Research in the field of bioelectrochemical systems is addressing the need to improve components and reduce costs for their large-scale application, leading to the study of innovative solid electrode separators made of biochar and terracotta. The tested biochar-based composites are produced from giant cane (Arundo Donax L.). Two different types of composite are used in this experiment: composite A, produced by pyrolysis from crushed chips of mixed clay A.donax L.; and composite B, produced by pyrolysis of already pyrolyzed giant cane (biochar) mixed with clay. The electrical resistivity, electrical capacitance, porosity, water retention and water leaching of the two types of composites (A and B) with 1, 5, 10, 15, 20 and 30 mass percentages of carbon (w/w) are characterized and compared. Less than 1 kΩ cm of electrical resistance is obtained for composite A with a carbon content greater than 10%, while the physical and electrical performance of composite B does not change significantly. SEM micrographs and 3D micro-computed tomography of different composite materials are provided, demonstrating a different structure of the carbon matrix in the terracotta matrix. The ability of composite A to duly reduce electrical resistance and increase water retention/leaching paves the way for a new class of resistive materials that can be used simultaneously as electrolyte separators and as external electrical circuits, enabling the compact design microbial fuel cells (MFCs). Proof of concept for this MFC design has been provided for several tested composites. Although all anolytes become anaerobic, only MFCs equipped with the A30% composite were able to produce energy, reaching maximum peak power with a resistance of approximately 1 kΩ. The low but significant power produced (around 40 mW/m² of cathode area) confirms that the proposed solution is particularly suitable for the recovery of nutrients and the bioremediation of environmental pollution, where energy collection is not required.