| Author: | Pan, Zhefei |
| Title: | Investigations on direct ethylene glycol fuel cells using hydrogen peroxide as oxidant |
| Advisors: | An, Liang (ME) Wen, Chih-yung (ME) |
| Degree: | Ph.D. |
| Year: | 2020 |
| Subject: | Ethylene glycol Hydrogen peroxide Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xxiv, 252 pages : color illustrations |
| Language: | English |
| Abstract: | Direct ethylene glycol fuel cells (DEGFCs), a clean and efficient power generation technology, have attracted great research interest as a promising power source, primarily because of excellent properties of ethylene glycol (EG), including high energy density and ease of transportation, storage as well as handling. Conventional alkaline fuel cells typically use ambient air as oxidant, but the ambient air containing carbon dioxide will lead to the carbonate issue in alkaline fuel cells, which refers to the reaction between carbon dioxide and hydroxide ions forming carbonates. It has been recently demonstrated that rather than using the ambient air or pure oxygen, using hydrogen peroxide (H₂O₂) as oxidant has attracted ever-increasing attention, primarily due to several unique characteristics of liquid hydrogen peroxide: (1) the use of hydrogen peroxide can substantially increase the theoretical voltage of EG fuel cells from 1.09 V to 2.47 V, potentially boosting the fuel cell performance; (2) the activation loss on the cathode can be lowered due to the two-electron-transfer process for hydrogen peroxide reduction reaction; (3) the serious water flooding problem occurring in air/oxygen-based fuel cells can be alleviated because of the intrinsic liquid state of hydrogen peroxide; and more impressively, (5) the use of hydrogen peroxide can achieve the operation of fuel cells in an oxygen-tight environment, such as outer space and underwater. The primary objective of this thesis is to investigate and understand the performance characteristics of EG fuel cells using hydrogen peroxide as oxidant through experimental and numerical approaches. Firstly, Nafion or polytetrafluoroethylene (PTFE) is typically used as binder in preparing porous electrodes, but the effective active sites are limited due to the fact that Nafion tends to be clad on the catalyst nanoparticles and PTFE tends to form inaccessible active sites, creating the barrier for mass/ion transport to active sites. A cost-effective poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is adopted as electrode binder, which tends to form a porous structure and adhere the catalyst nanoparticles onto the nickel foam skeleton but not to isolate the catalyst nanoparticles, achieving a higher effective active area. Meanwhile, it contains more amorphous domains capable of trapping a large amount of liquid electrolyte, creating more effective active sites. At the electrode level, the electrochemical surface areas of the three electrodes using PVDF-HFP, Nafion, and PTFE are 24.10, 18.62, and 16.44 m² g⁻¹, respectively. At the cell level, using the PVDF-HFP-based electrode exhibits the best performance with an open-circuit voltage (OCV) of 1.47 V, a maximum current density of 300 mA cm⁻², and a peak power density of 120.0 mW cm⁻² at 60°C, which shows an improvement of 13.7% and 58.1%, respectively, comparing to the fuel cell performance achieved by using Nafion and PTFE as the electrode binder. Secondly, an active fuel cell using EG as fuel and hydrogen peroxide as oxidant is designed, fabricated, and tested, which theoretically offers a theoretical voltage as high as 2.47 V. This active fuel cell can experimentally output an OCV of 1.41V and a peak power density of 80.9 mW cm⁻² at 60°C, which is 20.8% higher than that of using oxygen (67 mW cm⁻²). The performance improvement is mainly attributed to the faster kinetics of the two-electron-transfer hydrogen peroxide reduction reaction. Thirdly, the addition of auxiliary devices, such as liquid pumps and gas compressors, makes the active fuel cell system bulkier and more complex, reducing not only the volumetric energy density but also the design flexibility. Hence, a passive fuel cell using EG as fuel and hydrogen peroxide as oxidant is demonstrated, which avoids the usage of auxiliary devices. Although the passive fuel cell generates a lower power density than does an active one, it is more structurally compact, no parasitic loss in power, and can be operated under ambient conditions, making it a suitable candidate for powering portable electronic devices. It is found that this passive fuel cell yields an OCV of 1.58 V and peak power densities of 30.3 mW cm⁻² and 65.8 mW cm⁻² at 23°C and 60°C, respectively, showing an impressive improvement comparing to a passive air-based fuel cell, which is more than two times higher in the OCV (0.7 V) and more than five times higher in the peak power density (12 mW cm⁻²). In addition, it is also found that the heat generated by hydrogen peroxide self-decomposition shows a negligible effect on the fuel cell operation over the discharging process. Fourthly, a passive fuel cell stack consisting of two single cells is developed to examine the feasibility of this fuel cell technology in practical applications and then demonstrated to power an electric fan in underwater condition. This passive fuel cell stack exhibits an actual OCV of 3.0 V, a maximum current of 860 mA, and a peak power of 1178 mW at room temperature. The individual cell in the passive stack exhibits a good consistency over the whole current region, indicating a high degree of reproducibility achieved by the appropriate electrode manufacturing and cell assembly processes. Moreover, the running time (per refueling) of an electric fan powered by this passive stack is 2 hours and 36 minutes in underwater condition, demonstrating that this passive fuel cell stack is a promising power source for airtight situations, such as underwater and outer space. Lastly, a mathematical model is developed to give the in-depth insights of physical and chemical processes occurring in this fuel cell, which incorporates mass/charge transport and electrochemical reactions. Previous models treat the local concentration as the actual reactant concentration participating in the electrochemical reaction, suggesting that EG molecules and OH- ions are completely adsorbed on active sites. For a specific active site, however, the reactant with a higher local concentration is more likely to be adsorbed, which may lead to active sites fully occupied. The other with a lower local concentration cannot be further adsorbed, hindering the electrochemical reaction. As such, the fuel cell performance is significantly affected by the fuel solution composition and their transport characteristics. By considering the competitive adsorption of reactants on active sites, the present model accurately predicts the voltage losses, electrode potentials, local concentrations, and thus fuel cell performance under various operating and structural design parameters. |
| Rights: | All rights reserved |
| Access: | open access |
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