NOVEL ELECTROCHEMICAL METHOD FOR PRODUCING HYDROGEN, AND DEVICE FOR IMPLEMENTING SAME

An electrolysis device including a bioanode and a cathode catalyst, a method for implementing the same, and the use thereof for producing hydrogen.

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Description

In the field of energy, the taking into account of the increase in needs, in safety of supply, of environmental hazards requires still more extensive research work to be developed on diversification and optimum use of primary resources (fossil, nuclear, renewable resources . . . ).

Hydrogen, which allows flexible storage and distribution of energy while being not very polluting, may be a carrier for supplying energy to different sectors of activities.

Hydrogen, the most abundant element on our planet, is however not directly available in nature. In order to be economically and ecologically viable, massive use of hydrogen as a source of energy (thermal and electrical energy) depends on the development of all the industrial steps from production to final use, via storage and distribution.

At the production level, three criteria have to be observed:

    • competitiveness: investment and production costs should not be too high;
    • energy yield: the energy consumption for production should be limited;
    • cleanliness: the method should not be polluting in order to keep one of the major assets of hydrogen.

Presently, 95% of the hydrogen (essentially used in the chemical and petrochemical industry) is produced by reforming from fossil fuels. This method does not meet all the criteria: the price cost is three times higher than that of natural gas and there is emission of “bad” CO2 from fossil compounds. The other methods are related to thermochemical or electrochemical decomposition of water and to direct production from the biomass. Hydrogen may also be obtained as a byproduct of cracking, steam cracking, catalytic reforming units, electrolysis of brines, coking.

Today, the production of hydrogen via an electrochemical route (water electrolysis) represents 4% of the total production (Alain Damien, Hydrogène par électrolyse de l'eau (Hydrogen by electrolysis of water), Techniques de l'Ingénieur, volume Génie des Procédés, J6 366) and is mainly conducted according to two techniques:

    • at atmospheric pressure followed by compression required for storage and transport;
    • at high pressure: less than or equal to 30 bars for industrial apparatuses and which may exceed 10 bars on certain versions intended for submarines.

Electrolysis of water in an alkaline medium (essentially potassium hydroxide in a concentration from 25% to 40% by mass) benefits from a long experience. Its development is based on the development of novel materials meeting several criteria: resistance to corrosion in this alkaline medium and catalysis of the electrode reactions (high current density and low overvoltage). At the cathode, the deposits based on nickel on an iron base are the most used. Presently novel research work is conducted in order to show that materials of less high quality may be used as a cathode and allow a reduction in the investment costs. (Marcelo et al., International Journal of Hydrogen Energy 33 (2008) 3041-3044; Olivares-Ramirez et al., International Journal of Hydrogen Energy 32 (2007) 3170-3173). At the anode: the base should be more noble (nickeled steel or massive nickel) and the deposition of the catalyst still remains a delicate point which is the subject of much research work.

Industrial application of electrolysis in an acid medium deals with the production of small amounts of very pure hydrogen for laboratories. The main characteristic lies in the use of a cationic membrane (Nafion type) and of catalysts based on precious metals for the cathode (platinum black) and the anode.

Electrolysis of water using PEM (Proton Exchange Membrane) technology is also developed and achieved by feeding the anode with pure water: in order to increase the energy yield, metal catalysts (Pt, Pd) are used in the form of deposits on the faces of the membrane.

Work is also conducted for achieving electrolysis of steam. High temperature electrolysis is more efficient than the method at room temperature since a portion of the energy required for the reaction is provided via heat, which is less expensive to obtain than electricity).

Electrolysis of water is an electrolytic process which decomposes water into oxygen and hydrogen gas by means of an electric current. At the cathode, a reduction reaction occurs with production of dihydrogen gas according to the reaction: 2H++2e−→H2 gas. At the anode, an oxidation reaction occurs: 2H2O→O2 gas+4H++4e−. The overall reaction is:


2H2O→O2+2H2.

Thus, the potential on the terminals of an electrochemical cell, achieving electrolysis of water, cannot be less than the thermodynamic value of 1.23 volt at standard temperature and pressure when the anodic and cathodic solutions are at the same pH.

The economic yield is related to the cost of the required electricity. 80% of the price of hydrogen produced via an electrochemical route is due to electricity consumption.

It is therefore desirable to reduce energy consumption of the electrochemical process while reducing the aggressivity of the media used (potash at 25-30% corresponds to a pH of about 15) for operation under mild pH conditions in the vicinity of neutrality.

