CO residence time modulates multi-carbon formation rates in a zero-gap Cu based CO2 electrolyzer

doi:10.21203/rs.3.rs-3535552/v1

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CO residence time modulates multi-carbon formation rates in a zero-gap Cu based CO₂ electrolyzer

https://doi.org/10.21203/rs.3.rs-3535552/v1

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Carbon dioxide (CO₂) electrolysis on copper (Cu) catalysts has attracted interest for its direct production of C₂₊ feedstocks. Using the knowledge that CO₂ reduction on copper is primarily a tandem reaction of CO₂ to CO and CO to C₂₊ products, we show that modulating CO concentrations within the liquid catalyst layer allows for C₂₊ selectivity of > 80 % at 200 mA cm^‑2over broad conversion conditions. The importance of CO pooling is demonstrated through residence time distribution curves, varying flow fields (serpentine/parallel/interdigitated), and flow rate. While serpentine flow fields require high conversions to limit CO selectivity and maximize C₂₊ selectivity, the longer CO residence times of parallel flow fields reach similar selectivity over broad flow rates. Critically, we show that parts of the catalyst area are predominantly reducing CO instead of CO₂ as supported by CO reduction experiments, transport modelling, and achieving a CO₂ utilization efficiency greater than the theoretical limit of 25 % for C₂₊ products.

Physical sciences/Engineering/Chemical engineering

Physical sciences/Chemistry/Catalysis/Electrocatalysis

Electrochemical CO₂ reduction (ECO₂R) using copper (Cu) based catalysts are attractive due to copper’s ability to produce hydrocarbons and oxygenates^1–7. As a result, extensive efforts have gone into unravelling the reaction pathways and intermediates in both H-cell and gas diffusion electrode (GDE) cell architectures to understand and improve, what enables such broad C₁ and C₂ product spectrums^8–13. These studies have shown that copper’s unique ability to moderately bind carbon monoxide (CO) facilitates the coupling of two CO species^14–19. Critically, the coupling of CO-species has been shown to occur through both surface dimerization of two adsorbed CO-species and tandem reactions where CO₂ is first converted to CO, desorbs into the aqueous environment as CO_(aq). Copper is then not simply a unique CO₂/CO to C₂₊ catalyst, but an excellent CO₂ to CO catalyst.

A number of ECO₂R experiments performed in fully-aqueous reaction environments have sought to understand the effect of CO_(aq) on multi-carbon product formation^20–22 through a variety of control experiments and modelling. The importance of local CO_(aq) concentration has further motivated tandem catalyst and bimetallic systems where one catalyst is included for CO₂ to CO conversion, and a copper catalyst further facilitates CO₂/CO reduction. A few studies for instance have shown that such spatial variations can be used to tune CO coverage over a Cu catalyst to achieve higher partial current densities of C₂₊ products^23–25. While this premise is attractive and shows increased oxygenate production rates as a result, these fully tandem systems commonly produce CO in excess amounts except at very precise flow rates and current densities. We conclude that a primary reason for this is because copper itself already reaches excessive CO_(aq) concentrations during ECO₂R, particularly at elevated current densities. We then posit that the need for local CO regulation can also be met by further understanding and modulating the residence times of CO_(aq) in copper-based systems.

Suitable platforms to examine local CO regulation are zero gap membrane electrode assemblies (MEA) using anion exchange membranes due to their elevated reaction rates (> 100 mA/cm²), low cell voltages and ability to maintain a favourable alkaline environment around the catalyst surface which reduces by-product H₂ production^26–28. A notable consequence of these flow-based systems is the spatial variation in the concentration of reactants (CO₂, CO and H₂O) and products (CO, C₂H₄) within the catalyst layer as flow rate and current density are varied²⁹. These spatial variations will impact the localized Faradaic efficiency, but cannot be resolved through common measurements which give averaged Faradaic efficiency due to external product measurement.

