Shilei
Zhang
ab,
Hang
Wang
ab,
Yang
Wang
ab,
Min
Zhang
ab,
Weimin
Wang
ab,
Liang
Zhang
ab,
Jun
Li
ab,
Xun
Zhu
ab,
Qian
Fu
*ab and
Qiang
Liao
*ab
aKey Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing 400044, China. E-mail: fuqian@cqu.edu.cn; lqzx@cqu.edu.cn; Fax: +8623 65102474; Tel: +86 23 65102474
bInstitute of Engineering Thermophysics, School of Energy and Power Engineering, Chongqing University, Chongqing 400044, China
First published on 26th August 2025
Elevating the operating pressure could significantly enhance CO2 accessibility on the catalyst in electrochemical CO2 reduction (CO2RR); however, achieving selective production of multi-carbon (C2+) products under pressurized conditions remains a critical challenge because elevated pressure could shift the selectivity towards C1 products. This work systemically investigated the variation of product selectivity induced by pressure and electrolyte composition, and demonstrates that a local pH drop induced by increased pressure is the key reason for hindered C–C coupling, as confirmed by in situ Raman and multi-physics simulation. The elevated pressure results in an increase of the electrolyte's buffering capacity, which neutralizes the alkaline microenvironment near the catalyst, thereby impeding the C–C coupling. Decreasing the buffering ability (using non-buffered KCl electrolyte) or increasing the current density (from 0.4 A cm−2 to 1.0 A cm−2) could maintain a high interfacial alkalinity under elevated pressure, promoting C–C coupling. By mapping pressure–current density operational windows, the optimal conditions were identified, achieving 70% C2 faradaic efficiency (50% for C2H4) at 5 bar and 1 A cm−2, demonstrating superior C2 product selectivity. This work establishes electrolyte engineering principles for industrial CO2RR, enabling carbon-neutral chemical production under practical pressurized conditions through microenvironmental regulation.
Green foundation1. This study presents a pressurized liquid-phase electrolyzer that enables high-rate conversion of CO2 to value-added C2 chemicals under industrial conditions. This technology offers an alternative route for scalable CO2 utilization and facilitates green chemical synthesis by replacing fossil fuel-derived feedstocks.2. The proposed electrolyzer achieves 70% faradaic efficiency (FE) for C2 products at 5 bar, with 50% selectivity towards ethylene. It attains a C2 partial current density of 724 mA cm−2, outperforming ambient-pressure benchmarks by 10 fold. 3. Future work will focus on scaling up the electrolyzers (including their key components) and enhancing the energy conversion efficiency of the proposed system. |
Although some studies applied flowing water to wash the cathode and remove salts from the reaction zone,6,7 this approach substantially increases the operational complexity. On the other hand, directly using cathodes immersed in flowing electrolyte and using dissolved CO2 species as reactants could essentially mitigate salt precipitation issues and avoid the extra washing procedure. The primary challenge stems from the limited accessibility of CO2 in the dissolved phase,8,9 which leads to significant side reactions (HER: 2H2O + 2e− → H2 + 2OH−), particularly at high current densities. Notably, enhancing the partial pressure of CO2 can significantly enhance the CO2 solubility,10,11 increase the CO2 coverage on the catalyst surface,12 and accelerate the reaction kinetics,13 thereby greatly improving the CO2RR performance. For example, Lamaison et al. carried out CO2RR on a Zn–Ag catalyst at 9.5 bar, demonstrating the enhanced mass transfer ability induced by pressure and achieving a selectivity of 72% for CO at a high current density of 0.4 A cm−2.14 In addition, pressurized CO2RR shows high compatibility with industrial CO2 capture processes (e.g., high-pressure water scrubbing, pressure swing adsorption), avoiding the energy-wasteful process of CO2 decompression steps.15,16 Therefore, CO2RR using dissolved CO2 at elevated pressures is a promising approach for industrial-scale applications.