The application FR 2 904 330 describes a device for electrolysis of water and its use for producing hydrogen, said device comprising an anodic and cathodic compartment, such that said cathodic compartment has an electrolytic medium comprising at least one weak acid capable of catalyzing the reduction and an electrolytic solution, the pH of which is comprised between 4 and 9.

Nevertheless, this device does not allow sufficient lowering of the potential and therefore of the required electricity consumption.

Torres et al. (Applied Microbiology and Biotechnology 77, 3, 2007, 689-697) describe electrolysis devices having only a graphite cathode; now such graphite cathodes do not have optimum characteristics for producing hydrogen by reduction of water.

Micro-organisms may spontaneously adhere on any types of surfaces and form films called biofilms consisting of said micro-organisms, of a matrix of exopolymeric substances (polysaccharides, proteins, macromolecules . . . ) which they excrete, or substances produced by microbial metabolisms and accumulated compounds from the medium or from degradation of the supporting surface. It was recently discovered that the biofilms developed on conducting surfaces are capable of using these surfaces for discharging the electrons stemming from their metabolism (D. R. Bond et al., Science 295 (2002) 483, and L. M. Tender et al., Nature Biotechnology 20 (2002) 821; H. J. Kim et al., Enzyme and Microbial Technology 30 (2002) 145).

Other biofilms have been shown to be capable of catalyzing reduction of oxygen on materials such as stainless steels (A. Berge) et al., Electrochemistry Communications, 2005, 7, 900-904; FR 02 10009) which, in their initial condition without any biofilm, are not known for ensuring high reduction rates of oxygen. These biofilms may be exploited for discharging from the colonized surface, the electrons of the system towards a dissolved compound, for example oxygen.

Whether they are capable of catalyzing electrochemical oxidation or reduction reactions, these biofilms will subsequently be called electrochemically active biofilms or EA biofilms.

These technologies have essentially been developed for battery cells, i.e. for producing electricity.

Recently, these microbial battery cells have been used for assisting with the production of hydrogen from fermentation of organic materials, such as glucose (Liu et al., Environ. Sci. Technol. 2005, 39, 4317-4320; Call et al. Environ Sci. Technol. 2008, 42, 3401-3406; Rozendal et al., International Journal of Hydrogen Energy 31 (2006) 1632-1640).

Nevertheless, all these technologies use platinum, notably as a deposit on a carbon or TI base with a load from 0.35 to 5 mg Pt/cm2 at the cathode in order to catalyze reduction of water. Because of the associated cost, these systems can only be contemplated for very specific uses, for example when very pure hydrogen gas is desired.

The present inventors have henceforth developed a novel electrolytic device comprising a cathode made in a non-noble material, a weak acid as a catalyst of reduction of water and a bioanode in a non-noble material. Unexpectedly, the thereby generated device allows potentialization of the synthesis of hydrogen and reduction of electricity consumption to levels much lower than those of existing technologies.

According to a first object, the present invention therefore relates to an enhanced electrolysis device intended to produce hydrogen by an electrochemical process, by joint use of a bioanode and of cathodic catalysis by weak acids.

Thus, the device according to the invention comprises:

    • a cathode immersed in an electrolytic solution comprising at least one weak acid and the pH of which is comprised between 4 and 9;
    • an anode immersed in an electrolytic solution capable of forming an electrochemically active biofilm at the surface of the anode, and comprising at least one biodegradable organic compound and capable of being oxidized at the anode.

The anode with an EA biofilm formed at its surface and/or in the electrolytic solution capable of forming an EA biofilm is designated here by the term of “bioanode” or microbial anode.

The device according to the invention also comprises the following preferential embodiments, as well as any of their combinations:

    • Said electrolytic solution in which the cathode is immersed and said electrolytic solution in which the anode is immersed, may be identical or distinct. Thus, when they are identical, the device consists of a single compartment; when they are distinct (for example in the case of different pHs), the device consists of two compartments, one anodic compartment comprising said anodic electrolytic solution in which the anode is immersed, and the other cathodic one comprising said cathodic electrolytic solution in which the cathode is immersed, said anodic and cathodic compartments then being separated by a separator element allowing migration of the irons between said anodic and cathodic compartments.
    • The material of the anode is selected from any conducting material on which biofilms may be formed, such as notably carbon, graphite, stainless steel, nickel, platinum, DSA . . . , and which may optionally be modified by catalytic deposition customarily used for improving conductivity (platinum, palladium, etc.).
    • The cathode is a conducting material or comprises such a conducting material, notably at least its surface is in such a conducting material or comprises it. This material is selected from any conducting material, preferably distinct from carbon and from all its forms, notably selected from the group formed by conducting polymers, either oxidized forms or not of Fe, Cr, Ni or Mo, and their different alloys, notably stainless steel, notably 304L, 316L, 254 SMO steels. By stainless steel is meant alloys comprising iron, nickel and chromium, comprising more than 12% chromium, notably selected from (chemical analysis in % by weight):

