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ACS Energy Lett
ACS Energy Lett
nz
aelccp
ACS Energy Letters
2380-8195
American Chemical Society

10.1021/acsenergylett.4c01256
Letter
Long Period Voltage Oscillations Associated with Reaction Changes between CO2 Reduction and H2 Formation in Zero-Gap-Type CO2 Electrochemical Reactor
Mikami Nagisa †
Morishita Kei †
Murakami Takeharu †
Hosobata Takuya ‡
Yamagata Yutaka ‡
Ogawa Takayo †
https://orcid.org/0000-0003-3057-2467
Mukouyama Yoshiharu §∥
https://orcid.org/0000-0002-3313-2689
Nakanishi Shuji ∥
https://orcid.org/0000-0001-9334-9751
Ager Joel W. III *⊥#
https://orcid.org/0000-0002-9022-9858
Fujii Katsushi *†
Wada Satoshi †
† Advanced Photonics Technology Development Group, RIKEN Center for Advanced Photonics, 2-1 Wako, 351-0198 Saitama, Japan
‡ Ultrahigh Precision Optics Technology Team, RIKEN Center for Advanced Photonics, 2-1 Wako, 351-0198 Saitama, Japan
§ Division of Science, College of Science and Engineering, Tokyo Denki University, Hatoyama, Saitama 350-0394, Japan
∥ Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
⊥ Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, United States
# Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 United States
* Email for J.W.A.: jwager@lbl.gov.
* Email for K.F.: katsushi.fujii@riken.jp.
01 08 2024
13 09 2024
9 9 42254232
07 05 2024
15 07 2024
17 06 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Zero-gap-type reactors with gas diffusion electrodes (GDE) that facilitate the CO2 reduction reaction (CO2RR) are attractive due to their high current density and low applied voltage. These reactors, however, suffer from salt precipitation and anolyte flooding of the cathode, leading to a short lifetime. Here, using a zero-gap reactor with a transparent cathode end plate, we report periodic voltage oscillations under constant current operation. Increases in cell voltages occur at the same time as the reactor switches from the hydrogen evolution reaction (HER) to predominant CO2RR; decreases in cell voltage occur with the switch from the CO2RR to HER. Further, real time visual observations show that salt precipitation occurs during the CO2RR, whereas salt dissolution occurs during the HER. Slow flooding triggers the transition from the CO2RR to HER. A number of processes combine to slowly reduce the water content in the microporous layer, which triggers the transition back to the CO2RR.

Basic Energy Sciences 10.13039/100006151 DE-AC02-05CH11231 New Energy and Industrial Technology Development Organization 10.13039/501100001863 JPNP18016 document-id-old-9nz4c01256
document-id-new-14nz4c01256
ccc-price
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pmcThe reduction of carbon dioxide (CO2) exhaust is an urgent problem for human beings. Electrochemical CO2 reduction reactions (CO2RR) to convert CO2 into useful chemicals is an approach to solve this problem,1,2 thus motivating its study over several decades.3 Research on the CO2RR has mainly focused on the control of product selectivity and reduction of the overpotential.4,5 Due to the low solubility of CO2 in aqueous electrolytes and the low CO2 diffusion coefficient,6,7 many types of reactors have also been investigated including H-type cells, gas-flow-type reactors (gas-flow-type reactors usually have flowing catholytes and anolytes and use a gas diffusion electrode such that CO2 gas can be supplied from the backside of the cathode electrode), and zero-gap-type reactors.8 The current density for the CO2RR is over 1 A/cm2 in recent reports using flow-type reactors,1,9 but the applied voltage is still high. Anion exchange membrane (AEM) based membrane electrode assembly (MEA) zero-gap-type reactors with gas diffusion electrodes (GDEs) shown in Scheme 1a are currently believed to be the most suitable approach to lower the operating voltage.2 Although the lifetime is relatively good when CO-producing Ag electrocatalysts are used,10 stability remains a challenge when Cu-based electrocatalysts making multicarbon products are used.

