Direct carbonate electrolysis into pure syngas †

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Introduction
Syngas is a commodity feedstock used in the production of hydrocarbons and oxygenates via methanol routes and Fischer-Tropsch synthesis. 1,2Syngas is currently produced from fossilfuels via coal gasification and/or methane reforming. 3,4These pathways are energy intensive and have high CO 2 emission intensities (1.5 tCO 2 e tsyngas À1 ). 5 Electrochemical syngas production methods use renewable electricity to produce syngas with a lower carbon footprint.8][19] As a result, both the anodic and the cathodic outlet streams require CO 2 separation. 20,21eactive capture is an electrolysis approach that shortens this process (Fig. 1A).3][24][25][26] In the electrolyzer, protons are generated by the anodic oxygen evolution reaction (OER) and transported to the cathode through a cation exchange membrane (CEM) (Fig. 1B).The protons then react with the carbonate ions to regenerate CO 2 in situ.Syngas is produced through co-synthesis of CO from the regenerated CO 2 and H 2 from the aqueous solution.Unreacted CO 2 is recaptured by hydroxide ions (OH À ), a by-product of CO and H 2 evolution, to form carbonate.
Previous studies of reactive capture using bicarbonate electrolyte have demonstrated high CO selectivity (CO faradaic efficiency (FE) 4 50%).7][28] Direct electrolysis of carbonate (rather than bicarbonate) electrolyte allows for the collection of high-purity gaseous products, evidenced by the lack of CO 2 (o400 ppm) detected in the gas stream. 29Carbonate electrocatalytic conversion into syngas has previously been achieved with a H 2 /CO ratio of 3 (CO FE of 25%) and energy intensity of 86 GJ tsyngas À1 .][32][33][34] Here, we present an adlayer strategy that modulates the cathode pH and maximizes CO 2 conversion to produce syngas with a H 2 /CO ratio in the industrially relevant range (1-2, and corresponding to a CO FE between 33% to 50%).We develop a composite CO 2 diffusion layer (CDL) that enables cathode alkalinity to favour CO 2 electrolysis and increases CO selectivity by limiting the diffusion of protons to the cathode.We achieve a H 2 /CO ratio of 1.16 (CO FE of 46%) at a current density of 200 mA cm À2 .An energy intensity of 52 GJ tsyngas À1 was achieved, resulting in a 39% energy saving compared to the previous carbonate electrolysis report.

Results & discussion
Increasing CO 2 conversion Previous (bi)carbonate electrolyzers have employed a bipolar membrane (BPM) to dissociate water and provide protons and OH À to the cathode and anode, respectively. 26,29,35To reduce the membrane overvoltage, thereby reducing the required energy input, we used a cation exchange membrane (CEM) to transport protons directly from the anolyte or anodic OER, resulting in a voltage reduction of ca.0.5 V (Fig. S1, ESI †).A zero-gap configuration was first investigated by assembling the electrolyzer with an Ag electrocatalyst directly in contact with the CEM.We found that the maximum FE towards CO was 17% (H 2 /CO ratio of 4.88) at 100 mA cm À2 (Fig. 2A).The only other carbon-based product detected was methane with o0.2% FE.Hydrogen evolution reaction (HER) accounted for the remaining FE.
][41] We confirmed that CO 2 was the electroreduction reactant, as opposed to the (bi)carbonate ions, by replacing the acidic anolyte with an alkaline electrolyte to suppress in situ CO 2 regeneration.The only product detected was H 2 (Fig. S2, ESI †).To increase CO 2 conversion, we hypothesized that a CO 2 diffusion adlayer between the CEM and the catalyst would limit proton diffusion to the cathode and separate the acidic CO 2 regeneration region from the alkaline CO 2 electrolysis region.
We developed multi-physics models of the carbonate electrolyzer with varying CDL thicknesses of 0 (zero-gap configuration), 10, 25, and 50 mm (Supplementary note 2, ESI †).We found that increasing the CDL thickness increased the pH at the cathode which favoured CO 2 reduction over HER (Fig. 2B). 42,43However, increasing the CDL thickness also reduced the CO 2 concentration at the cathode due to the recapturing of in situ CO 2 within the extended alkaline region (Fig. 2C).The CDL must achieve high local cathode alkalinity and CO 2 concentration to produce syngas with sufficient CO content for direct industrial application.