304L: C, 0.02%, Cr: 17-19%, Ni: 9-11%;

316L: C, 0.02%, Cr: 16-18%, Ni: 11-13%, Mo (molybdenum): 2%,

430: C, 0.08%, Cr: 16-18%,

409: C, 0.06%, Cr: 11-13%, Ti (titanium),

254 SMO, etc. . . .

    • Said electrolytic solution in which the cathode is immersed contains at least one weak acid capable of catalyzing the reduction. Preferably, the pH is thus comprised between 7 and 8.5, more preferentially about 8;
    • As weak acids, those which have the acid form at the selected pH of the solution in which the cathode is immersed, are preferred; mention may notably be made of the different acido-basic species of the phosphate, such as orthophosphoric acid, dihydrogenphosphate, hydrogenphosphate, lactic acid, gluconic acid and/or mixtures thereof. Dihydrogenphosphate or hydrogenphosphate and more preferentially dihydrogenphosphate or a mixture of hydrogenphosphate/dihydrogenphosphate are preferred;
    • The concentration of said weak acid may be comprised between 0.1 and the solubility of said weak acid in the electrolytic solution in which the cathode is immersed. Thus, in the case of dihydrogenphosphate, the concentration is generally less than 1.5 mole per liter. More preferentially about 0.5 mole per liter.
    • Advantageously, a hydrogenphosphate/dihydrogenphosphate mixture is used for obtaining a buffer medium at a pH of about 8;
    • Said electrolytic solution in which the anode is immersed, generally has a pH comprised between 4 and 9; preferentially, the pH of the electrolytic solution in which the anode is immersed, is identical with that of the electrolytic solution in which the cathode is immersed, more preferentially the pH is neutral, comprised between 6 and 8.
    • The reactions at the anode and at the cathode may be conducted at a close pH; preferentially equal to about 7; also, it is possible to do without any separator element between both compartments. Nevertheless, if required, such a separator element may separate both compartments; said separator element may then be an electrochemical bridge known to one skilled in the art, such as a proton exchange membrane (PEM), a cationic membrane, a ceramic, an outer filtration membrane (UF), an anion exchange membrane (AEM), a bipolar membrane, or further a simple polymeric separator or any other separation allowing ion conduction, known to one skilled in the art; as an illustration mention may thus be made of the commercial membranes Nafion® 117 or 1135, or CM1-7000S.
    • Said electrolytic solution in which the anode is immersed advantageously comprises one or more micro-organisms or consortia of micro-organisms stemming from natural media (water effluents, muds, compost, sediments . . . ), known to form EA biofilms at the surface of the anode (Du et al, Biotechnology Advances 25 (2007), pp. 464-482, Rozendal et al, Trends in Biotechnology 26 (2008), pp. 450-459); thus, this is the case for example of micro-organisms present in marine sediments. In an original way, no electrolysis cell had not yet applied marine sediments as an electrolytic solution at the anode for producing dihydrogen. These marine bioanodes have two advantages: i) they give the possibility of operating at high salinities, which is very beneficial since the internal resistance of the electrolyzer is thus reduced (the anodes formed from effluents are known to not accept strong salinities, which increases the internal resistance to the cells) and ii) the fact of operating at high salinities may give the possibility of reducing parasitic methanogenesis reactions, notably since the micro-organisms producing methane do not appreciate strong salinities.
    • Said electrolytic solution in which the anode is immersed, advantageously contains marine sediments, and is preferentially seawater or an electrolytic solution of similar salinity;
    • As a biodegradable organic compound capable of being oxidized at the anode, mention may be made of any natural or synthetic organic compound, derived from biomass for example or as an illustration, such as acetic acid or an acetate, such as sodium acetate. The term of biodegradable indicates that the organic compound is capable of being transformed by micro-organisms. Advantageously, the organic compound may be selected from the weak acids described above.

The device according to the invention associating catalysis of the reduction of water by weak acids, an anodic oxidation reaction of organic materials by micro-organisms gives the possibility of obtaining a theoretical potential of the electrolysis cell of 0.11 volt at pH 7, in the case of the acetate, and therefore much smaller than the potential of 1.23 V required by electrolysis of water.

Said device may notably be an electrolyzer.

