In situ characterization technology reveals the efficient catalytic CO2 electroreduction reaction of metal copper nanograin boundaries

On February 9, 2023, Beijing time, Academician Yang Peidong’s research group from the Department of Chemistry at the University of California, Berkeley published the research results entitled “Operando studies reveal active Cu nanograins for CO2 electroreduction” in the journal Nature.

This work uses a variety of advanced in situ characterization techniques (operando methods) to reveal the complex structural evolution process of copper nanocatalysts under the conditions of carbon dioxide electroreduction reaction. This systematic in situ experimental study provides key experimental support for the thesis that the actual efficient reactive sites in the process of CO2 electroreduction come from metallic Cu nanograins.

The corresponding author of the paper is Yang Peidong, and the co-first authors are Yang Yao, Sheena Louisia and Sunmoon Yu.

Climate change caused by excessive carbon dioxide emissions is a severe challenge faced by major economies around the world, which has profoundly affected social and economic development, and actively addressing climate change has become a global consensus. In the field of carbon dioxide recovery and reuse, carbon dioxide electrocatalytic conversion is one of the most promising directions. Since the 70s of last century, scientists have discovered that copper can catalyze the reduction of carbon dioxide to produce a series of valuable multi-carbon basic chemical raw materials, such as ethylene, ethanol, propanol, etc. However, the catalytic mechanism of copper-based catalysts and their catalytic active sites have been poorly studied and intuitively studied.

In this work, under the leadership of Yang Yao, Academician Yang Peidong’s team combined a number of in situ characterization techniques through a variety of in situ characterization methods, including in situ electrochemical scanning transmission electron microscopy (EC-STEM) and four-dimensional STEM (4D-STEM) under liquid phase conditions, and in situ high-energy resolution fluorescence detection hard X-ray absorption spectroscopy (HERFD-XAS). In situ resonance soft X-ray scattering spectroscopy (RSoXS) and time-resolved in situ differential electrochemical mass spectrometry (DEMS) provide comprehensive quantitative analysis of the valence state and chemical environment of copper nanocatalysts under electrochemical reaction conditions, and provide clear and intuitive experimental evidence to reveal the evolution of copper nanocatalysts before and after the entire CO2 reduction reaction (Figure 1a).

The systematic in situ characterization technology comprehensively reveals the evolution of copper nanocatalysts during the reaction.Academician Yang Peidong’s team found that small-sized spherical copper nanocatalysts have excellent carbon dioxide reduction activity and multi-carbon product selectivity, but due to their highly active nature during electrochemical reactions, the traditional offline characterization technique (ex situ) can only observe the initial state of copper@cuprous oxide(Cu@Cu2O) core-shell structure and single-crystal cuprous oxide (Cu2O) nanocubes (P) generated after contact with air and water after carbon dioxide reduction reaction (P. Yang, PNAS, 2017, 114, 10560; PNAS, 2020, 117, 9194)。 In this work, the authors used the system in situ characterization technique (operando/in situ) to comprehensively reveal the evolution of copper nanocatalysts during the reaction using copper nanoparticles of 7-18 nanometers as model catalysts. This evolutionary process includes the initial desorption of the active agent ligand on the catalyst surface and the reduction of the Cu2O oxide layer on the surface, to the initial agglomeration of nanocrystals and further structural reforming, and finally the generation of zero-valent nanograin-bounded-rich polycrystalline metal copper catalysts, which contain a large number of reactive sites of low coordination copper. These structurally defective nanograin boundaries are closely related to catalytic reactivity, which can efficiently reduce carbon dioxide to multi-carbon products such as ethylene, ethanol, and propanol during the catalytic process. However, after the reaction termination catalyst is exposed to air and water, the structure rich in nanograin boundaries will also activate the double bonds of oxygen, promoting the rapid dissociation and insertion of oxygen atoms on the copper lattice, thereby enabling the rapid reconstruction of the actual catalytic structure to generate a single crystal Cu2O cube. The rapid occurrence of this process is an important reason why it is difficult to observe the actual reaction site of copper catalysts in a large number of previous ex situ experiments. In situ electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) can track the morphological changes of copper nanocatalysts under CO2 reduction conditions while ensuring reliable electrochemical reaction conditions while providing a spatial resolution of 1 nm in the liquid phase (Figure 1b-m).

Figure 1: (a) In situ characterization techniques can provide detailed characterization of the evolution of copper catalysts in electrochemical reactions that traditional offline cannot provide. (b-m) Dynamic agglomeration of nanoparticles of 7 nm copper under 0 V and -0.8 V vs. RHE conditions.

In situ liquid phase 4D-STEM characterization techniques.A key contribution to this work is the further development of the structural characterization technique of in situ liquid phase 4D-STEM based on EC-STEM (Figure 2). 4D-STEM technology acquires a two-dimensional electron diffraction at each pixel of a two-dimensional STEM image at the same time, and thanks to the single-electron sensitivity of the electron microscope pixel array camera (EMPAD), 4D-STEM can obtain high-quality electron diffraction at very low electron doses. Based on the analysis of experimental results, the authors found that the copper nanocatalyst reduces the surface cuprous oxide of monodisperse Cu@Cu2O nanoparticles at the initial mild reaction potential (about 0 V) and initially agglomerates into loosely bound metallic copper polymers (Figure 2e, f). Next, under the electric potential (-0.8 V) of actual CO2RR operation, the polymer is further agglomerated into a compact polycrystalline metallic copper (Figure 2j). After the end of the reaction, once exposed to air, the polycrystalline metallic copper will rapidly oxidize into a single-crystal cuprous oxide cube.

Figure 2: In situ electrochemical 4D-STEM dynamic tracking of morphology and structural changes of copper nanocatalysts in and after CO2RR reaction under liquid phase conditions.

In situ X-ray spectroscopy characterization reveals low coordination active copper sites.In order to accurately measure the chemical valence states of copper nanocatalysts during the reaction, hard X-ray near-edge absorption spectroscopy (XANES) in situ high-resolution fluorescence detection mode (HERFD) was used to trace the reaction valence states of 7- and 18-nanocatalysts (Figure 3a-d). 7 nm copper nanocrystals can be changed to a metallic state at 100% during the reaction (experimental error of 1% and below). In contrast, only 30% of the copper oxide on the surface of 18 nm copper nanocrystals can be turned into activated metallic copper. The CO2RR reaction products corresponding to the metal copper nanograin boundaries produced by the former are 6 times more selective than the latter (Figure 3f-g). In addition, in situ extended edge X-ray fine spectroscopy (EXAFS) demonstrated that the coordination number of polycrystalline copper nanograin boundaries during the reaction was only about 8, fully confirming that the formation of polycrystalline nanograin boundaries can provide active copper sites with low coordination (Figure 3e).

Figure 3: In situ HERFD XANES and EXAFS tracking changes in the chemical valence and coordination environment of copper nanocatalysts during and after CO2RR reactions. High-precision HERFD-XANES quantification can establish a quantitative structure-activity relationship with the selectivity of multi-carbon products produced by CO2RR.

In summary, this study is a milestone work for the in-situ characterization and tracking of catalyst structures under liquid phase electrochemical reaction conditions. Further combining in situ electron microscopy and in situ synchrotron radiation X-ray technology, this study provides a paradigm for the comprehensive analysis of the morphology, structure, chemical valence state and coordination environment of catalysts under reaction conditions. The researchers believe that the combination of multiple in situ characterization techniques will provide a powerful method for characterizing the actual active sites and their fine evolution under electrochemical reaction conditions relevant to other energy applications. (Source: Science Network)

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