Comment on "unbiased solar H₂ production with current density up to 23 mA cm^-2 by Swiss-cheese black Si coupled with wastewater bioanode" by L. Lu, W. Vakki, J. A. Aguiar, C. Xiao, K. Hurst, M. Fairchild, X. Chen, F. Yang, J. Gu and Z. J. Ren, Energy Environ. Sci., 2019, 12, 1088

A hybrid microbial photoelectrochemical (MPEC) system recently reported by Lu et al. achieved a high photocurrent density of up to 23 mA cm^-2 on the basis of area of a small photocathode matched to a larger bioanode. This current density number followed the tradition of photoelectrochemical (PEC) systems but didn't report performance of the total system based on the cross-sectional area of the reactor, as preferred by researchers working on microbial electrochemical (MEC) systems. The overall current supplied by the bioanodes oxidizing organic matter in wastewater was 1.3-1.9 mA, which corresponded to 0.18-0.27 mA cm^-2 based on cross-sectional area or 44-69 A m^-3 based on catholyte volume. It is important to recognize that the performance of the system can be limited by either the electrodes or reactor configuration, so data presentations based on multi-parameters are necessary to advance similar interdisciplinary research.