Scheme 1 (a) Schematic Structure of the Cathode Region in the Zero-Gap-Type CO2R Reactor and (b) Schematic Diagram of a Whole Zero-Gap-Type Reactor

Abbreviations: CC, current collector and serpentine gas flow with transparent end plate; GDE, gas diffusion electrode; CL, catalyst layer; AEM, anion exchange membrane. The anolyte is in the GDE anode.

A CMOS camera was utilized to track the time progression of salt precipitation in the flow field channels.

The short lifetimes of zero-gap-type reactors with Cu electrochemical catalysts are caused by salt precipitation and electrolyte flooding of the cathode.11−13 Both processes block access of CO2 gas to the catalyst. Further, it has been suggested that both phenomena are caused by the electromigration of hydrated K+ ions and electrolyte migration from the anode to the cathode.14

The full-cell voltage for CO2RR of the zero-gap reactor is usually constant under constant current operations.15,16 However, voltage oscillations have been observed in some cases, depending on the operating conditions. Analysis of the voltage oscillation phenomenon should be useful for understanding the CO2R mechanism in zero-gap reactors. A recent study of voltage oscillation used X-ray tomography to visualize the salt precipitation and the resultant flooding in the electrode.17,18 The relative rates of the CO2RR and HER reactions were also associated with changes in cell potential, and a model linking changes in the ion transport capacity of the membrane with cell potential was proposed. However, it was not possible to develop a more detailed mechanism like the order of the processes due to limitations of the employed observation methods.

In this study, the salt precipitation and electrolyte flooding were observed visually using a usual-sized zero-gap-type cell with transparent cathode end plate to clarify the order of the processes (Scheme 1a) with a 0.1 M KHCO3 anolyte and constant current operation of 200 mA/cm2 for a 5 cm2 electrode area. As shown in Scheme 1b, this arrangement allows real-time observation of the cathode electrode gas-flow region from the backside. Previous studies have employed a similar approach,19,20 but our experimental system is equipped with a CMOS camera which can track the conditions at the cathode backside in real time with data collected every 10 s to 1 min. For example, the image from the CMOS camera shown in Figure 1a clearly shows the backside of the gas diffusion electrode (GDE) of the cathode through the serpentine gas channel. To the right is an image taken after 5 h of operation; salt precipitation (white areas) on the backside of the GDE is clearly visible (Figure 1a, right).

Figure 1 (a) Photographs of the cathode through the transparent end plate: (left) reactor as fabricated; (right) after 5 h of operation. The dotted rectangle denotes the region of interest (ROI) used for the kymograph in (b) and integrated intensity versus time data in (c). The kymograph, which spans from 5 h after the initiation of the experiment to 19 h, demonstrates the periodic oscillation in intensity. To clearly see the periodic oscillation in intensity, the pseudo color was applied, with the yellow and black colors representing high intensity (salt precipitation) and low intensity (electrolyte flooding with salt dissolution), respectively. (d) Frequency analysis of (c). The green (upper) and orange (lower) colors represent the frequencies at 5–10 and 15–19 h from the start of the experiment, respectively. The current density was 200 mA/cm2 at RT using a 5 cm2 electrode with a Sustainion membrane. The anolyte was aqueous 0.1 M KHCO3. The flow rate of CO2 was 70 mL/min.

Under long-term, constant current operation (200 mA/cm2), periodic voltage oscillations in our MEA cell were frequently observed (Cu on Sigracet GDE, 0.1 M KHCO3 anolyte; more details of operating conditions are in section S.1, Reactor structures and Experimental conditions). Image analysis establishes a clear link between the cell voltage and salt formation: salt precipitation is associated with increases in full-cell voltage, whereas salt dissolution triggers a transition to lower voltage operation (Supplementary Movie 1). This kind of oscillation phenomenon requires slow negative feedback or time-delayed negative feedback. The analyses described below will show that the electrolyte ingress into the GDE during the CO2RR and the change in gas generation rate at the cathode when the dominant reaction is the HER are the reasons for the voltage oscillation under constant current conditions.