In this work, we study the influence of CO residence time on C₂₊ production rates for a Cu catalyst coated on carbon gas-diffusion layer in an alkaline MEA cell. We use pulse and negative tracer gases to generate residence time distribution curves under various flow rate conditions and flow field patterns (serpentine, parallel and interdigitated). When contrasting these findings with electrochemical data we are able to infer both local and spatial phenomena related to CO concentrations throughout the MEA cell. We find that C₂₊ production rates increase at decreased CO₂ flowrates because of near-complete CO consumption, achieving a maximum selectivity of 84% at 200 mA/cm². We further demonstrate how modulating CO concentrations via the choice of flow rate and flow field can enable regions of greater CO reduction instead of CO₂ reduction, allowing for elevated CO₂ utilization efficiency.

Within a MEA CO₂ electrolyzer, CO and other products are produced within the liquid-immersed catalyst layer. These products can then either diffuse across a gas-liquid interface and through the gas diffusion layer (GDL) into the gas channel, or in the case of CO they can further be reduced on the catalyst (Supplementary Fig. 1). To better understand the statistical amount of time that products reside within the liquid layer, we performed residence time distribution experiments of an ex-situ CO₂ electrolysis cell.

As illustrated in Fig. 1, a non-reactive tracer gas of 5% CO and 95% He is injected into an assembled electrolyzer, and a time-resolved output signal of CO is measured by a mass spectrometer (See Supplementary Fig. 2). By using defined input profiles of the tracer gas such as a pulse or negative tracers, the profile and delay of the output signal, known as a residence time distribution (RTD) curve, will give us information about the convective and diffusive properties of the MEA electrolyzer. Inside a CO₂ electrolyzer employing a gas diffusion electrode, however, contributions from the gas flow channel, the GDE and the liquid filled catalyst layer all affect the RTD and the dominant transport factor needs to be determined.

In this regard, previous studies from fuel cells have shown that pulse RTD tests predominantly show the gas flow field channel characteristics as gases pass through the system components (see Fig. 1a center)^30–32. In contrast, a negative tracer test saturates the entirety of the system for longer periods of time and provides information on the time required for gases to leave all parts of the cell³³. A negative tracer then gives a good perspective of products that are formed in the liquid phase leaving as the gas channel partial pressure is decreased³⁴. Here, the eventual release of the tracer is then maximized at the tail.

We began our non-electrochemical RTD tests by first examining the importance of the liquid layer in an MEA system as liquid diffusion and liquid-to-gas diffusion of CO are expected to be the largest transport barriers. For the experiments we used the most common flow field pattern (FFP), a serpentine channel which has a single fluid flow path from the inlet to outlet, creating a significant pressure drop in the reactor. There are then two scenarios in Fig. 1b. For the non-wetted case, we assembled the MEA cell as usual but without applying a potential, which means that no water wets the anion exchange membrane or is present on the cathode GDL. For the wetted case, we used a porous Zirfon membrane which is wetted by the anolyte flow. When we compare a 30 s pulse RTD test for the two cases, we see a stark difference in the RTD curves (Fig. 1c). For the non-wetted case, we see a CO RTD profile almost identical to the pulse input with a mean residence time of 28 s, whereas the wetted case shows a substantially longer mean residence time of 118 s. This control experiment confirms the impact of the water layer on transport properties and confirms its importance in future tests.

We then performed pulse and negative RTD measurements on the Zirfon wetted system for varied inlet flow rates (Fig. 2a, 2b). Here a clear difference is observed between a 10 sccm flow (representative of a high CO₂ conversion scenario) and 50 sccm (a low CO₂ conversion scenario). Consequently, the mean residence time for the tracer at 10 sccm was 118 s, higher than at 50 sccm (110 s). If we relate this finding to a CO₂ electrolyzer in operation, it indicates that a CO molecule in either the liquid layer or the gas channel will on average reside there for 8 s longer at 10 sccm than 50 sccm.

