Recent Progress in Electrocatalytic CO 2 Reduction to Pure Formic Acid Using a Solid-State Electrolyte Device

: The electrocatalytic CO 2 reduction reaction (CO 2 RR) to formic acid has gained signiﬁcant attention as a potential environmentally friendly approach to reducing CO 2 emissions and producing carbon-neutral liquid fuels. However, several challenges must be addressed to achieve the production of high-purity and high-concentration formic acid through CO 2 RR. One major challenge is the formation of a formate mixture instead of pure formic acid in conventional reactors. This requires costly downstream puriﬁcation and concentration processes to obtain pure formic acid. To overcome this problem, a three-compartment reactor design has been proposed where a solid-state electrolyte (SSE) is inserted between the anode and cathode compartments to recover pure formic acid directly. This reactor design involves the use of an anion exchange membrane (AEM) and a cation exchange membrane (CEM) to separate the anode and cathode compartments, and a center compartment ﬁlled with high-conductivity SSE to minimize ohmic resistance. Several studies have implemented this reactor design for continuous CO 2 RR and have reported remarkable improvements in the concentration and purity of the formic acid product. In this review, we summarize the recent progress of the SSE reactor design for CO 2 RR to produce pure formic acid (


Introduction
Anthropogenic activities, particularly the excessive usage of fossil fuels, have led to a significant increase in the level of CO 2 in the atmosphere since the beginning of the industrial period [1]. This rise in CO 2 levels has contributed to a climate crisis, evident from the recent occurrence of extreme weather events such as floods and forest fires. To address these challenges and reduce greenhouse gas emissions, substantial efforts have been directed toward finding ways to convert CO 2 into value-added chemicals or fuels, including methanol (CH 3 OH), formate (HCOO − )/formic acid (HCOOH), carbon monoxide (CO), syngas (CO and H 2 ), and ethylene (C 2 H 4 ) [2]. Among the numerous CO 2 conversion approaches such as biochemical, thermochemical, photochemical, and electrochemical methods, electrocatalytic CO 2 reduction reaction (CO 2 RR) is attracting significant attention as an environmentally friendly approach. CO 2 RR can be coupled with intermittent renewable energies due to its mild operating conditions with room temperature and atmospheric pressure. It enables the conversion of CO 2 into value-added products without additional emissions ( Figure 1) [3][4][5][6]. Due to their high selectivity (>90%) and activity (>100 mA cm −2 ) for industrial applications, the cost of certain CO 2 RR products, particularly C 1 chemicals such as CO and formic acid, has become economically competitive with traditional chemical processes [7][8][9][10][11][12]. This trend has been further supported by the decreasing price of renewable electricity.
Liquid products such as HCOO − /HCOOH offer significant advantages over gasphase CO2RR products due to their high energy densities and ease of storage and distribution [13,14]. Formic acid, in particular, is a crucial chemical feedstock with a global production of approximately 800,000 metric tons per year, used in a wide range of applications, including chemical production, cleaning, the textile industry, and antiseptics [15][16][17][18]. Moreover, formic acid has gained attention as a promising hydrogen carrier due to its high volumetric hydrogen density (53 g H2 per liter of HCOOH), low toxicity, and the fact that it is in the liquid phase under ambient conditions [19,20]. It is also considered an energy carrier for fuel cell applications due to its stability, low volatility, and high volumetric capacity [21,22]. Thus, producing formic acid through electrochemical CO2RR can serve as a carbon-neutral liquid fuel, effectively offsetting carbon emissions [23]. However, several challenges must be addressed for the practical applications of CO2RR in renewable formic acid production: (i) improving the selectivity, activity, and durability of the catalysts; (ii) reducing the energy consumption and cost of the process; (iii) scaling up the technology and integrating it with renewable energy sources; (iv) separating the product from aqueous mixture. Additional purification steps are often required, increasing production costs [23][24][25][26]. To address the limitations in CO2RR for formic acid production, extensive research efforts have been focused on developing efficient electrocatalysts, suitable electrolytes, and reactor designs to achieve high productivity. In particular, catalyst development has received significant attention to improving the selectivity and activity toward HCOO − /HCOOH production. Recent studies have made notable progress in identifying and designing efficient electrocatalysts for the selective production of HCOO − /HCOOH. For example, studies on the development of nanostructured and bimetallic catalysts have shown promising results in achieving high selectivity and activity for CO2RR to HCOO − /HCOOH [27][28][29][30][31][32][33][34][35][36]. In addition, single-atom catalysts (SACs) for CO2RR to HCOO − /HCOOH have been investigated due to their high surface area and atomic-level dispersion of active sites, which can enhance their electrocatalytic performance [12].
Nevertheless, there are other practical challenges that require technological advancements beyond the scope of electrocatalyst design. One prominent challenge is the purity and concentration of the liquid products [37]. In recent years, research efforts have been focused on developing efficient and scalable reactors for CO2RR to formate/formic acid with a growing demand for technologies and devices that enable direct formic acid To address the limitations in CO 2 RR for formic acid production, extensive research efforts have been focused on developing efficient electrocatalysts, suitable electrolytes, and reactor designs to achieve high productivity. In particular, catalyst development has received significant attention to improving the selectivity and activity toward HCOO − /HCOOH production. Recent studies have made notable progress in identifying and designing efficient electrocatalysts for the selective production of HCOO − /HCOOH. For example, studies on the development of nanostructured and bimetallic catalysts have shown promising results in achieving high selectivity and activity for CO 2 RR to HCOO − /HCOOH [27][28][29][30][31][32][33][34][35][36]. In addition, single-atom catalysts (SACs) for CO 2 RR to HCOO − /HCOOH have been investigated due to their high surface area and atomic-level dispersion of active sites, which can enhance their electrocatalytic performance [12].