Said device may assume diverse geometries, notably depending on the size of the installation. The geometry of the device is not critical and may be optimized by one skilled in the art according to techniques known per se.

Generally, the anodic and cathodic compartments are separated by one or more partition membranes, ensuring the passage for ions between electrolytic solutions. Thus, it is possible to mention the embodiment according to which the cathodic compartment is a tube plunging into the electrolytic solution of the anodic compartment and wherein one or more membranes are laid out either in the base of the cathodic tube, and/or in the side walls of the cathode tube immersed in the electrolytic solution of the anodic compartment. The designs 1 and 2 of the examples which follow, illustrate these embodiments.

Advantageously, the shape and the structure of the system according to the invention may be designed so as to generate exchange surface areas as large as possible for each of the functional areas. Mention may notably be made of porous structures, of the foam or felt type, or of any type of structure with a large specific surface area or a high degree of voids known in the state of the art. Also, shapes of the helix, dendrite, grid type etc. which increase the surface area of each element for a given volume may be favorable to its effectiveness. The shape may also be designed in correlation with the hydrodynamics of the medium for the circulating or stirred liquid environments. The geometry of the anode may be optimized depending on known constraints of electro-microbial catalyses, in particular for promoting adhesion of bacterial cells and electron transfer between these cells and the electrode. The cathode may be optimized for promoting gas evolvement and rapid discharge of the bubbles. On this point, it will be possible to benefit from different developed technologies for the design of electrodes with gas evolvement.

Said device may also further comprise any element customarily used in installations for electrolysis of water in an alkaline medium, such as for example means for collecting the thereby formed hydrogen such as a flow meter, a valve, a compressor and/or one or more reference electrodes such as the calomel-saturated electrode (CSE), ammeters, voltmeters, a voltage or current generator, etc.

Said micro-organism(s) forming an EA biofilm at the surface of the anode may spontaneously exist in the electrolytic solution of the anode. Alternatively or cumulatively, sowing the electrolytic solution with suitable micro-organism(s) in any possible forms (inocula, culture broths, lyophilizates, etc) may be contemplated. For this, it is possible to use as an inoculum, samples of media known to contain micro-organisms which easily form EA biofilms, such as muds of aqueous effluents (sewage works for example), sediments or biofilms for example marine sediments, composts, pure cultures of micro-organisms or any other medium known to one skilled in the art for giving EA biofilms. Advantage may be taken by sowing with samples of EA biofilms collected earlier on anodes of any system applying EA biofilms, such as microbial fuel cells for example. It is actually known that EA biofilms are good inocula for reforming EA biofilms. The first subcultures often ensure a significant increase in the catalytic activity. It is also possible to use pure cultures of micro organisms known for their capacity of forming EA biofilms, such as Geobacter, Desulfuromonas, Shewanella, Geopsychrobacter, Rhodoferrax, Geothrix, etc., and any EA strain known in the state of the art.

The sowing with one or several inocula may be carried out upon starting operation of the device, it may also optionally be renewed during operation in order to reactivate the device for example, for example in order to compensate for a reduction in its efficiency or after an operational incident.

The present invention also relates to the electrochemical process for producing hydrogen, by means of the device according to the invention. Thus, the process comprises the passing of a current into a device according to the invention, such that the anode and the cathode are connected to the opposite terminals of a current or potential source. For this purpose, any device may be used which allows delivery of a current or the maintaining of a potential difference, for example a potentiostat, a stabilized power supply or a photovoltaic panel. Generally, the potential range is comprised between 0.1 and 5V, notably between 0.2 and 2 V.

The intensity notably depends on the desired production and on the surface area of the electrodes.

Said method may be applied in a wide range of temperature and pressure operating conditions; operating at a temperature comprised between room temperature and 80° C. and at a pressure comprised between atmospheric pressure and 30 bars is notably preferred.

The water used for the cathodic electrolytic solution should not necessarily observe any particular purity criteria; thus, the water of the electrolytic solution may be seawater or distilled water, from the moment that the weak acid used and optionally electrolyte ion supports (such as KCl, NaCl, etc. . . . ) are in a sufficient amount in order to ensure conductivity of the reaction medium.

The method according to the invention may also comprise the preliminary biasing step by applying an imposed current or by maintaining a voltage of either one or both of the electrodes at an imposed potential, corresponding to the theoretical thermodynamic potential of the oxidation and/or reduction reaction to be initiated.

There again, these elements are within the capacity of one skilled in the art.

The present invention also relates to an installation, notably for producing hydrogen, comprising a device according to the invention.