To contrast these non-electrochemical RTD measurements with electrochemical data, we performed CO₂ electrolysis experiments in an MEA cell using a Ni foam anode, Cu sputtered carbon GDE (5 cm²) and humidified CO₂ as the reactant. The measurement techniques and instrumentation are shown in Supplementary Figure S8. For our tests we used a fixed geometric current density of 200 mA/cm² with CO₂ fed at various flowrates ranging from low to high CO₂ conversion conditions.

As shown in Fig. 2b and 2c, we find that the product distribution varies as CO₂ flowrates decrease from 50 sccm to 5 sccm. Notably, we see a decrease in the Faradaic efficiency (FE) of CO from 12.6% at 50 sccm to just 3% at 10 sccm. In contrast, the FE of ethanol increases substantially from 20.3% at 50 sccm to 30.8% at 10 sccm. The shift is even more stark when observing the oxygenate (ethanol + acetate) trend in Fig. 2c. In particular, the product spectrum where oxygenates outcompete ethylene FE is more indicative of ECOR on copper than that of ECO₂R^35,36. When coupled to the non-electrochemical RTD data it implies that the production rate of CO in both the 10 and 50 sccm cases may actually be similar, but the CO produced in the 10 sccm case remains longer in the gas channel and liquid layer such that it is further reduced, providing a more oxygenated product spectrum. Interestingly, rates of ethylene production remain unchanged (FE ~ 40%) across all flow rates, implying that any decrease in CO_(aq) predominantly contributed to ethanol product pathways (Equation S38 in Supplementary Notes).³⁷ When flowrates were further decreased (5 sccm), competing HER took over (FE_H2 of 40%) due to mass transport limitations of CO₂ reaching the entire 5 cm² Cu GDE (Fig. 2c). Here, however, the oxygenate to ethylene trend continues.

A single pass conversion efficiency of 24% (Fig. 2d) and a maximum C₂₊ selectivity of 84% (j_C2+ of 168 mA/cm²) are then achieved at 10 sccm, due to the higher residence time of CO in the reactor. A higher residence time ensures that there is sufficient time for dimerization of two ^*CO molecules, thereby achieving a CO utilization of 87.6% for C₂₊ production at 10 sccm (See Equations S28-S30 in Supplementary Notes and Supplementary Table 2). Notably then we can say that the highest combined Faradaic efficiency and single-pass conversion at a fixed current density both occur at low flow rates for a serpentine channel. At a higher flowrate of 50 sccm however, the C₂₊ selectivity drops to 65% with a single-pass conversion of only 4%. Additionally, a very low stoichiometric CO₂ excess of 1.13 (See Supplementary Table 2) is obtained at 10 sccm, which is beneficial for achieving product rich gas streams and reducing downstream gas separation costs as shown in a recent study³⁸.

Lastly, for the serpentine results, we found during our experiments that we lost less CO₂ to OH⁻ interactions than expected at 10 and 20 sccm. In particular, only 75% of OH⁻ ions generated at the cathode are converted to CO₃²⁻/HCO₃⁻ due to buffer reactions with CO₂ at 10 sccm, whereas all OH⁻ is reacted above 30 sccm (Fig. 2e). Assuming that these 75% of OH⁻ ions reacting with CO₂ are converted to CO₃²⁻ ions at these higher reaction rates (local pH > 12), this would mean that the ions transported across the AEM (towards the anode) is a mixture of CO₃² and OH⁻ ions. Typically, we would expect such a result only in CO₂ depleted cases where high H₂ FE’s are seen. However, the total ECO₂R FE are 9 % and H₂ FE is only %. This finding implies that we have regions in the reactor where reactions can proceed to C₂₊ products without parasitic reactions of CO₂ and OH⁻ ions. We then conclude that regions of our 5 cm² electrode are CO₂ depleted but not CO depleted.