Nevertheless, there are other practical challenges that require technological advancements beyond the scope of electrocatalyst design. One prominent challenge is the purity and concentration of the liquid products [37]. In recent years, research efforts have been focused on developing efficient and scalable reactors for CO 2 RR to formate/formic acid with a growing demand for technologies and devices that enable direct formic acid pro-duction. One promising approach is the use of an additional compartment of solid-state electrolyte (SSE) inserted between the anode and cathode compartments [23,26,38,39]. In this reactor design, the cathode and anode are in contact with an anion exchange membrane (AEM) and a cation exchange membrane (CEM), respectively. The HCOOand H + ions generated at the cathode and anode migrate across the AEM and CEM, respectively, into the center compartment, in which pure HCOOH molecules are formed via ionic recombination. In this review, we emphasize the significance and challenges associated with the direct production of high-purity formic acid by reviewing the recent progress of SSE devices for CO 2 RR and discussing strategies to enhance selectivity, activity, and purity for the direct production of pure formic acid. This work provides valuable insights and guidelines for the design of electrolyzers capable of producing pure liquid fuels through CO 2 RR, bringing us closer to large-scale applications in the future. In early CO 2 RR studies, numerous research groups have utilized reactors such as 3-electrode and H-type cells to produce formate in aqueous solutions. Figure 2 illustrates the typical configuration of an H-type cell, where the cathode and anode chambers are separated by an ion exchange membrane. This type of reactor has been widely employed for investigating various CO 2 RR catalysts due to its versatile setup. In these experiments, the catholyte is saturated with CO 2 by continuous bubbling using a glass sparger before electrolysis. The CO 2 gas is supplied continuously to the catholyte throughout the experiment to improve the absorption of the CO 2 gas in the aqueous solution [27]. Researchers have utilized this reactor configuration to develop highly active and selective electrocatalysts for CO 2 RR [27,[29][30][31][32][33][34][35][36]40]. Since the previous works by Hori et al. described the classification of metal catalysts according to product selectivity for CO 2 RR [41], extensive efforts have been made to develop various heterogeneous metal-based catalysts for selective CO 2 RR to formate. Table 1 summarizes some of the metal catalysts that have been investigated for their potential in CO 2 RR to formate, including tin (Sn), lead (Pb), bismuth (Bi), and indium (In). production. One promising approach is the use of an additional compartment of s state electrolyte (SSE) inserted between the anode and cathode compartm [23,26,38,39]. In this reactor design, the cathode and anode are in contact with an a exchange membrane (AEM) and a cation exchange membrane (CEM), respectively HCOOand H + ions generated at the cathode and anode migrate across the AEM CEM, respectively, into the center compartment, in which pure HCOOH molecule formed via ionic recombination. In this review, we emphasize the significance and lenges associated with the direct production of high-purity formic acid by reviewin recent progress of SSE devices for CO2RR and discussing strategies to enhance select activity, and purity for the direct production of pure formic acid. This work provide uable insights and guidelines for the design of electrolyzers capable of producing liquid fuels through CO2RR, bringing us closer to large-scale applications in the futu

CO2RR to Formate Using CO2-Dissolved Liquid Feed
In early CO2RR studies, numerous research groups have utilized reactors such electrode and H-type cells to produce formate in aqueous solutions. Figure 2 illus the typical configuration of an H-type cell, where the cathode and anode chamber separated by an ion exchange membrane. This type of reactor has been widely emp for investigating various CO2RR catalysts due to its versatile setup. In these experim the catholyte is saturated with CO2 by continuous bubbling using a glass sparger b electrolysis. The CO2 gas is supplied continuously to the catholyte throughout the e iment to improve the absorption of the CO2 gas in the aqueous solution [27]. Resear have utilized this reactor configuration to develop highly active and selective electro lysts for CO2RR [27,[29][30][31][32][33][34][35][36]40]. Since the previous works by Hori et al. described the sification of metal catalysts according to product selectivity for CO2RR [41], extensi forts have been made to develop various heterogeneous metal-based catalysts for sele CO2RR to formate. Table 1 summarizes some of the metal catalysts that have been i tigated for their potential in CO2RR to formate, including tin (Sn), lead (Pb), bismuth and indium (In).   Choi et al. studied the CO 2 RR to formate in an aqueous solution using tin-lead (Sn-Pb) alloys [27]. The study found that the Sn 56.3 Pb 43.7 alloy exhibits high faradaic efficiency (FE formate of 79.8%) and current density (j formate of 45.7 mA cm −2 ) for formate synthesis; 16%, and 25% improved FE formate and j formate compared to the single metal electrodes ( Figure 3).  Choi et al. studied the CO2RR to formate in an aqueous solution using tin-lead (Sn-Pb) alloys [27]. The study found that the Sn56.3Pb43.7 alloy exhibits high faradaic efficiency (FEformate of 79.8%) and current density (jformate of 45.7 mA cm −2 ) for formate synthesis; 16%, and 25% improved FEformate and jformate compared to the single metal electrodes ( Figure 3). Tan et al. reported a highly efficient CO2RR to formate on a tin (IV) oxide/zinc oxide (SnO2/ZnO) composite electrocatalyst with a grainy hollow nanofiber (HNF) structure [29]. The faradaic efficiency (FE) of formate on the SnO2/ZnO composite HNF reaches as high as 97.9% at −1.34 V (vs. RHE), outperforming many Sn-based catalysts. At −1.54 V (vs. RHE), the SnO2/ZnO HNF exhibits 2 times and 4 times higher current density for formate generation than those of SnO2 HNF and nanoparticles (NPs), respectively. This superior catalytic performance is attributed to its one-dimensional continuous structure as well as to the synergistic effects between SnO2 and ZnO, which facilitate faster electron transfer and improve the conductivity of SnO2/ZnO composite HNF ( Figure 4).