CDL design strategy
We first inserted commercially available hydrophilic microporous filters of different thicknesses between the catalyst and the CEM to separate the acidic CO 2 regeneration region from the alkaline electrolysis region.We found that the filters increased the selectivity towards CO (Fig. S6, ESI †).However, the performance of the inserted filters was inconsistent due to trapped CO 2 bubbles and material incompatibilities with the high and low pH extremes in this system (11 o pH o 2).We were motivated to design a robust and tuneable composite CDL.The engineered CDL needed to facilitate (bi)carbonate diffusion, hinder proton transport to the catalyst, sustain both high and low pH conditions, and have porous networks that allow mass transfer of in situ generated CO 2 .We selected TiO 2 particles as the main substrate in view of their chemical stability and hydrophilic nature. 44,45For a substrate binder, we chose a hydrophilic ionomer that is anionpermeable to transport (bi)carbonate ions to the CEM. 46,47The wettability of the substrate and ionomer is important because hydrophobic elements will hinder ion transport, limiting the availability of carbonate ions in the CO 2 regeneration region. 48hen a hydrophobic substrate or ionomer was incorporated into the CDL, the CO FE was lower than the fully hydrophilic system (Fig. S7A and B, ESI †).The TiO 2 and ionomer mixture was airbrushed evenly onto an Ag-catalyst layer (Fig. 3A and B) with corresponding energy dispersive X-ray (EDX) spectroscopy images confirming a distinct and uniform CDL (Fig. 3C).
To optimize the CDL for high CO FE, we screened TiO 2 particle sizes (5, 25, 200, and 1500 nm) and TiO 2 /ionomer weight ratios between 5-25 (Fig. S7C and D, ESI †).We found that a combination of 25 nm TiO 2 and a TiO 2 /ionomer ratio of 15 balanced the diffusion of (bi)carbonate ions and protons and enabled the local generation of CO 2 to result in peak CO FE.The size of TiO 2 nanoparticles and the ionomer volume fraction contribute to the permeability of the CDL.A high permeability failed to sufficiently hinder proton transport and resulted in hydrogen generation.A low permeability resulted in insufficient in situ regeneration of reactant CO 2 . 49e varied the CDL thickness between 10 to 50 mm and achieved a maximum CO FE of 46% (H 2 /CO ratio of 1.16) at 200 mA cm À2 with a 25 mm CDL (Fig. 3D).Achieving a high CO FE requires both a sufficiently alkaline local pH and adequate CO 2 availability.Thinner CDLs have a smaller gap between the catalyst and the CEM which shortens the proton diffusion distance and results in a lower cathode pH.Despite having the highest CO 2 concentrations in our simulations, the selectivity of the system with thinner CDLs was not optimal and approached the performance of the zero-gap configuration due to insufficient cathode alkalinity (Fig. 2B).As the CDL thickness is increased, the pH at the cathode increased but the local CO 2 concentration decreased (Fig. 2C).With the cathode pH plateauing at thicknesses greater than 25 mm, this CDL thickness provided sufficient CO 2 availability while maintaining an alkaline cathode environment to suppress HER.
We compared the difference in iR-compensated voltage of a zero-gap configuration to an otherwise identical system with a 25 mm CDL, with both experiments using an identical hydrogen-evolving catalyst (Supplementary note 3, ESI †).At 200 mA cm À2 , a voltage increase of 117 mV was observed for the CDL system which corresponded to an increase in pH of 2, consistent with the multi-physics model which predicted a pH increase of 1.7 (Fig. 2B and Fig. S8, ESI †).These findings suggest the CDL increases the pH of the cathode environment to favour CO 2 conversion and thereby yielded a higher CO FE.
We screened the selectivity and full cell voltage of the optimized 25 mm CDL at current densities between 50 to  S5).Error bars represent the standard deviation of at least three samples measured under identical conditions.
300 mA cm À2 and found that the CDL resulted in minimal voltage penalties while significantly increasing CO FE compared to the zero-gap configuration (+0.23 V and +35.3% CO FE at 200 mA cm À2 ) (Fig. 3E).Above 200 mA cm À2 , the CO selectivity decreased, and the CO partial current plateaued, which indicated a CO 2 mass transfer limit at the cathode (Fig. S9, ESI †).We hypothesized that the decrease in CO FE at the lower and higher current densities were due to an imbalance of cathode alkalinity CO 2 concentration at these extremes.The multi-physics model showed that compared to operating at 200 mA cm À2 , the pH is lower at 50 mA cm À2 , while the CO 2 concentration is lower at 300 mA cm À2 (Fig. 3F and G).
To investigate further, we decreased the carbonate flow rate while operating at 50 mA cm À2 and found that the CO FE increased (Fig. 3H).A slower flowrate increases the local pH due to the accumulation of OH À .At 300 mA cm À2 , the CO FE increased as the carbonate electrolyte flowrate increased up to 30 mL min À1 ; however, further increases in flowrate resulted in similar, or slightly decreased, CO FE (Fig. 3I).This result suggests that at high current densities, the local environment is excessively alkaline, and CO 2 availability is low.Increasing the carbonate electrolyte flowrate lowered the local pH and thereby increased the availability of CO 2 for reaction.
To assess the long-term stability of the engineered CDL, we operated the DCE system with continuous CO 2 capture and Fig. 4 Long-term operation of DCE over 23 hours with CO FE, H 2 /CO ratio, full cell voltage, and capture solution pH noted.Experiment conducted at a constant current density of 100 mA cm À2 with a 1 cm 2 active area.Schematic and picture of the experimental set-up are provided in the ESI † (Fig. S10A and B).S3).(E) CO 2 e emissions to produce one tonne syngas using rWGS, CE-WE, and DCE.The CO 2 e associated with the energy input in each process is considered.Detailed breakdown available in the ESI † (Table S4).

