High‐Concentration Electrosynthesis of Formic Acid/Formate from CO2: Reactor and Electrode Design Strategies

The electrochemical CO2 reduction reaction (CO2RR), driven by renewable energy, provides a potential carbon‐neutral avenue to convert CO2 into valuable fuels and feedstocks. Conversion of CO2 into formic acid/formate is considered one of the economical and feasible methods, owing to their high energy densities, and ease of distribution and storage. The separation of formic acid/formate from the reaction mixtures accounts for the majority of the overall CO2RR process cost, while the increment of product concentration can lead to the reduction of separation cost, remarkably. In this paper, we give an overview of recent strategies for highly concentrated formic acid/formate products in CO2RR. CO2RR is a complex process with several different products, as it has different intermediates and reaction pathways. Therefore, this review focuses on recent study strategies that can enhance targeted formic acid/formate yield, such as the all‐solid‐state reactor design to deliver a high concentration of products during the reduction of CO2 in the electrolyzer. Firstly, some novel electrolyzers are introduced as an engineering strategy to improve the concentration of the formic acid/formate and reduce the cost of downstream separations. Also, the design of planar and gas diffusion electrodes (GDEs) with the potential to deliver high‐concentration formic acid/formate in CO2RR is summarized. Finally, the existing technological challenges are highlighted, and further research recommendations to achieve high‐concentration products in CO2RR. This review can provide some inspiration for future research to further improve the product concentration and economic benefits of CO2RR.