Despite these significant results, the kinetics of CO2RR into formate is limited below 50 mA cm −2 (Table 1) due to the polarization losses caused by the low solubility of CO2 in the electrolyte, and the mass transfer limitation of OH − near the electrodes, inevitably Tan et al. reported a highly efficient CO 2 RR to formate on a tin (IV) oxide/zinc oxide (SnO 2 /ZnO) composite electrocatalyst with a grainy hollow nanofiber (HNF) structure [29]. The faradaic efficiency (FE) of formate on the SnO 2 /ZnO composite HNF reaches as high as 97.9% at −1.34 V (vs. RHE), outperforming many Sn-based catalysts. At −1.54 V (vs. RHE), the SnO 2 /ZnO HNF exhibits 2 times and 4 times higher current density for formate generation than those of SnO 2 HNF and nanoparticles (NPs), respectively. This superior catalytic performance is attributed to its one-dimensional continuous structure as well as to the synergistic effects between SnO 2 and ZnO, which facilitate faster electron transfer and improve the conductivity of SnO 2 /ZnO composite HNF ( Figure 4).
Despite these significant results, the kinetics of CO 2 RR into formate is limited below 50 mA cm −2 (Table 1) due to the polarization losses caused by the low solubility of CO 2 in the electrolyte, and the mass transfer limitation of OH − near the electrodes, inevitably limiting the productivity of CO 2 RR [42]. In particular, the low solubility of CO 2 in the aqueous catholyte significantly limits the current density, thus hindering the large-scale application of the CO 2 RR. limiting the productivity of CO2RR [42]. In particular, the low solubility of CO2 in the aqueous catholyte significantly limits the current density, thus hindering the large-scale application of the CO2RR.  [29]. Copyright 2020 American Chemical Society).

CO2RR to Formate Using Gaseous CO2 Feed
There have been experiments in CO2 electroreduction that involve supplying gaseous CO2 to the cathode, allowing for sufficient CO2 delivery to the catalyst even at elevated current densities. Some research groups achieved remarkable improvement in the current density of CO2RR to formate using gas diffusion electrodes (GDEs) typically composed of a backing layer, a microporous layer (MPL), and a catalyst layer (CL) [42][43][44][45]. Several research groups have successfully utilized GDE-type CO2RR reactors ( Figure 5a) and achieved high current densities for formate production, exceeding 200 mA cm −2 and even up to 1000 mA cm −2 , with faradaic efficiencies (FE) higher than 90% as summarized in Table 2 [46][47][48][49][50][51][52][53]. However, one challenge with GDE-type electrolyzers is that they typically use aqueous electrolytes, such as KHCO3, K2CO3, or KOH, for the collection of liquid products. The presence of these aqueous electrolytes can dilute the liquid products, such as formate/formic acid, which increases the costs associated with product recovery and separation [54].  [29]. Copyright 2020 American Chemical Society).

CO 2 RR to Formate Using Gaseous CO 2 Feed
There have been experiments in CO 2 electroreduction that involve supplying gaseous CO 2 to the cathode, allowing for sufficient CO 2 delivery to the catalyst even at elevated current densities. Some research groups achieved remarkable improvement in the current density of CO 2 RR to formate using gas diffusion electrodes (GDEs) typically composed of a backing layer, a microporous layer (MPL), and a catalyst layer (CL) [42][43][44][45]. Several research groups have successfully utilized GDE-type CO 2 RR reactors ( Figure 5a) and achieved high current densities for formate production, exceeding 200 mA cm −2 and even up to 1000 mA cm −2 , with faradaic efficiencies (FE) higher than 90% as summarized in Table 2 [46][47][48][49][50][51][52][53]. However, one challenge with GDE-type electrolyzers is that they typically use aqueous electrolytes, such as KHCO 3 , K 2 CO 3 , or KOH, for the collection of liquid products. The presence of these aqueous electrolytes can dilute the liquid products, such as formate/formic acid, which increases the costs associated with product recovery and separation [54].  To overcome this challenge of GDE-type reactors, some research groups have studied CO2RR to formate in a zero-gap type electrolyzer, which eliminates the need for an aqueous electrolyte [54][55][56][57]. Lee et al. reported a facile strategy of catholyte-free electrocatalytic CO2 reduction (CF-CO2R) that avoids the solubility limitation by supplying an appropriate amount of water vapor with gaseous CO2 as a cathode reactant [54]. In this electrolyzer, the water vapor. In the CF-CO2R electrolyzer, water vapor acts as a carrier for supplying dissolved CO2 to the cathode by forming a CO2-saturated aqueous film on the catalyst surface as shown in Figure 5b. The advantage of this approach is that the consumed CO2 in the film can be immediately replenished from the bulk gas stream, enhancing the mass transfer of CO2 and improving the reaction kinetics. Lee et al. achieved a formate current density (jformate) of 51.7 mA cm −2 at a low cell voltage of 2.2 V using this CF-CO2R approach. They also obtained a significantly high concentration of formate solution, reaching 116.2 g-formate L −1 , with a formate faradaic efficiency (FEformate) of 77.7% by reducing the vapor supply from 12.58 to 0.65 mg min −1 cm −2 at 2.2 V and 323 K.
