Electrochemical CO₂ reduction with earth-abundant metal catalysts

Energy production and CO₂ emissions are strictly connected. One of the most efficient and straightforward mitigations to the climate change is the conversion of CO₂ into chemicals that may play the role of energy vectors in a carbon-based energy cycle. Practical aspects of using abundant and low-cost materials are crucial for real-world applications, in particular redox catalysts with high turnover number, selectivity, and efficiency to overcome the CO₂ stability. The difficult process of achieving new technologies benefits from the knowledge and experience of previous achievements, guiding researchers toward new discoveries. This article briefly illustrates and discusses the main findings that have paved the way of CO₂ conversion to fuels or fuel precursors by electrochemical methods using redox catalysts that may find concrete applications.

Introduction

About 85% of the world's primary energy supply is currently obtained by the combustion of fossil fuels. As a result, anthropogenic CO₂ emissions into the atmosphere have reached the value of about 37 Gt/y [1], which is one of the primary causes of global warming and climate change. A level of 450 ppm of CO₂ in the atmosphere has been predicted by many models as the concentration necessary to stabilize the increase of the global average temperature to 2 °C at the end of 21st century; to keep CO₂ below this limit, between 40% and 70% of the total anthropogenic emissions, compared with 2010 emissions, must be reduced by 2050 and no anthropogenic emissions by 2100, as reported in the Paris Agreement [2,3]. The European Union strongly promotes the research toward the so-called “decarbonization” of the energy production sector through several projects, like the “European Green Deal.” Renewable approaches, such as photovoltaic, have seen a dramatic boost to real-world applications in recent years, but the nature and intermittency of the sunlight source make the technology less directly applicable in some sectors, such as transport and heating, where the combustion of fossil fuels is still dominant. There is a strong need for sustainable (sustainability is defined as “the ability to meet the current needs without compromising the ability of future generations to meet their needs” [4]) and environmentally friendly technologies able to provide energy vectors for human activities. In recent years, the need for a clean and green approach becomes mandatory [5, 6, 7]. The energy source for all renewable technologies is, directly or indirectly, the sun. The conversion and storage of sunlight into energy can occur via different pathways. A possible approach of minimizing carbon emissions and closing the carbon cycle as energy vector would be the conversion of CO₂ into fuels or fuel precursors by means of sunlight. Although the direct artificial photosynthetic approach [8, 9, 10, 11] is extremely interesting and potentially very efficient, but still far from real-world applications, separating the light-harvesting from CO₂ reduction processes is a suitable path. Indeed, the photocatalytic and electrochemical reduction share, in principle, almost an identical catalyst or catalyst precursor. The main difference is the way in which the radical anion is produced: while the electrochemical methods afford it directly at the electrode surface, photochemical ways require a subsequent quenching of the excited state by an external electron donor, usually a sacrificial reagent. This article covers the electrochemical reduction of CO₂ by means of selected earth-abundant metals. In very recent years, a significant number of papers dealing with the electrochemical reduction of carbon dioxide appeared, as well as many excellent reviews [12∗∗, 13, 14, 15∗, 16∗∗, 17, 18, 19, 20, 21∗]. We are trying to provide here an overview, covering complementary approaches and catalysts appeared in the field.