Introduction
The excessive carbon dioxide (CO 2 ) emission from the combustion of fossil fuels and other industrial processes has caused severe global warming problems including extreme weather, and glacial ablation. [1,2]Converting waste CO 2 into valuable chemicals and feedstocks has recently attracted much attention. [3- 5]Electrochemical CO 2 reduction reaction (CO 2 RR) technology can be coupled with renewable resources such as solar and wind, which have intermittent supplies as a means of storing energy or converting CO 2 into valuable products.[8] Depending on electrocatalysts and electrolyzer design, a great variety of chemicals and feedstocks can be obtained in CO 2 RR. [9,10][13] The possible reaction routes for the products and their relevant equations are presented in Table 1 and Figure 2. A variety of chemicals, including carbon monoxide (CO), [14][15][16] formic acid/formate (HCOOH/HCOO − ), [17][18][19] methanol (CH 3 OH), [20][21][22] ethanol (C 2 H 5 OH), [23][24][25] acetate/acetic acid (CH 3 COO − /CH 3 COOH), [26,27] methane (CH 4 ), [28][29][30][31] ethylene (C 2 H 4 ), [32][33][34] and other vital chemicals, are produced during CO 2 RR via stepwise transfer of protons (H + ) and/or electrons (e − ).It has been shown that CO 2 RR is a multiprotoncoupled, multielectron transfer process, as shown in Table 1, and different numbers of electron transfers can result in different chemical products. [35][41] For example, CO 2 is usually reduced to formic acid/formate by using Sn/Bi-based electrocatalysts.In brief, in an electrochemical CO 2 reduction process, CO 2 molecules firstly are attracted to the electrocatalyst surface to gain an electron to form Ã CO 2 ÁÀ radical intermediates, followed The electrochemical CO 2 reduction reaction (CO 2 RR), driven by renewable energy, provides a potential carbon-neutral avenue to convert CO 2 into valuable fuels and feedstocks.Conversion of CO 2 into formic acid/formate is considered one of the economical and feasible methods, owing to their high energy densities, and ease of distribution and storage.The separation of formic acid/ formate from the reaction mixtures accounts for the majority of the overall CO 2 RR process cost, while the increment of product concentration can lead to the reduction of separation cost, remarkably.In this paper, we give an overview of recent strategies for highly concentrated formic acid/formate products in CO 2 RR.CO 2 RR is a complex process with several different products, as it has different intermediates and reaction pathways.Therefore, this review focuses on recent study strategies that can enhance targeted formic acid/formate yield, such as the all-solid-state reactor design to deliver a high concentration of products during the reduction of CO 2 in the electrolyzer.Firstly, some novel electrolyzers are introduced as an engineering strategy to improve the concentration of the formic acid/formate and reduce the cost of downstream separations.Also, the design of planar and gas diffusion electrodes (GDEs) with the potential to deliver high-concentration formic acid/formate in CO 2 RR is summarized.Finally, the existing technological challenges are highlighted, and further research recommendations to achieve high-concentration products in CO 2 RR.This review can provide some inspiration for future research to further improve the product concentration and economic benefits of CO 2 RR.
by different stepwise transfers.The adsorbed Ã CO 2 ÁÀ radical intermediates form an *OCHO intermediate via a protonation step, and with further proton/electron transfer to form HCOOH as the major product, shown in the top-left mechanism of Figure 2, Pathway 1. [42,43] In addition, CO 2 can insert into the electrocatalyst surface (metal-H bond) to form Á OCHO intermediate or Á COOH intermediate (Dashed Box in Figure 2), and then, yield formate via proton/electron transfer steps as shown in the top-right mechanism of Figure 2, Pathways 2 and 3. [44][45][46] Therefore, converting CO 2 to formic acid/ formate is economically feasible because it is a simple two-electron transfer reaction with high selectivity and easy separation from the input CO 2 stream.
The conversion of CO 2 in CO 2 RR is a considerably complex reaction process due to its multiple reaction pathways and various intermediates corresponding to different final products.Moreover, splitting CO 2 molecules into intermediates with active sites requires a high energy input, for example, high overpotentials, and activation barriers.Hence, there are several challenges for CO 2 RR, as shown in Figure 3.
The first challenge is the amount of energy input to break CO 2 molecular bonds.Using renewable energy as a driving force in CO 2 RR to synthesize chemicals is a potential method to reduce our reliance on traditional fossil fuels.It also is a promising avenue for renewable energy to long-term storage of electricity as stable chemical bonds. [2]dopting proper electrocatalysts with high intrinsic catalytic activity can effectively reduce the activation energy of CO 2 RR, which leads to lower overpotentials and more efficient CO 2 conversion.Several comprehensive reviews have been published on the advances of electrocatalyst materials (such as alloys and transition metals), [49][50][51] reaction kinetics, [52,53] active sites, [13,54,55] and intrinsic activity, [56][57][58] for CO 2 RR to reduce the energy input.
