Syngas Production: Diverse H 2 /CO Range by Regulating Carbonates Electrolyte Composition from CO 2 /H 2 O via Co-Electrolysis in Eutectic Molten Salts

We present a novel sustainable method for the direct production of syngas (H2 + CO) from CO2/H2O co-electrolysis using a hermetic device, to address the continuously increasing level of environmental carbon dioxide (CO2). All experiments were conducted using a two-electrode system with a coiled Fe cathode and coiled Ni anode in eutectic mixtures of binary and ternary carbonates with hydroxide in a 0.1 : 1 hydroxide/carbonate ratio. With an applied voltage of 1.6–2.6 V and an operating temperature of 500–600 °C, the H2/CO product ratio was easily tuned from 0.53 to 8.08 through renewable cycling of CO2 and H2O. The Li0.85Na0.61K0.54CO3–0.1LiOH composite had the highest current efficiency among those tested, with an optimum value approaching ∼93%. This study provides a promising technique for the electrochemical conversion of CO2/H2O to a controllable syngas feedstock that can be used in a broad range of industrial applications.


Introduction
The ever-increasing combustion of non-renewable fossil fuels due to industrial development is releasing a large amount of carbon dioxide (CO 2 ) into the atmosphere, which has led to a serious greenhouse effect. As CO 2 is the main component of greenhouse gases, effectively controlling CO 2 generation and emission are urgent issues. [1][2][3] Using photochemical and electrochemical methods for the chemical reduction of CO 2 to reverse oxidative degradation is a huge challenge. 4-10 Among current technologies, chemical conversion and utilization of CO 2 is the most promising because it is both an economic and environmental friendly option. Since the last century, there has been gratifying progress in research on CO 2 chemical conversion, especially with respect to the electrode materials, electrolytes, and operating conditions required for electrochemical reduction in molten salts. [11][12][13][14][15][16][17][18] Syngas, a mixture of carbon monoxide (CO) and hydrogen (H 2 ), has been cited as an essential precursor to a wide range of high value-added industrial products, such as olens, fuels, and additives.
Conventional syngas production methods include natural gas conversion, heavy oil conversion, and folding airow bed gasi-cation technology. [19][20][21] However, the high temperatures (over 800 C) required inevitably consume heat and promote reactor corrosion. 22 In comparison, the molten salt electrolysis technique reported herein provides a low-temperature, stable, and safe route to syngas production.
In this method, the source of hydrogen (H 2 ) in syngas is LiOH, while carbon monoxide (CO) is sourced from carbonates. The co-electrolysis of CO 2 /H 2 O in eutectic molten salts provides a feasible way to produce syngas, which can be used in the Fischer-Tropsch (F-T) process to convert electrical energy to chemical energy. 23 Syngas with a H 2 /CO ratio of 1.7-3.1 was obtained by controlling the H 2 O/CO 2 feed ratio, as reported by Lee. 24 Recently, Sastre et al. developed an electrochemical method for converting CO 2 and H 2 O into syngas using a nanostructured Ag/g-C 3 N 4 catalyst, with H 2 /CO ratios ranging from 100 : 1 to 2 : 1. 25 We also demonstrated that, by rational design of the molten salt mixture, a desirable lower temperature (such as 600 C) led to the highly efficient one-pot generation of syngas via CO 2 /H 2 O coelectrolysis with a current efficiency of $92% and a H 2 /CO ratio of 1.96-7.97 in Li 1.07 Na 0.75 Ca 0.045 CO 3 / 0.15LiOH electrolyte. 22 This result demonstrates that CaCO 3 addition affects the composition of syngas. Using these methods, the CO 2 /H 2 O-derived generation of syngas has been achieved. Although syngas has successfully been produced in previous studies using a molten salt medium, H 2 is the favored product, oen resulting in a H 2 /CO molar ratio greater than 1.
However, there are specic reactions in which a H 2 /CO molar ratio of less than 1 is needed, such as in alcohol synthesis with a H 2 /CO ratio of 0.5-2 using K/Cu/Co/Zn/Al catalyst. 26 To broaden the utilization range, increasing the selectivity for CO in the syngas would be a signicant development.
