Electrocatalytic property of Zn-Al layered double hydroxides for CO2 electrochemical reduction

ABSTRACT Electrocatalytic CO2 reduction reaction (CO2RR) has attracted considerable attention as a technology to recycle carbon dioxide (CO2) into raw materials for chemicals using renewable energies. In this study, the electrocatalytic CO2RR activity of Zn-Al layered double hydroxides (LDHs) was studied. Zn-Al LDHs loaded carbon sheets were prepared, and CO2 RR was performed using CO2-saturated KHCO3 electrolyte to confirm the catalytic ability of Zn-Al LDH. Zn-Al LDHs intercalated with CO3 2− anion were synthesized using the mixture of metal nitrates with the different molar ratio of Zn2+/Al3+ by the co-precipitation process, whose corresponding products were named as Zn2Al1 LDH, Zn3Al1 LDH, and Zn4Al1 LDH, respectively. Except for Zn2Al1 LDH, ZnO was observed to exist as an impurity. The synthesized Zn-Al LDHs exhibited the electrocatalytic CO2RR activity for CO formation. In the case of the Zn2Al1 LDH, the current density of 15 mA cm−2 was obtained with 77% selectivity for CO and 94% selectivity for (CO + H2) at − 1.4 V vs. RHE. Furthermore, Zn3Al1 and Zn4Al1 LDHs showed a significant change relating to ZnO impurities in the XRD patterns and SEM images before and after the CO2RR whereas Zn2Al1 LDH did not show it. These results indicate that Zn-Al LDH is promising as a CO2RR electrocatalyst for CO formation.


Introduction
In recent years, carbon dioxide (CO 2 ) reduction reactions (CO 2 RR) have been attracting attention as a technology to achieve carbon neutrality, using surplus electricity generated from renewable energy sources such as solar and wind power [1,2]. Processes of CO 2 RR include photochemical, biochemical, and electrochemical reduction [3]. Among these processes, the electrochemical reduction is expected to have higher productivity, higher energy conversion efficiency, and higher sustainability than the other processes because surplus electricity can be used directly as an energy source [4]. Furthermore, the products of CO 2 RR include various reduction products such as carbon monoxide (CO), methane (CH 4 ), methanol (CH 3 OH), formic acid (HCOOH), and C 2 products, and selectively obtaining products is also considered an important function. Among these, CO can be used as a raw material for various chemicals: for example, the Fischer-Tropsch method can be used to synthesize liquid hydrocarbon fuels. Equation (1)-(3) shows the reaction of electrochemical reduction of CO 2 to CO under alkaline conditions. As shown in Equation (1), the formation of CO by CO 2 RR on the cathode side involves the formation of hydroxide ions (OH − ), and its activity tends to increase under alkaline conditions [5]. The CO 2 RR electrocatalyst must have high durability against alkaline conditions. Current research on CO-forming CO 2 RR has shown that catalysts using precious metals such as silver and gold have high catalytic efficiency [6]. However, precious metals are a limited resource, and there is a need to develop inexpensive electrocatalysts that do not use precious metals.
We focused on layered double hydroxide (LDH) as a material to solve this problem. LDH has the general composition [M 2+ 1−x M 3+ x (OH) 2 ] x+ [A n− x/n ·yH 2 O] x− as shown in Figure 1, which is composed of positively charged metal hydroxide layers containing divalent and trivalent metal ions (M 2+ and M 3+ ) and charge compensating anions (A n− ) and water inserted between the layers. The advantages of using LDH as a catalyst include high stability in alkaline solutions [7], high hydroxide ion conductivity [8], various metal compositions [9], and large specific surface area [9]. These properties are also preferable to electrocatalysts for different reactions, and thus, transition metal-containing LDHs have been studied as electrocatalysts for oxygen evolution [7,10,11] and oxygen reduction reactions [7,10,12]. Recently, the application of Cu-Al LDHs as CO 2 RR electrocatalysts has been reported, showing that Cu-Al-based LDHs can reduce CO 2 to CO and HCOOH [13]. Moreover, M-Al LDHs (M = Ni, Co) [14] and Zn-M LDHs (M = Al, Ti, Ga) [14,15] have also been reported to exhibit CO 2 RR catalytic activity for "photocatalysis", and Zn-Al LDH was found to have the highest CO selectivity (90%) [15].
In this study, the electrocatalytic CO 2 RR activity of Zn-Al LDHs was studied. Zn-Al LDHs loaded carbon sheets were prepared, and liquid-phase CO 2 RR was performed to confirm the catalytic ability of Zn-Al LDH.