Since the salt precipitation did not occur uniformly throughout the electrode, data taken from a specific area would not yield a quantitative analysis (Figure S3). Therefore, we enclosed the entire width from inlet to outlet of the serpentine-shaped flow field with the square in Figure 1a, right, and tracked the time course of its mean brightness (see section S.1, Reactor structures and Experimental conditions, and Figure 1b). By using a pseudocolor analysis of the intensity profile (kymograph), we can visualize the rates of salt precipitation and dissolution on the electrode (Figure 1c). Focusing on the local area, the salt precipitation and dissolution start from the edge of the metal electrode (Figure S4).

Via Fourier analysis, the oscillation period in the first 5–10 h of small-amplitude oscillation is about 40 min (Figure 1d, green), and it continues during 15–20 h of operation with increasing amplitude (Figure 1d, orange). Similar periodic oscillations were repeatably observed under the same experimental conditions (Figure S5). The voltage oscillation phenomenon was also observed when a metal end plate (not transparent) was used (Figure S6). This shows that the fluctuations are independent of the material used for the cathode plate.

To further characterize the oscillations in salt precipitation/dissolution and the voltage, we compared the Faradaic efficiency of the cathode products of C2H4, CO, and H2 with the intensity of the GDE image (Figure 2a). Although the products of the CO2RR were mainly C2H4, CH4, CO, and H2 in the gas phase and alcohols and aldehydes in the liquid phase, here the focus was on major gas phase products C2H4 and CO. High- and low-voltage operation corresponds to predominant CO2RR and HER, respectively (Figure 2a, top and upper middle). Further, high/low full-cell voltages correspond to high intensity of the image (salt precipitation) and low intensity of the image, respectively (Figure 2a, top and lower middle).

Figure 2 (a) (top) Repetitive oscillations in the reactor full-cell voltage. (upper middle) Faradaic efficiency (FE) of ethylene (C2H4), carbon monoxide (CO), and hydrogen (H2), represented by dark green, light green, and gray colors, respectively. (lower middle) Intensity of the entire image of MEA. (bottom) CO2 gas pressure in the cell. The right graphs represent magnified images of the gray area in the left graphs. Comparison between the upper and lower middle graphs reveals that high-intensity (salt precipitation) areas indicate CO2 reduction (CO and C2H4 formation observed), while low-intensity areas represent HER occurrence. Furthermore, the comparison between the intensity of the image (lower middle) and the full-cell voltage (top) graph exhibits a positive correlation that the voltage decrease corresponds to a decrease in intensity (salt dissolution) while the voltage increase results in an intensity increase (salt precipitation). (b) Dwell time analysis where the time course of cell voltage was depicted, with the orange and pink arrows indicating the dwell time at high and low voltages, respectively, corresponding to the data on (c). Dwell time was defined as a duration without the voltage change. (c) Dependence of dwell time on current density (n = 5). (d) Dependence of oscillation intervals (1/frequency) on current density (n = 5). The error bars in (c) and (d) show the time between 10% low from maximum values and 10% high from minimum values for all oscillations in the condition shown. The current density was 200 mA/cm2 at RT using a 5 cm2 electrode with a Sustainion membrane. The anolyte was aqueous 0.1 M KHCO3. The flow rate of CO2 was 70 mL/min.

The period of the voltage oscillation becomes shorter with increasing current density from 100 to 400 mA/cm2 (Figure S7). To quantitatively analyze this phenomenon, we estimated the dwell time (Figure 2b) of the CO2RR (dCO2RR: Figure 2c, pink) and the dwell time of the HER (dHER: Figure 2c, orange), corresponding to the duration of high-voltage and low-voltage regions, respectively. dCO2RR and dHER became shorter with increasing current density. The period of salt precipitation/dissolution was consequently decreased as well (Figure 2d).

To investigate whether the oscillations were specific to constant current operation, we changed the operation mode from constant current (200 mA/cm2) to constant voltage (3.5 and 3.7 V). In this mode, periodic oscillations in cell current were observed whose frequency increased with voltage (Figure S8). Clearly, the oscillations are not associated with the way the electrolysis cell is powered.