Comparison to alternative syngas production methods
We compared the energy intensity of syngas production via three CO 2 electrolysis pathways: thermocatalytic reverse water gas shift (rWGS) (Fig. 5A); low-temperature CO 2 electrolysis combined with water electrolysis (CE-WE) (Fig. 5B); and DCE (Fig. 5C) (Supplementary note 4, ESI †).In all cases, the feedstock CO 2 is captured from the atmosphere using an alkaline capture liquid. 13The rWGS pathway produces syngas with H 2 / CO ratio of 1 and requires 57 GJ tsyngas À1 (Fig. 5D).The CE-WE and DCE pathways produce syngas with H 2 /CO of 1.16, requiring 60 and 52 GJ tsyngas À1 , respectively.Syngas production via DCE circumvents intensive upstream and downstream processes and thereby results in a 13% energy saving compared to the CE-WE pathway and 8% energy savings compared to the rWGS method.The capture and dehydration steps in the DCE required only 0.85% of the energy demand for the DCE pathway, because the most energy intensive step of the capture process (regeneration) is avoided, and the output syngas stream is pure.
Of the three syngas production methods, DCE is the only pathway that is fully electrically driven, whereas CE-WE requires thermal energy input during CO 2 capture and rWGS requires thermal energy in two major processes.Comparing the operational CO 2 e emissions of the three syngas production pathways, DCE was the only method that offered a low CO 2 intensity (0.36 tCO 2 e tsyngas À1 ), while both CE-WE and rWGS exhibited high net CO 2 emissions (1.39 and 2.48 tCO 2 e tsyngas À1 , respectively) even when using renewable electricity (Fig. 5E).Compared to the fossil-based method (1.5 tCO 2 e tsyngas À1 ), DCE offers a 75% reduction in CO 2 e emissions.