The second challenge is effectively improving the reaction rate and achieving a high conversion rate.The limited solubility of CO 2 in aqueous media (33 mM in water, 25 °C, 100 kPa) and the Energy Environ.Mater.2023, 6, e12596 2 of 17 restricted gas/liquid mass transfer decrease the CO 2 RR reaction rate. [59,60]Meanwhile, a suitable electrode and improved mass transfer can remarkably achieve high-rate conversions, reaching industrially relevant current densities (> 200 mA cm −2 ) and increasing its practical application. [2,61,62][65] Compared to non-GDE configurations, GDEs can increase the current density by 2-3 orders of magnitude, achieving over 1 A cm −2 . [63]GDEs are attracting much attention recently due to their ability to obtain high current densities and we have lately published a comprehensive review on recent GDEs for various anhydrous electrolysis reactions including the electrochemical conversion of CO 2 , CO, and N 2 . [63]he third challenge for CO 2 RR is product selectivity.The electrode's structure, electrolyzer design, electrocatalyst type, and reaction conditions can affect the final products of CO 2 RR.As seen in Figure 1, the same intermediates are involved in the production of different products, which makes product selectivity more difficult.For example, Pb, Sn and In catalysts usually favor formate production during CO 2 RR, whereas Au, Pd, and Ag catalysts are selective toward CO. [12,50] An important performance metric for CO 2 RR is Faradaic efficiency (FE), which describes the selectivity of the electrons consumed for the targeted product.However, owing to the existence of competing reactions (e.g., H 2 evolution in aqueous media), a higher reaction rate (e.g., higher current density) generally results in a lower selectivity, resulting in the consumption of input power toward the unwanted products. [66]On the other hand, the CO 2 RR to formic acid/formate exhibits a perfect FE of nearly 100% compared to other chemicals, such as ethylene and acetate with FE of approximately 60% and 40%, respectively. [67]he stability of CO 2 RR is the fourth challenge as it guarantees its commercial adaption; however, most studies have reported limited stability (usually <100 h). [63,68]The decreasing selectivity and  [44,47,48] Copyright 2015 (2019), American Chemical Society.activity of electrocatalysts after a long-time reaction can lead to the declining stability of CO 2 RR.The electrocatalyst will be contaminated/deactivated during the reaction because the active sites are occupied by unreactive intermediates or other groups. [58,69]Moreover, flooding by aqueous electrolyte [64,70,71] and precipitating with carbonate salt, [72][73][74][75] which occurs frequently in a GDEs system, also can result in the electrocatalyst failure in CO 2 RR.The concentration of target products in CO 2 RR cannot be enriched and meet the requirements of industrial applications.Therefore, it is necessary to develop stable electrocatalysts and electrolyzers to increase the performance of stability.
The fifth challenge is producing high-concentration products.As the reaction mechanisms for CO 2 RR are shown in Figure 2, a variety of chemicals could be produced in an electrochemical reduction reaction, meaning that the concentration of the targeted product is limited, and the separation cost of products will dominate the whole CO 2 RR industrial process.As shown in Figure 4a, much of the operating costs (60%) in formic acid production were associated with the distillation process, which was used to separate formic acid from the mixture of CO 2 RR products. [76]Besides, the capital costs of production separation, including the equipment design and major components also take a large proportion of the whole process.Furthermore, the separation energy of the target product in CO 2 RR dramatically increases with the decrease in concentration (Figure 4b).For example, it needs 45 MJ kg −1 of separation energy for a 1 wt.%C 2 H 5 OH mixture in the water while just only 4.7 MJ kg −1 for 10 wt.%. [77]Meanwhile, the separation cost will reduce with the increase in the target product's concentration.For example, concentrating formic acid from 5 to 85 wt.% costs 900 $ ton −1 , while from 50 wt.%costs only 170 $ ton −1 , as shown in Figure 4b. [75]The step of the purification of target products in CO 2 RR contributes to a significant portion of the overall cost. [78,79]Therefore, a cost-intensive and complicated target product separation process is required and it limits their economic feasibility and large-scale industrial applications. [78,80]To achieve the possibility of CO 2 to economic chemical fuels (formic acid/formate), a solid-state reactor without liquid electrolytes (electrolyte-less system) was designed to eliminate the need for a liquid separation step. [17]For an electrolyte-less system, the reactive products can be collected effectively without any impurities, more details will be discussed in Section 2.1.On the other hand, a stable reaction over a long period for an electrode electrocatalyst in an electrolyzer can result in product enrichment, obtaining a high concentration of products.Despite the recent efforts to maximize product concentration, further research is needed for commercial applications.
Several recent review papers have focussed on CO 2 RR including electrocatalyst types, [81][82][83] reaction mechanisms, [50,68,84] and electrode material design. [63,85,86]Furthermore, formic acid/formate as important commodity materials with high industrial demand as a chemical feedstock are promising products of CO 2 RR.Formic acid and formate are two different forms of CO 2 reduction products, depending on the pH of the electrolyte.Generally, the CO 2 RR yields formic acid operating at acidic conditions below the pKa of formic acid (3.8) and formate productions at highly alkaline electrolyte systems (pH > 8). [87]Some excellent reviews also have summarized the state-of-art electrocatalysts for electrocatalytic conversion of CO 2 into formic acid/formate. [35,42,88]Nevertheless, it is also critical to obtain highly concentrated formic acid/formate from CO 2 RR, as a highconcentration liquid product in downstream separation processes will require low energy consumption, thereby compromising the overall CO 2 RR economic feasibility.Herein, this review focuses on the current strategies that can enhance the concentration of formic acid/formate in CO 2 RR and increase the implementation of commercial value.In the following section, we have summarized the state-of-the-art electrolyzer configurations that can contribute to the yield of high-concentration products, including reactors design and electrodes (planar electrode and gas diffusion electrode) design.This review concludes with a discussion of future opportunities and research needs toward high concentration and commercially viable rate of liquid products. .Reproduced with permission. [76]Copyright 2018, American Chemical Society; b) The separation energy inputs required for different concentrations of target products including CH 4 and C 2 H 5 OH.Reproduced with permission. [77]Copyright 2018, Elsevier Inc. and the separation cost of different concentrations of formic acid.Reproduced with permission. [75]Copyright 2019, American Chemical Society.