To evaluate the CO2 gas effect, we switched the injection gas from CO2 to Ar (with 200 mA/cm2 constant current) under conditions with partial salt precipitation in the channels. After 4–10 min later, the salt dissolved into the electrolyte. The electrolyte accumulated partially at the top of the metal electrode plate (bottom-side of the CO2 gas pathway) after 20 min. The electrolyte disappeared after 30 min, at which point the salt was pushed out of the cell. After that, salt precipitation and flooding were not observed (Supplementary Movie 2 and Figure S9).

As a result, the periodic voltage oscillations stopped and the voltage stayed in the low-voltage state, which is the signature of the HER (Figure 3a, top and middle). Furthermore, all the observable salt precipitation disappeared (Figure 3a, bottom, and Figure 3b,c). We observed the electrode for 150 min after the gas was switched from CO2 to Ar but did not see any further salt precipitation (see also Supplementary Movie 3). When the gas supply was returned from Ar to CO2, the voltage increased rapidly, and the voltage oscillation started again (Figure S10). The full-cell voltage is higher under CO2 supply than with Ar at the same current density (e.g., ∼3.7 V vs ∼3.3 V at 200 mA/cm2; see Figure S11). From this, we infer that the lowest overpotential HER sites are occupied, probably by CO, during the CO2RR.

Figure 3 (a) Full-cell voltage (top), Faradaic efficiency (FE, middle) of ethylene (C2H4), carbon monoxide (CO), and hydrogen (H2), represented by dark green, light green, and gray colors, respectively, and the intensity of the entire image of MEA (bottom). After 24.5 h, the background color changed from green to orange, indicating the switching from CO2 to Ar supply to the cathode. The maximum FE under Ar supply conditions is lower than the real value because of the viscosity correction of base gas being used for CO2. (b) Photograph of an MEA, with the chosen location for the kymograph (c) creation marked by a dotted rectangle. (c) Kymograph displaying the last 4.5 h of the 27 h experiment. A pseudocolor was applied, with black and white indicating high and low intensity, respectively. The arrow indicates the time at which CO2 was switched to Ar. (d) Impedance measurement with the colors being represented by orange, pink, and purple lines, which correspond to high voltage (CO2RR), low voltage (HER) during CO2 supply, and replacement from CO2 to Ar, respectively. ReZ and ImZ indicate the real and imaginary components of the impedance, respectively. The oscillating amplitude was 40 mA (8 mA/cm2) from 100 kHz to 1 Hz at a current of 1000 mA (200 mA/cm2). The current density was 200 mA/cm2 at RT for the 5 cm2 electrode with the Sustainion membrane. The anolyte was a 0.1 M KHCO3 aqueous solution. The flow rate of CO2 was 70 mL/min.

We performed impedance measurements to investigate the resistance losses in the cell.21 Galvanic electrochemical impedance spectroscopy (GEIS) under current control shows the resistance of the membrane (x-intercept of the semicircle) and the other resistance (diameter of the semicircle) by the overpotentials due to electrode–electrolyte interfaces and the pH difference between the anode and cathode in the entire reactor (Figure 3d). Although the impedance spectrum for the high voltage (orange, t = 22.73 h) is different from the low voltage in CO2 supply (pink, t = 24.69 h), the spectrum for the low voltage coincided with that measured with Ar supply (purple, t = 27.40 h) in Figure 3d. This indicates that the HER in CO2 supply was the same condition in Ar supply, that is, the CO2 gas was not appropriately supplied to the catalytic region. The series resistances were also different in the cases of high and low voltages. This probably indicates that the resistivity of the electrolyte in the cathode and/or the AEM changes; that is, the low resistivity is expected in the low-voltage period of the HER.

At this point, we suspected that the oscillations were associated with liquid electrolyte ingress into the GDE. We found that the oscillation period increased when the CO2 pressure in the cell was reduced (Figure S12). The oscillations persisted but became aperiodic when the pressure of the anode side was increased by using a clamp to restrict the anolyte circulation (Figure S13). The electrolyte concentration and the current density may also affect the electrolyte migration speed and direction from the migration model experiments (see section S.3, Electrolyte migration dependence on the pressure between the anode and the cathode, and Figures S14–S16).

