Enhancing carbon dioxide reduction

Newswise — Scientists at Kanazawa University unveil in ACS Nano the ability of ultrathin sheets of tin disulfide to expedite the chemical conversion of carbon dioxide. This discovery holds immense significance in our pursuit of a carbon-neutral civilization.

The imperative objective of attaining a sustainable, carbon-neutral society necessitates the recycling of carbon dioxide (CO2) emitted from industrial processes. In this regard, there is extensive ongoing research on electrocatalysts capable of effectively transforming CO2 into alternative chemical substances with reduced environmental impact. Among the potential electrocatalysts are two-dimensional (2D) metal dichalcogenides, but their efficiency is often compromised by concurrent reactions. However, Yasufumi Takahashi and colleagues from Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, have now identified a 2D metal dichalcogenide that exhibits remarkable proficiency in converting CO2 into formic acid—an organic compound occurring naturally and serving as an intermediate product in chemical synthesis.

In their study, Takahashi and his team conducted a comparative analysis of the catalytic capabilities of two-dimensional (2D) sheets of disulfide (MoS2) and tin disulfide (SnS2), both belonging to the class of metal dichalcogenides. Of particular interest was SnS2, as pure tin has a well-known catalytic effect on formic acid production. Electrochemical assessments of these compounds unveiled that while MoS2 promoted the hydrogen evolution reaction (HER) instead of CO2 conversion, SnS2 exhibited favorable CO2 reduction activity while concurrently suppressing HER. The researchers further conducted electrochemical measurements on bulk SnS2 powder, which demonstrated comparatively lower catalytic activity in CO2 reduction.

To gain insights into the location of catalytically active sites within SnS2 and understand why the 2D material exhibits superior performance compared to the bulk compound, the researchers employed a technique known as scanning electrochemical cell microscopy (SECCM). SECCM utilizes a nanopipette to create a nanoscale electrochemical cell with a meniscus-shaped sensing probe on the sample surface, enabling the detection of surface reactivity. The measurements conducted using SECCM unveiled that the entire surface of the SnS2 sheet demonstrated catalytic activity, not just specific "terrace" or "edge" features within the structure. This observation provides an explanation for the enhanced activity observed in 2D SnS2 compared to bulk SnS2.

Through detailed calculations, the researchers gained additional insights into the underlying chemical reactions. These calculations substantiated that the formation of formic acid is an energetically favorable pathway when employing 2D SnS2 as a catalyst. This finding further supports the efficacy of 2D SnS2 in facilitating the reduction of CO2 to formic acid.

The findings of Takahashi and his team represent a significant advancement in the utilization of 2D electrocatalysts for electrochemical CO2 reduction applications. In the words of the scientists themselves, "These findings will provide a better understanding and design strategies for metal dichalcogenide-based 2D electrocatalysis for electrochemical CO2 reduction to produce hydrocarbons, alcohols, fatty acids, and olefins without by-products." This statement highlights the potential impact of their research on enhancing our comprehension of and ability to develop effective strategies for utilizing metal dichalcogenide-based 2D electrocatalysts in the production of various valuable compounds from CO2, while minimizing the formation of unwanted by-products.

Background

Two-dimensional metal dichalcogenides

Two-dimensional (2D) metal dichalcogenide sheets, also known as monolayers, consist of materials with the chemical formula MX2, where M represents a metal atom such as molybdenum (Mo) or tin (Sn), and X represents a chalcogen atom like sulfur (S). The structure of these sheets can be visualized as a layer of X atoms atop a layer of M atoms, which is then followed by another layer of X atoms. These 2D metal dichalcogenides are classified within the broader category of 2D materials, which also includes graphene, and are characterized by their remarkable thinness. Compared to their bulk (3D) counterparts, 2D materials exhibit distinct physical properties owing to their unique dimensional characteristics.

Historically, 2D metal dichalcogenides have been extensively investigated for their electrocatalytic activity in the hydrogen evolution reaction (HER), a process that generates hydrogen. However, a recent discovery by Yasufumi Takahashi and his team from Kanazawa University has revealed a unique behavior in the 2D metal dichalcogenide SnS2. Contrary to its counterparts, SnS2 does not exhibit any HER catalytic activity. Instead, it acts as a catalyst for the electrochemical reduction of carbon dioxide (CO2) to formic acid—a highly significant property in the context of implementing strategies to mitigate the global CO2 footprint. This finding opens up new possibilities for utilizing SnS2 as an efficient catalyst for CO2 reduction and highlights its potential role in advancing carbon dioxide conversion technologies.