However, the selective synthesis of multi-carbon (C2+) products remains a significant challenge under pressurized conditions. Current studies reveal that Cu-based catalysts under high CO2 pressures exhibit formate-selective behavior due to the CO2 coverage-driven elevation of *OCHO intermediate adsorption energy barriers.17,18 While the majority of research emphasizes intermediate transformations, the variation of electrolyte contents induced by elevated CO2 concentrations (e.g., 1 bar CO2 in pure water, leading to pH ≈ 3.95) and its effect on CO2RR remain underexplored. Maintaining an alkaline interfacial pH (>10) is widely recognized as essential for facilitating C–C coupling via enhanced *CO dimerization.19–21 However, neutral and acidic electrolytes would boost the coupled proton–electron transfer step, enhancing C1 production (e.g., HCOOH, CH4).22–24 Under pressurized conditions, the dissolved CO2 would generate bicarbonate and H+, buffering the generated OH− during CO2RR (e.g., HCO3− + OH− → H2O + CO32−), creating an unfavorable microenvironment for C–C coupling.25–27 This unresolved conflict between pressure-enhanced mass transfer and pH-governed selectivity constitutes the key bottleneck for industrial-scale C2+ production.
In this work, we aim to achieve high rates of C2 production with the assistance of elevated pressure, proposing a strategy to minimize the pH decay through electrolyte design and pressure−current adaptation. By comparing a buffering electrolyte (KHCO3) and a non-buffering electrolyte (KCl), we demonstrate the significant influence of the buffering ability of electrolytes on C2 production selectivity. Complemented by multi-physics simulation and operando Raman spectroscopy at a wide range of current densities (0.2–1.0 A cm−2), we validate that C2 production performance is in accordance with the CO2 concentration-dependent modulation of interfacial OH− distribution. Besides, modifications of the applied current density on the interfacial OH− were quantitatively studied. Through optimizing the current density window at elevated pressures, we simultaneously regulated the local CO2 concentration and interfacial pH, achieving enhanced C2H4 current density and selectivity. As a result, the system achieved record high current densities of 554 mA cm−2 for C2H4 and 724 mA cm−2 for total C2 products, exceeding 70% FE for C2 production at 1.0 A cm−2. This work underscores interfacial pH management and microenvironment control as critical factors for pressurized CO2RR toward industrial application.
X-ray diffraction (XRD) was carried out on a BRUKER D8 XRD system (BRUKER Company, Germany) equipped with a Co Kα source over a range from 10° to 90°. The X-ray photoelectron spectroscopy (XPS) results were acquired using a ThermoFisher ESCALAB 250Xi spectrometer (ThermoFisher Scientific, USA).
Liquid MEA tests were carried out in an MEA cell, in which 1 M KOH was used as the anolyte and the catholyte saturated with CO2 was injected into the cathode at a flow rate of 60 mL min−1. Nickel foam was used as the anode. A Nafion 115 membrane (DuPont, USA) was used to separate the chambers. All gas products were analyzed using a gas chromatograph (8890, Agilent Technologies, Santa Clara, CA, USA). The faradaic efficiency was calculated according to eqn (1) taking CO as an example:
![]() | (1) |
In the equation, ICO is the partial current of CO (A), I is the total current obtained at the constant potential tests or the value set for chronopotentiometry in L-MEA tests (A), x is the CO ratio obtained from the gas chromatograph, v is the total gas volume, n is the electron number consumed for every CO2 molecule (n = 2 for CO2 → CO and HCOO−; n = 8 for CO2 → CH4; n = 12 for 2CO2 → C2H4 and C2H5OH), F is the faradaic constant (96485 C mol−1), Vm is the molar volume at 298 K (24.5 L mol−1), t is the test time (s), and Q is the total charge consumed in the test (C).
The liquid products were analyzed using a 1H nuclear magnetic resonance (NMR) spectrometer (Bruker Avance 400 dir) with DMSO (Sigma-Aldrich, ≥99.5%) as an internal standard. The 1H NMR spectra were collected at ambient temperature, operating at 400 MHz for 1H nuclei under water suppression mode. The water signal was suppressed using the Watergate W5 pulse sequence with double gradient echoes. Using this method, we determined the concentration of the liquid products in the catholyte solution and calculated the total FE of liquid products. The FE was calculated according to eqn (2) taking HCOO− as an example:
![]() | (2) |
In the equation, x is the molar concentration of the HCOO− and v indicates the electrolyte volume.
The operando Raman experiment was carried out on a custom-made cell equipped with an optical window made of sapphire glass under a wide range of current densities (0–1 A cm−2). The flow rate and back pressure were set the same as in the CO2RR process.