Conclusions
We developed a strategy for the efficient electroproduction of syngas with sufficient CO-content and purity for direct industrial application.Through a one-dimensional multi-physics model, we found that the selectivity towards CO can be improved by separating the acidic CO 2 regeneration region from the alkaline CO 2 electrolysis region.We engineered a composite CDL positioned between the cathode and CEM of a DCE system to modulate the local pH and improve local CO 2 conversion.The CDL was comprised of TiO 2 nanoparticles bound by hydrophilic ionomer and was conformally coated onto the Ag catalyst.By determining the optimal CDL thickness that balanced the cathode alkalinity with CO 2 concentration, a H 2 /CO ratio of 1.16 (CO FE 46%) was achieved with an energy intensity of 52 GJ tsyngas À1 and a CO 2 intensity of 0.36 tCO 2 e tsyngas À1 .Syngas production via DCE can provide a 13% energy saving compared to conventional CO 2 electrolysis methods, and a 75% CO 2 e emissions reduction compared to fossil-fuel methods.These savings suggest that DCE is a promising pathway toward energy efficient syngas -a foundational feedstock for renewable chemicals and fuels in a net-zero emissions future.

Reagents
Potassium hydroxide (KOH) (485%) and sulfuric acid (H 2 SO 4 ) were purchased from Bioshop.All reagents were of analytical grade and used without further purification.All solutions were prepared using Milli-Q grade water (18.2MO).

Electrode preparation
The cathode was fabricated by air-brushing an Ag nanoparticle ink onto commercially available hydrophilic carbon paper (AvCarb MGL190, Fuel Cell Store) on a hot plate at 75 1C to achieve a loading of approximately 3 mg cm À2 .The loading was measured by weighing the carbon paper before and after air-brushing.The catalyst ink was prepared with 150 mg of Ag nanoparticle (99.99%, 20 nm, metal basis, US Research Nanomaterials), 150 mg of Nafion dispersion (5 wt%, Fuel Cell Store), and 6 mL of methanol for a 25 cm 2 substrate and sonicated for 1 hour prior to air-brushing.A commercially available titanium-based anode was used (Magneto Special Anodes, Evoqua Water Technologies).
The CDL was fabricated by air-brushing a TiO 2 nanoparticle ink onto the fabricated cathode to achieve the desired thickness.For the optimized CDL, the ink was prepared with 50 mg of 25 nm TiO 2 , 333 mg of the prepared Aemion dispersion, and 4 mL of ethanol for a 6.25 cm 2 cathode and was sonicated for 1 hour prior to air-brushing.The CDL coated cathode was cut to a 1 cm 2 size prior to electrolyzer assembly.CDLs were characterized using SEM at the Centre for Nanostructure Imaging at the University of Toronto using an FEI Quanta FEG 250 environmental SEM.
Two hydrophilic microporous membrane filters were used: 125 mm PVDF (Filter 1, 0.45 mm pore size) was purchased from Sigma Aldrich and 100 mm nylon (Filter 2, 5 mm pore size) was purchased from Sterlitech.Both filters were used as received.

Operation of the electrochemical cell
The carbonate electrolysis experiments were performed in a 1 cm 2 electrolyzer with serpentine flow channels ingrained in both the stainless-steel cathode and the titanium anode current collectors.The was assembled by placing a CEM (Nafion 117) over the cathode, then placing the anode on the membrane.In all experiments, unless otherwise specified, the cathode feedstock was a carbonate electrolyte prepared by purging CO 2 at 80 sccm into 85 mL of 2 M KOH for 40 minutes, similar to a previous report. 29The anode was fed with 0.05 M H 2 SO 4 .After starting the experiment, the carbonate electrolyte was continuously purged with Ar gas flowing at 20 mL min À1 .The first gas sample is typically collected 20 minutes after the start of the experiment to ensure complete purging of excess CO 2 from the electrolyte preparation process and even mixing of gaseous products.The electrochemical measurements were performed with a potentiostat (Autolab PGSTAT204) and the full cell voltages reported are not iR compensated unless otherwise specified.

Product analysis
The cathode gas outlet stream was analyzed in 1 mL sample volumes by a gas chromatograph (PerkinElmer Clarus 590) coupled with a thermos conductivity detector (TCD) and flame ionization detector (FID).The gas chromatograph used Ar gas as the carrier (99.999%,Linde) and was equipped with a Molecular Sieve 5A Capillary Column and a packed Carboxen-1000 Column.Liquid product detection was performed using proton nuclear magnetic resonance spectroscopy ( 1 H NMR) on an Agilent DD2 600 spectrometer in D 2 O using water suppression mode, with dimethyl sulfoxide (DMSO) as the internal standard.Calculation of FE and energy efficiency are included in Supplementary note 1 in the ESI.† pH measurements were conducted using Apera Instruments AI311 Premium Series PH60.