Strategies for Obtaining High-Concentration Products
Enhancing the selectivity and concentration of products could enhance the practicality of CO 2 RR for real applications.The production of formic acid/formate has higher selectivity and energy utilization efficiency in CO 2 RR compared to other reduction chemicals.Besides, it is an important substance that has relatively high commercial value.Thus, some studies have focused on the increment of the yield of formic acid/formate in CO 2 RR.Researchers have applied a variety of techniques to increase the concentration of formic acid/formate in CO 2 RR.In this section, we focus on two main strategies that are used to enhance the concentration of formic acid/formate in CO 2 RR: designing reactors and designing electrodes.

Reactor Design to Increase the Concentration of Formic Acid/Formate
The purity of products for electrochemical CO 2 RR is significantly influenced by the design of reactors.Conventional CO 2 reactors usually use liquid electrolytes, such as KHCO 3 and KOH, to increase ionic transportation between the cathode and anode and to collect liquid products. [43,89]However, the products are generated with low concentration and mixed with other impurities from the electrolyte.Therefore, extra separation and concentration processes are required, resulting in energy-intensive purification steps and increasing the overall cost. [75]Besides, as discussed in Section 1, the products generally are diverse (Figure 2).To resolve this problem, and achieve relatively pure and high-concentration products, novel electrolyzer configurations such as three-compartment electrolyzers, [90,91] catalyst-coated membrane electrodes, [92] catholyte-free electrolyzers, [93] and solid-state reactors, [17,94] have been designed.
It has been well documented in the literature that the concentration of formic acid and formate in CO 2 RR is influenced by the choice of electrolyzers.The use of GDEs electrolyzers in CO 2 RR has shown excellent conversion reactivity (current densities close to 1A cm −2 ). [63,86,95,96]Because the gaseous feeds can directly contact with electrocatalysts and improve the mass transfer, leading to higher local CO 2 concentrations on the electrocatalyst layer. [17]However, carbonate precipitation on GDEs leading to electrolyte flooding into GDEs often occurs in CO 2 RR, which reduces the GDEs' lifespan and performance stability. [63,70,71,97]To solve these issues, Yang et al. [90] designed a three-compartment electrochemical cell configuration to directly produce pure formic acid.They modified the GDE structure with an anion exchange membrane to operate the system without an electrolyte to avoid flooding and control the pH between 7-11 to suppress the hydrogen evolution.The designed electrolyzer was composed of three parts (as shown in Figure 5a): a cathode compartment using GDE with a Sustainion™ membrane (pH of 7-11), a center flow compartment with a strong acid environment (pH of 1-5) and inputting deionized water (DI) stream; an anode compartment operating with DI water feed and cation exchange membrane.By designing the three-compartment electrochemical cell configuration, the cell voltage was considerably stable (3.3-3.4V for 500 h), and the concentration of formic acid product could reach a range of 15-18 wt.% with a FE of about 30%.Further studies were done by the same group in 2020, which remarkably improved the stability of the reaction and the yield of a formic acid product via the three-compartment design electrolyzer. [91]They examined the CO 2 RR performance to produce formic acid under different reaction conditions in a three-compartment electrochemical cell.The electrolyzer was reacted at different current densities: 100, 200, and 250 mA cm −2 , as shown in Figure 5b.The results demonstrated that the concentration of formic acid increased with current densities, such as 1.5 M formic acid at 100 mA cm −2 (3.23 V), 2.7 M formic acid at 200 mA cm −2 (3.52 V), and 2.9 M formic acid at 250 mA cm −2 (3.76 V) while FE stayed steady about 73%.Then, they changed the rate of DI water flow in the center compartment and found that it can significantly affect the concentration of formic acid (Figure 5c).Lastly, long-term tests were carried out and the electrolyzer presented 1000 h stability at 200 mA cm −2 with formic acid FE above 70% and 11 wt.% (~2.4 M) concentration, as can be seen in Figure 5d.
The membrane, which can separate anodes and cathodes in electrolytes, is an important component in CO 2 RR electrolyzers since formate can be oxidized in the anode. [98]According to a different ion transport pathway between the anode and cathode sides of the electrolyzer, membranes in CO 2 RR reactor systems can be divided into two categories in terms of their ionic permeability: cation exchange membrane (CEM), which cations, such as protons or other positively charged ions in the anolyte can be transported; anion exchange membrane (AEM), which anions in the catholyte, such as OH − or other negatively charged ions can be transported. [57]Designing electrolyte-less systems with suitable membranes (CEM or AEM) can effectively avoid the limited solubility of CO 2 and reduce the cost of downstream separation of formic acid/formate.It is a good strategy to gain high-concentration formic acid/formate.Diaz-Sainz et al. developed Sn catalyst coating membrane electrodes with a continuous filter-press cell, which worked as a solid electrolyte to avoid using liquid catholyte. [92]They achieved a much higher formate concentration of 19.2 g L −1 compared with their former results of Sn-GDE configuration (2.5 g L −1 ). [92,99]Furthermore, Lee et al. reported a facile method of a catholyte-free electrolyzer (shown in Figure 6a) to resolve the issue of low CO 2 aqueous solubility. [93]They replaced the catholyte with a CO 2 -saturated thin liquid film on the catalyst surface.Sufficient dissolved CO 2 could be supplied to the cathodic reaction system by using the "catholyte-free" method, which remarkably improved the formate concentration.The results demonstrated a high formate concentration of 41.5 g L −1 with a FE of 93.3% at 2.2 V (Figure 6b).
In contrast to a common liquid electrolyte, using a solid-state electrolyte (SSE) for CO 2 RR helps to gain pure products without mixing with the electrolyte.Xia et al. [94] designed a solid electrolyte CO 2 RR device (Figure 6c).They used catalyst-coated gas diffusion layer (GDL) electrodes in the cell cathode and anode, which are separated by anion and cation exchange membranes.The SSE in the middle is either an anion or cation conductor.The HCOOH was formed via the anioncation recombination of solid ion-conducting electrolytes in between and spread out through deionized water/gas flow.Using a 2D Bi catalyst at the cathode for CO 2 -to-HCOOH conversion, the concentration of pure HCOOH product could be up to 12 M with a formate FE over 90%.At the same time, the reaction was sustained for 100 h continuous operation with over 80% FE and a stable generation of 0.11 M HCOOH with negligible degradation (Figure 6d).There are various solid electrolytes that SSE can be applied in other liquid products and other electrocatalyst reduction reactions.Therefore, further increasing the ion conductivity and stability in solid-state devices will significantly improve the concentration of the targeted product in the CO 2 RR process.
The above-mentioned solid electrolyte layer still works with liquid/ vapor water due to the use of the DI water flow stream and liquid electrolyte on the anode side. [94]This was resolved by using N 2 vapor flow instead of deionized water or vapor in the solid-state reactors. [17]The products can be effectively removed via an inert gas stream without any downstream separation or purification processes.For example, Fan et al. reported all-solid-state electrochemical reactors to convert CO 2 into formic acid in the form of vapor. [17]In their study, an all-solidstate reactor was designed to convert CO 2 to formic acid vapor, which was fed by an H 2 gas stream for the oxidation reaction on the anode side and used N 2 gas flow to carry away produced formic acid vapor.A high concentration of formic acid was gained through the coldcondensation process and the N 2 stream was recycled, as shown in Figure 7a.According to the results of the all-solid-state reactor (Figure 7b), the conversion of CO 2 to formic acid in CO 2 RR has shown outstanding characteristics.The ultra-high concentration of HCOOH (nearly 100 wt.%) was obtained, which presented slightly higher than the commercial product of >96 wt.%. [17]It is so far the best-reported performance of converting CO 2 to formic acid in electrochemical CO 2 reduction.
On the other hand, there is an essential yet overlooked issue that can greatly limit the concentration of formic acid/formate in CO 2 RR: massive carbon losses from carbonate crossover.During CO 2 RR, a large number of hydroxide ions generated at the cathode-electrolyte interface rapidly reacted with CO 2 to form carbonate or bicarbonate. [100,101]hen they migrate to the anode side of the reactor through the cathode-anode interface driven by the electric field and reform CO 2 by reacting with the locally generated protons from the oxygen evolution process (Figure 7c). [100]The produced CO 2 is mixed with the oxygen from oxygen evolution; therefore, it cannot be recycled directly, leading to a large carbon loss and reducing the overall energy efficiency in CO 2 RR.Kim et al. [100] demonstrated an efficient recovery of crossover Figure 5. a) 3-compartment designed electrolyzer composed of three parts: a cathode compartment using GDE structure with a Sustainion™ membrane and controlling the PH of 7-11; a center flow compartment with strong acid cation exchange media (PH of 1-5) and deionized water stream; an anode compartment operating with deionized water feed and cation membrane.Reproduced with permission. [90]Copyright 2017, Elsevier Ltd. b) The targeted product of FE and concentration at different current densities: 100, 200, and 250 mA cm −2 ; c) the targeted product FE and concentration under different center compartment DI water flow rate at 200 mA cm −2 , and d) electrolyzer voltage and the concentration and FE under operating 1000 h.Reproduced with permission. [91]Copyright 2020, Elsevier Ltd.
CO 2 in CO 2 RR via using the PSE reactor, which could gain a highconcentration product in the conversion of CO 2 to formate. [17]They used the silver nanowire electrocatalyst as a model study.The results showed that more than 90% of the crossover CO 2 gas was consistently recovered with more than 99% O 2 purity on the anode side.Meanwhile, CO 2 RR maintained catalytic performance with over 90% CO selectivity and showed excellent stability, which was operated continuously for 750 h without obvious change in CO selectivity and the recovered CO 2 remained relatively constant (Figure 7d).Besides, the authors also demonstrated that this strategy for crossover CO 2 recovery could be applicated to other electrocatalysts of CO 2 RR and desired products.This successful strategy recovered the lost CO 2 gas during CO 2 RR while maintaining high catalytic performance.It provides a promising strategy to obtain high-concentration products via designing a PSE reactor to improve CO 2 utilization efficiency and improving energy efficiency.
Having a solid-state electrolyte device in CO 2 RR is a promising and prospective strategy to obtain high-concentration formic acid/formate as it eliminates the mixture of the liquid products with the electrolyte.Following the same mechanism, other liquid products also could be produced with high purity by changing the type of catalysts.Further work will be focused on the designing of electrocatalysts for different value-added fuel products. [17,94]Moreover, all-solid-state devices could be applied in high-pressure systems owing to the absence of electrolytes and it provides a potential route to transform CO 2 into chemicals and fuels in large-scale industrial production. [79]Therefore, improving the stability of solid-state devices and the ion conductivity will be focused on further studies as it can effectively increase the energy efficiency of CO 2 RR. [94]

