Correlating the catalyst activity, selectivity, and stability with its structure and composition is of the utmost importance in advancing the knowledge of heterogeneous electrocatalytic processes for chemical energy conversion. Well-defined colloidal nanocrystals with tunable monodisperse size and uniform shapes are ideal platforms to investigate the effect of these parameters on the catalytic performance. In addition to translating the knowledge from single-crystal studies to more realistic conditions, the morphological and compositional complexity attainable by colloidal chemistry can provide access to active catalysts which cannot be produced by other synthetic approaches. The sample uniformity is also beneficial to investigate catalyst reconstruction processes via both ex situ and operando techniques. Finally, colloidal nanocrystals are obtained as inks, a feature which facilitates their integration on different substrates and cell configurations to study the impact of interactions at the mesoscale and the device-dependent reaction microenvironment on the catalytic outcome. In this Review, we discuss recent studies in selected electrochemical reactions and provide our outlook on future developments on the use of well-defined colloidal nanocrystals as an emerging class of electrocatalysts.

In this short critical perspective, we outline the serious problems caused by air pollution in Europe. Using two types of metrics, level assessment and trend assessment, we quantify the contribution of ammonia, NO x , SO x , non‐methane volatile organic compounds, and particulate matter in terms of years of life lost per capita and explain the connection between the various pollutants and their effects on human health and the environment. This is done on the basis of data collected by individual European Union (EU) member states as well as by the EU as a whole. We examine general emission trends as well as sector‐specific emissions and discuss the effectiveness of current legislation in reducing health risks and environmental damage. By combining these results with a cost–benefit analysis, we show that a further reduction in NO x emissions is the most urgent and potentially the most beneficial.

With the electrochemical CO2 reduction reaction (CO2RR), CO2 can be used as a feedstock to produce value-added chemicals and fuels while storing renewable energy. For its enormous potential, an extensive research effort has been launched to find the most active electrocatalyst. The reduction of catalyst size has been tested and proven as a key approach to increase the activity of CO2RR while reducing capital cost. However, the catalytic selectivity is not linearly related to the catalyst size due to the influence of many other structural factors. Thus, in-depth knowledge of structure-performance relationships of metal catalysts with different sizes aids in designing efficient electrocatalysts for CO2RR. This Review surveys three decades of research on CO2RR and categorizes various metal catalysts into four size regimes, namely, bulk materials in the form of single crystals, nanoparticles, clusters, and single-atom catalysts. The effects of different structural factors, including crystal facet, coordination environment, metal–support interactions, etc., on the catalysts in each size regime are discussed. Finally, general conclusions are provided with perspectives on future directions for better understanding and further development of active and selective catalysts for CO2RR.

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