The zero-gap electrolyzer has several advantages over the GDE-catholyte method, including (1) lower cell voltage and improved energy efficiency by eliminating a large ohmic resistance of the catholyte compartment; (2) reduced cost by removing the cost of the catholyte; and (3) zero-gap configuration, which makes the system simple and easy to scale up. However, the lower current density observed in this configuration is indeed due to the challenges associated with the discharge of liquid products and the formation of solid precipitates.
In the zero-gap type electrolyzer, the liquid products have to be discharged in the opposite direction of the CO2 supply, and ions (HCOO − , HCO3 − , CO3 2− , K + , and Na + , etc.) in the liquid products form solid precipitates, which limits the supply of CO2. Additionally, the flow of aqueous liquid products through the gas diffusion electrode (GDE) can cause a decrease in the hydrophobicity of the electrode over time, a phenomenon known as electro-wetting. This leads to flooding of GDE that significantly inhibits the migration of CO2 to the catalyst layer, reducing the selective reduction of CO2 to formates while promoting the hydrogen evolution reaction (HER) as shown in Figure 6.
Recently, Díaz-Sainz et al. achieved a high jformate of 300 mA cm −2 by continuous operation using an anion exchange membrane (AEM, Sustainion TM , Dioxide Materials) with a formate concentration of 6.7 g L −1 at a cell potential of 3.6 V [57]. In this configuration, the formate solution could be obtained at the anode because the HCOO − ions generated in the cathode are transferred to the anode side via the AEM. They even obtained higher jformate up to 600 mA cm −2 with a formate concentration of 10.8 g L −1 at a cell potential of 4.7 V. When AEM is used in a zero-gap type electrolyzer for formate production, the cathodic To overcome this challenge of GDE-type reactors, some research groups have studied CO 2 RR to formate in a zero-gap type electrolyzer, which eliminates the need for an aqueous electrolyte [54][55][56][57]. Lee et al. reported a facile strategy of catholyte-free electrocatalytic CO 2 reduction (CF-CO 2 R) that avoids the solubility limitation by supplying an appropriate amount of water vapor with gaseous CO 2 as a cathode reactant [54]. In this electrolyzer, the water vapor. In the CF-CO 2 R electrolyzer, water vapor acts as a carrier for supplying dissolved CO 2 to the cathode by forming a CO 2 -saturated aqueous film on the catalyst surface as shown in Figure 5b. The advantage of this approach is that the consumed CO 2 in the film can be immediately replenished from the bulk gas stream, enhancing the mass transfer of CO 2 and improving the reaction kinetics. Lee et al. achieved a formate current density (j formate ) of 51.7 mA cm −2 at a low cell voltage of 2.2 V using this CF-CO 2 R approach. They also obtained a significantly high concentration of formate solution, reaching 116.2 g-formate L −1 , with a formate faradaic efficiency (FE formate ) of 77.7% by reducing the vapor supply from 12.58 to 0.65 mg min −1 cm −2 at 2.2 V and 323 K.
The zero-gap electrolyzer has several advantages over the GDE-catholyte method, including (1) lower cell voltage and improved energy efficiency by eliminating a large ohmic resistance of the catholyte compartment; (2) reduced cost by removing the cost of the catholyte; and (3) zero-gap configuration, which makes the system simple and easy to scale up. However, the lower current density observed in this configuration is indeed due to the challenges associated with the discharge of liquid products and the formation of solid precipitates.
In the zero-gap type electrolyzer, the liquid products have to be discharged in the opposite direction of the CO 2 supply, and ions (HCOO − , HCO 3 − , CO 3 2− , K + , and Na + , etc.) in the liquid products form solid precipitates, which limits the supply of CO 2 . Additionally, the flow of aqueous liquid products through the gas diffusion electrode (GDE) can cause a decrease in the hydrophobicity of the electrode over time, a phenomenon known as electro-wetting. This leads to flooding of GDE that significantly inhibits the migration of CO 2 to the catalyst layer, reducing the selective reduction of CO 2 to formates while promoting the hydrogen evolution reaction (HER) as shown in Figure 6.