Using the above findings, we can then predict the dominant electrochemical reaction occurring along the flow channel length for low, moderate, and high CO₂ conversion cases (Fig. 3a). In green regions the primary reactant is CO₂, whereas in purple regions, CO is more in abundance than CO₂. As discussed elsewhere³⁹, the maximum CO₂ utilization efficiency to ethylene and ethanol products for the green region is 25% (Fig. 3b). However, the purple region performing primarily ECOR has no such limitation as there may not be enough CO₂ present to react with the formed OH⁻ (Fig. 3c). Indeed when we calculate the CO₂ utilization efficiency across various flow rates, we reach a value of 31% (Fig. 3d), breaking the limit of 25% obtained for pure C₂₊ product formation when CO₃²⁻ ions act as the sole charge carrier.

To further test the above conclusion regarding CO residence times, we performed further RTD and electrochemical tests on parallel flow fields. In contrast to serpentine channels, a parallel FFP has channels divided into parallel paths with a very low pressure drop between the inlet and outlet⁴⁰. Mass transport through the GDE is then dominated predominantly by diffusion. Due to fundamental differences in GDE transport between FFP’s, we performed negative tracer RTD measurements to compare the release of gases from each system (Fig. 4a and Supplementary Figure S6). Again, all components of our standard electrochemical MEA cell were assembled except with a pretreated Zirfon membrane pressed against the carbon GDL to mimic the wetted catalyst in real ECO₂R conditions (See Supplementary Information for details).

Observing the normalized RTD curves in (Fig. 4a), the negative tracer experiments show a large difference in the serpentine and parallel FFP curves at 50 sccm. Despite being at identical flow rates, there is 16.7 s delay for the tracer to exit the reactor using a parallel FFP in comparison to the serpentine FFP, illustrating an increased residence time of the tracer gas inside the reactor. The higher residence time shows that the use of a parallel FFP creates the likelihood of higher reactant pooling in the wetted regions of the GDL surface. These results can be anticipated as the parallel flow field has lower channel velocities than the serpentine channel (Supplementary Table 4 ), which impacts concentration gradients between the gas channel and liquid layer, thus slowing gas removal from the liquid.

By combining the flow rate and flow field RTD data together we can compose the qualitative graph in Fig. 4b. Here we see that the serpentine channel can have long or short residence times depending on the flow rate inputted. Conversely, the parallel channel has a lower sensitivity to flow rate as the fluid velocity is always at a substantially lower value than the serpentine channel at equivalent volumetric rates. These conclusions then lead to a representative image of CO pooling during electrochemical CO₂ reduction for each of the different cases as shown in Fig. 4c.

We then performed ECO₂R using varied gas flow field patterns (FFP) at the cathode Fig. 4d shows the product distribution using all three different FFP’s at 200 mA/cm² and a CO₂ inlet flowrate of 10 sccm. While all three FFPs show a similar selectivity of CO (3–4%), there were differences in the individual C₂₊ product distribution. For both the serpentine and interdigitated FFPs, FE of ethylene (40%) and oxygenates (40%) remain quite similar, achieving a C₂₊ selectivity of 82–84% with a low CH₄ (FE ~ 1%) and H₂ (FE ~ 8%). However, when a parallel FFP is used at the cathode, the FE of acetate doubles to 16% and CH₄ increases to 9%. This comes at the expense of decreased ethylene (FE ~ 32%) and ethanol (FE ~ 22%), leading to a drop in total C₂₊ selectivity of 72% (Fig. 3b). The selectivity switch from ethylene/ethanol to acetate for the parallel FFP suggests that a higher local alkalinity around the catalyst is more likely, due to CO₂ depletion within the GDE⁴¹.

The higher CH₄ production also shows that an increased ^*H coverage (from ^*H₂O) occurs within the catalyst layer due to depleted CO₂ in some parts of the catalyst layer. An increased ^*H coverage is plausible since CH₄ formation is well known to occur through a surface recombination of ^*CO and ^*H via a Langmuir-Hinshelwood mechanism⁴². Overall, the use of a parallel FFP at the cathode at 10 sccm results in decreased CO₂ access at some portions of the Cu catalyst layer and a subsequent increase in local alkalinity, producing higher CH₄ and acetate respectively. Previous studies on CO reduction on Cu have also attributed the increased acetate production to the abundance of OH⁻ ions, which leads to a higher local pH around the catalyst surface^43,44.

