Accelerating electrochemical CO₂-to-CO conversion with Zn–Al layered double hydroxide amorphized by grinding treatment

Carbon dioxide (CO2) electrolysis is one of the promising technologies to convert CO2 into value-added chemical compounds and has attracted attention in recent years from the perspective of energy crisis and carbon neutrality. For efficient CO2 electrolysis, numerous attempts have been made to develop superior electrocatalysts that accelerate the CO2 reduction reaction (CO2RR), and their performance has been enhanced by controlling structure, morphology, and elemental composition. However, the effect of grinding treatment of catalysts on their structure, morphology, and CO2RR activity is frequently underestimated, which is performed for grain refining of catalyst particles to achieve optimum activity in general. In the present paper, we focused on Zn–Al layered double hydroxide (LDH) as an electrocatalyst which has CO2RR activity for carbon monoxide (CO) evolution. Zn–Al LDH and its ground samples for 10, 20, 30, and 60 min (Gx-LDHs, x = 0, 10, 20, 30, and 60, respectively) were prepared using a facile co-precipitation method and simple grinding with a mortar and pestle. Remarkably, Gx-LDHs exhibited superior activity for CO2-to-CO conversion as grinding proceeded. In particular, the growth rate in partial current density of CO was 3.2-fold and 1.6-fold from G0-LDH (as-prepared) to G10-LDH and from G10-LDH to G60-LDH, respectively. XRD and TEM analysis showed that the crystal structure of Zn–Al LDH collapsed due to amorphization as grinding proceeded. XAFS analysis indicated that Zn sites in the amorphous phase are mainly in the low-valent Zn state (Znδ+, 0 < δ < 2), which is a favorable state for CO2-to-CO conversion. Our results highlighted that grinding treatment clearly affected the structural and morphological properties of Zn–Al LDH, enhancing its CO2RR activity.