In the model, we propose that changes in the electrolyte volume in the GDE cause the oscillatory CO2RR and HER behavior in our MEA cell (Figure 4; see also section S.4 ,Voltage oscillation analysis, and Supplementary Movie 3). This clearly shows that the complete electrolyte covering of the cathodic catalyst causes the CO2RR to change to the HER, and the partial CO2 supply recovery to the cathodic catalyst causes the change from the HER to the CO2RR. That is, the physical conditions of the electrolyte and the cathodic catalyst are probably the reason for the reaction changes. This means that the order of the voltage oscillation process is different from the previous model.18 That is, the electrolyte flooding at the cathode occurs before the HER, and not the water flux to the cathode increasing after the HER starts.

Figure 4 Schematic depiction of the model for the voltage oscillation. A detailed description of states (a–d) and of the transitions between them are given in the main text.

The oscillations between the HER and CO2RR are comparable to those reported by Seger and co-workers for similarly constructed AEM-based electrolysis cells.18,22 The remarkable periodicity of the oscillations observed here is evident in Figure 2a and is emphasized in the phase portrait shown in Figure S17 (Supplementary Movie 4). Clearly, the system is bistable and supports a mechanism which can trigger the transition between the two metastable states. However, this bistable phenomenon will not continue indefinitely, because the periodic salt precipitation and dissolution will differ in position and amount for each cycle. Still, since these bistable phenomena clarify each condition, especially the electrolyte existence in the cathode for the CO2RR and HER; the analysis of these states is useful to obtain the guidelines of the stable CO2RR. Oscillations of either current or potential are frequently seen in electrochemical systems. Here, we adapt concepts used to understand oscillatory behavior in chemical systems of relevance to living systems to analyze the present case.22,23

We begin by referring to Figure 1a and Scheme 1a, which depicts the porous layer of the GDE being saturated with CO2, leading to a steady CO2RR at the cathode. However, possibly due to electrowetting caused by the very negative potential on the current collector (CC) of the cathode plate (CP), there is an increasing amount of liquid electrolyte ingress into the GDE (Figure 4a). K+ ions are also transported into the GDE porous layer. The flooding probably does not occur in the hydrophobic microporous layer (MPL) but occurs in the hydrophilic mesoporous substrate (MPS) via the water path of (maybe invisible) crack regions in the MPL. (The SEM photographs of GDE with voltage oscillation and no voltage oscillation are shown in Figure S18.) Initially the diffusion of the CO2 within the MPL layer is sufficiently fast to supply the Cu catalyst and CO2RR is minimally affected because the gas paths remain in the MPS. At the same time, the salts of KHCO3 begin to precipitate in the flow field channels (as seen by the camera) and possibly within the water-front regions of MPS in GDE (this possibility has been suggested by Moss et al.18) because the K+ ions react with the supplied CO2 gas and the salt concentration of the electrolyte reaches the precipitation concentration due to the water consumption by reductions (Figure 4b). At a certain point, CO2 access to the Cu catalyst is abruptly stopped and the HER becomes the dominant reaction because the electrolyte flooding covers the whole MPL region and prevents most of the CO2 supply to the Cu catalyst region, as shown in Figure 4c.