The present invention, allowing production of hydrogen via a low temperature electrochemical route is therefore distinguished by:

    • low electricity consumption,
    • operation under milder pH conditions than with the use of potash (present industrial process),
    • the use of a homogeneous catalyst (weak acids) for the cathode reaction which allows replacement of the noble metals customarily used for the cathode (Pt, Ni used in present processes) with less noble metals such as steels, and
    • the use of the biomass for lowering the potential of the anodic reaction.

Other objects, features and advantages of the invention will become better apparent upon reading the following description, given as an illustration and not as a limitation.

FIGURES

FIG. 1 schematically illustrates a device according to the invention wherein (1) illustrates the cathodic compartment, (2) illustrates a permeable membrane, (3) illustrates the anode, (4) illustrates a device for measuring the flow rate of the hydrogen formed and (5) illustrates a valve.

FIG. 2 illustrates the production of hydrogen depending on the voltage on the terminals of the electrolysis cell (design 1)

FIG. 3 illustrates the biasing curves plotted from the results obtained upon biasing the electrodes with Ecell varying from 0 to 2V. For each Ecell, a current is obtained; further the cathodic potential is measured relatively to a reference electrode, the anodic potential is then inferred therefrom (Ecell−Ecath).

FIG. 4a illustrates the energy consumption versus the production capacity for the devices according to the invention and those of the prior art.

FIG. 4b is a zoom of FIG. 4a for low productions.

 EXAMPLES

1. The Experimental Set-Up

The experiments were conducted in an electrochemical reactor including two distinct compartments as described below:

Cathodic Compartment where Hydrogen Production Occurs

A tube in Plexiglas with a diameter of 6 cm with a height of liquid of 30 cm, i.e. a cathodic volume of 0.85 L. The cathode consists of two plates of stainless steel 254 SMO corresponding to a total geometrical surface area of 50 cm2, it is immersed into the cathodic medium consisting of phosphate buffer (0.5M pH 8).

Two compartment designs were tested:

Design 1: a membrane (Nafion® 117) is attached to the bottom of the tube defining an exchange surface area of 28 cm2 (diameter: 6 cm).

Design 2: a membrane (CM1-7000S—Membranes International Inc. USA) is attached in 3 rectangular windows (20×1.5 cm2) dug at the periphery of the bottom of the tube defining a total exchange surface area of 90 cm2 (20*1.5 cm).

The upper end of the tube is hermetically sealed and the produced gas is collected via a bubble flowmeter in order to evaluate the production of hydrogen.

Anodic Compartment where Oxidation of the Acetate Catalysed by a Marine EA Biofilm Occurs

A tube in Plexiglas with a diameter of 20 with a height of liquid of 80 cm, i.e. an anodic volume of 25 L.

The anode is carbon felt (Carbone Lorraine) with a geometrical surface area of 2,500 cm2 (¼ m2) immersed in natural seawater (pH 7) in which sodium acetate (2 g/L) was dissolved.

The bioanode of this compartment was prepared by using an inoculum consisting of marine biofilm harvested at La Tremblade (Atlantic Ocean). The carbon electrode was biased at −0.2 V/CSE for 15 days during which the current strongly increased (up to 0.4 A/m2) demonstrating the efficiency of the bioanode for oxidation of the acetate.

The end of the cathodic compartment including the membrane is immersed in the anodic compartment. A potential difference, designated by Ecell (potential on the terminals of the electrolysis cell), is imposed between both electrodes via a potentiostat (VMP2 from Biologic) or a solar panel coupled with a resistor of variable value and the output current is measured. A reference electrode immersed in the cathodic compartment is also used for tracking the performances of both electrodes (measurement of the potential of the cathode and deduction of that of the anode).

2. The Results of Design 1+Potentiostat

The results obtained with the cathodic compartment according to design 1, i.e. a membrane at the bottom of the cathodic tube, are grouped in Table 1.

TABLE 1 design 1 Energy Current H2 Energy consumption Ecell I in density production Faradic consumption kWh/ (V) mA (A/m2) (mL/h) yield kWh/m3 H2 Nm3 H2 0.0 0.0 0.0 0 — — — 0.2 2.5 0.5 Visible — — — 0.4 5.0 1.0 Visible — — — 0.6 14.0 2.8 4.8 74 1.8 1.9 0.8 29.0 5.8 11.4 85 2.0 2.3 1.0 44.0 8.8 19.8/9.9* 97 2.2 2.5 1.2 59.0 11.8 31.2 114 2.3 2.5 1.4 75.0 15.0 42.0 121 2.5 2.8 1.6 91.0 18.2 49.2 117 3.0 3.3 1.8 107.0 21.4 54.6 110 3.5 3.9 2.0 125.0 25.0 60.0 104 4.2 4.6 *value for a surface area reduced to 25 cm2

FIG. 2 illustrates the production of hydrogen versus the potential on the terminals of the electrolysis cell (Ecell). Finally, the biasing curves were plotted for observing the individual performances of each of the electrodes (FIG. 3).

