DIRECT LIQUID FUEL CELLS
4.12 FUEL CELLS USING INORGANIC LIQUIDS AS FUELS
Sodium borohydride (NaBH4) is a substance with a relatively high hydrogen content (10.6 wt%). It is a strong reducing agent. In strongly alkaline aqueous solutions, sodium borohydride is stable but undergoes hydrolysis, forming NaBO2and evolving hydrogen gas in neutral and acidic solutions:
NaBH4ỵ2H2O!NaBO2ỵ4H2 ð4:17ị Because of this feature, the compound is sometimes used as a convenient hydrogen source for small PEMFCs (Wainright et al., 2003).
The first data concerning a possible direct use of this compound as a reducing agent (fuel) in fuel cells of the DBHFC type appeared in the early 1960s (Indig and Snyder, 1962; Jasinski, 1965). In 1999, Amendola et al.
reported building a fuel cell with an anion-conducting membrane using a solution with 5% NaBH4+ 25% NaOH. At a temperature of 701C, this cell yielded a power density of about 60 mW/cm2. Work in this direction was not followed up, apparently due to the fact that so far, anion-exchange membranes stable enough in concentrated alkaline solutions are not available. In 2003, Li et al. suggested that a proton-conducting membrane be used to this end. This variant is the one generally adopted at present.
Usually, a solution containing 10 to 30 wt% NaBH4and 10 to 40% NaOH is supplied to the anode compartment of borohydride–oxygen fuel cells of the membrane type. The reactions occurring in a borohydride–oxygen cell and their thermodynamic parameters are:
Anode: NaBH4ỵ8OH!NaBO2ỵ6H2Oỵ8e E0 ẳ 1:25 V ð4:18ị Cathode: 2O2ỵ8eỵ4H2O!8OH E0ẳ ỵ0:40 V ð4:19ị Overall: NaBH4ỵ2O2 !NaBO2ỵ2H2O E0ẳ1:65 V ð4:20ị
DGẳ 1272 kJ=molẳ1:65 eV DGẳ 1360 kJ=molẳ1:77 eV Theoretically, sodium borohydride yields eight electrons upon oxidation;
that is, it is a reactant with a very high energy content (9.3 kWh/kg). As a
* Since the solution is alkaline, the electrode reactions are formulated with hydroxyl ions OHrather than with hydrogen ions, and the electrode potentials are referred to the alkaline variant of the SHE. This explains the difference between equations (4.19) and (4.2), which from a thermodynamic point of view, are completely equivalent.
94 DIRECT LIQUID FUEL CELLS
matter of fact, fewer electrons are involved in the current-producing reaction.
This is due to the fact that the electrode potential of borohydride is more negative than the potential of the hydrogen electrode in the same alkaline solution (0.828 V). When borohydride solution is getting into the fuel cell, cathodic hydrogen evolution is possible at the anodic metal catalyst because of the negative potential, and this is attended by the coupled reaction of borohydride oxidation (which is not current-producing). This is analogous to the corrosion of an electronegative metal under the effect of ‘‘local elements’’
on which hydrogen evolution is facilitated. As a result, part of the borohydride is lost unproductively. At nickel catalysts for the anodic reaction, the effective number of electrons in the current-producing reaction is about four. At platinum catalysts, this number is even lower. High values of this number (6.9) were observed for gold (Amendola et al., 1999). Different intermetallic compounds, including some containing rare-earth elements, have also been suggested as anodic catalysts.
Even when taking into account the lower number of electrons, the specific energy content of borohydride remains rather high. Under the assumption of six electrons, it is close to 7 kWh/kg. This substance therefore is rather promising for the development of small fuel cells as a power supply for portable equipment.
So far it is not clear how to solve the following problem. Upon contact with NaOH solution, H+ ions in the ion-exchange membrane are exchanged (at least in part) for Na+ ions. During current flow, these sodium ions start to migrate from the anode toward the cathode. When reaching the cathodic zone, hydrogen ions become involved in oxygen reduction and then are eliminated as water vapor. However, the sodium ions will accumulate in the cathodic zone as NaOH, which must then be eliminated from the fuel cell separately.