Previous works demonstrated that halide ions play a significant role in the modification of the catalyst structure and valence state through specific adsorption, which must be carefully considered.23,30 To avoid structure reconstruction differences in different electrolytes, 0.5 M KI was added to the electrolyte in L-MEA tests.31,32 Transmission electron microscopy (TEM) analysis was carried out on the Cu NNA after CO2RR (0.6 A cm−2, 5 bar for 15 min for all post-mortem samples) in KHCO3 and KCl electrolytes. The TEM images (Fig. 1d and Fig. S7a) reveal similar needle-like structures in both samples, with an average diameter of approximately 50 nm at the base and gradually tapering towards the tip. Elemental distribution (Fig. 1e and Fig. S7b) of the reduced Cu NNA demonstrated homogeneous Cu/O distribution across both KHCO3- and KCl-reduced samples. The morphologies and elemental distributions of Cu NNA reduced in the different electrolytes are similar, which ensures that the performance in the two electrolytes will not be affected by structure reconstruction and valence state variation.
X-ray diffraction (XRD) was carried out to identify the species and crystal patterns. All samples delivered strong Cu (PDF#04-0836) peaks (Fig. S8), which represent the backbone of the Cu mesh. After the thermal treatment, Cu(OH)2 (PDF#13-0420) was fully dehydrated to CuO (PDF#45-0937), which mainly consists of CuO (002) and (111) facets (Fig. 1g). The characteristic Cu2O (PDF#05-0667) peak confirms the preservation of Cu+ species in both the KHCO3- and KCl-reduced samples, as evidenced by X-ray diffraction analysis. The X-ray photoelectron spectroscopy (XPS) analyses (Fig. 1h) suggest that the precursor exhibits distinct Cu2+ features with a strong satellite peak and a higher binding energy of Cu 2p. Samples reduced in KCl electrolyte show a slightly higher Cu2+/Cu0/+ ratio than that of Cu NNA reduced in KHCO3 electrolyte, indicating that a slightly more Cu2+ sites are preserved in the samples reduced in KCl electrolyte. The Cu0 and the Cu+ species were identified from the Cu Auger LMM transition spectrum (Fig. 1i). An intense peak for Cu+ species (∼916.6 eV) was detected in CuNNA reduced in KHCO3 electrolyte, whereas a 0.2 eV positive shift (∼916.8 eV) was observed in CuNNA reduced in KCl electrolyte. This Cl−-induced chemical shift suggests potential Cu–Cl coordination.33–35 Given that the additive (KI) could more efficiently stabilize the Cu oxidation state,31,32 the impact of the small amount of Cu–Cl coordination on the catalytic performance is negligible. This comprehensive characterization confirms that the KI-stabilized Cu NNA maintains structural and chemical consistency across electrolytes, enabling isolated study of pH effects without interference from morphological or valence state variations.
Initially, a current density of 0.4 A cm−2 was applied to assess the influence of pressure on the electrochemical performance. Fig. 2b shows the results obtained in the 1.5 M KHCO3 electrolyte (all subsequent experiments using KHCO3 electrolyte were conducted with 1.5 M KHCO3/0.5 M KI unless otherwise specified). Under ambient pressure (1 atm), the HER dominated the reaction and the FE for total CO2RR products was only 13.6%. As the operating pressure increased, the FE for total CO2RR products gradually increased due to the elevated concentration of the reactant (CO2) near the electrode surface.10,17,18 At 4 bar CO2 pressure, the FE for total CO2RR products reached 53.8%. Notably, the FEC2H4 value increases from 5.2% to 19.3%, representing a fourfold increase. However, H2 is still the major product, accounting for approximately 40% at 0.4 A cm−2 and 4 bar.