Recently, Díaz-Sainz et al. achieved a high j formate of 300 mA cm −2 by continuous operation using an anion exchange membrane (AEM, Sustainion TM , Dioxide Materials) with a formate concentration of 6.7 g L −1 at a cell potential of 3.6 V [57]. In this configuration, the formate solution could be obtained at the anode because the HCOO − ions generated in the cathode are transferred to the anode side via the AEM. They even obtained higher j formate up to 600 mA cm −2 with a formate concentration of 10.8 g L −1 at a cell potential of 4.7 V. When AEM is used in a zero-gap type electrolyzer for formate production, the cathodic GDE flooding could be significantly reduced because HCOO − moves to the anode through AEM, and the crossover of water and the migration of cations (K + , Na + , etc.) from the anode is almost completely prevented. GDE flooding could be significantly reduced because HCOO − moves to the anode through AEM, and the crossover of water and the migration of cations (K + , Na + , etc.) from the anode is almost completely prevented.

Three-Compartment Configurations
While the research and development in GDE and zero-gap designs have resulted in remarkable advances in both faradaic efficiency and current density for formate production, these devices still have a crucial challenge to overcome. The product obtained from these reactors (GDE and zero-gap types) consists of HCOO − mixed with other impurity ions such as K + , HCO3 − , OH − , and so on. This mixture is required to be separated and acidified to obtain formic acid solutions, which require a considerable amount of energy and cost [19,23,38,39,58]. Zhu et al. reported that the additional costs associated with downstream purification and concentration, which greatly affect the economic viability of liquid products. This emphasizes the necessity of achieving high-purity and high-concentration liquid products directly from CO2 electrolyzers [37].
Therefore, the demand for technologies and devices for direct CO2 reduction to formic acid has increased. The utilization of an additional compartment of solid-state electrolyte (SSE) in a 3-compartment reactor design presents a promising approach for the direct recovery of pure formic acid from CO2 reduction as shown in Figure 7 [23,26,38,39]. In this design, the GDEs are employed on both the cathode and anode sides, which are in contact with an anion exchange membrane (AEM) and a cation exchange membrane (CEM), respectively. At the cathode, CO2 reacts with water (H2O) generating formate (HCOO − ) and hydroxide (OH − ) anions: Simultaneously, the oxidation of water occurs at the anode supplied with deionized water, humidified H2, or acidic solution, forming oxygen gas and protons (H + ): Both the HCOO − and OH − ions migrate through the AEM into the center compartment where they meet the H + ions produced from the anode compartment which pass

Three-Compartment Configurations
While the research and development in GDE and zero-gap designs have resulted in remarkable advances in both faradaic efficiency and current density for formate production, these devices still have a crucial challenge to overcome. The product obtained from these reactors (GDE and zero-gap types) consists of HCOO − mixed with other impurity ions such as K + , HCO 3 − , OH − , and so on. This mixture is required to be separated and acidified to obtain formic acid solutions, which require a considerable amount of energy and cost [19,23,38,39,58]. Zhu et al. reported that the additional costs associated with downstream purification and concentration, which greatly affect the economic viability of liquid products. This emphasizes the necessity of achieving high-purity and highconcentration liquid products directly from CO 2 electrolyzers [37].
Therefore, the demand for technologies and devices for direct CO 2 reduction to formic acid has increased. The utilization of an additional compartment of solid-state electrolyte (SSE) in a 3-compartment reactor design presents a promising approach for the direct recovery of pure formic acid from CO 2 reduction as shown in Figure 7 [23,26,38,39]. In this design, the GDEs are employed on both the cathode and anode sides, which are in contact with an anion exchange membrane (AEM) and a cation exchange membrane (CEM), respectively. At the cathode, CO 2 reacts with water (H 2 O) generating formate (HCOO − ) and hydroxide (OH − ) anions: Simultaneously, the oxidation of water occurs at the anode supplied with deionized water, humidified H 2 , or acidic solution, forming oxygen gas and protons (H + ): Both the HCOO − and OH − ions migrate through the AEM into the center compartment where they meet the H + ions produced from the anode compartment which pass through the CEM. In the center compartment, ionic recombination occurs, leading to the formation of HCOOH and H 2 O: The produced pure HCOOH in the center compartment can be recovered by passing deionized water or a humidified N 2 stream through the porous SSE layer.