Preparation of the LDH-loaded carbon sheet as a working electrode
The LDH-loaded carbon sheet was prepared by the following procedure. First, 7.5 mg of the Zn-Al LDH, 7.5 mg of the conductive aid (Vulcan XC72, Cabot Corp.), and 6.0 μL of the binder (Teflon solution: 60 mass% ethylenechlorotrifluoroethylene copolymer (ECTFE)) were mixed by grinding, resulting in the film- like mixture. The obtained mixture was pressed onto a carbon sheet at 3 MPa for 10 s and 5 MPa for 30 s, and the excess area of the mixture was cut. The coating area was 1.0 cm 2 with a 1.0 cm square shape.

Electrocatalytic CO 2 RR experiment
Electrocatalytic CO 2 RR was carried out by using a threeelectrode setup composed of a two-chamber cell (H-type cell), as shown in Fig. S1. The cathodic compartment and anodic compartment were separated by a piece of the anion exchange membrane to avoid the unexpected influence of the oxidation reaction taking place on the counter electrode.  [17].
Where V was the volume concentration of CO or H 2 in the produced gas from the reaction cell. I (mA) was the average current during the reaction, and r was the CO 2 flow rate (m 3 s −1 ) at ambient temperature and pressure. For the other constants in the formula, p was 1.013 × 10 5 Pa, F was 96,485 C mol −1 , R was 8.3145 J mol −1 K −1 , and T was 298 K.  the peaks of zinc oxide (ZnO) were observed as impurity peaks [18].

Characterization of Zn-Al LDHs
From the FE-SEM images, as shown in Figure 3, plate-like crystals, characteristic of LDH, with a size of 0.1-1 μm were observed in all the systems, but nanometer-order fine particles were also observed in Zn 3 Al 1 and Zn 4 Al 1 LDHs, which appeared to be the most abundant in Zn 4 Al 1 LDH. These fine particles could be assigned to be ZnO, considering the XRD results. The SEM-EDX mapping showed that Zn 2+ and Al 3+ were homogeneously distributed on the particles in the micrometer-order in Zn 2 Al 1 and Zn 3 Al 1 LDH as shown in Figures S2 (a) and (b). On the other hand, a localized presence of Zn was observed in Zn 4 Al 1 LDH as shown in the green circle of Figure S2 (c). This also indicates the presence of ZnO, considering the XRD results. The elemental analysis by ICP-AES showed the chemical composition with Zn 2+ /Al 3+ molar ratios of 2.16, 3.11, and 4.17 for Zn 2 Al 1 , Zn 3 Al 1 and Zn 4 Al 1 LDH, respectively. These ratios are almost the same as the starting composition ones.
These results are accordance with the previous studies with the urea method [19], as well as coprecipitation method [20,21]. One of them postulated that the crystallization of a hydrotalcite with a Zn 2+ /Al 3+ molar ratio of 2:1 occurred more preferentially rather than that of 3:1 [21]. Therefore, in the present study, it was considered that a hydrotalcite with a Zn 2+ /Al 3+ molar ratio of 2:1 was a main product even in Zn 3 Al 1 and Zn 4 Al 1 LDH systems, where excess Zn 2+ ions were not incorporated into the hydroxide layer and were precipitated as ZnO.

