Patent Application
20050255341
The present invention relates to a direct borohydride fuel cell (DBFC) which uses hydrogen peroxide as oxidant. More particularly, the present invention relates to direct borohydride fuel cell which uses hydrogen peroxide as oxidant in conjunction with aqueous sodium borohydride as hydrogen-carrying liquid fuel, hydrogen-storage alloy as anode and Na+-form of Nation™-117 as membrane electrolyte.
A fuel cell is an electrochemical device that continuously converts chemical energy directly into electrical energy for as long as fuel, such as hydrogen, and oxidant, such as oxygen, are supplied to it. There are six generic fuel cell systems, namely (i) phosphoric acid fuel cells, (ii) alkaline fuel cells, (iii) molten carbonate fuel cells, (iv) solid oxide fuel cells, (v) polymer electrolyte fuel cells, and (vi) direct methanol fuel cells.
Among the aforesaid fuel cell systems, polymer electrolyte fuel cells (PEFCs) are considered as front-runner for portable-power applications. Although PEFCs have advanced substantially in terms of their development, their commercialization is still limited owing to the problems related to carbon monoxide poisoning of anode while using a reformer with the PEFC, and hydrogen storage while using a directly fueled PEFC. Therefore, alternative hydrogen-carrying liquid fuels such as methanol, which has a capacity of 5.6 Ah/g and a hydrogen content of 12.5 wt. %, has attracted the attention of fueling PEFCs directly with methanol. Such fuel cells are referred to as direct methanol fuel cells (DMFCs). But DMFCs have limitations of low open-circuit-voltage, low electrochemical-activity, and methanol crossover.
As an obvious solution to the above mentioned problems associated with DMFCs, other promising hydrogen-carrying liquid fuels such as sodium borohydride, which has a capacity value of 5.67 Ah/g and a hydrogen content of about 11 wt %, have been explored. The U.S. Pat. No. 5,599,640, entitled, “Alkaline fuel cell”, issued to Lee et al. (hereafter “Lee”) was the first report of a fuel cell comprising an aqueous alkaline solution of electrolyte containing a hydrogen-releasing agent selected from the group consisting of NaBH4, KBH4, LiAlH4, KH and NaH, an oxygen electrode as cathode and a hydrogen storage alloy electrode as an anode. This fuel cell, however, did not have any membrane electrolyte to restrict the reactants and products from one electrode to diffuse to the other. Amendola et al. were the first to use an OH−-ion conducting anion exchange membrane-based borohydride-air fuel cell with a power density close to 60 mWcm−2 at 70° C., as described in the article entitled, “A novel high power density borohydride-air fuel cell” published in the Journal of Power Sources 84 (1999) pp 130-133 (hereafter “Amendola”). However, the borohydride-air fuel cell described by Amendola suffers from borohydride crossover as the BH4−-ions can easily permeate through the anion exchange membrane. Besides, it would be mandatory to scrub CO2 from air inlet of such a fuel cell to avoid its carbonate fouling. Li et al. mitigated BH4− crossover problem by adopting a fuel cell structure using Nafion membrane as electrolyte to separate the fuel from the cathode and could achieve a power density as high as 160 mWcm−2 at 70° C. with such a fuel cell as described in the article entitled “A Fuel Cell Development for Using Borohydrides as the Fuel” published in the Journal of The Electrochemical Society, 150 (7) A868-A872 (2003) (hereafter “Li”). But even in the borohydride-air fuel cell proposed by Li, it would be mandatory to scrub CO2 from air both to avoid carbonate fouling as well as to prevent accumulation of alkali in the catholyte to facilitate oxidant flux at the cathode.
Thus, it is essential to come up with an improved direct borohydride fuel cell which does not require constant scrubbing of CO2 thereby completely avoiding carbonate fouling. Also, it is required to device direct borohydride fuel cells which can work even in the absence of air such as under-water conditions.
The main object of the present invention is to provide a direct borohydride fuel cell which uses hydrogen peroxide as oxidant.
Another object of the present invention is to provide a DBFC which does not require constant scrubbing of CO2.
Yet another object of the present invention is to provide a DBFC which completely avoids carbonate fouling.
Still another object of the present invention is to provide a DBFC which can be used in the absence of air such as under-water conditions.
The present invention will be described with reference to the accompanying drawings, wherein:
Accordingly, the present invention provides, a direct borohydride fuel cell (DBFC) using Hydrogen peroxide as a liquid oxidant, said DBFC comprising:
In an embodiment, the membrane is a pretreated polymer membrane electrolyte.