Catalysts 2023, 13, x FOR PEER REVIEW 9 of 18 through the CEM. In the center compartment, ionic recombination occurs, leading to the formation of HCOOH and H2O: The produced pure HCOOH in the center compartment can be recovered by passing deionized water or a humidified N2 stream through the porous SSE layer. Side reactions that can occur on the cathode [26]: The bicarbonate ion (HCO3 − ) can move to the central compartment through the AEM where it reacts with protons produced on the anode to produce CO2: Formic acid oxidation reaction that can occur on the anode [26]: The 3-compartment cell design requires thin spacing in the central compartment and a high-conductivity SSE to minimize the cell IR drop. It is important to note that the conductivity of aqueous formic acid alone is relatively low, ranging between 5 and 10 mS cm −1 for formic acid concentrations of 5 to 20 wt% [26]. Therefore, the choice of a high-conductivity SSE is crucial to ensure efficient ionic transport and minimize energy losses within the electrolyzer system. Yang et al. investigated the ionic conductivity of four types of ion exchange resin including Amberlite ® IR120, Amberlite ® IRN77, Dowex ® 50WX2, and Side reactions that can occur on the cathode [26]: The bicarbonate ion (HCO 3 − ) can move to the central compartment through the AEM where it reacts with protons produced on the anode to produce CO 2 : Formic acid oxidation reaction that can occur on the anode [26]: The 3-compartment cell design requires thin spacing in the central compartment and a high-conductivity SSE to minimize the cell IR drop. It is important to note that the conductivity of aqueous formic acid alone is relatively low, ranging between 5 and Catalysts 2023, 13, 955 10 of 17 10 mS cm −1 for formic acid concentrations of 5 to 20 wt% [26]. Therefore, the choice of a high-conductivity SSE is crucial to ensure efficient ionic transport and minimize energy losses within the electrolyzer system. Yang et al. investigated the ionic conductivity of four types of ion exchange resin including Amberlite ® IR120, Amberlite ® IRN77, Dowex ® 50WX2, and Duolite C433 in water and in 5, 10, 15, and 20 wt% formic acid solutions [26]. The results of the study indicated that the use of cation ion exchange resins as SSE materials led to a significant increase in ionic conductivity compared to aqueous formic acid alone. The ion exchange resin/formic acid matrix mixtures exhibited an enhancement in ionic conductivity by a factor of approximately 6-10 compared to formic acid solutions alone. Among the SSEs tested, hydrogen-form sulfonic acid resins such as Amberlite ® IR120 and Dowex ® 50WX2 demonstrated high ionic conductivities. However, Dowex ® 50WX2, with its fine particle size range of 100-200 mesh (75-150 µm), showed a notable pressure drop in the center flow compartment during evaluation.

Strategies for High-Purity Formic Acid Recovery
Several studies used three-compartment reactor configurations for continuous CO 2 electroreduction to recover high-purity HCOOH solution (Table 3) [23,26,38,39]. Yang et al. introduced a new approach by incorporating an acidic center compartment in their system, allowing for the direct production of formic acid instead of formate salts [26]. In this configuration, only deionized (DI) water is introduced into the center flow compartment to recover the formic acid product from the cell. The cell design included an anode compartment, a center compartment containing a cation exchange SSE enclosed by a CEM (Nafion ® ) on the anode side and an AEM (Sustanion™) on the cathode side, and a cathode compartment with Sn-based GDE.
The details of the configuration of their cell assembly are illustrated and summarized in Figure 8a and Table 4. They investigated the cell performances with a different flow rate of DI water in the center compartment and various ion exchange resins for the center compartment. In addition, they compared two modes of cell operation, center compartment single-pass, and recirculation modes, and found that the single-pass mode was preferred due to differences in faradaic efficiency. The study achieved a high FE HCOOH of up to 94% at a current density of 140 mA cm −2 during 142 h or operation at a cell voltage of about 3.5 V. The HCOOH product concentration reached up to 9.4 wt% (Figure 8b). These results demonstrate the potential of the three-compartment reactor configuration for the continuous production of high-purity formic acid with improved efficiency and concentration.
Catalysts 2023, 13, x FOR PEER REVIEW 10 of 18 Duolite C433 in water and in 5, 10, 15, and 20 wt% formic acid solutions [26]. The results of the study indicated that the use of cation ion exchange resins as SSE materials led to a significant increase in ionic conductivity compared to aqueous formic acid alone. The ion exchange resin/formic acid matrix mixtures exhibited an enhancement in ionic conductivity by a factor of approximately 6-10 compared to formic acid solutions alone. Among the SSEs tested, hydrogen-form sulfonic acid resins such as Amberlite ® IR120 and Dowex ® 50WX2 demonstrated high ionic conductivities. However, Dowex ® 50WX2, with its fine particle size range of 100-200 mesh (75-150 μm), showed a notable pressure drop in the center flow compartment during evaluation.

Strategies for High-Purity Formic Acid Recovery
Several studies used three-compartment reactor configurations for continuous CO2 electroreduction to recover high-purity HCOOH solution (Table 3) [23,26,38,39]. Yang et al. introduced a new approach by incorporating an acidic center compartment in their system, allowing for the direct production of formic acid instead of formate salts [26]. In this configuration, only deionized (DI) water is introduced into the center flow compartment to recover the formic acid product from the cell. The cell design included an anode compartment, a center compartment containing a cation exchange SSE enclosed by a CEM (Nafion ® ) on the anode side and an AEM (Sustanion™) on the cathode side, and a cathode compartment with Sn-based GDE.