Previously, Chery et al. studied the nature of electrolytes Li 2 CO 3 -Na 2 CO 3 (52 : 48 mol%), Li 2 CO 3 -K 2 CO 3 (62 : 38 mol%), Na 2 CO 3 -K 2 CO 3 (56 : 44 mol%), and a ternary mixture of Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 (43.5 : 31.5 : 25 mol%) by thoroughly analyzing reoxidation and reduction. 27 Herein, the CO 2 reduction mechanism at a gold electrode in molten carbonates is investigated using cyclic voltammetry. The present work is a systematic exploration of changes in the H 2 /CO ratio in various binary or ternary carbonates (Li 1.51 K 0.49 CO 3 , Li 1.07 -Na 0.93 CO 3 , Li 1.43 Na 0.36 K 0.21 CO 3 , and Li 0.85 Na 0.61 K 0.54 CO 3 ) mixed with LiOH that favor syngas formation, but inhibit metal deposition, 28 with the goal of broadening the H 2 /CO ratio in syngas. During electrolysis, alkali oxides, which are produced from the decomposition of monovalent alkali carbonate and LiOH, can combine with CO 2 and H 2 O to renew the electrolyte. This regeneration of the carbonate electrolyte affords an advantageous circulation system to give syngas as the nal product of CO 2 /H 2 O reduction via co-electrolysis in molten salts. Furthermore, the electricity needed for this electrolysis is measured to assess whether electrolysis proceeds with a relatively high current efficiency. In this study, CO 2 /H 2 O is synergistically converted into valuable chemicals by electrolyzing molten salts, providing an alternative route to resolve global excessive CO 2 emissions and convert conventional electricity to chemical energy.

Experimental methods
The electrolysis cell consists of an alumina crucible (Al 2 O 3 > 99.9%, 440 mm, 85 mm in height) lled with binary or ternary mixed carbonates (Li 1.51 K 0.49 CO 3 , Li 1.07 Na 0.93 CO 3 , Li 0.85 Na 0.61 -K 0.54 CO 3 , and Li 1.43 Na 0.36 K 0.21 CO 3 ) and LiOH for the CO 2 /H 2 O co-electrolysis experiments, with a total mixed molten salts mass of 80 g. The thermal energy for electrolysis was provided by a specially customized ceramic heating sleeve. Due to inevitable corrosion caused by the electrolytes and high-temperature oxidation, an affordable and corrosion-resistant electrode material was investigated for its long-term stability. Metallic materials Ni (41.6 mm, 39.7 cm in length, 20 cm 2 , Hebei Steady Metal Products Co., LTD, China) and polished Fe (41.6 mm, 39.7 cm in length, 20 cm 2 , Hebei Steady Metal Products Co., LTD, China), both in the form of spiral wires, were used as the anode and cathode, respectively. When the mixed salts reached the pre-set temperature, the two-electrode system was placed into the electrolyte and completely sealed with a sealant and sealing bolt. All electrolysis was performed in the voltage range 1.6-2.6 V. DC power (BK PRECISION 1715A) was used as the power supply for electrolytic production of carbon-based fuels in the electrolyte. The mean gas collection rate was near 120-140 mL min À1 , controlled by a volumetric owmeter. The gaseous products were expelled into a sampling bag through a topside gas-guide tube under argon, which also protected the electroactivity of the electrode.
The molar ratio of hydroxide to carbonate in the LiOH and LiNa (LiK, LiNaK) eutectic electrolyte was dened as n H :n C . With n H :n C ¼ 0.1 : 1, the experiments are performed at temperatures of 500-600 C, with voltage of 1.6-2.6 V applied at each temperature. Table 1 shows the experimental electrolytic conditions in detail.

