Investigation of Bimetallic silver-copper BTC MOF for electrochemical CO 2 reduction in Zero-gap Membrane electrode assembly (MEA) configuration

Bimetallic Metal-Organic Frameworks (MOFs) of silver, copper and the ligand benzene 1,3,5-tricarboxylate (AgCu-BTC MOFs), derived from Cu-BTC (HKUST-1), have been synthesized by fast co-precipitation method and investigated for CO 2 reduction reaction (CO 2 RR). Three AgCu-BTC MOF variants were synthesized with varying Ag content: AgCu-1 (9.4 at.%), AgCu-2 (12.5 at.%), and AgCu-3 (16.5 at.%). A range of structural characterization techniques, including SEM-EDS, XRD, FTIR, and XPS, were utilized, revealing the formation of low-crystalline AgCu-BTC MOF with Ag + 1 in an ionic state coordinated to the BTC framework. The investigation focused on CO 2 reduction using humidified CO 2 gas with bimetallic AgCu MOFs as electrocatalysts in a zero-gap MEA setup. The setup included a gas diffusion electrode (GDE) with a Sustainion Anion exchange membrane and bicarbonate as the anolyte. Cyclic Voltammetry (CV) and Linear Sweep Voltammetry (LSV) showed that the AgCu-3 MOF, with the highest silver content (16.5 at.%), exhibited a lower onset potential at -0.65 V vs Ag/AgCl compared to pristine Cu-BTC MOF owing to the better activity with Ag inclusion. Constant potential (CP) experiments combined with product analysis indicated that AgCu-3 MOF predominantly produced CO and H 2 as the main products

• The presence of Ag in the BTC framework improved CO2 reduction activity and suppressed H2 evolution.
• Adjusting the relative humidity of the CO2 gas stream can improve the selectivity of CO formation over H2 during CO2 reduction.

Introduction
The goal of net-zero emissions by 2050 to tackle climate change needs major innovation efforts to provide conceivable technologies for carbon capture and utilization.One ambitious challenge involves the direct conversion of atmospheric carbon dioxide into valuable chemical products using renewable sources [1].Electrochemical conversion of CO2 stands out as a compelling alternative due to its scalability to industrial levels compared to other techniques such as thermal catalysis [2,3], microbial conversion [4,5], and photocatalysis [6,7].
Effective CO2 electrochemical conversion, however, faces limitations related to the activity and selectivity of the electrocatalyst.Extensive research has been dedicated to understanding the CO2RR mechanisms and developing novel materials to enhance activity and selectivity [8].
Interestingly, mechanistic studies have shown that introducing a second metal to form a heterometallic BTC MOF can significantly enhance CO2 adsorption due to the change in electrostatic interaction strength by altering the metal atom.[38].This has been experimentally shown by Perfecto et al., who introduced Zn(II), Ru(III), and Pd(II) as doping metals in the pristine HKUST-1.Notably, electrocatalysts based on Ru(III) bimetallic MOF achieved FE as high as 47.2% for methanol.
Besides Cu-BTC MOF, Ag-BTC MOF was reported by Shuang et al. and was employed as an efficient catalyst with 95% FE for CO2 reduction to CO in an H-cell setup.The reduced activation energy for CO-forming intermediates on Ag-BTC MOF is responsible for the enhanced activity and selectivity for CO production [32].Moreover, by combining Ag and Cu metals, AgCu binary alloys have exhibited remarkable catalysis for ethanol production due to their synergistic effect and capacity to engage in tandem reactions [39].In this work, silver-copper AgCu-BTC MOF was successfully synthesized, and their electrocatalytic CO2 reduction performance was explored.
Electrocatalytic evaluations of MOFs were generally conducted using H-cell or gas-feed flow-type electrolyzers, where the MOF electrodes were immersed in liquid electrolytes for catalytic reactions [40,41].The Zero-gap MEA configuration has recently garnered attention for CO2 electrolysis [42].Unlike traditional H-cell setups, this configuration reduces the inter-distance between the gas diffusion electrode (GDE) and membrane, thus reducing the overall resistance and can be operated with a humidified CO2 gas stream rather than having an electrolyte at the cathode side.Furthermore, it effectively mitigates mass transport limitations, allowing for the attainment of current densities exceeding 100 mA cm -2 .This setup facilitates the investigation of (1) MOF's selectivity for specific targeted CO2 reduction products over hydrogen formation, (2) the potential influence of the absence of an aqueous electrolyte in the MEA configuration on the stability of MOF materials, and (3) the impact of variations in humidity levels in the CO2 gas stream on the selectivity of CO2 reduction products.The zero-gap MEA configuration offers a unique opportunity to observe how MOFs perform with gas-phase CO2 reduction in this setup.
In this work, the synthesis, characterization and electrochemical testing of bimetallic AgCu-BTC MOFs with varying concentrations of Ag at.% aimed to clarify the role of silver in the BTC structure and investigated if the bimetallic Ag-Cu MOF could promote the coupling of C-C bonding, enable the selective synthesis of hydrocarbons, and suppress the competing hydrogen evolution.Further, electrochemical studies were conducted to understand the activity and selectivity of the AgCu-BTC MOF catalyst towards CO2RR.The electrochemical studies were performed using a tailor-made gas-diffusion electrode half-cell setup (GDE half-cell) with bimetallic AgCu MOF-based electrode and anion exchange membrane (Sustainion), for product quantification and assess the role of humidity on the competing reactions between HER and CO2RR.Finally, the morphological changes of the AgCu-BTC MOF electrode during CO2RR in the gas phase and under humid conditions have been investigated by SEM-EDS and FTIR.Gaining insights into the performance of MOF-based electrodes tested in an MEA configuration is crucial for further developing scalable and efficient electrocatalysts for the direct electrochemical reduction of CO2.