The details of the configuration of their cell assembly are illustrated and summarized in Figure 8a and Table 4. They investigated the cell performances with a different flow rate of DI water in the center compartment and various ion exchange resins for the center compartment. In addition, they compared two modes of cell operation, center compartment single-pass, and recirculation modes, and found that the single-pass mode was preferred due to differences in faradaic efficiency. The study achieved a high FE HCOOH of up to 94% at a current density of 140 mA cm −2 during 142 h or operation at a cell voltage of about 3.5 V. The HCOOH product concentration reached up to 9.4 wt% (Figure 8b). These results demonstrate the potential of the three-compartment reactor configuration for the continuous production of high-purity formic acid with improved efficiency and concentration.    Yang et al. also demonstrated the long-term stability of pure formic acid production by 1000 h operation of the 3-compartment design at industrially relevant current densities [39]. The electrolyzer (Figure 9a) can be operated at a current density as high as 300 mA cm −2 with a cell voltage below 4.0 V, and no significant change in FE HCOOH was observed at current densities between 100 and 250 mA cm −2 . Typically, higher DI water flow rates led to higher FE HCOOH but lower HCOOH concentrations as shown in Figure 9b. The electrolyzer directly produced a 6.03-12.92 wt% formic acid product at FE HCOOH range of 73.0-91.3%, depending on the operating conditions. The highest formic acid FE of 91.3% was achieved using a center compartment DI water flow rate of 0.17 mL min −1 with an HCOOH concentration of 6.03 wt%. In addition, they confirmed that the FE HCOOH was mostly above 70% and HCOOH concentration above 11 wt% during the 1000 h long-term test at 200 mA cm −2 (Figure 9c). Yang et al. also demonstrated the long-term stability of pure formic acid production by 1000 h operation of the 3-compartment design at industrially relevant current densities [39]. The electrolyzer (Figure 9a) can be operated at a current density as high as 300 mA cm −2 with a cell voltage below 4.0 V, and no significant change in FEHCOOH was observed at current densities between 100 and 250 mA cm −2 . Typically, higher DI water flow rates led to higher FEHCOOH but lower HCOOH concentrations as shown in Figure 9b. The electrolyzer directly produced a 6.03-12.92 wt% formic acid product at FEHCOOH range of 73.0-91.3%, depending on the operating conditions. The highest formic acid FE of 91.3% was achieved using a center compartment DI water flow rate of 0.17 mL min −1 with an HCOOH concentration of 6.03 wt%. In addition, they confirmed that the FE HCOOH was mostly above 70% and HCOOH concentration above 11 wt% during the 1000 h long-term test at 200 mA cm −2 (Figure 9c). Xia and Wang's group also demonstrated the production of high-purity formic acid using the 3-compartment cell design using an ultrathin two-dimensional Bi (2D-Bi) catalyst for CO2RR to HCOOH [38]. They used a porous styrene-divinylbenzene (SVB) copolymer as SSE consisting of sulfonic acid functional groups for H + conduction. They obtained a 0.112 M (~0.5 wt%) pure HCOOH solution with a peak FEHCOOH of 93.1% and a partial current density of 32.1 mA cm −2 at 3.08 V and at a DI water flowrate into the center compartment of 12 mL h −1 . Xia and Wang's group also demonstrated the production of high-purity formic acid using the 3-compartment cell design using an ultrathin two-dimensional Bi (2D-Bi) catalyst for CO 2 RR to HCOOH [38]. They used a porous styrene-divinylbenzene (SVB) copolymer as SSE consisting of sulfonic acid functional groups for H + conduction. They obtained a 0.112 M (~0.5 wt%) pure HCOOH solution with a peak FE HCOOH of 93.1% and a partial current density of 32.1 mA cm −2 at 3.08 V and at a DI water flowrate into the center compartment of 12 mL h −1 .
However, they pointed out a few challenges that remain impeding its practical applications: (i) the concentration of the product was limited due to the use of DI water in the SSE layer; (ii) the rate of product generation was not sufficiently high to meet the demands of industrial applications; (iii) the system still relied on the use of a liquid electrolyte on the anode side for water oxidation [23]. To achieve a high concentration of formic acid solutions, it is necessary to reduce the flow rate of DI water. However, it should be noted that increasing the concentration of formic acid in the SSE layer can lead to a significant decrease in faradaic efficiency. This is because higher formic acid concentrations can increase the rate of the reverse reaction and result in increased crossover to the anode. In this concern, they suggested an all-solid-state reactor, where an inert gas (N 2 ) flow is supplied to recover high-purity HCOOH vapor from the center compartment instead of DI water. High-purity HCOOH can be obtained by a simple cold condensation process. As a result, they accomplished HCOOH concentration up to 12.1 M (~49.3 wt%) using 10 sccm of humidified N 2 gas [38], and nearly 100 wt% pure HCOOH using 20 sccm of dry N 2 gas [23] instead of DI water flow to recover the generated HCOOH. This alternative approach of utilizing an all-solid-state reactor with N 2 gas flow offers several advantages. It enables higher formic acid concentrations without sacrificing faradaic efficiency, as the dilution effect from DI water is eliminated. Additionally, the recovery of high-purity formic acid vapor through a cold condensation process simplifies the product separation step.

Opportunities and Challenges
The 3-compartment electrolyzer design using SSE offers a promising pathway for the direct production of high-purity formic acid (HCOOH) and represents a significant step towards commercializing CO2RR technologies. One key advantage of the SSE reactor design is its ability to prevent the crossover of cathode-generated carbonate ions [59]. In the typical 2-compartment electrolyzer, hydroxide ions (OH − ) generated at the cathode during CO 2 RR react rapidly with the supplied CO 2 to form carbonate (CO 3 2− ) or bicarbonate (HCO 3 − ) ions, which move across the AEM to the anodic side, resulting in carbon loss. On the other hand, in the SSE reactor design, the CO 3 2− or HCO 3 − ions migrate through AEM into the center compartment and then be recombined with protons from the anode to form CO 2 gas again, which could be easily recirculated to the cathodic reactant.