Product characterization
Aerwards, the syngas obtained from electrolysis was characterized by gas chromatography (GC, Agilent 7890B) equipped with a thermal conductivity detector (TCD) and hydrogen ame ionization detector (FID) to determine the content of each component. Aer obtaining the concentration of each substance from the chromatogram, only the cathode fuel gas (methane, hydrocarbons, hydrogen, and carbon monoxide) was calculated. Fourier transform infrared spectroscopy (FTIR, Tensor27) was used to characterize the molecular structure of the products. The current-voltage relationship of different cathode materials (0.5 cm 2 surface area) was measured using a Ni wire anode (20 cm 2 surface area). Additionally, the current efficiency was calculated from the charge (in Faradays) passed during electrolysis compared to the charge required to form each measured mole of the gaseous products using the following equation: 28 where h i is the current efficiency contributed by the "i-th" product (%), m i is the mass of product i (g), MW i is the molecular weight of product i (g mol À1 ), and n i is the number of electrons transferred per molecule of product i. Far is the charge

Results and discussion
Theoretical analysis of hydroxide selection The reduction of CO 2 /H 2 O via co-electrolysis of a molten mixture of carbonates and hydroxide can be driven by applying an external force eld to the electrolysis unit. As shown in Scheme 1, using a hermetic device, syngas and hydrocarbons are generated from the reaction of OH À and CO 3 2À on the cathode surface, and oxygen is formed by oxidation of O 2À on the anode. The intermediate product (metal oxide) in the reaction process can absorb the incoming carbon dioxide and water, generating carbonates and hydroxides to regenerate the electrolyte, which completes the construction of a circulation system. The target product, CO and H 2 , can be obtained by the coelectrolysis of ionized OH À and CO 3 2À via reaction (2). The generated O 2À can be consumed in the following two ways: (i) reaction with CO 2 or H 2 O, regenerating CO 3 2À or OH À according to reactions (3) or (4), and (ii) the oxidation of O 2À to produce oxygen via electron loss (reaction (5)). (3) The reacted OH À in the electrolysis comes from hydroxide. It is necessary to control the source of OH À to keep the produced syngas mixture at the desired H 2 /CO ratio.
Compared to divalent molten salts, monovalent salts have higher conductivity, lower energy consumption, and give better electrical conductivity for the reduction of carbon dioxide at high temperature. 29,30 Basic data was obtained from NIST Chemistry WebBook 31 Fig. 1a shows the potential of metal deposition with three kinds of hydroxides. Unlike lithium hydroxide, pure sodium or potassium hydroxide tended to reduce the alkali cation to the alkali metal because of the relatively low metal deposition potentials. When KOH serves as the hydrogen source, K metal required a lower potential than H 2 , meaning that K metal deposition could become a side reaction. 32 Fig. 1b shows the calculated thermodynamic electrolysis potential of various hydroxides as a function of temperature for syngas formation. The electrolysis potential was calculated from the thermochemical enthalpy and entropy of individual species. The formulae can be written as: 33 H À H 298.15 where n B is the stoichiometric number, B is a component of the reaction, H is the standard enthalpy (kJ mol À1 ), S is the standard entropy (J mol À1 K À1 ), G is the standard Gibbs free Scheme 1 Schematic diagram of experimental process. energy (kJ mol À1 ), Far is the Faraday constant (96 485C mol À1 ), n is the number of transferred electrons, t ¼ T/1000, and T is the temperature in K, and A-G are thermodynamic parameters. 33 For ease of discussion, the absolute value of the electrolysis potential was used to represent the calculation value. As shown in Fig. 1b, the theoretical electrolytic voltage in the equations of various MOH (M ¼ Li, Na, K) for syngas generation decreased with increasing electrolytic temperature. At 700 K, the energy required for syngas production corresponded to a voltage of 1.57 V in the LiOH electrolyte, which was lower than those in NaOH electrolyte (1.70 V) and KOH electrolyte (1.80 V). In comparison to the NaOH and KOH systems, the LiOH system required a lower potential, and Li deposition was relatively low. As the KOH system showed contrary behavior at the same electrolytic temperature, the LiOH electrolyte was chosen as the optimal system.

