WO2009026640A1

WO2009026640A1 - Hydrogen production by the solar driven electrolysis of sulfurous acid

Info

Publication number: WO2009026640A1
Application number: PCT/AU2008/001273
Authority: WO
Inventors: James Trevor Hinkley; Sten-Eric Lindquist
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2007-08-28
Filing date: 2008-08-28
Publication date: 2009-03-05
Also published as: AU2008291700A1

Abstract

A method and system for producing hydrogen comprises the electrolysing of a solution comprising SO2 and H2SO4 in an electrolysis cell 12 so as to produce hydrogen. The electrolysis cell makes use of the photovoltage generated from one or more single solar cells (e.g. on a panel 10).

Description

Hydrogen Production by the Solar Driven Electrolysis of Sulfurous Acid

Technical Field

A method and system are disclosed for producing hydrogen by the solar driven electrolysis of sulfurous acid. The method and system make use of the photo voltage of one or more single solar cells.

Background Art

Alkaline electrolysis is a known technology for producing hydrogen, but requires a considerable cell potential (around 2 Volts) and therefore has a high energy demand. The Hybrid Sulfur (or "Westinghouse") process also includes an electrolysis step for hydrogen production; however, the process uses a solution of sulfur dioxide in sulfuric acid to reduce the cell potential.

A reference herein to the prior art is not an admission that the prior art forms part of the common general knowledge of a person of ordinary skill in the art in Australia or elsewhere.

Summary of the Disclosure

In a first aspect there is provided a method for producing hydrogen comprising the step of electrolysing a solution comprising SO₂ and H₂SO₄ so as to produce hydrogen, using the photovoltage generated from one or more single solar cells.

Such a method can provide a more efficent system for hydrogen production. IQ this regard, the method can make use of (and can adapt) the electrolysis step in the Hybrid Sulfur process, but which can be operated to reduce the electrolysis cell potential to around 0.6V. It has now surprisingly been recognised that this voltage is comparable to that which can be produced by certain single solar cells. Furthermore, it has now surprisingly been recognised that a much greater current efficiency can be produced out of a solar cell by optimising the performance of the cell to maximise current rather than power at a given potential, which can then be used to drive the Hybrid Sulfur electrolysis.

In this regard, in one form, the one or more single solar cells are configured so as to produce as high as possible a photocurrent for electrolysis.

Furthermore, known photovoltaic (PV) solar cells are operated to maximise power and, for the most part, are connected in series to attain useful voltages. In one form of the present method two or more single solar cells are interconnected in parallel to produce the highest possible photocurrent (ie. to increase to a maximum the amount of photocurrent produced by a given area of collectors). However, in a variation on this configuration the method can comprise a further step of switching between series and parallel connection arrangements (with the latter being preferred when maximising current density). This switching can relate to (or depend on) the photovoltage being produced by the solar cells, and can be made so as to optimise the current density produced by the cells. For example, in low light intensity, a series configuration of solar cells can be employed.

In one form the solar cell is tailored to the operating requirements and characteristics of the electrolysis cell, to enable utilisation of a larger proportion of the solar spectrum than a known silicon PV cell. This tailoring enables higher photon conversion and produces higher current densities

In one form the electrolysis of the solution can take place in a cell comprising one or more negative and positive electrodes, each located in a respective compartment separated by a membrane. The hydrogen can be cathodically generated at the (or each) negative electrode and then removed from the cell. Sulfate ions can be anodically produced from sulfite ions at the (or each) positive electrode. In this electrolysis step the hydrogen is typically collected as an evolved gas, whereas the resultant sulfuric acid and unreacted sulfur dioxide are typically recycled within the method. The electrolysis step of the method can also be operated in an open loop, "once-through" configuration, when e.g. directly using SO₂ from a waste stream or off-gas. While a number of different photovoltaic cells can be suitable, in one embodiment the photovoltaic cell is a dye-sensitized solar cell (DSSCs). DSSCs show particular promise commercially as these cells have the unique property of being able to maintain photovoltage even at low light intensities.

In a variation the SO₂ can be sourced from the off-gas of an industrial process that comprises SO₂. Such off-gases include those produced by metal smelting operations and/or power station emissions. Sourcing SO₂ in this way can simplify and economise the method.

In a second aspect, a system for producing hydrogen is provided comprising: - one or more single solar cells arranged to generate a photovoltage;

- an electrolysis cell in which a solution comprising SO₂ and H₂SO₄ is electrolysed to produce hydrogen, using the photovoltage generated from the one or more single solar cells.

In an embodiment the one or more single solar cells and the electrolysis cell can be integrated. For example, the one or more single solar cells can be directly mounted a top of the electrolysis cell. This can eliminate circuiting and thus electrical losses between the solar and electrolysis cells. Also, the electrolysis cell can operate at the same current densities as the solar cell(s).