Chemicals and Materials
All chemicals were of analytical grade and used without any further purification.Deionized water

Synthesis of bimetallic AgCu-BTC MOF
The AgCu-BTC MOF was synthesized at room temperature via a fast co-precipitation method in a procedure similar to HKUST-1 synthesis [43].The atomic ratio of Ag and Cu was adjusted by modulating the precursor molar concentration during synthesis, as detailed in Table S1.Typically, several g of AgNO3 and Cu(NO3)2.3H2O, depending upon the desired atomic ratio, was mixed in 75 ml of methanol using a magnetic stirrer.H3BTC were added to 75 ml of methanol to prepare the ligand solution and stirred with 10 ml of diethylamine for 10 minutes.Subsequently, the completely dissolved metal solution is mixed with ligand solution to form AgCu-BTC MOF immediately.The precipitate obtained is stirred for 15 minutes and centrifuged at 4500 rpm to collect the product.The product is repeatedly washed with ethanol to remove unreacted ligands.
Finally, the product is collected in a petridish and dried in a vacuum oven at 323 K for 12 h.The resulting samples were named AgCu-1, AgCu-2, and AgCu-3 MOFs.

Analytical techniques
The morphology of the AgCu-BTC MOF samples was analyzed by Scanning Electron Microscopy (SEM), employing the Zeiss Merlin FEG-SEM operated at 2 kV-15 kV.The same SEM instrument conducted an Energy Dispersive Spectroscopy (EDS) analysis to characterize the element distribution and ratio in the samples.The MOF sample's powder X-ray diffraction (PXRD) pattern was obtained using the Rigaku Miniflex instrument equipped with a Cu-Kα radiation source.The scanning range spanned from 5 to 60 degrees with a step size of 0.02 degrees.Fourier transform infrared (FTIR) spectra were recorded using a Perkin Elmer Spectrum Two FTiR spectrometer.
For the assessment of the electronic states of elements within the synthesized AgCu-BTC MOF material, X-ray photoelectron spectroscopy (XPS) was performed utilizing the ESCALAB 250Xi by Thermo-scientific XPS instrument.Agilent 5977B GC/MSD instrument was employed, featuring Thermal Conductivity Detector (TCD) and Mass Spectrometry (MS) detectors for both analysis and quantification of gaseous products.