Further, this depletion in CO₂ access for a parallel FFP, suggests that a significant portion of catalyst surface is predominantly used for electrolysis of CO (produced from CO₂) to C1 (methane) and C₂₊ products. Supporting this hypothesis are the results from our empirical numerical transport model which shows that about 18% of GDE has no CO₂ access when a parallel FFP is used at the cathode (Supplementary Fig. 12). The total C₂₊ selectivity is however lower (72%) for the parallel FFP due to depleted CO₂ and increased ^*H coverage as is evident from the increased CH₄ selectivity. This would then imply that, if excess CO₂ is fed into the system to ensure no mass transport limitations, the parallel FFP should maximize C₂₊ production due to the increased residence time of CO within the GDE as shown earlier (Fig. 4a).

To assess some of the above statements, we operated the serpentine and parallel FFPs at 200 mA/cm² and at an excess CO₂ flowrate of 50 sccm to ensure sufficient CO₂ is available for both the cases to prevent CO₂-deplete regions. Interestingly, we find a switch in the product distribution, with a significantly higher C₂₊ selectivity for the parallel FFP (75.2%), compared to the serpentine case (68%). As shown in Fig. 4e, the FE of CO was then more than twice lower (5%) for the parallel FFP compared to the serpentine case (FE 12.6%). This increased CO utilization to C₂₊ products for the parallel FFP shows the benefit of an increased residence time within the GDE for dimerization of two CO molecules.

The calculated CO utilization rate towards C₂₊ products then reached 83.6% for the parallel FFP (See Supplementary Table 2) at 50 sccm, significantly higher than 65% obtained for the serpentine case. Operating electrolyzers using a parallel FFP is then beneficial at higher flowrates, but comes at a cost of lower single pass conversion of CO₂ fed into the reactor. Recent studies have however shown that operating at lower single pass conversion efficiencies (5–10%) are sufficient since the energy required for gas separation is 100 times lower than the actual electrolyzer energy requirements⁴⁵. Considering this aspect, a parallel FFP might be beneficial at higher and a broader range of flowrates due to its inherent ability to increase reactant residence time inside the liquid filled catalyst layer. In addition, a parallel FFP also benefits from a very low pressure drop in the reactor (Supplementary Table 4), which might be beneficial as CO₂ electrolyzers are scaled to larger areas (> 100 cm²).

ECOR experiments

While much of the work here showcased the influence of residence time of CO on C₂₊ production, the Faradaic efficiency results of individual C₂ products (ethylene, ethanol and acetate) also showed distinct trends. For instance, the use of a parallel FFP at the cathode produced the highest acetate (FE 15–16%) at both 10 and 50 sccm inlet flowrates, which was twice higher than the serpentine and interdigitated FFPs. We hypothesized that the local catalyst microenvironment, specifically the local alkalinity as a result of differences in CO₂ availability might be altered due to the FFP used, which may explain selectivity differences. We then performed CO electrolysis at 200 mA/cm².

Figure 5a shows the product distribution obtained from electrochemical CO reduction (ECOR) for the three FFPs. As the reactant feed is switched from CO₂ to CO, we see a clear selectivity switch from ethylene/ethanol to acetate for all the three FFP. Acetate production in these conditions is similar to existing ECOR literature but it is interesting that the differences we observed in ECO₂R have mostly been removed here. When we consider that most of the ECO₂R differences for serpentine vs parallel channels are explained to be a result of CO pooling and tandem reactions, it then makes sense that we do not see a stark serpentine-parallel difference for CO electrolysis in Fig. 5a where no products can be further reduced.