Determination of optimal temperature and operating voltage
The eutectic points of pure lithium, sodium, or potassium carbonates are 723, 851, and 891 C, respectively. The relatively high melting temperature of the individual carbonates increases both the reaction energy consumption and heat loss. Low carbonate melting points are achieved by eutectic mixtures of alkali carbonates, such as Li 1.51 K 0.49 CO 3 , Li 1.07 Na 0.93 CO 3 , Li 0.85 Na 0.61 K 0.54 CO 3 and Li 1.43 Na 0.36 K 0.21 CO 3 at 490, 499, 375, and 390 C, respectively. [34][35][36] To achieve the controlled synthesis of syngas at relatively low temperatures, a binary carbonate mixture of Li-Na or Li-K or a ternary carbonate mixture of Li-Na-K were chosen as electrolytes to reduce the reaction temperature in this study. The highest eutectic point of the electrolytes studied was 499 C (Li 1.07 Na 0.93 CO 3 ) and the lowest eutectic point of the electrolytes studied was 375 C (Li 0.85 -Na 0.61 K 0.54 CO 3 ). As interpreted in our previous study, a high electrolysis temperature leads to an increased hydrogen yield due to enhanced reactivity. 28 Due to slower ionic migration, poor conductivity at low electrolytic temperature, difficulty of operation, and corrosion resistant performance at a high operating temperature, an appropriate temperature range was shown to be necessary for stable, continuous, and efficient electrolysis. Furthermore, according to previous theoretical and electrochemical reports, the selective electroreduction of CO 2 to CO likely occurs in Li-Na and Li-K molten salts at #650 C by cyclic voltammetry. 37,38 Furthermore, temperatures of over 600 C favor methane formation, achieving a methane yield of 64.9% in a eutectic mixture of carbonates as electrolyte in electrochemically reducing H 2 O/CO 2 . 28 In our previous study, 33 when the temperature was about 550 C, the methane content was less than 25%. Aer conducting a systematic experimental study of experimental data, the range of electrolysis temperatures studied herein was chosen as 500-600 C.
The electrolytic voltages required to form the reduction products in various reactions were calculated using the Gibbs energy at temperatures ranging from 400 K to 900 K in molten lithium carbonate. As shown in Fig. 2, obvious downward trends were observed for cathodic product generation. Beyond that, theoretical calculation also demonstrated that electrolysis voltage was another critical factor for controlling reaction selectivity. The electrolysis temperatures investigated ranged from 773 K to 873 K, and the electrolytic potential of the mixtures of carbonates and hydroxide further decreased. Therefore, the potentials of pure lithium carbonate decomposition were below 1.6 V. However, 1.6 V was taken as the starting voltage in this study because of the concentration overpotential, electrochemical overpotential, and resistance overpotential. 39,40 The actual decomposition voltage was greater than the theoretical value, E(decomposition potential) ¼ E(theory decomposition voltage) + E(overpotential) + IR. According to our previous study, 32,33 metal deposition occurs at higher voltages and, when the voltage exceeds 2.5 V, the CO content showed a signicant downward trend. Therefore, an electrolysis voltage range of 1.6-2.6 V was determined.

Optimization of electrodes for molten carbonate/hydroxide conversion
In Li x Na y K z CO 3 -LiOH electrolytes, O 2 is produced from the oxidation of O 2À and CO 3 2À at the anode, according to reactions (5) and (10): To prevent electrolyte corrosion and high-temperature oxidation of these ions, a cost-effective and corrosion-resistant electrode material with long-term stability is necessary. In our previous study, a Ni electrode showed a lower anodic overpotential by polarization analysis. 28 Therefore, Ni wire was used as the anode in this electrochemical study. To select cathode materials with excellent chemical stability, Ni, Fe, and Ni-Cr were tested as applicable cathodes. The electrochemical performance of these materials was evaluated using polarization tests, as shown in Fig. 3.
As the current density increased, the overpotential also increased, and the working voltage of the electrolytic cell exceeded the rest potential voltage. Minimizing this overpotential was essential to achieve maximum energy efficiency during electrolysis. The lowest cathodic overpotential and low cost made Fe a preferable cathode material. Therefore, a Fe cathode was a viable choice for long-term electrolysis in Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 /LiOH. We believe that the low overpotential observed for the Fe cathode presents an analogous electrocatalytic opportunity for syngas production via simultaneous splitting of hydroxide and carbonate on a similar surface. Therefore, iron and nickel were selected as the cathode and anode materials, respectively.

