Application of a five level central composite design to optimize operating conditions for electricity generation in a microbial fuel cell

Peer review under responsibility of Taibah University.

Figures & data

Fig. 1 Schematic structure of a two-compartment microbial fuel cell.

Table 1 Levels of the investigated factors in the central composite design.

Factors	Levels
	Symbol	−−	−	0	+	+ +
Temperature	Y1	12	17	22	27	32
Initial anodic compartment pH	Y2	5	6	7	8	9
Salt bridge component concentrations (agar and KCl)/100 ml	Y3	(2.1, 2.0)	(2.2, 3.0)	(2.5, 5.0)	(2.8, 7.0)	(2.9, 8.0)

Table 2 The central composite design for three independent variables: temperature (Y1), initial anodic compartment pH (Y2), and salt bridge component concentrations (agar and KCl) (Y3).

Trails	Factors
	Y1	Y2	Y3
1	−	−	−
2	−	−	+
3	−	+	−
4	−	+	+
5	+	−	−
6	+	−	+
7	+	+	−
8	+	+	+
9	− −	0	0
10	+ +	0	0
11	0	− −	0
12	0	+ +	0
13	0	0	− −
14	0	0	+ +
15	0	0	0
16	0	0	0

Fig. 2 The response of voltage yield (mV) as a function of initial pH and temperature (°C) based on the central composite design results.

Fig. 3 The response of voltage yield (mV) as a function of salt bridge component concentrations (g/100 ml) and temperature (°C) based on the central composite design results.

Fig. 4 The response of voltage yield (mV) as a function of salt bridge component concentrations (g/100 ml) and initial pH based on the central composite design results.

Fig. 5 The relationship between the response (voltage yield) and the different independent variables, showing the predicted optimal values based on the central composite design results.

Table 3 Observed responses of three independent variables: temperature, initial anodic compartment pH, and salt bridge component concentrations (agar and KCl).

Run	Voltage (mV)	Current (mA)	Current density (mA/m^2)	Power (mW)	Power density (mW/m^2)	Volumetric power density (mW/m^2)	Improvement ratio of control (%)	Coulombic efficiency (%)	Internal resistance (Ω)
1	638	21.26	1.62	13,563.9	1035.73	6784.06	−13.07	17.86	26.54
2	669	22.3	1.7	14,918.7	1138.83	7459.35	−8.85	18.73	26.39
3	550	18.33	1.39	10,081.5	769.72	5041.66	−25.06	15.4	27.72
4	727	24.23	1.84	17,615.2	1344.85	8808.81	−0.95	20.35	26.39
5	704	23.46	1.79	16,518.4	1261.1	8260.26	−4.08	19.71	26.39
6	749	24.96	1.9	18,695	1427.48	9350.01	2.04	20.97	26.38
7	689	22.96	1.75	15,819.4	1207.94	7912.01	−6.13	19.29	26.39
8	761	25.36	1.93	19,299	1473.59	9652.01	3.67	21.3	26.34
9	475	15.83	1.2	7519.25	574.1	3765.41	−35.28	13.3	27.08
10	809	26.96	2.05	21,810.6	1665.34	10,908	10.21	22.65	20.96
11	712	23.73	1.81	16,895.8	1289.93	8449.06	−2.99	19.93	26.39
12	785	26.16	1.99	20,535.6	1568	10,270.4	6.94	21.98	25.58
13	379	12.63	0.96	4786.77	365.49	2394.01	−48.36	10.61	27.89
14	793	26.43	2.01	20,959	1600.12	10,480.8	8.03	22.2	24
15(C)	732	24.4	1.86	17,860.8	1363.41	8930.4	−0.27	20.5	26.38
16(C)	734	24.46	1.86	17,953.6	1370.88	8979.26	0	20.55	26.38

Fig. 6 Current vs. time curve of a typical batch operation.