Preparation of Bimetallic MOF electrode for CO2 reduction
The bimetallic Ag-Cu BTC MOF catalyst ink was prepared by mixing the synthesized MOF material with a 5 wt.% Nafion dispersion as an ionomer and ethanol as a solvent.This mixture was placed in a glass vial and subjected to bath ultrasonication at 350 W for 30 minutes to achieve a homogeneous slurry.Subsequently, the catalyst ink was manually coated onto H23C2 carbon paper with 5 cm x 5 cm size by airbrush spraying.During the airbrush coating process, the carbon paper was mounted onto a hot plate set at 80°C.Finally, a catalyst loading of approximately 1.0 mg cm -2 on GDE was achieved for all the samples.This study employed a GDE half-cell to assess the electrochemical activity of the synthesized bimetallic AgCu-BTC MOF.This setup enables the testing of catalysts in zero-gap MEA configuration, replicating realistic mass transport conditions, and facilitates conventional threeelectrode experiments [44], [45].The cell assembly comprises a stainless steel bottom body with a flow-field design.At the same time, the top part is composed of Teflon, and it features a 3 mm diameter hole designed to accommodate the electrolyte, reference electrode, and counter electrode, as illustrated in Fig. 1(a).The gas diffusion layer coated with the catalyst ink was punched into a disk with a 3mm diameter and placed on an uncoated carbon paper with a punch-out hole of the same 3mm diameter.Subsequently, a 20 mm diameter sustainion membrane was cold-pressed onto the GDE, forming the MEA.This step ensured proper adhesion between the electrode and membrane, reduced contact resistance, and prevented bulging due to flooding during operation.

Assembly of GDE half-cell setup
Finally, the upper Teflon part was clamped together with the bottom part, with the membraneelectrode assembly positioned in between.The anolyte compartment was filled with a 0.5 M KHCO3 electrolyte, allowing the MEA contact with the electrolyte.A leak-free Ag/AgCl electrode, which was regularly tested for potential drift against a master reference electrode (saturated calomel electrode (SCE)), was utilized as the reference electrode for all measurements, while a Pt wire counter electrode was housed within a glass frit tube to prevent oxygen contamination of the anolyte.During the measurements, a CO2 stream with varied water content was continuously purged into the GDE half-cell at a flow rate of 20 mL/min.The outlet of the GDE-cell was directly connected to the GC/MS system to detect the products formed during the CO2 reduction process.

Electrochemical measurements
In all electrochemical measurements, a Gamry Reference 3000 potentiostat was used.To ensure a proper assembly of the GDE half-cells, impedance measurements at high frequencies were conducted before each electrochemical measurement, consistently resulting in a resistance below 100 Ohms.The electrochemical behavior of the synthesized bimetallic AgCu-BTC MOF and HKUST-1 was investigated using cyclic voltammetry at a scan rate of 50 mV s -1 under dry and humid conditions.Additionally, linear sweep voltammograms (LSV) were recorded at a scan rate of 20 mV s -1 to obtain polarization curves for screening the catalyst with a low onset potential.
Constant Potential (CP) experiments were conducted for an hour at different applied potentials in a humid CO2 environment to observe products from CO2RR.For comparison, HKUST-1 was also tested using the GDE half-cell for product analysis under similar conditions.In addition, constant current (CC) experiments were performed at 100 mA cm -2 under different %RH of CO2 gas stream to understand the influence of water on the distribution of CO2R products.
Moreover, electrochemical surface area (ECSA) investigations were carried out to study the degradation of the MOF under electrochemical conditions.Cyclic voltammograms were obtained at several scan rates (200, 150, 100, 50, and 25 mV s −1 ) before and after a one-hour constant potential (CP) experiment at -1.7 V vs Ag/AgCl.In addition, by plotting a peak current in the nonfaradaic region Δj/2 (Δj = janodic − jcathodic) versus scan rate, the capacitance Cdl is calculated from the slope of the straight line.Freshly prepared anolyte and catalyst-coated electrodes were used for each new experiment unless otherwise specified.Equations for calculating FE and reaction rates can be found in the Supplementary Information (SI).