Motivated by the excellent performance of KCl electrolyte achieved in similar electrochemical systems,36–38 we evaluated the performance using 1.5 M KCl as the non-buffering catholyte (all subsequent experiments using KCl electrolyte were conducted with 1.5 M KCl/0.5 M KI unless otherwise specified). As shown in Fig. 2c, the FE value for total CO2RR products in KCl electrolyte (48.3%) shows a nine-fold increase compared to that in KHCO3 electrolyte (5.2%) under ambient pressure. When the operating pressure was increased to 2 bar, the FECO2RR value in KCl electrolyte significantly increased to 80.8%, and the dominant product shifted from H2 to C2H4 (40%). However, as the pressure further increased from 2 bar to 4 bar, the FEC2H4 started to decline as well as the overall FE for CO2RR in KCl electrolyte. On the other hand, the performance in KHCO3 electrolyte maintained a positive correlation between FECO2RR and pressure (1–4 bar), and KCl exhibited volcano-type dependence peaking at 2 bar. The different trends in buffering electrolyte and non-buffering electrolyte are consistent with the change in electrolyte composition induced by CO2 pressurization, with the acidification induced by pressurization being more severe in non-buffering electrolyte than in buffering electrolyte (Tables S2 and S3).
Furthermore, the product distribution analysis revealed a significant pressure-dependent selectivity shift in KCl electrolyte systems. As illustrated in Fig. 2d, the ratio of C2 production to C1 production (C2/C1) exhibits significant growth from 0.8 (1 bar) to 3.4 (4 bar) under KCl conditions, representing a 325% enhancement. In the KHCO3 systems, the C2/C1 ratio maintains a constant value of 0.9–1.1 across the same pressure range. The selectivity difference in the two electrolytes demonstrates that the intrinsic activity of the catalyst in the two electrolytes undergoes significant transition through intermediate modulation.39,40
The performance enhancement further reveals the modification of Cl− with the intrinsic activity of the Cu catalyst. To gain insights into this mechanism, in situ Raman spectroscopy was carried out to reveal the detailed spectra of intermediate vibrations induced by KCl electrolyte.42 As shown in Fig. 3c, distinct peaks for adsorbed CO (*CO) can be observed in the range 2000–2200 cm−1. Peak fitting and deconvolution at this range yields a low-frequency band (LFB) at ∼2055 cm−1 and a high-frequency band (HFB) at ∼2115 cm−1, which can be attributed to *CO intermediates adsorbed on terrace and step sites, respectively.29,39,43,44 In addition, another broad band at a lower Raman shift of ∼1940 cm−1 was observed, which arises from the interaction between *CO intermediates.39,45
The LFB *CO peak remains near constant as the potential increases, while the HFB *CO peak exhibits potential-dependence. The HFB *CO intensity increases from −0.3 to −0.5 V, indicating its accumulation at lower potentials. At higher potentials (more negative than −0.5 V), the HFB *CO intensity starts to decline, likely due to its consumption in CO–CO coupling and C2+ production. Thus, HFB *CO is the preferred reactive intermediate for C–C coupling. In contrast, LFB *CO occupies Cu surface sites, showing limited contribution to C2+ production. The high percentage of HFB in total *CO, as depicted in Fig. 3d, confirms that KCl electrolyte promotes the formation of active HFB *CO. In KHCO3 electrolyte, however, LFB *CO dominates, increasing the C–C coupling barrier and exacerbating HER side reactions.39 Besides the intermediate change, the bicarbonate/carbonate composition on the electrode is quite different in the two electrolytes, suggesting distinct differences in the local pH of the electrodes.
To accurately access the bicarbonate/carbonate composition in the two electrolytes within L-MEA cells, we conducted operando Raman studies based on a custom-designed MEA cell with an optical window (Fig. 4a). Recent works demonstrate that in situ pH microelectrodes46 and fluorescence47 can obtain an accurate pH near the electrode, yet operando Raman48–50 spectroscopy shows better compatibility with the pressurized MEA. This setup enables the real time acquisition of Cu2O and bicarbonate/carbonate Raman signals under conditions of 4 bar CO2 pressure and 0.2–1.0 A cm−2, as shown in Fig. 4b and c. The obvious signals of Cu2O (417 cm−1) prove that the Cu+ species remains stable during the electrolysis, which could be ascribed to the KI additive. The operando Raman spectroscopy reveals distinct interfacial pH environments in the two electrolytes. In KHCO3 electrolyte, a high bicarbonate concentration on the interface was observed. At current densities exceeding 0.4 A cm−2, a weak peak corresponding to carbonate (v = 1058 cm−1) emerged. However, in KCl electrolyte, carbonate is the main species on the interface with a weak bicarbonate peak at OCP. In KHCO3 electrolyte, the weak carbonate peak indicates a weakly alkaline interface, which is unfavorable for C–C coupling.49 Through ionization equilibrium, it can be deduced that the interface pH is much higher in KCl electrolyte than in KHCO3.