Analysis and characterization of electrochemical products
Gaseous products prepared from the Li 0.85 Na 0.61 K 0.54 -0.1LiOH electrolyte at a temperature of 550 C, using 20 cm 2 of Fe wire as the cathode and 20 cm 2 of Ni wire as the anode, were monitored by gas chromatography (GC) and IR spectroscopy. Gas chromatography detection consisted of a hydrogen ame ionization detector (FID) and thermal conductivity detector (TCD). As shown in Fig. 4a (FID), small by-products, such as propane and n-butane, were generated during electrolysis. TCD signals also indicated a small amount of CH 4 , and these alkanes are collectively represented as C x H y . The presence of CO, H 2 , and the anodic product were also shown in the TCD signals. As shown in Fig. 4b, peaks in the range 3000-3100 cm À1 were associated with unsaturated C-H stretching vibrations, 41,42 while peaks from 2800 cm À1 to 3000 cm À1 clearly indicated saturated C-H stretching vibrations, including -CH 3 and -CH 2 -. Trace amounts of CO 2 were present in the gaseous products. CO 2 has four modes of vibration, with two being infrared-active. The presence of CO 2 was conrmed by the stretching vibrations at 2349 cm À1 and the exural vibration at 667 cm À1 . 43 The stretching vibration of C^O was present in the range 2200-2250 cm À1 , which proved the presence of CO. The peaks from 1300-1400 cm À1 corresponded to C-H bending vibrations, and peaks from 1300-1700 cm À1 were associated with C-C stretching vibrations. The peaks at around 750 cm À1 corresponded to C]C out-of-plane exural vibrations in cis-olens. The above IR observations demonstrated that the products of CO 2 /H 2 O coelectrolysis contained a large amount of CO and hydrocarbons in this work.

Effect of mixed molten carbonate compositions on syngas product selectivity
As shown in Fig. 5, under all investigated electrolytic temperatures with voltage ranging from 1.6 V to 1.8 V, CO was the main product. However, a further increase in applied voltage (1.8-2.6 V) seemed to favor H 2 generation, indicating that 1.8 V resulted in the optimum CO fraction. To illustrate the dependence of syngas composition on temperature, CO, H 2 , and C x H y fractions at 1.8 V were calculated at ve temperatures, as shown in Fig. 6. Fig. 6 shows the gaseous product contents in the Li 1.07 -Na 0.93 CO 3 -0.1LiOH system at various electrolytic temperatures. When the applied temperature was increased from 500 to 550 C, the CO content increased gradually under the same electrolysis voltage. For instance, the yield of CO rose from $43.2% at 500 C and 1.8 V to $55.7% at 550 C and 1.8 V. In contrast, under the same electrolysis conditions, the H 2 content gradually decreased with increasing electrolysis temperature (500-550 C), showing that increasing temperature led to an increase in current density and favored CO generation. As the electrolysis temperature was further increased (550-600 C), the H 2 content increased gradually while the CO content gradually decreased at the same electrolysis voltage. This could be ascribed to the reduction potential required by H 2 and CO decreasing at elevated temperature. However, the rate of decrease in H 2 production was faster than that of CO, meaning that 550 C was the optimum electrolysis temperature for this system. This result showed the dependence of the CO fraction on the applied temperature, with higher temperatures found to not favor targeted CO production. The change in C x H y byproduct content was within 10%. The above-mentioned experimental results showed that 550 C was the optimum electrolysis temperature for the Li-Na system. Specically, the H 2 /CO Fig. 3 Polarization curves of various cathode materials during electrolysis. Fig. 4 Results of gaseous product analysis from (a) gas chromatograph with FID and TCD, and (b) IR spectra under the same electrolytic conditions. molar ratio of 0.62-9.60 was gained by adjusting the electrolysis voltage and operating temperature of the Li-Na system. Compared with previous research, 23-25 syngas was generated simultaneously with a H 2 /CO ratio of less than 1.
