Plasma-based CO2 conversion: How to correctly analyze the performance?

Plasma-based CO2 conversion is promising for carbon capture and utilization. However, inconsistent reporting of the performance metrics makes it difficult to compare plasma processes systematically, complicates elucidating the underlying mechanisms and compromises further development of this technology. Therefore, this critical review summarizes the correct definitions for gas conversion in plasma reactors and highlights common errors and inconsistencies observed throughout literature. This is done for pure CO2 splitting, dry reforming of methane and CO2 hydrogenation. We demonstrate that the change in volumetric flow rate is a critical aspect, inherent to these reactions, that is often not correctly taken into account. For dry reforming of methane and CO2 hydrogenation, we also demonstrate inconsistent reporting of energy efficiency, and through numerical examples, we show the significance of these deviations. Furthermore, we discuss how to measure changes in volumetric flow rate, supported by data from two experimental examples, showing that the sensitivity inherent to a standard component and a flow meter is essential to consider when deriving the performance metrics. Finally, some general recommendations and good practices are provided. This paper aims to be a comprehensive guideline for authors, to encourage more consistent calculations and stimulate the further development of this technology.