Structural characterization of AgCu-BTC MOF
AgCu-BTC MOF with different ionic radii has been synthesized to harness the synergistic effects of two dissimilar metals.The actual loading of the metal ions in the final AgCu-BTC MOF structures differs from the initially introduced silver precursor concentration and depends on the metal ions' reaction kinetics with the ligand [46].This difference in the Ag to Cu ratio in the starting materials and the products is shown in Table S1.octahedral structure of HKUST-1 (Fig. S2) to irregularly shaped AgCu-BTC MOF particles was due to the use of the deprotonating agent, which, in turn, facilitates the formation of combined Ag and Cu BTC framework.In Fig. S3, the EDS images of AgCu-BTC MOFs reveal a uniform distribution of silver throughout the observed area in all samples.However, as the Ag content rises from AgCu-1 to AgCu-3, EDS reveals the formation of small, nanosized Ag clusters.This ought due to the formation of localized Ag-BTC with less Cu in those areas.As shown in Table S1, the increase in the atomic weight percentage of silver is inversely related to the copper precursor content.Here, the AgCu-3 MOF has the highest Ag content among other MOF samples, around 16.5 at.%, and the rest is Cu in the MOF structure.
The PXRD patterns of the AgCu-BTC MOFs exhibited broad peaks, signifying a reduction in crystallinity, as shown in Fig. 3(a).These observed patterns have some similarity to the PXRD This is consistent with findings from a previous study by Kwang Soo et al., where a bimetallic Ag-Cu BTC MOF, synthesized through a different route, displayed comparable initial peaks.In their study, they observed Ag nanoparticles formed within the MOF structure.This occurrence was attributed to the use of DMF as a stronger reducing agent, which reduces the silver ions to nanoparticles during the synthesis process [49].However, in this work, the typical XRD peaks for Ag nanoparticles were not observed in the Ag-Cu MOF prepared in this work.This indicates that the fast crystallization method can prevent the formation of Ag nanoparticles.Specific peaks at 2θ values 7° and 14° for HKUST-1 were not observed in the AgCu-BTC MOF patterns of AgCu-MOF 1 to 3. The absence of these peaks can be attributed to the formation of AgCu-BTC MOF structures characterized by low crystallinity.This phenomenon is a consequence of the fast nucleation process adopted during the synthesis.Moreover, the defects resulting from the low crystalline AgCu-BTC framework may enhance the electrochemical CO2R activity of the catalyst by providing additional active sites and promoting improved charge transfer dynamics [50].
FTIR analysis was conducted to further analyze the structure of the AgCu-BTC MOF (Fig. 3(b)).
The FTIR spectrum reveals that the AgCu-BTC MOFs do not exhibit any broad peak in the 3100-3700 cm -1 range, which is typically associated with adsorbed H2O [35].Additionally, characteristic peaks corresponding to the C-H bending, C-O stretch and -C=O (symmetric and asymmetric bands) vibrations of the BTC ligand were observed at approximately 729 cm -1 , 1114 cm -1 and 1300-1700 cm -1 , respectively, similar to the HKUST-1 structure [37,51].The peaks from 1000-1250 cm -1 generally correspond to the stretching of C-O bonds [52].An additional vibration peak at 1050 cm -1 emerged for AgCu-BTC MOFs, attributed to the C-O binding with Ag ions, for AgCu-2 and AgCu-3 MOFs (with higher loading of Ag at.%) [52,53].Furthermore, the stretching vibrations of the Ag-O and Cu-O bonds with carboxylate groups were identified at 480 cm -1 [54,55].Cu +2 in coordination with BTC ligand as reported for HKUST-1 [35].The electronic state of Cu is predominantly in the +2 state, leading to additional satellite peaks at 939.5 eV, 943.5 eV and 962.6 eV.In Fig. 4(d), characteristic peaks of Ag +1 valence states are observed at 368.3 eV (Ag + 3d5/2) and 374.2 eV (Ag + 3d3/2) for all AgCu-BTC MOFs, which were in agreement with PXRD and FTIR that silver is in Ag +1 ionic state in the BTC framework rather than being present as a nanoparticles [49].to Cu 0 state transitions, respectively [57,58].Under the humid condition, the cathodic peak intensity for Cu +2 to Cu +1 was still visible, while the other transition to the 0 state was suppressed.