While the SSE approach offers advantages in terms of product separation energy and cost, it introduces the challenge of increased ohmic resistance, which can reduce the energy efficiency of CO 2 RR. Thus, the energy penalty by increased ohmic resistance should be compared with the energy required to separate the liquid fuels from the electrolyte. It is possible to reduce the resistance of the solid-state electrolyte (SSE) by engineering ultrathin SSE layers [38]. This can be achieved by decreasing the thickness of the SSE, which would enhance the ion conductivity within the solid electrolyte. Additionally, improving the contact interface between the SSE layer and the membranes can also contribute to reducing resistance [37]. Beyond the conventional formate production technology, there was remarkable progress in high-purity formic acid production via adopting a 3-compartment electrolyzer using SSE, which exhibited nearly 100 wt% pure formic acid. However, it is important to note that high-purity formic acid has not yet been produced at commercially relevant partial current densities (>200 mA cm −2 ). This highlights the need for further research and development efforts to optimize the SSE reactor design, enhance the current density, and improve the energy efficiency to make high-purity formic acid production economically viable at larger scales. Figure 10 illustrates the current status of high-purity formic acid production, indicating that there is still room for improvement in achieving commercially relevant partial current densities while maintaining high purity. Future research should aim to overcome these challenges and explore innovative strategies to enhance the performance and scalability of the SSE-based 3-compartment electrolyzer for efficient and commercially viable formic acid production.

Summary and Outlooks
In summary, recent research has been reviewed in reactor design for formic acid production to address the challenges of impurity and low concentration in conventional CO2RR reactors. Early studies focused on producing formate in aqueous solutions using 3-electrode and H-type cells. However, the limited solubility of CO2 in the aqueous catholyte restricted the current density and hindered the large-scale application of CO2RR. To overcome this limitation, researchers have introduced reactor designs that ensure sufficient CO2 supply to the catalyst. GDE-type reactors and zero-gap electrolyzers have shown significant improvements in formate current density, reaching up to 930 mA cm −2 , and achieving concentrated formate products of up to 33.7 wt%. However, these products contain impurity ions, requiring downstream processes such as separation and acidification to obtain pure formic acid. One promising approach to obtaining pure formic acid is the incorporation of an additional SSE compartment between the anode and cathode. This 3-compartment electrolyzer has demonstrated the production of nearly 100 wt% pure formic acid. However, high-purity formic acid has not yet been produced at commercially relevant partial current densities.
Future research should focus on developing cost-effective and scalable technologies for formic acid production from CO2RR. This includes exploring catalyst materials that are abundant, low-cost, and highly efficient. Additionally, optimizing reactor designs and process parameters to maximize productivity and minimize energy consumption will be crucial for cost-effective large-scale production. To enhance the economic viability of the liquid products, it is crucial to achieve high-purity and high-concentration formic acid directly from the CO2 electrolyzer. This would eliminate or minimize the need for extensive downstream processing, reducing energy consumption, operational costs, and capital investments associated with purification and concentration steps. Future progress in this approach also provides a potential route to store H2 in liquid HCOOH and could be extended to the production of C2+ liquid fuels. This review contributes to the advancement of electrolyzer technology, bringing us closer to large-scale applications of CO2RR for the production of high-purity formic acid and other liquid fuels in the future.

Summary and Outlooks
In summary, recent research has been reviewed in reactor design for formic acid production to address the challenges of impurity and low concentration in conventional CO 2 RR reactors. Early studies focused on producing formate in aqueous solutions using 3-electrode and H-type cells. However, the limited solubility of CO 2 in the aqueous catholyte restricted the current density and hindered the large-scale application of CO 2 RR. To overcome this limitation, researchers have introduced reactor designs that ensure sufficient CO 2 supply to the catalyst. GDE-type reactors and zero-gap electrolyzers have shown significant improvements in formate current density, reaching up to 930 mA cm −2 , and achieving concentrated formate products of up to 33.7 wt%. However, these products contain impurity ions, requiring downstream processes such as separation and acidification to obtain pure formic acid. One promising approach to obtaining pure formic acid is the incorporation of an additional SSE compartment between the anode and cathode. This 3-compartment electrolyzer has demonstrated the production of nearly 100 wt% pure formic acid. However, high-purity formic acid has not yet been produced at commercially relevant partial current densities.
Future research should focus on developing cost-effective and scalable technologies for formic acid production from CO 2 RR. This includes exploring catalyst materials that are abundant, low-cost, and highly efficient. Additionally, optimizing reactor designs and process parameters to maximize productivity and minimize energy consumption will be crucial for cost-effective large-scale production. To enhance the economic viability of the liquid products, it is crucial to achieve high-purity and high-concentration formic acid directly from the CO 2 electrolyzer. This would eliminate or minimize the need for extensive downstream processing, reducing energy consumption, operational costs, and capital investments associated with purification and concentration steps. Future progress in this approach also provides a potential route to store H 2 in liquid HCOOH and could be extended to the production of C 2+ liquid fuels. This review contributes to the advancement of electrolyzer technology, bringing us closer to large-scale applications of CO 2 RR for the production of high-purity formic acid and other liquid fuels in the future.

Data Availability Statement:
The data presented in this study are available in the corresponding references listed.