Catalyst characterization and performance
The I-V measurements were performed from (0 to 28) hours cycle to study the electrocatalytic activity of UH-CSMFC, which is shown in fig. 2. To avoid any unnecessary electrochemical reaction by the metal catalyst and to promote stability, the same material (Graphite) was used as an anode and cathode. For continuous operation of the device, and for cleaning of the excess of nitrogen compounds from wastewater, the constant state is the most desirable, where the device behaviour is entirely irreversibly, which helps to feed the device.
From fig.3 (a) and (b), it confirms the catalytic activity for both the samples as Fig. 3(a) showing the comparison between fuel 0.5g/ml in a liquid state urea hydrogen microbial fuel cell (UH-MFC)and UH-CSMFC with urea fuel 0.5g/ml for checking the performance between them. The potential redox peak for urea bipolar CV measurements was in the range of 0 to ±1, similar to the values reported in the literature. The catalytic activities were found to be at ± 0.1 to 0.6 V range for urea and ± 0.5 V for the ammonium ions [5]. Both urea and ammonium ions are related to each other as sources of nitrogen and as fuel for accelerating the process of power generation used this time [3, 4, 9-12].
As shown in fig.3 (b), the EIS difference between the two samples. EIS measurements were performed to investigate further the electrochemical behaviour of the compost soil were real, and imaginary impedance studied in the frequency range from 0 Hz to 10,000 Hz for the applied field. Shows the electrocatalytic activity study by using bipolar CV measurements with the comparison of the urea liquid state UH-MFC and the effect of UH-CSMFC.
The comparative studies show that electrocatalytic activity increases gradually, as UH-CSMFC with urea fuel 0.5g/ml and its redox potential is higher in comparison to the urea liquid state UH-MFC with fuel in both the voltage polarities. The corresponding EIS measurement data, which matches the CV, trend fully. EIS measurements were performed to investigate the electrochemical behaviour of the compost soil. The high-frequency region in the semicircle shows the charge transfer resistance (Rct) between the working electrode/electrolyte interfaces that is caused by the faradaic-redox reaction of the electrode. In the case of compost soil with urea fuel, Rct is significantly decreased, which correlates with the increase electrocatalytic activity those results in a gradually reducing impedance.
The soil was known itself working as an electrocatalyst [4]. Similar to bacteria and enzymes, the soil may also catalyse the oxidation of urea. Due to the addition of the urea nitrogen source in the soil, the chemical reaction enhances the ph. The Vmax for the high-affinity response of reaction (N2O→NO→N2) showed a relatively small peak, followed by first a decline peak and then a sharp increase. Urea is always a portion of food for the bacteria; urea stimulates bacteria to release urease [21, 26]. When urea is hydrolysed, it generates ammonia, transforms to ammonium ions (NH3 to NH4 ions) which are not going to volatilize. Following this, volatilization ammonification, by following the nitrogen cycle fixation lead to nitrification and denitrification. Eventually release the last product to nitrogen (N2) from UH-CSMFC through the process of nitrification and denitrification.
Electrochemical measurements of UH-CSMFC
Fig. 4(a) shows the strength of the fuel cell concentration from 0.1 g/ml urea to 0.5 g/ml urea sample. The highest catalytic activity observed at 14 hours with the inert Gr/Gr electrodes. Then, the device stability was checked by adding the 0.5 g/ml urea fuel at regular intervals of time. The power density was 18.26 mW/m2, as evident from Fig. 4(b). Fig. 4(c) shows the effects of urea concentration on the power density of the cell [5, 29]. Electrocatalytic activity and electro-oxidation of urea showed the same trend for both the polarities of the redox potential. The urea fuel at higher concentration of 0.5 g/ml, a maximum oxidation peak generates power, and minimum over potential of the urea oxidation reaction was obtained. Thus, it was inferred that the current density was concentration-dependent, and the higher electrocatalytic activity was reported at the highest concentration of urea fuel. Which directly affects the power in compost soil performance.
The sustainability of the pH is confirmed at the beginning first running cycle (0 to 28 hrs) the pH compost was at lower 9.2 to slowly increase 9.7 in the fuel cell as compared with the liquid state UH-MFC while fuelling continually after every 28 hrs of the cycle in the Fig. 4(d).
The role of the pH is crucial at the liquid and the soil state of the compost fuel cell for the power output. Generally, microorganisms require the natural atmosphere for the optimal growth of the microbes for the generation of power. The biological and electrochemical reaction of the MFC changes with the pH level by consumption of urea. The new catalyst is cheap and used for cleaning process industrial wastewater, urea, and urine rich wastewaters with the generation of the energy from the waste products with UH-CSMFC.
When the fuel is feed at the beginning in the liquid state, the pH is near about 6.8 to 7.2 has a lower generation of electricity and still maintained the same Vs time for single cycle from 0 to 28 hrs with a single shot of fuel as shown form the Fig. 3(d). Yet, as compared urea fuel 0.5g/ml feed to the UH-CSMFC, this enhanced the power generation. The increase in the pH is due to the proton consumption and O.H. Generation by the anodic and cathodic side reactions, mostly indicating the effect of bacteria [7-9].
In fig. 5(a), (b), the power density reached its maximum peak at 14th hours and decrease to (0 to 28) hour’s cycle. The I-V measurements study explains the sustainability of fuel cell, while the power generation the pH also shows the stable behaviour as we optimized for a long time vs hours the fuel supply continues. The sustainable study was shown in fig.5 (a). A commercial fuel cell has been refuelled several times after every 28 hours. Accordingly, the power generation was monitored to assess its sustainability. The results show that the stable functioning of the device continues until the fuel supplied to the UH-CSMFC fuel cell.
