effect of variable operating parameters for hydrocarbon fuel formation from CO2 by molten salts electrolysis

The emission of CO2 has been increasing day by day by growing world population, which resulted in the atmospheric and environmental destruction. Conventionally different strategies; including nuclear power and geothermal energy have been adopted to convert atmospheric CO2 to hydrocarbon fuels. However, these methods are very complicated due to large amount of radioactive waste from the reprocessing plant. The present study investigated the effect of various parameters like temperature (200–500 oC), applied voltage (1.5–3.0 V), and feed gas (CO2/H2O) composition of 1, 9.2, and 15.6 in hydrocarbon fuel formation in molten carbonate (Li2CO3-Na2CO3-K2CO3; 43.5:31.5:25 mol%) and hydroxide (LiOH-NaOH; 27:73 and KOH-NaOH; 50:50 mol%) salts. The GC results reported that CH4 was the predominant hydrocarbon product with a lower CO2/H2O ratio (9.2) at 275 oC under 3 V in molten hydroxide (LiOH-NaOH). The results also showed that by increasing electrolysis temperature from 425 to 500 oC, the number of carbon atoms in hydrocarbon species rose to 7 (C7H16) with a production rate of 1.5 μmol/h cm2 at CO2/H2O ratio of 9.2. Moreover, the electrolysis to produce hydrocarbons in molten carbonates was more feasible at 1.5 V than 2 V due to the prospective carbon formation. While in molten hydroxide, the CH4 production rate (0.80–20.40 µmol/h cm2) increased by increasing the applied voltage from 2.0–3.0 V despite the reduced current efficiencies (2.30 to 0.05%). The maximum current efficiency (99.5%) was achieved for H2 as a by-product in molten hydroxide (LiOH-NaOH; 27:73 mol%) at 275 oC, under 2 V and CO2/H2O ratio of 1. Resultantly, the practice of molten salts could be a promising and encouraging technology for further fundamental investigation for hydrocarbon fuel formation due to its fast-electrolytic conversion rate and no utilization of catalyst

Electrochemical production of sustainable hydrocarbon fuels from CO2 co-electrolysis in eutectic molten melts

Because of the heavy reliance of people on limited fossil fuels as energy resources, global warming has increased to severe levels because of huge CO2 emission into the atmosphere. To mitigate this situation, a green method is presented here for the conversion of CO2/H2O into sustainable hydrocarbon fuels via electrolysis in eutectic molten salts [(KCl-LiCl; 41:59 mol %), (LiOH-NaOH; 27:73 mol %), (KOH-NaOH; 50:50 mol %), and (Li2CO3-Na2CO3-K2CO3; 43.5:31.5:25 mol %)] under the conditions of 1.5-2 V and 225-475 °C depending on the molten electrolyte used. Gas chromatography (GC) and GC-mass spectrometry (MS) techniques were employed to analyze the content of gaseous products. The electrolysis results in hydrocarbon production with maximum 59.30, 87.70, and 99% Faraday efficiencies in the case of molten chloride, molten hydroxide, and molten carbonate electrolytes under the temperatures of 375, 275, and 425 °C, respectively. GC with a flame-ionization detector and a thermal conductivity detector and GC-MS analysis confirmed that H2 and CH4 were the main products in the case of molten chlorides and hydroxides at an applied voltage of 2 V, while longer-chain hydrocarbons (>C1) were obtained only in molten carbonates at 1.5 V. In this way, electricity is transformed into chemical energy. The heating values obtained from the produced hydrocarbon fuels are satisfactory for further application. The practice of using molten salts could be a promising and encouraging technology for further fundamental investigation of sustainable hydrocarbon fuel formation with more product concentrations because of their fast electrolytic conversion rate without the use of a catalyst

Sustainable conversion of carbon dioxide into diverse hydrocarbon fuels via molten salt electrolysis

In recent decades, the unlimited use of fossil fuels mostly for power generation has emitted a huge amount of carbon dioxide in the atmosphere which in return has led to global warming. Here we use green technology, the molten salt electrochemical system comprising of titanium and mild steel as a cathode with graphite anode whereas molten carbonate (Li2CO3-Na2CO3-K2CO3; 43.5:31.5:25 mol%), hydroxide (LiOH-NaOH; 27; 73 and KOH-NaOH; 50:50 mol %) and chlorides (KCl-LiCl; 41-59 mol%) salts as electrolytes This study investigates the effect of temperature, feed gas ratio CO2/H2Oand use of different cathode materials on hydrocarbon product along with current efficiencies. Gas chromatography and mass spectroscopy have been applied to analyze the gas products. According to GC results, more specific results in terms of high molecular weight and long chain hydrocarbons were obtained by using titanium cathodic material rather than mild steel. The results revealed that among all the electrolytes, molten carbonates at 1.5V and 425˚C produced higher hydrocarbons as C7H16 while all other produced CH4. The optimum conditions for hydrocarbon formation and higher current efficiencies in case of molten carbonates were found to be 500oC under a molar ratio of CO2/H2O of 15.6. However, the current efficiencies do not change on increasing the temperature from 425 to 500oCand is maintained at 99% under a molar ratio of CO2/H2O of 15.6. The total current efficiency of the entire cathodic product reduced clearly from 95 to 79% by increasing the temperature under a CO2/H2O ratio of 9.2 due to the reduction of hydrocarbon generation in this case, despite the formation of C7H16. Therefore, due to its fast electrolytic conversion rate and low cost (no use of catalyst) the practice of molten salts could be an encouraging and promising technology for future investigation for hydrocarbon fuel formation