Electrode Design to Increase the Concentration of Formic Acid/Formate
[104] Several excellent reviews give detailed summaries of electrodes in CO 2 RR in recent years, focusing on advances in types of electrocatalysts, [105,106] reaction mechanisms [50,107] and targeted products (e.g., C 2+ ) [108,109] via comparing their FE, energy efficiency, current densities as well as the potential onset.However, less effort has been devoted to discussing the high concentration of the targeted product in CO 2 RR.Hence, we will not introduce the details about reaction mechanisms and parameters of electrocatalysts for CO 2 RR here.Although few reports have been focused on the studies of high-concentration products (formic acid/formate) in electrode electrocatalysts of CO 2 RR, we will review especially electrode design strategies that produce highconcentration of formic acid/formate via increasing FE, current densities, and keeping stable long-term operation simultaneously.
According to the structure of electrode electrocatalysts, there are two types of electrodes in CO 2 RR: planar electrode and gas diffusion electrode (GDE), as shown in Figure 8. Planar electrodes, like conventional electrochemical reaction electrodes, usually are coated with electrocatalysts on the carbon surface and immersed in the electrolyte (Figure 8a). [110]The selectivity and reactivity of CO 2 RR can be improved by designing types of electrocatalyst materials and modifying Figure 6.a) Schematic illustration of the catholyte-free CO 2 RR system for formate synthesis via forming CO 2 -saturated thin liquid film on the electrocatalyst surface and supplying water/CO 2 vapor stream; b) The electrolytic performance of CF-CO 2 R compared with the traditional electrochemical CO 2 reduction using liquid KCl catholyte, including the FE of formate, partial current densities of formate and formate concentration.Reproduced with permission. [93]Copyright 2018, Wiley-VCH.c) Schematic illustration of the CO 2 reduction electrolyzer with a solid electrolyte.Electrodes were catalyst-coated GDL separately by AEM and CEM; SSE in the middle as an anion or cation conductor (based on the type of ion-conducting polymers' functional groups), the product was formed via H +conductor and HCOO − -conductor recombination in between and spread out through deionized water/ gas flow; d) The stability of electrochemical CO 2 reduction to pure HCOOH by using a solid electrolyte.The reaction could continuously operate for 100 h with a FE of over 80% and stable generate 0.11 M HCOOH with negligible degradation.Reproduced with permission. [94]Copyright 2019, Springer Nature Ltd.
Energy Environ.Mater.2023, 6, e12596 their structure on planar electrodes to enhance the concentration of a targeted product.On the other hand, GDEs have a highly porous structure by which CO 2 can be directly fed into the reactor and react with electrocatalysts (Figure 8b,c). [111]GDEs can effectively reduce the mass transport constraints of CO 2 and provide sufficient CO 2 in the vicinity of the catalyst layer, which is crucial to enhancing the reaction rate and gaining a high-concentration-targeted product.Here, we will review the state-of-the-art design of planar electrodes and GDEs for potential high-concentration formic acid/formate in CO 2 RR.

