Fuel and Electrolyte

As discussed previously, the substrate oxidized within the EFC is not integrated within the electrodes but is supplied directly as fuel. The fuel is typically an aqueous solution of the substrate with sufficient salts to form an electrolyte. The kinetics and stability of enzymes immobilized to EFC electrodes are strongly dependent on pH and, therefore, fuel is buffered to maintain the required pH level. Enzyme kinetics and stability vary 

The rate of enzyme-catalyzed reactions follows Michaelis-Menten kinetics, defined as 

ENZYMATIC FUEL CELL DESIGN, OPERATION, AND APPLICATION 

FIGURE 16.2 Michaelis-Menten kinetics of an enzyme-catalyzed reaction rate dependence on substrate concentration. The example is plot for Vniax=4 mol 1 s and K = 1 mol 1 .  

In summary, optimal saturated substrate concentrations, the need for by-product removal, and the need for flow-through agitated hydrodynamic conditions arc the main aspects for optimal fuel selection for each specific EFC design. Future design may address recycling water from the reaction waste for on-board formation of fresh fuel, from a concentrated solution or solid phase stock. To our knowledge, such system has yet to be bmlt and demonstrated with EFCs. 

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Figures 5.20 and 5.21 represent the measured and predicted mole fractions of CO from the fuel and electrolyte channel exhaust, respectively, as a function of flow rate. All the models fail to predict the measured values for CO, possibly also due to inaccuracy of the elementary mechanism used here. However, all the models predict decreasing CO content in the exhaust with increasing flow rate, which is consistent with the experiments.

Tominaka et al. [39] developed the first monolithic microfluidic fuel cell with air-breathing capabilities. The planar architecture of the silicon-based device is shown schematically in Fig. 4.3. In this case, a singular fuel and electrolyte containing I-shaped nficrochannel is employed that is completely open to the ambient air on the top side and in contact with two cathodes on the side walls and an anode on the... 

A prototype system-integrated microfiuidic fuel cell stack based on the air-breathing direct methanol laminar flow fuel cell technology [42] has been reported by INI Power Systems. A combination of planar and vertical stacking methods was employed to scale the system and increase its power output. With respect to fuel utilization, a fuel and electrolyte separation and recirculation system was proposed at the cost of added complexity and reduced energy density of the complete fuel cell system. 

The anode (fuel electrode) must provide a common interface for the fuel and electrolyte, catalyze the fuel oxidation reaction, and conduct electrons from the reaction site to the external circuit (or to a current collector that, in turn, conducts the electrons to the external circuit). 

Figure 9.5 Membraneless LFFCs with air-breathing cathodes, (a) Schematic illustration for the arrangement of fuel and electrolyte streams in the channel. Adapted from Ref. [32]. (b) Cross section and carrier...

Adsorbed species that desorb when the pressure of the bulk species is greatly reduced by pumping, in gas phase studies, are designated weakly bonded. The analogous procedure in electrochemical studies consists in replacing the mixture of fuel and electrolyte by pure electrolyte in the case of liquid fuels or by reducing the pressure of gaseous fuels... 

As mentioned before, factors other than electrochemical must be considered in the production of fuel cells and batteries. For example, the cost of corrosion resistant casing for the cell, as well as the cost of electrodes, fuel and electrolyte, must be considered. For space applications, cost is not as important as reliability and long life for the motor industry the prime considerations are power per unit weight and unit volume, as well as cost. 

Note that the temperature dependence of the standard OCP and DP is very important for estimating the efficiency of the electrochemical systans vs. efficiency of a heat engine. For example, the theoretical (thermodynamic) efficiency of Hj/Oj fuel cell decreases when temperature increases, and the theoretical efficiency of C/O2 fuel cell does not significantly change over a wide temperature range. In contrast, the thermodynamic efficiency of a heat engine always increases when temperature increases. The efficiency of fuel and electrolytic cells will be considered later in Chapter 8. 

The potential-current curve for an electrolyzer can be described nsing a similar equation with slightly different parameters due to different materials and designs nsed in fuel and electrolytic cells. Certainly, Ep, should be used instead of EQ when this equation is employed for electrolysis, and a small correction with a sign should be made taking into account the sign convention adapted in this book and described in Chapter 2. 

There are a number of electrochemical energy conversion systems such as fuel cell, electrolyzer, battery, flow cell, etc. In this book, we cover the fundamentals of the electrochemical energy conversion considering only fuel and electrolytic cells. 

The total efficiency and its components for fuel and electrolytic cells are considered in detail. 

A heat balance approach is introduced to be used for both fuel and electrolytic cells. 

The anode of a fuel cell is the interface between the fuel and electrolyte. The main functions of the anode are ... 

Fig. 14 (a) Schematic diagram of direct alcohol or borohydride alkaline fuel cell. 1. Fuel-electrolyte mixture storage 2. Exhausted-fuel-electrolyte mixture storage 3, 4. Peristaltic pump 5. Load 6. Anode terminal 7. Cathode terminal 8. Air 9. Cathode electrode 10. Anode electrode 11. Fuel and electrolyte mixture 12. Magnetic stirrer 13. Anode shield, (b) Experimental set-up for direct alchol or sodium borohydride alkaline fuel cell. 

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