Conversion of carbon dioxide gas to hydrocarbon fuels by electrolysis in molten salts

Using fossil fuels in power generation has not been considered a serious issue until recent justification of the depletion of fossil resources in addition to the rising atmospheric temperature as a result of increasing CO2 emission from fossil fuel consumption. Technologies that can absorb CO2 from the emissions and, more substantially, convert CO2 economically into useful products, e.g. materials or fuels, with lower carbon intensity are urgently needed and more desirable than simply storing the gas underground. Performing such conversion at high temperatures (200-600 oC) can offer both thermodynamic and kinetic advantages. Molten salts are ideal media for high temperature reactions but their use for the conversion of CO2 and H2O to beneficial products has not yet been examined properly. Thus, this research aims to investigate the feasibility of producing hydrocarbons by the electrochemical reduction of CO2 and H2O (steam) in molten salts at the atmospheric pressure. Two different mechanisms were suggested in literature for hydrocarbon formation after the co-electrolysis of CO2 and H2O to CO or C (carbon) and H2 in molten salts. The first one is the partial oxidation of CH4 that is produced feasibly in molten salts (e.g. C+2H2=CH4). Due to the sufficient availability of oxide ions (O2-) in molten salts, where the partially reduced species of oxygen (O2-, O22-) are obtained, the produced CH4 can be transferred to C2 and other hydrocarbons by the catalytic oxidative coupling in liquid phase. However, there is a possibility of CH4 reaction with O2 to CO2 and H2O. The second mechanism is the direct reaction of carbon with atomic hydrogen adsorbed primarily on the cathode producing different hydrocarbons. However, some other studies detected CH4 attached with slight amounts of long-chain hydrocarbons in the cathodic gas product during the direct electrolysis of molten carbonates mixed with hydroxides. The electrolyser used in this work resembled that for CO2 reduction to CO in lithium containing molten carbonates at 900 oC using a cell with partitioned cathode and anode compartments. However, in addition to the ternary molten carbonates (Li2CO3-Na2CO3-K2CO3) of (43.5:31.5:25 mol%), this work also studied the molten chloride salts of KCl-LiCl of (41:59 mol%) and molten hydroxides of LiOH-NaOH (27:73 mol%) and KOH-NaOH (50:50 mol%). The electrolyser was employed at different temperatures (220-600 oC) depending on the molten salt applied. Various cathodic gases were produced during the electrolysis as confirmed by gas chromatography. At the specified temperatures in this work, olefin hydrocarbon species between (C2-C5) rather than paraffins were found (as the reaction of CO with H2 is feasible) by a total production rate of 0.06 mmol/h of the whole product associated with H2 and CO in molten carbonate electrolysis at 1.5 V and 425 oC under a feed gas of 15.6 molar ratio of CO2/H2O. The priority of olefin formation can be confirmed also by the mechanism of partial oxidation of CH4. The summation of current efficiencies for different cathodic products was close to 100%. The CH4 gas was the predominant hydrocarbon fuel produced during the electrolysis in molten hydroxide in general. No significant indication of hydrocarbon formation was found in the molten chlorides from CO2 reduction or conversion even at 1.3 bar of CO2. The effect of the molten salt temperature, applied electrolysis voltage and the CO2/H2O ratio of the feed gas were also examined during the electrolysis in molten carbonates and hydroxides. By increasing the electrolysis temperature from 425 oC to 500 oC, the number of carbon atoms in the hydrocarbon species produced in the cathodic gas rose to 7 (C7H16) with a production rate of 1.5 μmol/cm2.h at a CO2/H2O ratio of 9.2 increasing the average molecular weight of the product and thus the calorific value. However, the hydrocarbon fuel content in the cathodic gas product in general was found to be higher in the case of high inlet gas CO2 content (CO2/H2O=15.6) by 18% at 425 oC and 41% 500 oC which can be considered as the optimum condition for hydrocarbon formation in this research. Due to the prospective carbon formation, the electrolysis to produce hydrocarbon in molten carbonates was more feasible at 1.5 V than that performed at 2 V. In molten hydroxide case, the CH4 production rate increased when the applied voltage was increased from 2.0 to 3.0 V despite the reduced current efficiencies. Because the electrolytic conversion can be very fast and achieved without using any catalyst, such as the precious metals used in other CO2 reduction routes in water, the results reported in this thesis are promising and encouraging for further fundamental investigation and technological development