In another embodiment, the pretreated membrane electrolyte is made up of H+/Na+-form of Nafion™-117 or any H+/Na+ conducting membrane.
In yet another embodiment the liquid hydrogen releasing agent used is aqueous sodium borohydride (NaBH4), KBH4, LiAlH4, KH or NaH or any other hydrogen releasing agent. In yet another embodiment, the anode is made up of a hydrogen storage alloy and wherein sodium borohydride is oxidized at the anode according to the following reactions,
8NaOH 8Na++8OH−
NaBH4+8OH− NaBO2+6H2O+8e−(E°a=−1.24 V vs. SHE)
In yet another embodiment, the hydrogen storage alloy is selected from the group consisting of AB5 Misch metallic alloy, an AB2 Zirconium (Zr) alloy and an AB2 Titanium (Ti) alloy or any hydrogen storage material with similar characteristics.
In yet another embodiment, the cathode is made up of a carbon electrode dispersed with a catalyst material and wherein the decomposition of H2O2 into O2 and H2O and electroreduction of H2O2 when pH converges to 0 takes place at the catalyst/electrode interface of the cathode according to the reactions,
4H2O2 4H2O+2O2
2O2+4H2O+8e− 8OH− (E°c=0.4 V vs. SHE)
4H2O2+8H++8e− 8H2O (E°c=1.78 V vs. SHE)
In yet another embodiment, the cathode catalyst material is made up of 60 wt. % Pt/C with platinum loading of 1 mg cm−2.
In yet another embodiment, the maximum power density attainable is about 136 mWcm−2 at a cell voltage of about 1V while operating with the hydrogen peroxide solution near zero pH at about 35° C.
In another embodiment of the invention, the maximum power density attainable is about 352 mWcm−2 at a cell voltage of about 1.2V while operating with the hydrogen peroxide solution near zero pH at about 70° C.
The direct borohydride fuel cell of the present invention has a power density of about 350 mWcm−2 at the cell voltage of almost 1.2 V at 70° C.; the open-circuit voltage of the DBFC is about 2V.
Working of the DBFC
In the following paragraphs, the working of the DBFC is explained.
In the fuel cell of the present invention, sodium borohydride is oxidized at its anode according to the following reactions.
8NaOH 8Na++8OH− (1)
NaBH4+8OH− NaBO2+6H2O+8e− (E°a=−1.24 V vs. SHE) (2)
At the cathode of the DBFC, hydrogen peroxide is decomposed into oxygen and water at the catalyst/electrode interface according to the reactions,
4H2O2 4H2O+2O2 (3)
2O2+4H2O+8e− 8OH− (E°c=0.4 V vs. SHE) (4)
Electroreduction of hydrogen peroxide is also highly likely according to the reaction,
4H2O2+8Na++8e− 8NaOH (E°c=0.87 V vs. SHE) (5)
As the pH of the H2O2 solution in the catholyte converges to 0, electroreduction of H2O2 will proceed as follows.
4H2O2+8H++8e− 8H2O (E°c=1.78 V vs. SHE) (6)
The variation in H2O2 reduction potential with pH is governed by,
E(H2O2)=1.78−0.059 pH (7)
The net cell reaction in such a DBFC is,
NaBH4+4H2O2 NaBO2+6H2O (8)
The cell potential for this DBFC will range between 1.64 V and 3.02 V depending on the pH of H2O2 solution in the catholyte. The specific energy of such a fuel cell will be as high as 17 kWh/kg. Similar to Li, we have also used a Nafion membrane to separate anode and cathode compartments of the fuel cell while selectively employing an AB5-group MmNi3.55Al0.3Mn0.4Cu0.75 hydrogen-storage alloy, where Mm stands for Misch metal comprising La-30 wt. %, Ce-50 wt. %, Nd-15 wt. %, Pr-5 wt. %, as the anode material. In the literature, both AB2 and AB5-group alloys have been successfully employed as negative electrodes in nickel-metal hydride batteries. Although AB2-group alloys yield superior energy storage density, the AB5-group alloys have higher hydrogen retention capacity.