These first results show that it is possible to produce hydrogen in an electrochemical cell by applying very low potentials between the cathode and the anode and this in a medium with a neutral pH (7-8): a current is detected for a potential of 0.2V for which low gas evolvement is observed. Next, when the potential of the cell increases (Table 1 and FIG. 2), the production of hydrogen increases.

As a comparison, for a production of the order of 20 mL/h (corresponding to 10 mL/h for a surface area reduced to 25 cm2) i.e. an output current of 8.8 A/m2, a cell potential of 1.0V is needed, while 1.90V is needed for operation in a concentrated potash medium and 1.67V for an electrolysis cell including a cathodic compartment containing phosphate (0.5M) and an anodic compartment containing concentrated potash (as described in FR 2 904 330 (Table 4)).

A reduction by more than 40% of the required potential is therefore observed, i.e. a reduction by 49% of the electricity consumption as compared with operation in a potash medium, by 40% as compared with the phosphate system with the cathode combined with the potash at the anode.

During the test, the potential of the cathode varies from −0.76 to −2.09 V/CSE while the potential of the anode varies from −0.56 to −0.09 V/CSE when the output current passes from 2.5 to 125 mA (FIG. 3): the time-dependent change of the potential of the anode shows that this bioanode is capable of providing a lot of current and is not the limiting factor. On the cathode side, the potential extends over a greater range, and the faradic yield is smaller at lower Ecell values and reaches (or even exceeds) 100% for Ecell values greater than or equal to 1.2V. In fact for Ecell below 1.2V, the current is relatively low and the proportion of the residual current (which would be obtained without any phosphate in the solution) over the total current is more significant. Yields greater than 100% may perhaps be explained by diffusion into the cathodic compartment of the carbon dioxide formed at the anode during oxidation of the acetate, which may notably reduce the purity of the collected hydrogen or modify the composition of the collected gas.

3. The Results of Design 2+Potentiostat

The results obtained with the cathodic compartment according to design 2, i.e. a membrane adhered to the three windows at the bottom of the cathodic tube are grouped in Tables 2 and 3: two cathodic solutions were tested, one consisting of seawater+phosphate (0.5M, pH 8) and the other one consisting of phosphate (0.5M, pH 8) dissolved in distilled water. With respect to the results obtained with design 1, the delivered currents for a given cell potential are still higher with this novel design (cf. tables and biasing curves, FIG. 3): a production of respectively 3.6 and 7.4 mL/h in seawater and distilled water is obtained from a cell potential of 0.4V. On the biasing curves (FIG. 3), it may be noticed that the anodic curves are very similar and that the design modification has consequences especially on the cathodic branch: the larger membrane surface area allows better distribution of the field lines and therefore an increase in the active surface area of the cathode. Moreover, modification of the cathodic medium (seawater or distilled water) entails significant differences: the increase in conductivity is actually far from having the expected effect—less losses via ohmic drop, therefore increased production efficiency—on the contrary, it would seem that the numerous constituents of seawater are an obstacle to the reduction of the hydrogen atoms of the phosphate species and therefore to the production of hydrogen (absorption/reaction competition). Pollution of the membrane may also be imagined, by the Na+ ions which would block the ion exchange sites of the membrane and would induce an ohmic drop reducing the performances of the system.

TABLE 2 design 2 seawater Current H2 pro- Energy Energy Ecell I in density duction consumption Consumption (V) mA (A/m2) (mL/h)* kWh/m3 of H2 kWh/Nm3 of H2 0.0 0.0 0.0 0.0 — — 0.2 1.5 0.3 visible — — 0.4 10.5 2.1 3.6 1.2 1.3 0.6 43.0 8.6 19.4 1.3 1.5 0.8 80.0 16.0 37.1 1.7 1.9 1.0 115.0 23.0 53.3 2.2 2.4 1.2 125.0 25.0 58.0 2.6 2.9 1.4 170.0 34.0 78.8 3.0 3.4 1.6 200.0 40.0 92.7 3.5 3.8 1.8 276.0 55.2 128.0 3.9 4.3 2.0 358.0 71.6 166.0 4.3 4.8 *Evaluated from measurements obtained with design 1 for I < 80 mA, for I > 80 mA, faradic yield taken to be equal to 100%.