Electrocatalytic CO 2 RR experiment with LDHloaded carbon sheet in 0.1 M aqueous KHCO 3 solution
In the electrocatalytic CO 2 RR with Zn-Al LDHs, 0.1 M aqueous KHCO 3 solution was used as a typical electrolyte. The applied potential dependence of current density (j) and Faradaic efficiency (FE) are shown in Figures 4 and 5, respectively. As shown in Figure 4, the current density increased with more negative applied potentials in all the samples, and catalytic currents were observed. The total current densities in all the systems are almost the same at − 0.6 to −1.2 V vs. RHE. The additional measurement at −1.4 V vs. RHE was performed only for Zn 2 Al 1 LDH, showing the maximum current density of −15.2 mA cm −2 at −1.4 V vs RHE. CO was observed as a major product and increased at more negative applied potentials in all the systems. For Zn 2 Al 1 LDH, as shown in Figure 5 (a), the FE for CO (FE CO ) was 1, 18, 38, 63, and 77% at − 0.6, −0.8, −1.0, −1.2, and −1.4 V vs. RHE, respectively. And then, the hydrogen (H 2 ) formation was observed as a major side-reaction with the FE of 17-41%, and electron consumption by other reactions gave the FE of 6-41%. CH 4 was detected as a minor product but its FE was less than 0.5% for all the cathodes. In summary, the partial current density for CO formation (j CO ) of 11.7 mA cm −2 was obtained with 77% selectivity for CO and 94% selectivity for (CO + H 2 ) at −1.4 V vs. RHE for Zn 2 Al 1 LDH. The highest FE CO of 77% for Zn 2 Al 1 LDH is lower than that of Au nanoparticles (the highest FE CO of 97% [22]) reported so far, but higher than that of Cu-Al LDH (the highest FE CO of 42% [13]) reported  previously, indicating that Zn 2 Al 1 LDH is promising as a non-precious CO 2 RR electrocatalyst for CO formation. For Zn 3 Al 1 and Zn 4 Al 1 LDHs, the FE of each product did not differ significantly to Zn 2 Al 1 LDH at all the applied potentials, suggesting that all of them acted as CO 2 RR electrocatalysts and ZnO in Zn 3 Al 1 and Zn 4 Al 1 LDHs did not have a significant contribution to CO formation. Zn is known to be the only earth-abundant monometallic electrocatalyst with high CO selectivity, but bulk Zn catalysts tend to show large overpotentials and slow reaction rates due to small numbers of active sites [23]. Since the monovalent Zn + (3d 10 4s 1 ) site has been found to be an active site in many cases due to its coordinatively unsaturated characteristics [24], the monovalent Zn + site could also act as an active site in the case of Zn-Al LDHs. Figure 6 shows the XRD patterns of Zn 2 Al 1 , Zn 3 Al 1 , Zn 4 Al 1 LDHs before and after CO 2 RR. In all the systems, the Zn-Al LDH peaks were observed even after CO 2 RR, where the peak positions did not change. On the other hand, the ZnO peaks disappeared in Zn 3 Al 1 and Zn 4 Al 1 LDHs, and new peaks corresponding to zinc carbonate (ZnCO 3 ) [25] appeared in Zn 4 Al 1 LDH, while no peaks other than Zn-Al LDH was observed in Zn 2 Al 1 LDH after CO 2 RR. Figure S3 shows the SEM images of the carbon sheet loaded with Zn-Al LDHs before and after CO 2 RR. In all the LDHs, significant changes in the size of particles were not observed at the major region. However, only in Zn 4 Al 1 LDH, new spherical particles of about 1 μm were locally observed after CO 2 RR as shown in Figure S4(d). These spherical particles were indicated to be ZnCO 3 derived from dissolved ZnO considering the XRD results. This is also supported by the EDX mappings as shown in Figure S4, where the local region occupied by the spherical particles clearly showed more zinc components than the major region. The above XRD and SEM-EDX results suggest that Zn-Al LDH is electrochemically more stable than ZnO under the present experimental conditions.