Selection of the optimum electrolytic voltage for the Li 1.51 K 0.49 CO 3 -0.1LiOH system is shown in Fig. S1. † At 1.6 V, the CO content increased with increasing electrolysis temperature (500-550 C), as shown in Fig. 7, indicating that the solubility of CO 2 increased with elevating temperature and the kinetics were enhanced. 44,45 The CO content decreased with a further increase in temperature (550-600 C). This might be attributed to CO oxidation occurring at 575 C in the Li-K system. 22 This showed that the best electrolysis temperature for the Li-K system was 550 C. As shown in Fig. S1, † the H 2 /CO molar ratios were well controlled in the range 0.76-5.04, 0.67-8.08, 0.59-4.33, 0.66-4.04, and 0.81-4.07. In particular, the adjustable range of H 2 /CO molar ratio range in the Li 1.51 K 0.49 CO 3 -0.1LiOH system was 0.59-8.08.
Selection of the optimum electrolytic voltage for the Li 1.43 -Na 0.36 K 0.21 CO 3 -0.1LiOH system under electrolytic temperatures is shown in Fig. S2, † and a detailed description can be found in the ESI. † Fig. 8 plots the gas concentration of the Li 1.43 Na 0.36 -K 0.21 CO 3 -0.1LiOH system at various temperatures. The CO concentration by electrolysis at 1.8 V in Li 1.43 Na 0.36 K 0.21 CO 3 -0.1LiOH increased to $61.6% as the electrolytic temperature was increased from 500 C to 550 C, indicating that the increase in the temperature favored the formation of CO. Concurrently, at the applied voltage of 1.8 V, the hydrogen concentration dropped to $32.8%, and other products stayed at around $5%. The CO content then gradually declined at temperature of 550-600 C, while the hydrogen content increased with increasing temperature (550-600 C). This phenomenon was caused by high-temperature activation of the Li 1.43 Na 0.36 K 0.21 CO 3 electrolyte and hydrogen formation was due to the activity of growth 32 The CO content of the system reached a maximum of $61.7% while the H 2 content was 32.8% at 1.8 V and 550 C. The adjustable range of the H 2 /CO molar ratio was 0.53-7.76 in the Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 system with a mass ratio of 61 : 22 : 17. Selection of the optimum electrolytic voltage for the Li 0.85 -Na 0.61 K 0.54 CO 3 -0.1LiOH system has also been interpreted in Fig. S3. † At an electrolysis voltage was 2.2 V, the compositions of the electrolysis gaseous products are shown in Fig. 9. The CO content gradually increased, the H 2 content gradually decreased, and the CO selectivity increased slightly when increasing the temperature from 500 to 550 C. With a further increase in temperature, the CO content began to decrease, while the H 2 content gradually increased. With a lower Fig. 6 Compositions of electrolysis gaseous products at temperatures of 500-600 C in the Li 1.07 Na 0.93 CO 3 -0.1LiOH electrolyte system. Fig. 7 Compositions of electrolysis gaseous products at temperatures of 500-600 C in the Li 1.51 K 0.49 CO 3 -0.1LiOH electrolyte system. Fig. 5 Compositions of electrolysis gaseous products in the operating voltage range 1.6-2.6 V at temperatures of 500 C, 525 C, 550 C, 575 C, and 600 C in the Li 1.07 Na 0.93 CO 3 -0.1LiOH electrolyte system. electrolysis potential required for the temperature rise, the higher temperature did not contribute to CO formation. These results conrmed that the CO content reached a maximum at 550 C and 2.2 V. The H 2 /CO molar ratio was 1.02, and the H 2 / CO molar ratio ranged from 1.02 to 7.42 in Li 0.85 Na 0.61 K 0.54 CO 3 by tuning the voltage and temperature.
In summary, the four electrolyte systems investigated presented varying H 2 /CO molar ratio ranges under different electrolytic conditions, but with the common feature that all maximum CO fractions were observed at 550 C. This was a desirably low temperature (vs. 800 C) 22 that could lead to the highly efficient one-pot generation of syngas by CO 2 /H 2 O via coelectrolysis in molten salts. In detail, compared with Li 0.85 -Na 0.61 K 0.54 CO 3 -0.1LiOH, the other three systems demonstrated an advantage in the generated H 2 /CO ratio, implying an enlarged application potential. Furthermore, Li 1.51 K 0.49 CO 3 -0.1LiOH and Li 1.43 Na 0.36 K 0.21 CO 3 -0.1LiOH provided maximum CO contents of more than 60%, which were superior to those of Li 1.07 Na 0.93 CO 3 -0.1LiOH and Li 0.85 Na 0.61 K 0.54 CO 3 -0.1LiOH. Therefore, it was concluded that a larger Li 2 CO 3 fraction favored CO generation, and that Li 2 CO 3 -induced modication at the interface between the cathode and electrolyte might be responsible for the observed changes in CO contents. By regulating the composition of the electrolytes, the synthesis of wide range of H 2 /CO ratios has been successfully realized, and the industrial application range of syngas has been expanded.