While the HER is the dominant reaction, there are concurrent phenomena that trigger the transition back to the CO2RR. Generation of H2 gas at the cathode and the consumption of the water will reduce the water amount in the cathode (Figure 4d). While gas is generated by the CO2RR, the volume of gas generated by the HER is larger because of +1/2 molecules per electron for the HER and −1/12 molecules per electron for the CO2RR to C2H4. The water consumptions for H2 formation and CO2RR to C2H4 are 1 and 2/3 molecule per electron, respectively (Supplementary Movie 3). Further, the liquid electrolyte in the GDE will be depleted in carbonate species since the CO2 supply is stopped under this HER condition.17,18 This will change the bulk pH of the cathode electrolyte from near-neutral to alkaline. (The bulk pH change model experiments changing with Ar and CO2 supply are shown in Figure S19. The pHs at the vent of the anolyte for CO2 reduction and H2 formation were around 7.30 and 7.65, respectively. This supports our assertion that the local pH around the cathode catalyst increases.) Since the precipitation concentration of K2CO3 (the solubility in water is ca. 8.1 mol/L at 20 °C), which is the existing form in alkaline solution, is much higher than that of KHCO3 (the solubility in water is ca. 3.3 mol/L at 20 °C), the liquid electrolyte accessing the cathode flow field channels is able to dissolve the salt precipitates (maybe also the salts we cannot see that are inside the GDE). These effects combine to restore, again rather abruptly, the access of CO2 to the cathode such that the CO2RR can restart due to the appearance of the CO2 gas paths. The chemical reaction details are discussed in section S.4, Voltage oscillation analysis.

This pH change is also supported by the series resistance of the HER being lower than that of the CO2RR discussed in the GEIS of Figure 3c because the resistivity of K2CO3 is generally lower the that of KHCO3 due to their ion concentration differences.

Our model explains some observed phenomena like abrupt changes from the CO2RR to HER and vice versa (Figure 3) and the observation of oscillations with (1) a different GDE (Figure S20), (2) use of a CO-forming Ag catalyst (Figure S21), and (3) supply of humidified CO2 to the cell (Figure S22),24−26 as shown in section S.6, Other supporting data for the voltage oscillation.

In summary, we conducted real-time observations of electrodes using a CMOS camera in a zero-gap type reactor with a transparent cathode end plate to understand the voltage oscillation with the transition from the CO2RR and HER. The salt precipitation and electrolyte flooding in the cathode occurs at the CO2RR and the amount of electrolyte decreases with the HER, respectively. From these observations, a model of the fluctuation is proposed. In this model, liquid electrolyte volume fluctuations in the cathode influence the switching between the dominant CO2RR and HER at the cathode electrode. The change from the CO2RR to the HER is caused by the complete electrolyte covering of the cathodic catalyst, and the opposite is caused by partial CO2 supply recovery to the cathodic catalyst. The results show that the order of the voltage oscillation process is different from that of the previous model.18 The effective mitigation of this voltage oscillation is thought to prevent liquid electrolyte flooding over the CO2RR catalyst region by controlling the electrolyte migration from the anode.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.4c01256.Salt precipitation and flooding on cathode (MP4)

Transition from salt precipitation to flooding on electrode and argon replacement from CO2 gas (MP4)

Correlation between the applied voltage and flooding (MP4)

Time course of the phase portrait of partial current densities for HER and CO2RR to ethylene (MP4)

Reactor structures and experimental conditions, supporting data for the voltage oscillation under constant current, electrolyte migration dependence on the pressure between the anode and the cathode, voltage oscillation analysis, model evaluation of the voltage oscillation analysis, other supporting data for voltage oscillation, and captions for Movies 1–4 (PDF)

Supplementary Material

nz4c01256_si_001.mp4

nz4c01256_si_002.mp4

nz4c01256_si_003.mp4

nz4c01256_si_004.mp4

nz4c01256_si_005.pdf

Author Contributions

N.M., K.M., and T.M. contributed equally to this work. N.M., T.O., S.W., and K.F. designed the research. T.H., T.M., and Y.Y. designed the transparent cell. N.M., K.M., and T.M. performed experiments and obtained data. N.M., K.M., T.M., Y.M., S.N., J.W.A., and K.F. established the reaction model. N.M., J.W.A., and K.F. wrote the paper.

The authors declare no competing financial interest.

Acknowledgments

We thank Ms. Y, Qin for preparing the electrode and Dr. Y. Kinosita for preparing the manuscript. This research was partially based on integrated electrochemical systems for scalable CO2 conversion to chemical feedstock projects performed as part of the Moonshot Research and Development Program funded by the New Energy and Industrial Technology Development Organization (JPNP18016). The work of J.W.A. (analysis of switching mechanism) was supported by the Clean Energy Manufacturing Program, U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, under contract no. DE-AC02-05CH11231.
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