To study the consumption of urea, we performed I-V measurements in which urea fuel in the liquid state first was injected as fuel with regular interval of time, and its current density, power density is calculated. Initially, we have injected the urea fuel and left for the activation. The first sample was activated and shows maximum peak power at the 14 hours in the single cycle, and power decreases. After refuelling it in the 2nd cycle with fuel, power again repeated to its maximum. This indicates that the urea is consumed in compost MFC device to generate power [21].
In the performance of MFC device pH, sustainability is measured at room temperature until 140 hrs in comparison to the working of a fuel cell and check the sustainability of the UH-CSMFC. From the results, pH in the liquid state is decreased while in the power generation process in compost soil starts higher up taking fuel. The balanced system was established within the range of pH 9.2 – 9.7 in the compost-based system. The higher pH does not affect the electricity generation due to the buffer effects of the bacterial activities in the fuel cell [21, 27, 28]. The fig. 5(b) mentioned the pH difference between the liquid and soil state that the soil state has stable and higher pH, which is helpful for the electricity generation for the compost fuel cells optimized and monitored regularly. The consumption of urea is to be used for cleaning process industrial wastewater, urea, and urine rich wastewaters with the generation of the energy by UH-CSMFC.
Performance ofbacteria
To study the role of bacteria, enzymes for generating hydrogen and electric power. Compare the power of compost soil standard sample before, and after killing the bacteria by doing the autoclaved sterilization study at 120 0C, [29].
The compost, soil demonstrates, the role of bacteria, enzymes in the functioning of the MFCs, the compost soil containing cells were sterilized by autoclave treatment, and the power generated by these cells were compared with those that were not sterilized. While the first sample contained bacteria in the compost soil sample, the second sample that was autoclaved at 1200C contained having no live bacteria. This was evident from fig. 6 (a) and 6(b), which shows the bacterial growth in plates after 28 hours. Bacterial colonies growth were visible in the plates, as shown in fig. 6(a), no colonies were found in the autoclaved sample shown in fig. 6(b). These results established the role of bacteria and enzymes in enhancing electricity production in the compost soil sample (fig. 6(c)). The Keithley I-V measurements studies shows that compost soil commercial device having a maximum power density of 18.26 mW/m2; the maximum power density observed in the autoclave treated sample was only 0.03 mW/m2. From these results, the role of microbes was demonstrated to be essential for the enhancement of power in the UH-CSMFC. In this compost soil system, MFC was found to produce enhanced energy and sustainability, due to the advantageous effects of different types of soil bacteria, enzymes (anaerobic and aerobic) [2, 21].
Mechanism discussion
An alkaline medium was used to carry out the urea electrolysis both for hydrogen production and direct electricity production:[5, 29]
The operating mechanism of UH-CSMFC is given below,
Anode reaction
The role of UH-CSMFC mechanism, as mentioned below.
CO (NH2)2 + H2O → 2NH3 + CO2
NH3 + O2 + 2e- → NH2OH + H2O …………. 1)
NH2OH + H2O→ NO2- + 5H+ + 4e- …………. 2)
NH4+ + NO2- → N2 + 2H2O
Cathode reaction
NH3 + H2O → NH4+ + OH-
The overall reaction for anode and cathode
2CO (NH2)2 + H2O → 3H2 + N2+ CO2
We have confirmed the combined mechanism for both compost soil, and urea fuel cell enhances the power generation due to urea fuel dissolved in a liquid state so that in soil bacteria, enzymes uptake, catalyze, then generate electricity and produce H2 + N2+ CO2 mixed gas in UH-CSMFC [5, 7, 28, 29].
Compost soil in operation performs ammonification by the process of nitrification and denitrification process to reach to release the last product (N2) as while supplying electron and protons. When urea was hydrolysed the urease enzyme releases in the soil is faster rate as compared to liquid, it generates ammonia, ammonium ions (NH4 + ions) later. Following further, the ammonification and volatilization lead to nitrification and denitrification process.
Reaction 1 conversion urea to ammonia, then hydroxylamine, is catalysed by enzymes ammonia monooxygenase. Reaction 2 converts the hydroxylamine to nitrite, catalysed by the enzymes hydroxylamine oxidoreductase [22, 23].
Hydrogen is separated from a hydrogen/nitrogen/carbon dioxide mixture by an electrochemical separation method. The apparatus for separating hydrogen was similar to that used in a polymer electrolyte membrane fuel cell for producing an electrical current. Pure hydrogen gas can be separated without pressurization, and the separation rate can be easily controlled by the applied current [25].
Oxidation from urea to nitrogen gas, carbon dioxide, and hydrogen by bacteria results in the generation of ammonia or transform to ammonium ions, which are converted to carbonic acid C.O. (OH) 2, or carbamate as reported in the literature before. Ammonification leads to (Nitrosomonas and Nitrobacter) to NO3 (nitrate) or directly NO2 (nitrite) in a process called nitrification, which eventually produces nitrogen (N2) [5, 21, 26, 28, 29,47-54].
Therefore, compost soil systems be a natural medium to transport electrons and protons easily in an eco-friendly and non-toxic manner for power and hydrogen generation. This study confirmed that the urea has a profound effect on the power and hydrogen generation from the UH-CSMFC. The focus is to get power from the UH-CSMFC in coming future by using waste like urea rich wastewater, urine, industrial wastewater, which contains much amount of urea and a huge source of hydrogen storage. [1-13,44-55].