Current efficiency is a signicant metric of CO 2 /H 2 O transformation selectivity. Regarding the volume of obtained gaseous products, the current efficiency was calculated, as shown in Fig. 10. An irregular change in the current efficiency was observed in the Li 1.07 Na 0.93 CO 3 -0.1LiOH and Li 1.51 K 0.49 CO 3 -0.1LiOH systems, and the current efficiency of each reduction product was lower than 60%. Presumably, this was due to the deposition of alkali metals at the cathodic surface. 46 CO 3 2À ions can also be reduced indirectly via the prior reduction of alkali metal ions to the metal (reactions (11) and (12)). 47 In another study on a Ni electrode under similar conditions to that stated earlier, the cathodic limit corresponded to the reduction of CO 3 2À ions to carbon, while the anodic limit was assigned to the oxidation of Ni according to reaction (13). 48 Ni 2+ + 2e À / Ni Of the four electrolytes investigated, the mixture of Li 0.85 Na 0.61 K 0.54 -0.1LiOH exhibited a higher current efficiency Fig. 8 Compositions of electrolysis gaseous products at temperatures of 500-600 C in the Li 1.43 Na 0.36 K 0.21 CO 3 -0.1LiOH electrolyte system. Fig. 9 Compositions of electrolysis gaseous products at temperatures of 500-600 C in the Li 0.85 Na 0.61 K 0.54 CO 3 -0.1LiOH electrolyte system. Fig. 10 Current efficiencies of total gas generation in various electrolytes measured in the two-electrode system (a) Li 1.51 K 0.49 CO 3 -0.1LiOH electrolyte system, (b) Li 1.51 K 0.49 CO 3 -0.1LiOH electrolyte system, (c) Li 1.43 Na 0.36 K 0.21 CO 3 -0.1LiOH electrolyte system, (d) Li 0.85 Na 0.61 K 0.54 -0.1LiOH electrolyte system. with an optimum efficiency approaching $93%. However, it produced a relatively low fraction of CO ($25%). The current efficiencies for fuel gas production are all over 79%, signifying that regulating applied cell voltages leads to syngas generation of various concentrations.

Conclusions
In summary, the one-pot generation of syngas with a wide range H 2 /CO ratios (0.62-9.6 vs. over 1) was achieved through rationally designed molten salt electrolysis system. Electrolysis was carried out at 500-600 C with an operating voltage of 1.6-2.6 V using a low-cost Fe cathode and Ni anode. CO 2 /H 2 O was directly transformed into syngas via electrolysis in the Li 0.85 Na 0.61 K 0.54 -0.1LiOH electrolyte, with a 93.2% current efficiency at a constant voltage of 1.8 V and temperature of 500 C. Moreover, the molar ratios of H 2 /CO were 0.62-9.60, 0.59-8.08, 0.53-7.76, and 1.02-7.42 for Li 1.07 Na 0.93 CO 3 -0.1LiOH, Li 1.51 K 0.49 CO 3 -0.1LiOH, Li 1.43 Na 0.36 K 0.21 CO 3 -0.1LiOH, and Li 0.85 Na 0.61 K 0.54 -0.1LiOH electrolytes, respectively. Syngas with diverse H 2 /CO ratios was obtained by regulating the electrolyte composition, applied cell voltage, and electrolytic temperature. The methanebased hydrocarbon content varied by within 10% in the four electrolytes. In this manner, this work provides a path for further enhancements of product selectivity of CO and H 2 , and demonstrates a new sustainable process for recycling H 2 O and CO 2 .

Conflicts of interest
There are no conicts to declare.