The higher acetate production observed during this reactant switch from CO₂ to CO for all FFPs also suggests a stronger dependence of product distribution on local alkalinity around the catalyst layer. The concentration of local OH⁻ ions during ECOR are more than one order of magnitude higher than for ECO₂R⁴¹, where neutralization by buffering reactions with CO₂ occurs. It has been shown before that these abundant hydroxide ions react with the CH₂CO intermediates (Equation S40 in Supplementary Notes) relevant for ethylene and ethanol, leading to a switch in product towards acetate.^46,47 A moderate interfacial pH, observed during ECO₂R (pH < 12–13)⁴⁸ is then beneficial to avoid this switch from ethylene/ethanol to acetate. However, modulating interfacial pH in these catholyte free MEA cells at higher current densities is quite challenging, as this would require modifications either in the type of the ion exchange membrane used or ionomers⁴⁹ within the catalyst coated GDL.

This interplay of product formation rates highlight important implications for CO electrolysis in zero gap MEA electrolyzers. Although ECOR is beneficial due to the absence of carbonate crossover and lower full cell voltages (Fig. 5b), the findings here show that these advantages comes at the expense of lower ethylene and ethanol formation rates. In addition, the calculated full cell energy efficiency (Supplementary Table 7) for a combined ethylene and ethanol production from ECOR is similar (23.4%) to CO₂ ER (29%), highlighting the main benefit of CO ER lies in the long term operational stability, due to absence of carbonate formation at the cathode. Overall, this shows that CO₂ electrolysis still has potential for producing high rates of ethylene and ethanol which have greater demand for replacement from current fossil fuel based production routes than acetic acid.

Finally, the major challenge that hinders commercialization of CO₂ electrolyzers using Cu based catalysts lies in the inability to selectively produce ethylene or ethanol with high selectivity (> 70%). Most studies have however shown that a combined 70–80% selectivity towards ethylene and ethanol can be obtained at industrially relevant current densities. While these branching pathways towards ethylene and ethanol cannot be well controlled as shown in a recent study⁵⁰, we posit here that researchers must look into a combined ethylene + ethanol selectivity as a performance metric. This is because, ethanol as a liquid product can be separated from the MEA reactor and further be used as a starting material to produce ethylene through catalytic dehydration reaction⁵¹.

Importantly, the energy requirements for this ethanol dehydration reaction to ethylene (45 kJ/mol of ethylene) are two orders of magnitude lower than a CO₂ electrolyzer producing ethylene (2900 KJ/mol of CO₂ )⁵². We then argue here that the research community should look into integrating catalytic dehydration of ethanol to ethylene as an additional process step to CO₂ electrolysis in order to make it energy efficient and industrially viable.

In conclusion, the residence time of CO in the liquid catalyst region greatly impacts product distribution from CO₂ electrolysis. Here we show that modulating CO residence time is possible with varied inlet flowrates and flow field patterns, and is an important consideration for both catalyst and system studies. We show that while both the interdigitated and serpentine flow patterns require higher single pass conversions to limit CO selectivity, a parallel flow pattern shows the highest C₂₊ selectivity at larger and a broader range of flowrates. Under lower flow conditions we also show that the electrolyzers begins to split into CO₂ and CO dominated regions, which has implications for selectivity, CO₂ utilization efficiencies, and local catalytic and component effects.

Acknowledgement

Thomas Burdyny and Siddhartha Subramanian would like to acknowledge the co-financing provided by Shell and a PPP-allowance from Top Consortia for Knowledge and Innovation (TKI’s) of the Ministry of Economic Affairs and Climate in the context of the TU Delft e-Refinery Institute. In addition, the authors thank Joost Middelkoop for technical support with flow field design fabrication and Herman Schreuders for technical support with magnetron sputter deposition technique.

Conflicts of interest

The authors declare no competing financial interest.

Author Contributions

Conceptualization: SS and TB

Experimental Investigation : SS, JK, PG, HPIvM, ASK

Transport model and simulations: SS

Supervision: TB

Writing – original draft: SS and TB

Writing – review & editing: SS, TB, JK, ASK, HPIvM, JM, PG, BD, RK, AU.

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CO residence time modulates multi-carbon formation rates in a zero-gap Cu based CO₂ electrolyzer

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