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Electrochemical CO2 reduction offers a promising avenue for the sustainable production of valuable chemicals and fuels. Considering the industrial value of chemical raw materials, the ideal is multi-carbon (C2+) products. The kinetics of the rate-determining step in the reaction determines the partial current density of the C2+ product, which depends on the adsorption energy of the adsorbed CO (*CO) and the subsequent *CO surface coverage (θ*CO). Since θ*CO is determined by the CO concentration near the catalyst, increasing the local CO concentration can increase the yield of C2+ products on Cu. In particular, for steam-fed systems unaffected by low CO solubility in the electrolyte, it has been demonstrated that an increase in CO partial pressure directly leads to an increase in C2+ production.

Given this, the team of Professor Wu Jingjie from the University of Cincinnati, Professor T. Bell, and Professor Adam Z. from the Lawrence Berkeley National Laboratory designed a segmented gas diffusion electrode (s-GDE), in which the CO selective catalytic layer (CL) inlet This section prolongs the residence time of CO in the subsequent C2+ selection section to increase the conversion rate. This phenomenon leads to higher CO utilization and C2+ current density than pure Cu and Cu/Ag s-GDE catalysts by increasing the *CO coverage in Cu CL. On this basis, the authors developed an s-GDE composed of Cu/Fe-NCs, and the C2+ FE exceeds 90% when the jC2+ exceeds 1 A cm-2.These results demonstrate the importance of transport and establish design principles to enhance C2+ FE and jC2+ tandem CO2 reduction.

In situ CO management using segmented tandem electrodes

Regulating *CO Surface Coverage

Among catalysts for CO2R, only copper-based catalysts have been demonstrated to have the ability to generate C2+ products, and many previous studies aimed at developing catalysts with higher C2+ generation rates. In the reaction from CO2 to C2+ on Cu, the kinetics of the rate-determining step that determines the partial current density (jC2+) of the C2+ product depends on the adsorption energy of adsorbed CO (*CO) and subsequently on the *CO surface coverage ( θ*CO). Therefore, recent works have attempted to increase the incorporation of *CO into Cu to enhance the conversion to C2+. Since θ*CO is determined by the CO concentration near the catalyst, the yield of C2+ products on Cu can be enhanced by increasing the local CO concentration. Especially for steam-fed systems that are not affected by the low solubility of CO in the electrolyte, an increase in CO partial pressure has been shown to directly lead to increased C2+ production.

A tandem CO2R system integrating two sequential CO2-to-CO and CO-to-C2+ steps on two different catalytic sites can enhance θ*CO on the Cu surface. In these systems, one catalyst material selectively converts CO2 to CO to provide an in situ CO source to enhance θ*CO, and another copper-containing catalyst performs CC coupling. Since θ*CO is usually the limiting factor, higher jC2+ values can be achieved by increasing the local CO partial pressure and increasing the CO production rate. However, if the CO generation rate exceeds the CC coupling rate, the CO utilization and C2+ FE decrease. This trade-off requires managing in situ CO formation to maximize C2+ FE while maintaining high jC2+.

Two s-GDE (stacked and coplanar) structures containing Ag and Cu CL segments were designed, as shown in Fig. 1a,b. In the stacked configuration, the Cu and Ag CL segments exist in different layers in length (y-axis) and in plane (z-axis) directions. In the coplanar configuration, the Cu and Ag CL segments are monolayers in the z direction, but still distinct in the y direction. In both cases, the Ag CL segment is aligned with the CO2 gas inlet to enable rapid conversion of the incoming CO2 into a supplemental CO supply that enhances θ*CO and can be converted to C2+ products in the subsequent Cu CL segment ( Figure 1d).

To gain further insight into the relationship between the partial pressure of CO (PCO) and CO conversion, we performed a series of experiments capable of tracking local PCO and jC2+ in various segments along the length of the s-GDE (y-direction). Using stacked s-GDEs as the model geometry, local PCO and C2+ production variations along the y-axis were indirectly mapped by measuring CO efflux rates and jC2+ for six modified s-GDEs. The resulting modified s-GDEs (E1 to E6) were 0.50 cm wide and condensed 0.2 cm long Ag CL stacked on Cu CL ranging from 0.2 (E1) to 2.0 cm (E6) (Fig. 2a).  . The CO2R performance of these six electrodes (E1 to E6) was evaluated in a membrane electrode assembly (MEA) electrolyzer.

Fig.1 Concept of segmented series gas diffusion electrodes

By plotting the FE enhancement observed by extending the length of the Cu CL (Fig. 2c), it is observed that as the Cu CL segment extends from E1 to E6, the CO FE gradually decreases from 65.5 to 6.2%, while the overall FE C2+ product rises from 33.2 to 82.0%. The FEs of specific C2+ products (C2H4, - and ) showed similar behavior to the overall C2+ FE. This trend suggests that the residence time of CO (produced in AgCL) increases with Cu CL for better conversion to C2+ products. However, the FE gradients for CO and C2+ products decrease with increasing Cu CL length, indicating that there is a length beyond which no further FE enhancement is observed because all the in situ generated CO has been consumed. This theory was further confirmed by plotting the CO utilization, defined as the percentage of CO produced in s-GDE converted to C2+ products, as a function of the Cu CL length. As shown in Fig. 2d, CO utilization for C2+ formation increases with Cu CL length and reaches a maximum of 0.82.

