Beyond the rate: A critical analysis of epistemological and methodological pitfalls in CO 2 reduction benchmarking for sustainable fuel production

Photocatalytic CO2 reduction (CO2RR) represents a promising route toward sustainable solar fuel generation and climate change mitigation. However, progress in this field is increasingly hindered by a fundamental disconnect: universal benchmarking through indirect quantification of trace reaction products rests upon physical and analytical assumptions that rarely withstand rigorous scrutiny. In this critical review, these hidden variables are systematically deconstructed. The CO2RR framework is contrasted with the well-established direct quantification paradigm of pollutant mineralization (e.g., benzene oxidation), with S-scheme heterojunctions being utilized as a consistent mechanistic platform for comparison. It is demonstrated that conventional rate-based comparisons, the dominant metric in the literature, systematically obscure two critical methodological pitfalls: (i) the Adventitious Carbon Paradox, wherein product signals originate from residual organic contaminants rather than true CO2 conversion and (ii) the Thermodynamic Illusion, where mass-transport limitations in batch reactors fundamentally distort apparent kinetic parameters. Through quantitative case analysis, it is shown that a high apparent CO2-to-CH4 conversion rate (e.g., 22.5 µmol g−1 h−1) can mask an actual hourly single-pass CO2 conversion efficiency of less than 0.01%. To address these epistemological gaps, a three-tiered framework for rigorous benchmarking is proposed, mandating: (i) standardized pre-cleaning protocols, (ii) isotopic validation as a community norm rather than exception, and (iii) a paradigm shift from isolated “rate” reporting toward practical “conversion” metrics that reflect scalable, real-world performance. This review provides both a diagnostic critique of current practices and a constructive pathway forward. It aims to align academic evaluation with the technological requirements of sustainable fuel generation.