Fig. 1
Fig. 1 Direct carbonate electrolysis to produce syngas enabled by a CDL.The chemical balance of carbon capture from air, anodic OER, CO 2 regeneration, CO evolution reaction, H 2 evolution reaction, and in situ CO 2 recapture by OH À are presented in eqn (1)-(6), respectively.(A) Schematic of DCE integrated with carbon capture in a reactive capture system.(B) Schematic of the conversion of carbonate into CO 2 facilitated by the CDL and protons, and subsequent conversion of CO 2 into CO via electroreduction on the Ag catalyst.Unreacted CO 2 is recaptured by OH À generated as a byproduct of CO and H 2 evolution.The carbonate electrolyte is recirculated.

Fig. 2
Fig. 2 CDL thickness modulates cathode pH and CO 2 concentration.(A) FE towards CO and H 2 in a zero-gap configuration at current densities between 50 to 200 mA cm À2 .Corresponding full cell voltages are noted on the secondary y-axis.Error bars represent the standard deviation of at least three samples measured under identical conditions.(B) One-dimensional multi-physics modelling of pH at distances from the cathode and current density of 200 mA cm À2 for CDL with thickness of 0 (zero-gap), 10, 25, and 50 mm.(C) One-dimensional multi-physics modelling of CO 2 concentration at distances from the cathode and current density of 200 mA cm À2 for CDL with thickness of 0 (zero-gap), 10, 25, and 50 mm.Accompanying models of HCO 3 À and CO 3 2À concentrations are provided in the ESI † (Fig. S4).

Fig. 3
Fig. 3 Optimization of the CDL for industrial H 2 /CO ratio.(A-C) Cross-sectional scanning electron microscopy (SEM) image of the CDL evenly airbrushed onto the Ag catalyst atop a silicon wafer (A), with seamless interfacial contact between the CDL and Ag catalyst (B), and corresponding energy dispersive X-ray (EDX) spectroscopy elemental mapping of Ti, O, and Ag (C).(D) FE towards CO and H 2 /CO ratio at 200 mA cm À2 with thicknesses of the CDL between 0 and 50 mm.(E) FE towards CO and H 2 at current densities between 50 to 300 mA cm À2 with 25 mm CDL.Corresponding full cell voltages are noted on the secondary y-axis.(F) One-dimensional multi-physics modelling of pH at distances from the cathode and current densities of 50, 200, and 300 mA cm À2 for 25 mm CDL.(G) One-dimensional multi-physics modelling of CO 2 concentration at distances from the cathode and current densities of 50, 200, and 300 mA cm À2 for 25 mm CDL.Accompanying models of HCO 3 À and CO 3 2À concentrations are provided in the ESI † (Fig. S5).(H) FE towards CO and H 2 /CO ratio at 50 mA cm À2 with carbonate electrolyte flowrates between 0.35 to 17.5 mL min À1 showing an increase in CO FE with decreasing flowrate.(I) FE towards CO and H 2 /CO ratio at 300 mA cm À2 with carbonate electrolyte flowrates between 10 to 65 mL min À1 showing an increase in CO FE with increasing flowrate.Typical carbonate electrolyte flowrates are provided in the ESI † (TableS5).Error bars represent the standard deviation of at least three samples measured under identical conditions.

Fig. 5
Fig. 5 Comparison of three syngas production methods.(A-C) Schematic showing process pathways, major chemical inputs and outputs, and energy source of rWGS (A), CE-WE (B), and DCE (C).(D) Energy intensity comparison to produce one tonne syngas using rWGS, CE-WE, and DCE.Syngas dehydration energies for CE-WE and DCE are too small to be seen in this figure.Detailed breakdown available in the ESI † (TableS3).(E) CO 2 e emissions to produce one tonne syngas using rWGS, CE-WE, and DCE.The CO 2 e associated with the energy input in each process is considered.Detailed breakdown available in the ESI † (TableS4).