TABLE 3 design 2 distilled water Current H2 pro- Energy Energy Ecell I in density duction consumption consumption (V) mA (A/m2) (mL/h)* kWh/m3 of H2 kWh/Nm3 of H2 0.0 0.0 0.0 0 — — 0.2 1.5 0.3 Visible — — 0.4 16.0 3.2 7.4 1.2 1.3 0.6 63.0 12.6 29.2 1.1 1.3 0.8 112.0 22.4 51.9 1.7 1.9 1.0 160.0 32.0 74.2 2.2 2.4 1.2 207.0 41.4 96.0 2.6 2.9 1.4 250.0 50.0 115.9 3.0 3.4 1.6 302.0 60.4 140.0 3.5 3.8 1.8 357.0 71.4 165.6 3.9 4.3 2.0 410.0 82.0 190.1 4.3 4.8 *Evaluated from measurements obtained with design 1 for I < 80 mA, for I > 80 mA, faradic yield taken equal to 100%.

4. The Results of Design 2+Coupling of Renewable Energy

Finally, tests were conducted by coupling the reactor provided with the cathodic compartment of design 2 (distilled water+0.5M phosphate, pH 8, cathode with a geometrical surface area of 25 cm2) with a photovoltaic panel. The voltage delivered by the panel to the electrochemical cell was regulated by introducing into the circuit a variable resistor (1 to 10Ω).

The results, grouped in Table 4, are by far the best in terms of production: 11 mL/h of hydrogen are produced with a cell voltage of 0.4V (bold line) i.e. a 0.6V reduction of the required potential, as compared with design 1 which, as a reminder, already allowed division by 1.7 (and more) of the potential required during electrolysis in a cathodic potash or potash+phosphate medium.

TABLE 4 design 2 + photovoltaic Current H2 pro- Energy Energy Ecell I in density duction consumption consumption (V) mA (A/m2) (mL/h)* kWh/m3 of H2 kWh/Nm3 of H2 0.00 0 0.0 0.0 — — 0.41 27 10.8 10.6 1.0 1.2 0.81 111 44.4 51.5 1.7 1.9 1.12 166 66.4 77.0 2.4 2.7 1.73 383 153.2 177.6 3.7 4.1 2.20 530 212.0 245.8 4.7 5.3

5. Comparative Results

In terms of energy consumption per Nm3 of produced hydrogen (N=Normal conditions=273 K), the gain is highly significant. For a production of the order of 10-11 mL/h on a stainless steel cathode (geometrical surface area 25 cm2), the following order is obtained from the most energy-consuming to the least energy-consuming (synthesis of Tables 1 to 4):

Present Industrial Technique:

Cathodic potash pH 15 and anodic potash pH 14: 4.9 KWh/NM3H2

FR2 904 330:

Cathodic phosphate (0.5M, pH8) and anodic potash pH15: 4.3 kWh/Nm3 H2.

Device according to the invention:

    • Cathodic phosphate (0.5M, pH8) and anodic biodesign 1 pH7: 2.5 kWh/Nm3 H2 (takes into account the geometrical surface area of 50 cm2)
    • Cathodic phosphate (0.5M, pH8) and anodic biodesign 2 pH7: 1.2 kWh/Nm3 H2

The device according to the invention allows a reduction in the energy consumption by more than 49% (design 1) and 75% (design 2) as compared with the industrial technique and by 40% (design 1) and by 72% (design 2) as compared with that of FR 2904330.

The device according to the invention was also compared with technologies for producing hydrogen from organic materials, using a cathode in a noble material, as described by Liu et al., Environ. Sci. Technol. 2005, 39, 4317-4320; Call et al., Environ. Sci. Technol. 2008, 42, 3401-3406; Rozendal et al., International Journal of Hydrogen Energy 31 (2006) 1632-1640.

It emerges therefrom that:

    • for a same cell potential over a range from 0 to 1.2V, the productions of hydrogen reported by these authors are all less than those obtained according to the invention. Moreover, according to the invention, the system may attain production rates of the order of 0.1 m3/h/m2 of cathode, therefore much more significant than those reported which are all less than 0.005 m3/h/m2 of cathode (see FIGS. 4a and 4b). Moreover, in order to produce 0.1 m3/h/m2, the energy consumption according to the invention is of the order of 5 kWh/Nm3 of H2. Under conditions in a concentrated potash medium, this consumption corresponds to a production rate per unit surface which is 20 times less.
    • Moreover, for the highest production rate reported in the literature (5.2 10−3 m3/h/m2, see Call et al., supra), the energy consumption is 2 kWh/Nm3 of H2; it passes to 1.3 kWh/Nm3 of H2 by using the device according to the invention (steel cathode and phosphate concentration of 0.5 M) i.e. a 35% reduction.