Electrochemical CO2 reduction using AgCu-BTC MOF
This behavior is attributed to the adsorption of water at the catalyst site in the potential range (-0.3 V to -0.5V), allowing the competitive hydrogen evolution reaction (HER) to occur [57].
The reduction peaks corresponding to Ag +1 to Ag 0 usually happen at -0.19 V, thus overlapping with the Cu +1 to Cu 0 transition [59].The oxidation peak currents under dry CO2 condition for all MOFs were seen at 0.22 V, owing to the desorption of CO2 reduction intermediate [60].However, these oxidation peaks shifted to lower potentials (from 0.22 V to 0.08 V) under humid CO2 for MOFs with low Ag content (HKUST-1 and AgCu-1 MOF).This peak shift could be related to Cu/Ag oxidation rather than the adsorption of CO2 reduction intermediates on the MOFs.
Meanwhile, the oxidation peaks for AgCu-2 and AgCu-3 MOF remain at 0.22 V, thus being attributed to CO2 intermediates on these MOF catalysts.
In addition, in Fig. 5(b), the onset of hydrogen evolution reaction (HER) is observed with the increase of current density in negative potentials (< -0.2 V vs Ag/AgCl) for all MOFs [61].
HKUST-1 MOF exhibits the higher current density amongst all MOFs, showing a preference for hydrogen evolution reaction.However, the current density for HER is progressively reduced from AgCu-1 to AgCu-3 MOF samples, implying that the AgCu-3 MOF favors CO2 reduction and suppresses the HER reaction.
Linear sweep voltammetry (LSV) conducted in the potential range from -0.2 to -1.8 V vs Ag/AgCl under humid CO2 condition for all MOFs were shown in Fig. 6.LSV results show a decrease in onset potential for the reduction reaction with increasing Ag content in the MOF.The AgCu-3 MOF with the highest Ag content has a low onset potential of about -0.6 V vs Ag/AgCl, comparable to the literature value [35].This is attributed to higher silver content in AgCu-3 MOF involving product analysis with a gas chromatograph at regular intervals to observe the selectivity of the product with respect to the applied potential under humid CO2 (80% RH) condition.The primary products observed were CO and H2 (syngas), and their FE at different potentials are depicted in Fig. 7(a).The maximum FE (CO + H2) obtained is around 70% at -1.8 V vs RHE potential with AgCu-3 MOF, as seen in Fig. 7(a).FE values for hydrogen were reduced to 5% at high negative potentials, and the catalyst was more selective towards CO production.A notable portion of the current density employed in the reaction seems to be consumed by other parasitic reactions, such as MOF degradation and salt formation, rather than contributing to the desired product formation [62].Thus, the total FE value in our system never reached 100%.However, in Fig. S5(a), it was observed that in addition to the increase in the rate of CO production with higher applied negative potentials, the hydrogen production rate also increased due to the excess availability of water at the catalyst site during these experiments with 80% RH CO2 gas stream.
Consistent with the CV observations, HKUST-1 generated more H2 compared to the AgCu-3 MOF.Interestingly, only syngas (CO + H2) were observed when the catalyst contained copper and silver.Even with HKUST-1, no other gaseous products like ethylene were identified in the zerogap MEA configuration.This phenomenon could be attributed to the local pH changes (high >9 pH under these conditions) in the zero-gap MEA configuration as observed for other design configurations, which significantly influences intermediates that determine product formation [63,64].

Influence of water on product formation
AgCu-3 MOF catalyst was tested under various humidity levels in a constant current experiment at 100 mA/cm², as shown in Fig. 7(b).The results revealed an increase in CO production rates with decreasing humidity from only 6.2 µmol s -1 cm -2 at 80% RH to 13 µmol s -1 cm -2 at 20% RH.
Moreover, the decrease in water content in the gas stream also suppresses H2, thereby increasing the local concentration of CO2 and leading to an enhanced CO2RR rate.This approach to reducing the RH of the CO2 gas stream is beneficial at the system level to improve the CO2RR selectivity.
Moreover, in both CP and CC experiments, no visible liquid droplets were observed coming from the cell; thus, no liquid products were reported.showing that during electrochemical reactions, OH − ions produced rapidly displace the coordinated linkers away from the metal nodes, forming Co-hydroxides [65].This is considered the underlying cause of AgCu-3 MOF catalyst's rapid morphological and structural changes.

