Synergistic Integration of Zr-MOF (UiO-66) and Bi Electrocatalysts for Enhanced CO2 Conversion to Formate

The utilization of renewable energy-driven CO 2 conversion technology has garnered considerable attention as a potential remedy for both the energy crisis and climate change. Among various methods, the electrocatalytic CO 2 reduction reaction (CO 2 RR) has received particular focus due to its mild reaction conditions and its ability to produce various valuable products. Specifically, formic acid holds great promise for CO 2 electrolysis due to its potential for energy storage and transportation, as well as its commercial viability as indicated by techno-economic assessments. Bi, In, and Sn are several metal catalysts that have been reported for formic acid production, with Bi catalysts demonstrating favorable properties in terms of both cost-effectiveness and selective production of formic acid. However, despite efforts to enhance the intrinsic catalytic activity of Bi through methods such as nanostructuring and alloying, it has yet to achieve the desired level of performance. In light of recent findings by Nam et al. on the ability of a metal-organic framework (MOF) to regulate reaction intermediates for Ag catalyst, resulting in higher CO production, we draw inspiration from MOF's versatility and demonstrate the successful coupling of Bi with UiO-66, a Zr-MOF, to achieve higher CO 2 reduction rates and thus increase formic acid production [1]. We synthesized MOF materials, UiO-66 and NH 2 -functionalized UiO-66 (UiO-66-NH 2 ), and deposited Bi catalysts on the MOF structures using the NaBH4 reduction method, resulting in Bi/UiO-66 and Bi/UiO-66-NH 2 samples. To compare the catalytic activity, we also synthesized Bi particle samples using the same method (Bi). Prior to CO 2 reduction examination, all electrocatalysts were pre-treated in a 1.0 M KOH solution for 5 minutes, and then CO 2 electrolysis was performed in a flow-cell reactor. Among the synthesized samples, Bi/UiO-66 demonstrated excellent CO 2 reduction properties, exhibiting about 5 times higher current density (-220 mA/cm 2 ) at an applied potential of -0.7 V vs. the reversible hydrogen electrode (RHE) than Bi alone (-44 mA/cm 2 ), despite the identical electrochemically active surface area (ECSA) for both samples. On the other hand, Bi/UiO-66-NH 2 showed an almost identical ECSA-normalized current density compared to Bi/UiO-66, indicating the negligible effect of NH 2 functionalization on UiO-66 for CO 2 RR. Nevertheless, it is evident that the utilization of Zr-MOF (UiO-66) is beneficial in increasing the CO 2 conversion rate of metallic Bi catalyst. To comprehend the reason behind the superior catalytic activity exhibited by the Bi/UiO-66 sample, we conducted various characterizations, such as SEM, TEM, FTIR, Raman, and XPS. Our results revealed that the structural evolution of UiO-66 occurs by the formation of carbonate-coordinated Zr-hydroxide during CO 2 electrolysis, contributing to the high CO 2 reduction current density. Moreover, the disappearance of the carbonate-relevant peak in the C 1s from XPS analysis after the decline in catalytic activity suggests that the carbonate species formed at Zr-MOF site, which is the captured form of CO 2 molecules, play a crucial role in efficient CO 2 capture and conversion. These findings suggest that Zr-MOF can be used for CO 2 capture and conversion with high efficiency. [1] Nam et al., J. Am. Chem. Soc. 2020, 142, 51, 21513–21521.