The fuel cell of the present invention will be further illustrated below with reference to
The membrane electrode assemblies (MEAs) form a seminal component of various DBFCs of this invention and were obtained by sandwiching the pre-treated Nafion®-117 polymer electrolyte membrane between the anode and cathode. To prepare the anode catalyst layer, a slurry of the alloy obtained by ultra-sonicating the required amount of alloy with 5 wt. % Vulcan XC-72R carbon and 7 wt. % of Nafion® solution in isopropyl alcohol was pasted on carbon paper (Toray TGP-H-090) of 0.28 mm thickness. The loading of alloy catalyst was 5 mgcm−2, which was kept identical for all the MEAs. The cathode comprises a backing layer, a gas-diffusion layer, and a reaction layer. A carbon paper (Toray TGP-H-090) of 0.28 mm thickness was employed as the backing layer for the cathode. To prepare the gas-diffusion layer, Vulcan-XC 72R carbon was suspended in water and agitated in an ultrasonic water bath. To this, 10 wt. % Nafion solution obtained from Aldrich was added with continuous agitation. The required amount of cyclohexane was then added to it drop wise. The resultant slurry was spread onto a teflonized carbon paper and dried in an air oven at 80° C. for 2 h. To prepare the reaction layer, required amount of the catalyst (60 wt. % Pt/C) was suspended in isopropyl alcohol. The mixture was agitated in an ultrasonic water bath, and 7 wt. % of Nafion® solution was added to it with continuing agitation for 1 h. The catalyst ink thus obtained was coated onto the gas-diffusion layer of the electrode. The cathode contained 60 wt. % Pt/C catalyst with platinum loading of 1 mgcm−2. A Nafion loading of 0.25 mg cm−2 was applied to the surface of each electrode. The membrane electrode assembly was obtained by hot pressing the cathode and anode on either side of a pre-treated Nafion®-117 membrane at 60 kg cm−2 at 125° C. for 3 min.
Liquid-feed DBFCs were assembled with various MEAs. The anode and cathode of the MEA were contacted on their rear with gas/fluid flow field plates machined from high-density graphite blocks in which channels were machined to achieve minimum.
Mass-polarization in the DBFCs. The ridges between the channels make electrical contact with the back of the electrode and conduct the current to the external circuit. The channels supply alkaline sodium borohydride solution to the anode and hydrogen peroxide to the cathode. Electrical heaters were placed behind each of the graphite blocks to heat the cell to the desired temperature. Aqueous sodium borohydride solution comprising 10 wt. % NaBH4 in 20 wt. % aqueous NaOH was pumped to the anode chamber through a peristaltic pump. Hydrogen peroxide 15% w/v solution with varying pH was introduced into the cathode chamber through another peristaltic pump. The graphite blocks were also provided with electrical contacts and tiny holes to accommodate thermocouples. After installing single cells in the test station, performance evaluation studies were initiated.
Galvanostatic-polarization data for the DBFC in the temperature range between 35° C. and 70° C. were recorded by circulating aqueous sodium borohydride solution in the anode chamber and 15% w/v hydrogen peroxide solution adjusted to various pH values ranging between 1 and 0 in the cathode chamber. Anode polarization data for the DBFC at various temperatures were also obtained employing an Hg/HgO, OH− (MMO) reference electrode. Cathode polarization data were derived by subtracting anode polarization values from the respective cell polarization data at various load current-densities.
The anode was fed with aqueous NaBH4 solution at a feed rate of 3 ml/min, and the cathode was fed with 15% w/v H2O2 solution with pH values close to 1 at a feed rate of 5.5 ml/min to the cathode. The cell performance data at various temperatures are shown in
A DBFC operating with the anode with aqueous NaBH4, solution at a feed rate of 3 ml/min, and the cathode with 15% w/v H2O2 solution with pH values close to 0.5 at a feed rate of 5.5 ml/min to the cathode was also studied in addition to Example 1. The cell performance data at various temperatures are shown in
A DBFC operating with the anode with aqueous NaBH4 solution at a feed rate of 3 ml/min, and the cathode with 15% w/v H2O2 solution with pH values close to 0 at a feed rate of 5.5 ml/min to the cathode was also studied in addition to Examples 1 and 2. The cell performance data at various temperatures are shown in
Table 1 below summarizes the electrical performance data of the DBFCs presented as Example 1, 2 and 3 above.
It has been possible to attain a maximum power density of 136 and 352 mWcm−2 at a cell voltage of 1 V and 1.2 V while operating such a DBFC employing hydrogen peroxide solution as oxidant with near zero pH at 35° C. and 70° C., respectively. The operational conditions for the DBFCs, however, are not yet fully optimized, and a further enhancement in its performance is highly likely. In order to obtain a large-sized cell, several unit cells could be connected in series as is conventionally done to form a fuel cell stack.
Single-electrode polarization curves at various operational temperatures for the catholyte with varying pH values are shown in
Advantages of the Present Invention