Conclusion
This study explores the synthesis and electrocatalytic performance of an AgCu-BTC MOF in an MEA configuration using a GDE half-cell.Three AgCu-BTC MOFs with increasing silver content were synthesized and characterized.Despite the MOF's nanosized, less crystalline structure with ionic silver, AgCu-3 exhibited a lower onset potential at -0.65 V vs Ag/AgCl than other catalysts and HKUST-1 in LSV.CO and H2 were primary reduction products, achieving a total faradaic efficiency of nearly 70%.AgCu-3 demonstrated higher CO production (65% FE) and suppressed hydrogen evolution compared to HKUST-1.
Additionally, water plays a crucial role in product distribution; when reducing the relative humidity of the CO2 gas stream to 20%, AgCu-3 MOF doubled the rate of CO production and suppressed hydrogen evolution.A post-electrochemical experiment investigation confirms that the AgCu-3 MOF experienced structural degradation under electrochemical reduction due to their chemical instability and potential increase in local pH.This work provides insights into MOF electrocatalysis, highlighting the significance of water in both product formation and reaction rates, thereby paving the way for future advancements in the field of MOF electrocatalyst for CO2RR application.

Fig. 1
Fig. 1 Sketch of (a) the GDE half-cell used to study AgCu-BTC MOF and (b) the membraneelectrode assembly (MEA) in contact with the anolyte (0.5M KHCO3).A sustainion membrane and GDL coated with AgCu-BTC MOFs are sandwiched between the Teflon and stainless steel parts.
Typically, when synthesizing MOFs with dissimilar materials like Cu and Ag, which possess different ionic radii (Cu +2 -0.73 Å and Ag +1 -1.15 Å) and binding affinities to the BTC ligand, separate crystalline phases are formed.However, introducing a weak base like diethylamine (with a pKa value of 11.02) could deprotonate the organic linker, leading to an increased nucleation rate during synthesis [47].This facilitates fast nucleation of Cu and Ag metal ions with BTC ligand forming a mixture of Ag-BTC and Cu-BTC networks in the same structure, rather than forming separate MOF phases completely.With increasing Ag precursor concentration, the colors of the sample (as shown in Fig. S1 changed from deep blue (HKUST-1) through turquoise blue (AgCu-1) to teal (AgCu-3).

Fig. 2
Fig. 2 shows the morphologies of the AgCu-BTC MOFs in SEM micrographs, revealing irregular

Fig. 3 (
Fig. 3 (a) PXRD patterns of AgCu-BTC MOFs and HKUST-1 MOF, along with reference patterns (solid lines) [48].(b) FTIR spectra of synthesized MOF particles and HKUST-1 MOF.The purple dotted lines show the location of the characteristic peaks observed for HKUST-1 MOF, and the red dotted line for additional peaks for AgCu-BTC MOFs.

Fig. S4 summarizes
Fig. S4 summarizes all the XPS survey scans of all AgCu-BTC MOFs and confirms the presence

Figure 5
Figure 5 illustrates cyclic voltammetry (CV) using a GDE half-cell in an MEA configuration,

Fig. 5
Fig. 5 CV curves of HKUST-1 and AgCu-BTC MOF catalysts in (a) dry and (b) humid CO2 conditions at a scan rate of 50 mV s −1 and CVs were reported after five scans.In both experiments, 0.5 M KHCO3 was used as anolyte.

Fig. 8 (
Fig. 8 (a) Calculated ECSA values were plotted for AgCu-3 MOF catalyst before and after the CP experiment at -1.7 V vs Ag/AgCl.(b) FTIR of the AgCu-3 MOF catalyst before and after the CP experiment at -1.7 V vs Ag/AgCl for an hour.To assess the chemical stability of the MOF under electrochemical conditions, ECSA and FTIR