The chemical reactions at the interface, shown in Fig. 4d, are critical for elucidating the local pH dynamics. During the dissolution of CO2, the electrolyte is acidified and the concentration of bicarbonate increases (Eq1). The high concentration of bicarbonate could inhibit the acidification but neutralize the CO2RR generated OH− (Eq2) as shown in Eq3. To quantify the pH balance in the bulk electrolyte and local interface, we conducted a multiphysics simulation based on the reactions and diffusion. The simulated data (Fig. S18) illustrate the concentration ratio of carbonate and bicarbonate in the bulk and the interface in the two electrolytes at 4 bar. It agrees well with the Raman result that little carbonate exists on the interface of the electrode in KHCO3 electrolyte and the opposite trend is seen in KCl electrolyte.
Based on this model, we further investigate the influence of pressure on the interface chemical environment. In KHCO3 electrolyte, the influence of pressure on the interface pH is subtle (Fig. 4e). Notably, increasing the pressure significantly reduces the surface OH− concentration in KCl electrolyte (Fig. 4f). As the pressure increases from 2 to 4 bar, the interfacial OH− concentration decreases from 33.8 mM to 23.0 mM. This reduction in alkalinity explains the decline in performance at 0.4 A cm−2 between 2 and 4 bar in KCl electrolyte. Moreover, the interface pH continues to decrease as the pressure increases from 4 to 5 bar. This can be attributed to the increasing amounts of H+, dissolved CO2, and bicarbonate generated during pressurization, which would neutralize the local alkaline environment and impede the C–C coupling. In summary, while the CO2 pressure can enhance the CO2RR performance, it also lowers the interface pH, leading to a performance decrease above certain pressures. At a current density of 0.4 A cm−2 and CO2 pressures exceeding 2 bar, the diminished pH begins to inhibit CO2RR performance.
In contrast, the KCl system demonstrates pressure-dependent optimization windows (Fig. 5c). While maintaining volcanic trends, the peak positions of FEC2H4 under fixed pressure shift gradually from 0.4 A cm−2 (1 bar) to 1.0 A cm−2 (5 bar). This increasing trend of the optimized current density (Δj = 0.6 A cm−2 across 1–5 bar) demonstrates strong correlation with interfacial pH change under increasing pressure (Fig. 5d), providing evidence that the interfacial pH change induced by pressure could lead to an inhibited C–C coupling mechanism. Furthermore, this dynamic behavior highlights the critical role of operational parameters on the local reaction environment. Through further optimizing the parameter of current density, the system achieves remarkable performance at 5 bar in KCl electrolyte (Fig. 5e). Notably, faradaic efficiencies of 50% for ethylene and 70% for total C2 products are obtained at a current density of 1.0 A cm−2, while the ethylene partial current density continues to increase, reaching 554 mA cm−2 at 1.2 A cm−2. The KCl based systems exhibit a 5-fold improvement in jC2H4 (554 vs. 118 mA cm−2) relative to bicarbonate-based systems under identical conditions.
In contrast to state-of-the-art high-pressure CO2RR systems (listed in Table S5),17,18 which demonstrate promoted formate selectivity under high pressure, this work reveals that pressure induced local pH drop plays a dominant role in decreased C2 selectivity. Through electrolyte engineering, we alleviated the neutralization effect of the CO2-derived species. The non-buffered KCl system sustains a pH of >10 (Fig. 5d), which is critical for C–C coupling.19–21 By adopting low pressure and high current density to preserve the high local pH, this work achieves efficient C2 selectivity, establishing new benchmarks as shown in the comparative analysis (Fig. 5f and Table S6). Besides, the pressurized L-MEA shows great compatibility with recent pulsed electrolysis51,52 and ionic liquid additives,53,54 which highlights its potential for synergistic enhancement. Notably, the pressurization energy accounts for less than 0.5% of the total electrolysis energy consumption (Fig. S35–S37), which further supports the energy feasibility of pressurized operations in the context of CO2 reduction systems.
Supplementary information is available. See DOI: https://doi.org/10.1039/d5gc02334c. Detailed computational methods, density functional theory (DFT), techno-economic analysis, and additional experiment results are included in the supporting information (Figure S1–S37 and Table S1–S5).
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