Planar Electrode
The electrode as an indispensable component of electrochemistry plays a vital role in CO 2 RR.In general, CO 2 is dissolved in the aqueous electrolyte and then reduced on the surface of electrocatalysts in a planar electrode (Figure 8a). [112]Owing to the high dependence on the local environment near the gas-solid-liquid electrode interface, designing the electrocatalyst morphology and interface on the surface of electrodes presents an effective strategy to increase the concentration of products. [72]Various strategies have been used by fabricating different electrocatalysts to achieve high conversion rates and energy efficiencies in CO 2 RR.Here, we particularly set our focus on reviewing promising strategies in designing electrocatalysts on planar electrodes to improve the concentration of formic acid/formate.
In general, one of the most critical factors determining the final product in CO 2 RR is the type of electrocatalysts, different mental electrocatalysts result in various products.An efficient electrocatalyst corresponding to the targeted reaction pathway and product could effectively improve the practicality of CO 2 RR.However, most monometallic electrocatalysts have their limitations, for example, Au, Zn, and Ag tend to convert CO 2 to CO instead of liquid products. [113]Pd, In, and Pb have shown high selectivity for the formation of formate.However, Pd is expensive while In and Pb are toxic and not friendly to the environment. [114,115]Bi and Sn show good performance in gaining high-yield formate products in CO 2 RR, which are summarized in Table 2.An et al. [35] provided a comprehensive overview of recent advances in Bi and Sn-based electrocatalysts in CO 2 RR to formic acid/ formate.However, it is still far from satisfactory for commercial largescale applications of CO 2 RR. [116,117]Hence, designing bimetallic electrocatalysts provides a favorable strategy to integrate the advantages of different properties of metals. [117]It can gain an economically highconcentration chemical with high selectivity, high current density and long-term stability at low overpotential in CO 2 RR for future practical application.
In addition, it has been shown that metal-metal bifunctional interfaces could stabilize the intermediate of *OCHO more than a monometallic surface shown in Figure 9a, which is the key step for formate production, as they contain more low-coordinated active sites. [118,119]ence, designing bimetallic or multicomponent electrocatalysts is an effective and promising strategy for converting CO 2 to formic acid/formate.Combining binary components and utilizing their synergistic effect to tune the electrocatalytic geometric and electronic structure can effectively perform the conversion of CO 2 to formic acid/formate with high concentration simultaneously.Wen et al. synthesized a bimetallic Bi-Sn electrocatalyst by decorating Sn nanosheets with Bi nanoparticles Comparison between the all-solid-state reactor and other works.Reproduced with permission. [17]Copyright 2020, Springer Nature.c) Schematic of the PSE reactor for CO 2 RR and the process of crossover CO 2 recovery.d) The stability test of crossover CO 2 recovery in CO 2 RR.Reproduced with permission. [100]opyright 2022, Springer Nature Limited.
Energy Environ.Mater.2023, 6, e12596 on carbon fiber (CF) (Figure 9b) for transforming CO 2 into formate in CO 2 RR. [115]There have abundant active sites through the orbital interaction of Bi-Sn and SnO and have a high electronegativity of Bi with facile electron transfer from Sn to Bi.Therefore, the interface of the Bi-Sn bimetallic electrocatalyst was inclined to adsorb *OCHO intermediate and facilitated the selectivity and stability of production for formate over CO and H 2 .The FE of formate reached 96% at −1.14 V versus RHE and the reaction rate of the product was 0.74 mmol h −1 cm −2 .Besides, the Bi-Sn bimetallic electrocatalyst maintained its initial activity over 100 h of continuous operation with unchanged formate current density and FE, as shown in Figure 9c.
The electrocatalyst type is a significant parameter to obtain a high-concentration product in CO 2 RR for a large-scale CO 2 reduction system.As low-cost and earth-abundant electrocatalysts, Cu and Cubased, are usually limited in their practical application owing to producing a variety of hydrocarbon products. [5,18]If we can improve the selectivity of Cu-based electrocatalysts, it can greatly reduce the cost of CO 2 electrochemical reactions and improve the industrial application of carbon dioxide reduction.The designing of binary Cu-based catalysts is a good strategy to increase the selectivity of Cu catalysts, as introducing different metals could modulate the electronic structure and stable key reaction intermediates. [36]For example, Peng et al. [117] synthesized a Bi-Cu bimetallic electrocatalyst on the defective copper foam by a simple electrochemical deposition for CO 2 RR to formate.The structure of the Bi-Cu bimetallic electrode could effectively enhance the adsorption of the OCHO* intermediate owing to the synergistic effect of Bi and Cu atoms.It remarkably increased the selectivity of the production of formate over other hydrocarbons, moreover, its durability could reach 58 h with a FE of 94.37%.
Combined with the strategy we discussed in part 2.1, solid electrolytes displayed an excellent performance for obtaining the highconcentration formic acid in CO 2 RR.Hence, designing a metallic electrocatalyst and then using it in the SSE system is another effective strategy to gain high-concentration products.Zheng et al. [18] synthesized a single-atom Pb-alloyed Cu electrocatalyst (Pb 1 Cu), shown in Figure 10a, to convert CO 2 into formic acid efficiently.In their study, the critical step was to control the formation of the intermediate of *HCOO through precise electronic/geometric manipulation using the single metal atom Pb on the metal Cu electrocatalyst.It could promote the carbon protonation of CO 2 to form *HCOO intermediate, which is the main path toward formic acid, as shown in Figure 2, and suppressed the oxygen protonation of CO 2 to form other C 1 and C 2+ products.The electrocatalyst of Pb 1 Cu exhibited excellent performance in CO 2 RR to form formic acid and the maximal FE could reach 96% with a partial current density of −800 mA cm −2 at −0.8 V versus RHE in 0.5 M KHCO 3 electrolyte.Then, they replaced the liquid electrolyte with a proton-conducting solid electrolyte to avoid the influence of the mixture.The peak FE could reach 94% with a partial current density of 375 mA cm −2 at −3.86 V, corresponding to a 0.16 M pure HCOOH solution.Meanwhile, the Pb 1 Cu electrocatalyst showed much more long-term stability than before, as shown in Figure 10b,c.The reaction could continuously operate over 180 h at 3.45 V (FE > 85%, current density: 100 mA cm −2 ), whereas, with the liquid electrolyte, the stability was just 22 h (FE around 90%, current density: 500 mA cm −2 ).Recently, Wang et al. [72] designed a facile synthesis of carbon-confined indium oxides (In 2 O 3 @C) electrocatalysts for converting CO 2 into formic acid by introducing SSE configurations.It also showed high selectivity and activity for gaining pure formic acid solution (0.12 M).This outstanding strategy provides a direction for future studies to parallel design high-efficient electrocatalysts and advanced electrolyzers, which can obtain high-concentration products in CO 2 RR.
The strategy of designing bimetallic electrocatalysts on planar electrodes can achieve high-concentration products and keep long-term stability.This strategy has shown high utilization value in other products in CO 2 RR.122] The maximum durability in this CO 2 RR system could reach 150 h (Figure 11b).They put ZnO and Ag phases into an individual ultrafine nanoparticle and then injected them inside nanopores of ultrahigh-surface-area carbon nanospheres.The designed structure of the electrocatalyst could enhance the stability of the *COOH intermediate favorable for CO production, owing to the electron delocalization between ZnO and Ag, resulting in electron density reconfiguration.Meanwhile, it could promote CO production and suppress HCOOH generation by limiting the rate step of forming HCOO* in a high thermodynamic barrier.The results demonstrated that the designed Zn-Ag-O catalyst realized high energy efficiency of 60.9% with a FE of up to 98.1% toward CO.Therefore, although the designing of planar electrode strategies focused on the production of formic acid/formate in this review, they could apply to other chemicals preparation in CO 2 RR.