This last result is to be compared with the 13% reduction (4.3 vs. 4.9 kWh/Nm3 of H2) obtained with the substitution with the steel cathode and the presence of weak acid alone.

This result actually demonstrates the synergy of the bioanode combined with the weak acid catalysis at the cathode.

    • Finally, these authors work with a cathode containing platinum unlike the device according to the invention which applies cathodes in stainless steel, an industrial material par excellence.

Claims

1-18. (canceled)

19. An electrolysis device comprising:

a cathode immersed in an electrolytic solution comprising at least one weak acid and the pH of which is comprised between 4 and 9;
an anode immersed in an electrolytic solution capable of forming an electrochemically active biofilm at the surface of the anode, and comprising at least one biodegradable organic compound capable of being oxidized at the anode,
such that the cathode is or comprises a material selected from conducting polymers, either oxidized forms or not of Fe, Cr, Ni or Mo, and their different alloys, notably stainless steel, notably 304L, 316L, 254 SMO steels.

20. The device according to claim 19, wherein said electrolytic solution in which the cathode is immersed and said electrolytic solution in which the anode is immersed, are identical.

21. The device according to claim 19, wherein said electrolytic solution in which the cathode is immersed, comprises at least one weak acid with a pH comprised between 7 and 8.5.

22. The device according to claim 19, wherein said electrolytic solution in which the anode is immersed, comprises one or several micro-organisms or consortia of micro-organisms stemming from natural media, known to form electrochemically active biofilms at the surface of the anode.

23. The device according to claim 22, wherein said micro-organism(s) is/are sown in said electrolyte solution.

24. The device according to claim 23, wherein said sowing is achieved by adding one or more inocula selected from muds of water effluents, marine sediments, biofilms, composts or pure micro-organism cultures.

25. The device according to claim 19, wherein the electrolytic solution in which the anode is immersed, contains marine sediments or contains a saline concentration, similar to that of seawater.

26. The device according to claim 19, wherein said weak acid is selected from one or more acids from the group comprising orthophosphoric acid, hydrogenphosphate, dihydrogenphosphate, lactic acid, gluconic acid and/or mixtures thereof.

27. The device according to claim 19, wherein the concentration of the weak acid is comprised between 0.1 mole per liter and the solubility of said acid in the electrolytic solution in which the cathode is immersed.

28. The device according to claim 19, such that said electrolytic solution in which the cathode is immersed and said electrolytic solution in which the anode is immersed, are distinct and separated by a separator element between the anodic and cathodic compartments allowing migration of the ions between said compartments.

29. The device according to claim 28 selected from a proton exchange membrane (PEM), a cationic membrane, a ceramic, an ultrafiltration (UF) membrane, an anion exchange membrane (AEM), a bipolar membrane or further a simple polymeric separator (gas).

30. The device according to claim 19, wherein said biodegradable organic compound capable of being oxidized is a natural or synthetic organic compound or derived from biomass.

31. The device according to claim 30, wherein the organic compound capable of being oxidized is selected from the group comprising acetic acid or an acetate.

32. The device according to claim 19, comprising a means or means for collecting the hydrogen formed.

33. The device according to claim 19, such that the material of the anode is selected from carbon, graphite, stainless steel, nickel, platinum, DSA (dimensionally stable anode).

34. An electrochemical method for the synthesis of hydrogen comprising the application of a device according to claim 19, wherein the anode and the cathode are connected to two opposite terminals of a current or potential source.

35. The method according to claim 34, wherein either one or both of the electrodes are biased beforehand.

36. An installation comprising a device according to claim 19.

Patent History
Publication number: 20110315562
Type: Application
Filed: Dec 23, 2009
Publication Date: Dec 29, 2011
Applicant: INSTITUT NATIONAL POLYTECHNIQUE DE TOULOUSE (Toulouse Cedex 4)
Inventors: Regine Basseyguy (Venerque), Alain Bergel (Toulouse), Benjamin Erable (La Tremblade), Luc Etcheverry (Montlaur), Serge Da Silva (Perpignan)
Application Number: 13/141,549
Classifications
Current U.S. Class: Utilizing Specified Electrode (205/638); Cells (204/242); Diaphragm Type (204/252); Gas Withdrawal (204/278)
International Classification: C25B 1/10 (20060101); C25B 1/02 (20060101); C25B 11/04 (20060101); C25B 1/04 (20060101);