Short communicationA wireless charger powered the extracellular electron transfer for hydrogen recovery from organics
Introduction
With the rise of microbial electrochemical technology (Chen et al., 2019a, 2019b), MECs burst out enormous potentiality for hydrogen recovery during wastewater treatments (Ki et al., 2016), since the model of MEC proposed in 2005 (Hong et al., 2005; Logan, 2006). The principle of microbial electrolysis cell producing hydrogen is that organic matter can be oxidized at anode while the protons can combine with electrons to produce hydrogen at cathode. The electrons generated by the oxidation of organics are transferred to the anode via an electron carrier located in the outer membrane of the cell and then pass through an external circuit to the cathode, where the electrons are combined with hydrogen ions (collected by the proton exchange membrane or directly through the electrolyte to the cathode) to produce hydrogen under the drive of a low external voltage (Gil-Carrera et al., 2013; Hong and Hu, 2010). Comparing with other methods, MEC hydrogen production has the following merits: 1) various substrate selection, where Biodegradable waste that can be theoretically used by microorganisms can be converted to hydrogen in MECs, such as, domestic sewage, distillery wastewater, corn straw fermentation liquid, activated sludge and other organic waste (Anders et al., 2011; Asztalos and Kim, 2015; Cotterill et al., 2017; Escapa et al., 2009; Nam et al., 2014), and the final metabolites of dark fermentation (such as acetic acid, propionic acid, butyric acid) can be further transformed as hydrogen (Logan et al., 2008); 2) the energy conversion rate of hydrogen production by MECs can reach 50%~70%, while the maximum energy conversion rate of hydrogen production by traditional fermentation is less than 15% (Nath et al., 2006). Furthermore, MECs hydrogen production has no secondary pollution compared with chemical hydrogen production (Nath et al., 2006); 3) MECs producing hydrogen requires lower potential difference (0.2–0.8 V) compared with traditional water electrolysis (>2.1 V)(Fornero et al., 2010; Shen et al., 2016).
Rozendal et al. estimated that the materials of MECs used in laboratory might have costs up to 800 times higher than anaerobic digestion (Rozendal et al., 2008). Therefore, the process of expanding the laboratory-designed MEC to an industrial scale needs to consider the cost of the enlarged reactor (Escapa et al., 2012). In addition to expensive cathode materials and membranes, field applications will inevitably mean handling relatively large currents (Escapa et al., 2016). Small equipment can be connected in parallel to solve this problem, however, large reaction facilities using parallel connection will greatly increase the cost of circuits and electrical equipment.(Escapa et al., 2012). Except the extra cost, wired connections have the following disadvantages: 1) security: the aging of electrical circuit because of humidity, bending, temperature and voltage stress is likely to destroy the insulation of the cable, resulting in potential safety hazards (Alshaketheep et al., 2016); 2) environmental cost: discarded cables will not only cause environmental pollution, but also increase additional labor and economic investment during the recycling process (Malcolm Richard et al., 2009).
Due to these disadvantages of the conventional wired power transmission (CWPT), people began to try the development of other alternative methods. Although numerous tests have been made in history to transmit electrical energy via the wireless power transmission (WPT), little success was achieved until Brown et al. developed a cross-field transmitter of 400 kW with 70% energy efficiency (Low, 2009). Subsequently, WPT attracted the interest of scholars around the world. Chang et al. developed a contactless transmission system that increased the coupling coefficient from 0.17 to 0.24, increasing the power of wireless power transmission between electric vehicles and charging stations to 88.7% (Chang et al., 2015). Xin et al. developed a wireless power transmission system based on inductive coupling that can transmit at least 310 mW of useable power in the human body and having A current density of 3.82 A/m2 at 25W of transmitted power (Xin et al., 2010). In general, WPT systems have been continuously applied to various fields including portable electronic equipment, medical, automotive, industrial and other fields (Li et al., 2014).
In view of the potential risks and obstacles that may exist in the above traditional circuit connections, this paper introduces a technical strategy to use wireless power supply to drive microorganisms to generate bioenergy from organic substrates. WPT can overcome the disadvantages of traditional wired circuits, enabling various devices to retain working by supplying voltage to them safely, efficiently and over a distance (Fang et al., 2018). So far there are no reports about wireless power driving MEC to produce hydrogen. This study firstly demonstrates that the wireless power supply mode can replace the existing line connection mode to drive microorganisms to transform organic substrates in hydrogen, which paves a new cost-effective avenue for the real application of MECs.
Section snippets
MEC design
The single module design and connection mode were shown in Fig. 1. Briefly, the wireless charging device was constructed by using a wireless transmitting module with a load Qi wireless charging standard, launched by the Wireless Power Consortium (WPC), and the terminal was connected to the power. The distance between the emitter port and the receiver port was ≤5 mm. The receiver contained a transformer module, and the output was connected to the MEC. The Single chamber MEC reactor was made of a
Calculation
Coulombic efficiency (CE) and electron recovery efficiency (ERE) of calculation methods in this study were based on previous description (Cai et al., 2016a).
The energy efficiency () was calculated by the equation as:where was the heat energy in the hydrogen (−237.1 kJ/mol).
The electricity () input was calculated by the equation as , where the U was the applied voltage.
The effective inductance (L), coupling coefficient (K) and transmission efficiency () of the WPT
Reactor performance
The performances of all reactors were similar to each other regardless of the change of power supply mode (Fig. 2). As shown in Fig. S1, after a week or so, the current steadily increases and then enters a stable state in two parallel experiments (a and b) of WPT. The startup speed of reaction device (a) was comparatively faster, and the current peak value was relatively lower after entering the stable period, which could only approach 6 mA. However, the reactor (b) enters a stable period after
Conclusion
In summary, we propose the wireless charging module as a power source to supply external voltage to the microbial electrolysis device. Its hydrogen production effect is comparable to that of MECs with the wired power supply under the same conditions. In addition, the wireless charging method could be cost-effective once the charging distance beyond specific value. Therefore, the wireless charging strategy showed considerable potentials to be implemented in the subsequent practical application
Notes
The authors declare no competing financial interest.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study is supported by Beijing Outstanding Young Scientist Program (BJJWZYJH01201910004016).
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