Dear, this trend is related to the polarization curve shape. As you know, the last part of this curve deals with concentration losses related to the limit of mass transfer between biofilm and anode or cathode and the Terminal Electron Acceptor (TEA). In this regime, the decrease of output voltage as function of current density is non-linear as the voltage dramatically drops at high currents up to the last value of current, namely short circuit current, when voltage is zero. Power Density  (PD) curve is obtained as the products of output voltage (V) and current density (J), PD=V*I. If you plot power density vs current density, the first part of the curve is quite linear, corresponding to the ohmic region of the polarization curve and, after reaching the Maximum Power Point (MPP), PD falls because in the above formula I increases but, as response of the MFC, V decreases due to concentration losses. In practice, the cell isn't capable to produce the current required at a high voltage, i.e. provide high electrical power. Polarization and power curves are used to determine the operative condition of a MFC under load.

Thanks for the clarification. So much to understand!