Fig. 2 Transformation of CO produced in s-GDe along the channel

To understand the improvement in conversion, it is critical to understand the distribution of PCO and C2+ mass activity (defined as the partial current of C2+ product, normalized by the mass loading of copper in the catalyst layer) in s-GDE.  The C2+ mass activity decreases with increasing Cu CL length, and more CO is converted to C2+ products (Fig. 2d). Thus, conversion of in situ generated CO to C2+ leads to the decay of PCO along the length of s-GDE (Fig. 2b). The corresponding decrease in the mass activity of C2+ products with increasing electrode length also indicates that jC2+ is largely dependent on the local concentration of CO, rather than the concentration of feed CO2 since the feed gas has a high stoichiometric ratio, jC2+ will Relatively constant throughout s-GDE. We demonstrate that in situ generation of CO with increased residence time in extended Cu CLs improves θ*CO and thus jC2+.

Fig.3 Multiphysics model of mass transfer and CO adsorption in s-GDE

Multiple studies on aqueous electrolytes have shown that the rate of CO to C2+ products has an approximate first-order relationship with the CO concentration at low CO partial pressures. This rate order is consistent with the linear trend observed in Fig. 2e. The adsorption model was implemented in a multiphysics simulation of gas-phase CO and CO transport in the s-GDE and flow channel (shown in Fig. 3a) to estimate the local θ*CO in the experimentally tested s-GDE. It is worth noting that the simulations assume that the behavior of s-GDE is a perfect cascade. In other words, all C2+ products come from CO generation on Ag, rather than direct reduction of CO2 on Cu. While indeed a simplification, this assumption is relatively consistent with the results shown in Figure 2, which suggest that the trend in jC2+ is primarily driven by the consumption of in situ produced CO. From l-GDE to s-GDE, the Ag layer with high concentration as shown in Fig. 3b increases the mean value of θ*CO realized within the catalyst layer. This phenomenon can be explained as follows: As the Cu: Ag area ratio increases, the formation of CO in Ag-CL occurs at the enrichment end near the inlet, thereby increasing the local concentration of CO, resulting in θ*CO near the inlet Ag/Cu boundary. Locally increasing, θ*CO decays along the length of Cu-CL as CO is consumed to form C2+.  The variation of the θ*CO curve with increasing Cu/Ag area ratio is reflected in the simulation shown in Fig. 3c, consistent with the above results. Figure 3d shows the average θ*CO in CL as a function of the Cu/Ag area ratio at constant jC2+, indicating that for the same jC2+, the average θ*CO in CuCL increases as the length of the Ag layer shrinks. An increase in the average θ*CO enhances jC2+ through mass action, corresponding to a decrease in the overpotential required to achieve the same C2+ current density. The simulation results provide evidence for the hypothesis that a higher average θ*CO is achieved in s-GDE.

Fig.4 CO flow curve in Cu/Ag s-GDE simulated at 700 mA cm-2

The simulations highlight the importance of optimizing the CO2 feed gas flow rate in tandem catalysis.  The CO produced by Ag is transported out of the CL and returned to the flow channel. In the flow channel, CO2 acts as a carrier gas, which convects CO along the channel and redistributes it along the length of the Cu-CL (Fig. 4). Therefore, if the molar flow rate of the CO2 feed is too low, there will not be enough convective flux to bring the generated CO to the channel length for adsorption and reaction on the Cu-CL segment. However, for too high a feed rate, CO will be preferentially expelled from the flow channel instead of being redistributed into CuCL.

In summary, the authors propose a segmented gas-diffusion electrode structure for the highly selective conversion of CO2 into C2+ products. The residence time of CO in the Cu-CL segment was maximized by optimizing the relative length and loading of Cu and Ag in Cu/Ag s-GDE. The 2D continuum model verified that the effects of CL area ratio, residence time, and feed flow rate on θ*CO jointly enhanced jC2+. The s-GDE architecture employed in this study offers unique opportunities for application in industrial systems for CO2 electrolysis. In addition to obtaining high C2+ current density and relatively low Ag loading, this study exploited channel gradients to enhance CO utilization in tandem catalysts. These downstream concentration gradients are more pronounced in industrial systems employing larger electrodes for large-scale CO2 reduction. Therefore, it is conceivable that the developed s-GDE scheme is more efficient at scale.

refer to:

Wu et al. And by in situ CO., 2022, 5:202-211.

DOI: 10.1038/-022--0.

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