Gas Diffusion Electrodes (GDEs)
Although the above strategies have demonstrated the potential of gaining a high concentration of formic acid/formate via modifying Reproduced with permission. [112]Copyright 2018, American Chemical Society.electrocatalysts on planar electrodes for CO 2 RR, the poor solubility and mass transport of CO 2 in aqueous electrolytes limit the conversion efficiency. [123]This can be circumvented by the design of GDEs, shown in Figure 8b,c.Gas diffusion electrodes (GDEs) can be distinguished from the conventional planar electrodes by the contact mode of CO 2 and electrolyte: with a planar electrode, CO 2 is dissolved in the bulk electrolyte, which is flowing by the surface of the electrodes, while GDE allows CO 2 to pass through a porous gas diffusion layer to the electrode interface.Typically, the cathode is deposited a porous electrocatalyst on the gas diffusion layer in a GDE.GDEs possess a highly porous structure that can control the mass transfer of feed CO 2 , electrolytes and products continuously to and from the electrocatalyst layer.It plays an important role in controlling the local environment around the electrocatalysts, shown in Figure 12. [73,124] Meanwhile, GDEs increase the electrocatalyst surface areas and intensify the contact between the gas, liquid (electrolyte) and solid (electrocatalyst) phases. [123]Thus, The threedimensional structure of GDEs is easy to modify the microenvironment and improve the current densities and FE in CO 2 RR, which provides a promising strategy to gain high-concentration formic acid/formate.As we discussed in Section 2.1, the solid-state reactors with highconcentration formate have utilized GDEs. [17,94]Meanwhile, Rabiee et al. and Wakerley et al. have reviewed the recent development of GDEs in CO 2 RR comprehensively. [63,73]he ultimate property of using a GDE in CO 2 RR is that modulates types of electrocatalysts on catalyst layers and the structure of GDEs.For example, Merino-Garcia et al. [125] designed a SnO 2 -based electrocatalyst GDEs to continuously convert CO 2 into formate via CO 2 single-pass electrochemical conversion.The GDEs allowed the feed gaseous directly to the interface of the gas diffusion cathode, which reacted with SnO 2 -based electrocatalysts.The formate concentration value could reach 27 g L −1 (0.6 M) with a FE of 44.9% at 300 mA cm −2 .Besides, Liu et al. [126] combined nano SnO-modified CuO foam with a carbon-based GDE to convert CO 2 into formate.The results showed ultrahigh rate formate production.The current density could increase to −1152 mA cm −2 at −1.2 V versus RHE in 1 M KOH.The FE of formate could reach ~99% at 0.6 V versus RHE.As the homogeneously modified Sn atoms suppressed the generation of H2 and hydrophobic carbon nanoparticles on the GDE penetrated the microporous structure of the Cu foam increased the reaction area in the three-phase interface. [126]Recently, an ultrahigh current density of 2.0 A cm −2 with a formate FE of 93% at −0.95 V versus RHE has been achieved by Lin et al. [127] They designed a Bi 2 S 3 -derived electrocatalyst via a solvothermal method loaded on carbon paper as GDE in a flow cell for CO 2 RR.The stability of formatting formate could maintain 100 h with an FE > 90% at an industrial-relevant current density of 250 mA cm −2 .Meanwhile, the author employed this electrocatalyst in a solid electrolyte cell, the concentration of formic acid was 3.5 M with an FE of 93% and a partial current of 1.1 A at 4.2 V. [127] Apart from the electrocatalyst layer, the structure design of GDEs also is a promising strategy to gain a high yield of formic acid/formate.A promising configuration of GDEs is hollow fiber (microtubular), which has natural structural advantages in CO 2 RR for its large area and low gaseous mass transfer resistance. [128]Herein, we briefly review hollow  the key pathway for the formation of formate production.Reproduced with permission. [119]Copyright 2019, American Chemical Society.b) a bimetallic Bi-Sn electrocatalyst by introducing Bi nanoparticles into Sn nanosheets on CF; c) the stability of the Bi-Sn bimetallic electrocatalyst and the performance of their partial current density and the FE during 100 h operation at −1.14 V. Reproduced with permission. [115]Copyright 2018, Wiley-VCH.and the proton-conducting solid electrolyte, respectively.Reproduced with permission. [18]Copyright 2021, Springer Natural Limited.
Energy Environ.Mater.2023, 6, e12596 formation and flooding is crucial.137] 3. Perspectives and Recommendations for Future Work Converting CO 2 into high-added-value chemicals via electrocatalyst CO 2 RR is an effective method to store renewable energy and implement sustainable chemical manufacturing. [141,142]Most attention in current is being paid to highly efficient electrocatalysts and the study of their reduction performance in CO 2 RR, including selectivity and activity.Nevertheless, producing high-concentration products through CO 2 RR is a significant parameter but is often overlooked for economic feasibility and industrial application, which is an efficient way to decrease the cost during the postpurification process.
According to an analysis of the cost of formic acid/formate separation in CO 2 RR, high concentrations of products can minimize downstream separation costs. [75]Lowconcentration-targeted product streams after CO 2 RR will require high-cost for gas-gas, gasliquid, or liquid-liquid separation processes.Thus, it is essential to obtain a high concentration of the targeted product in an early reaction step in CO 2 RR from an economic point of view.Converting CO 2 into formic acid/formate has shown attractive application prospects for large-scale industrialization.Therefore, in this review, we outlined some advanced strategies for CO 2 RR for gaining high-concentration formic acid/formate.
In a typical electrochemical CO 2 RR reactor, the CO 2 is reduced at the cathode chamber and water is oxidized at the anode chamber.The two chambers are separated by an ionexchange membrane, which allows ion transport. [143]Electrolyte-less systems generally use an ion-exchange membrane as the electrolyte, and the CO 2 can be reacted with no aqueous electrolyte.It reduces the influence of gaseous mass transport and reaction rate limitation in an aqueous electrolyte.Furthermore, it can reduce the cost of product separation.The electrolyte as an ions transport media can be replaced by ion-exchange membranes in a CO 2 RR electrolyzer, through which the type of ions passing can be controlled.On the other hand, it is a good idea to design a membrane-less system, which may effectively reduce the electrolyzer's overall cost for fewer components.Membrane-less reactors can facilitate the coupling of cathode and anode products directly.It is anticipated that membrane-less electrolyzers will have a great influence on the behavior of CO 2 RR.A series of new electrolyzers may be an additional booster to realize industrial application, in which the concentration of products increases while the overall costs decrease.
Changing the structure of reactors and designing special electrodes can effectively increase the concentration of formic acid/formate in CO 2 RR.Compared with the existing system, using a solid-state device in CO 2 RR showed excellent performance in gaining highconcentration products.Especially, the all-solid-state reactor exhibits perfect performance in CO 2 RR to generate high-concentration products, which reach nearly 100 wt.% of the formate.They can gain a  [120] Copyright 2021, American Chemical Society.
Energy Environ.Mater.2023, 6, e12596 higher concentration of product and higher pure fuels with a lower cost than conventional electrolyzers because it reduces the product's downstream separation and purification processes.However, there are still challenges, for instance, the membrane and solid electrolyte may gradually degrade under dry conditions.Therefore, future research can improve the performance of ion-exchange membranes under low water conditions, improve the capability of electrocatalysts, elevate operation temperature and the stability of solid electrolytes, and reduce product crossover.
There is a limited amount of literature on the study of gaining high-concentration products in CO 2 RR.As the product in CO 2 RR is generally formed in low concentration and mixed with by-products in the actual reaction system.Gaining a targeted product with high concentration and easily separating from downstream is still a challenge for large-scale applications.For example, the solid-state reactor is not suitable for all high-concentration products in CO 2 RR.However, most studies in current research have been focused on increasing the reactive rate, selectivity and stability of electrocatalysts in CO 2 RR.Those strategies will be devoted to the development of gaining high-concentration products for CO 2 RR in future.Therefore, the designing of electrodes is a promising strategy to gain a high concentration of products in CO 2 RR.Utilizing the difference in metal electron structure to design bimetallic electrocatalyst electrodes can effectively control the key intermediate of the targeted product and change the reaction pathway to obtain the highly concentrated product.Besides, it is a good method to directly use electrocatalysts as reaction electrodes such as Cu hollow fiber GDEs, which can effectively reduce the mass transport constraints and increase reaction active sites.Future progress in electrodes for CO 2 RR to highconcentration products can be focused on improving the selectivity and reactive sites with high stability.
In general, the increase in product concentration is desired to reduce the cost of the CO 2 RR downstream separation processes.In addition, the species of other by-products should also be considered as they may determine the separation efficiency and downstream separation process complexity.For instance, even though the high-concentration liquid product (formic acid/formate) can be obtained, other liquid product azeotropes were observed. [75,144]In addition, the capital and operating cost analysis of electrolyzers also need to be carried out by considering the reactor design and material selection, such as the electrolyzer design, electrodes and membranes.Therefore, to control the product separation cost, more factors need to consider, and the separation process needs to be customized based on the product composition.
This review expected that the design of electrolyzers and electrode strategies could inspire other researchers' interest in further gaining high concentrations of targeted products in CO 2 RR.Further, these strategies would enable extended to other liquid and even gaseous chemicals to produce various valueadded products.shows the performance of Cu hollow fiber in CO 2 RR compared with Cu foil and Cu foam, including the current density, faradaic efficiency and formate yield.The results demonstrated that Cu HF has better reactivity as electrons and catalysts than the other two.The formate yield increased obviously, over 2500 μmol h −1 cm −2 .Reproduced with permission. [129]Copyright 2021, Elsevier B.V.