An interesting proposal is that of building a borohydride fuel cell using hydrogen peroxide (H2O2) instead of oxygen as the oxidizing agent (Raman et al., 2004; Ponce de Leo´n et al., 2007). In such a cell, both electrodes work with liquid reactants, which simplifies the design and manipulation since the need for sealing that always arises when working with gaseous reactants is eliminated. An alkaline hydrogen peroxide solution is rather stable.
The thermodynamic value of the electrode potential of the peroxide electrode, 0.87 V, is markedly more positive than the potential of the oxygen electrode, 0.4 V. Accordingly, the EMF the cell rises to 2.11 V, which in turn leads to a higher working voltage in the fuel cell. The possibility pointed out by Raman et al. (2004) of being able to reach voltages of more than 3 V in such a cell refers to a hypothetical case in which an alkaline borohydride solution is combined in the cell with an acidic hydrogen peroxide solution. Such a cell cannot be operated for any length of time, since the large pH gradient of the solutions inevitably leads to interdiffusion and mixing of the solutions.
A problem associated with borohydride fuel cells is the fact that both reactants fundamentally may undergo catalytic decomposition, which leads to a lower utilization efficiency. Also, gaseous decomposition products may hamper access of the liquid reactants to the catalyst.
4.12 FUEL CELLS USING INORGANIC LIQUIDS AS FUELS 95
In a review by Wee (2006), the parameters of DBFCs are compared with those of DMFCs. It was shown that a DBFC system is superior to that of a DMFC system in terms of cell size and fuel solution consumption. From an economic point of view (total operating costs) a DBFC system is more favorable in specific portable applications such as miniaturized or micro power systems with short operational time spans.
4.12.2 Direct Hydrazine Fuel Cells
In the 1970s, a number of studies were published dealing with the design of hydrazine–oxygen fuel cells with an alkaline electrolyte. Several models of such batteries were actually built for portable devices and for various military objects, but owing to the high toxicity and the high price of hydrazine, this work was not developed further (see Chapter 6). Hydrazine is a strong reducing agent, both in a thermodynamic sense (the equilibrium potential of its oxidation reaction is rather negative) and from the viewpoint of an uncompli- cated electrochemical reaction. The theoretical value of the EMF of a hydrazine–oxygen cell is 1.56 V.
In 2003, Yamada et al. suggested using this substance in a fuel cell with a proton-conducting (i.e., acid) membrane. Hydrazine was used as a 10%
aqueous solution of hydrazine hydrate (N2H4H2O). In the aqueous solution, because of its strong alkaline properties, hydrazine dissociates into the ions N2H5+ and OH. The anodic oxidation of hydrazine can be written
N2Hỵ5 ỵ2OH!N2ỵHỵỵeỵ2H2O ð4:21ị the only reaction products being nitrogen and water.
Disperse powders of platinum and Pt–Ru were used as catalysts for the anodic reaction in the experimental cell. When working at a temperature of 801C, this DHFC had the following parameters (the parameters for a DMFC of analogous design working at 1001C are given in parentheses): open-circuit voltage (OCV), 1.2 V (0.7 V); voltages at current densities of 50, 100, and 150 mA/cm2: 0.95 V (0.58 V), 0.57 V (0.5 V), and 0.48 V (0.46 V). It can be seen that under these conditions at current densities below 50 mA/cm2, a hydrazine fuel cell has definite advantages with respect to voltage (owing to the high OCV). At higher current densities these advantages disappear. In the gases evolving during operation of the cell, noticeable quantities of ammonia have been detected. This is due to the fact that apart from the current-producing reaction, catalytic decomposition of hydrazine occurs:
N2H4!N2ỵ2H2 ð4:22aị and/or
3N2H4!N2ỵ4NH3 ð4:22bị 96 DIRECT LIQUID FUEL CELLS
The first of these reactions does not lead to a decrease in hydrazine utilization, since the hydrogen that is formed can take part in the anodic reaction. The second reaction, hydrazine decomposition, leads to a lower utilization factor.
In the opinion of the authors, the basic problem arising in fuel cells of this type is free penetration of the N2H5+
ions through the proton-conducting membrane to the cell’s cathodic zone, which leads to those consequences that had been described above with respect to methanol crossover.
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CHAPTER 5