Figure 1 .
Figure 1.Schematic illustration of the CO 2 RR technology to value-added fuels and chemical feedstocks and achieve a carbon-neutral energy cycle.

Figure 2 .
Figure 2. Schematic illustration of reaction mechanisms for Electrochemical CO 2 reduction.Various pathways are shown for CO 2 RR to form formate, CO, hydrocarbons and alcohols, based on different electron transfer and intermediates.[44,47,48]Copyright 2015 (2019), American Chemical Society.

Figure 3 .
Figure 3. Main challenges for the application of electrochemical CO 2 reduction.

Figure 4 .
Figure4.a) Operating costs in CO 2 RR for formic acid (dotted line).Reproduced with permission.[76]Copyright 2018, American Chemical Society; b) The separation energy inputs required for different concentrations of target products including CH 4 and C 2 H 5 OH.Reproduced with permission.[77]Copyright 2018, Elsevier Inc. and the separation cost of different concentrations of formic acid.Reproduced with permission.[75]Copyright 2019, American Chemical Society.

Figure 7 .
Figure7.a) Schematic illustration of an all-solid-state electrochemical CO 2 RR electrolyzer: the cathode/anode coated different selective catalysts with AEM/ CEM and were separated by a porous solid electrolyte (PSE).The CO 2 was reduced to formate ions via a particular catalyst in the cathode phase, while the anode phase was fed in an H 2 gas stream to release protons.Then the formate ions and protons were driven by the electric field across AEM or CEM into the PSE layer and directly recombined to form a formic acid molecule, finally removed out of the reactor into the product-collector via N 2 vapor flow; b) Comparison between the all-solid-state reactor and other works.Reproduced with permission.[17]Copyright 2020, Springer Nature.c) Schematic of the PSE reactor for CO 2 RR and the process of crossover CO 2 recovery.d) The stability test of crossover CO 2 recovery in CO 2 RR.Reproduced with permission.[100]Copyright 2022, Springer Nature Limited.

Figure 8 .
Figure 8. Schematic illustration of different types of CO 2 RR electrodes: a) planar electrode; b) gas diffusion electrode.Reproduced with permission.[112]Copyright 2018, American Chemical Society.

Figure 9 .
Figure9.a) The mechanism includes the different pathways to produce CO and formate in CO 2 RR, and a schematic illustration of the intermediate of *OCHO on three different metal surfaces, which is the key pathway for the formation of formate production.Reproduced with permission.[119]Copyright 2019, American Chemical Society.b) a bimetallic Bi-Sn electrocatalyst by introducing Bi nanoparticles into Sn nanosheets on CF; c) the stability of the Bi-Sn bimetallic electrocatalyst and the performance of their partial current density and the FE during 100 h operation at −1.14 V. Reproduced with permission.[115]Copyright 2018, Wiley-VCH.

Figure 10 .
Figure10.a) Schematic illustration of the mechanism of converting CO 2 into formic acid using a Pb 1 Cu electrocatalyst; b,c) the stability of converting CO 2 into HCOOH in a 0.5 M KHCO 3 electrolyte and the proton-conducting solid electrolyte, respectively.Reproduced with permission.[18]Copyright 2021, Springer Natural Limited.

Figure 11 .
Figure 11.a) Schematic illustration for designing "two ships in a bottle"; b) Comparison of the stability of different samples.Reproduced with permission.[120]Copyright 2021, American Chemical Society.

Figure 13 .
Figure 13.a) Schematic illustration of the reaction mechanism for Cu hollow fiber in CO 2 RR; b)shows the performance of Cu hollow fiber in CO 2 RR compared with Cu foil and Cu foam, including the current density, faradaic efficiency and formate yield.The results demonstrated that Cu HF has better reactivity as electrons and catalysts than the other two.The formate yield increased obviously, over 2500 μmol h −1 cm −2 .Reproduced with permission.[129]Copyright 2021, Elsevier B.V.

Table 1 .
List of the most commonly electrochemical CO 2 RR products with equilibrium standard potential in the aqueous electrolyte at pH 7, 1 atm, 25 °C and 1 M solute concentration.

Table 2 .
Summary of strategies to increase the concentration of formic acid/formate in CO 2 RR and their mechanisms.