The system of the second aspect can be otherwise configured and operated in accordance with the method as defined in the first aspect.

Brief Description of the Drawings

Notwithstanding any other forms which may fall within the scope of the method and system as set forth in the Summary, specific embodiments of the method and system will now be described, by way of Example, and with reference to the accompanying drawings in which:

Figure IA plots the current density- voltage (iV) characteristics of an electrolytic process, with respect to the Standard Hydrogen Electrode (SHE) (with the electrode reactions at the positive (+) and negative (-) electrodes also being indicated); Figure IB plots the iV characteristics of a single photovoltaic (PV) solar cell

(with the scaling factor /being denoted on the Y-axis (/being defined in the Symbols and Definitions table on page 16));

Figure 2A plots three iV curves (a, b, c) for different electrode materials used in the electrolyzer; Figure 2B plots the iV characteristics of a single PV solar cell at different light intensities, showing how both V_oc and i_sc increase with increasing light intensity;

Figure 3 A plots three iV curves (a, b, c) having the same cathodic behaviour, but different anodic behaviour with increasing overpotential (in the order a, b, c); Figure 3B plots the i V characteristics of a single PV solar cell when output voltage Vy₁ of the PV cell at the working point is correspondingly increased;

Figure 4 shows a schematic process flow diagram for an adaptation of the Hybrid Sulfur process in accordance with the present method and system;

Figure 5 plots i V curves that show the effect of varying insolation on the working point for a directly coupled SNElL type electrolyzer and a module of DSSC solar cells connected in pairs;

Figure 6 plots i V curves that show an increase in system efficiency by incorporation of voltage control to enable a PV module to operate at a maximum power point; and

Figure 7 plots i V curves to demonstrate a voltage control strategy applied to two of the lower insolation cases from Figure 6, showing the increase in power efficiency through voltage control under variable insolation conditions. Detailed Description of Specific Embodiments

Prior to summarising the electrochemistry and design of the present method and system for producing hydrogen by the solar-powered electrolysis of sulfurous acid, the Hybrid Sulfur process will first be described, by way of background. In addition, it should be noted that the table on page 16 sets forth and provides further explanation for the symbols used in the equations and Figures of this specification.

Hybrid Sulfur Process

The Hybrid Sulfur (or Westinghouse) process is a two step process which generates hydrogen through the dissociation of water into hydrogen and oxygen using thermal and electrical energy inputs.

The Hybrid Sulfur process was developed in the 1970s for the production of hydrogen using thermal energy and electricity from a nuclear reactor. Research on the Hybrid Sulfur process was discontinued in 1983 in favour of a more cost-effective (conventional) process based on the steam reforming of methane. Recent concern about the impact of fossil fuel use on climate change has reignited interest in thermochemical processes for producing hydrogen.

The Hybrid Sulfur process can be divided into two major energy consuming steps. In the first step, thermal energy is used to dissociate SO₃ at an elevated temperature (approximately 950°C) as follows:

2SO₃ → 2SO₂ + O₂ (1)

In the second step, the SO₂ is dissolved in water to form sulfurous acid (H₂O + SO₂ → H₂SO₃), which is then oxidised in an electrolysis process. This produces hydrogen as a result of the overall reaction:

SO₃ ²- + H₂O → SO₄ ²- + H₂ (2)

To close the cycle, water has to be removed from the sulfuric acid solution, and the resulting sulfuric acid decomposed into SO₃ and water again:

H₂SO₄ → SO₃ + H₂O (3)

The net reaction for the Hybrid Sulfur process cycle is thus:

2H₂O → O₂ + 2H₂ (4) Ideally, all sulfur compounds are regenerated in the cycle. In one form of the solar driven Hybrid Sulfur cycle disclosed herein Reaction (1) can utilize highly concentrated light delivered by a solar dish or solar tower. Reaction (3), which occurs at around 600⁰C, can utilise lower temperature solar energy or thermal energy from heat recovery.

H₂ Production from a Sulfite Water Solution by an Electrolyzer Driven by a Solar Panel with the Electrical Potential of Single PVs (e.g. in Parallel) It has now been surprisingly discovered that an electrolysis cell (herein

"electrolyzer") of the Hybrid Sulfur process can be driven by a photovoltaic (PV) solar cell (or panel) using the output voltage of a single solar cell. More particularly, it has been discovered that Reaction (2) (above) can be powered by solar energy. It has further been noted that a solar driven promotion of Reaction (2) may be implemented in a cost effective manner.

The basic principle of the method and system disclosed herein is to drive Reaction (2) in an electrolyzer making use of current and voltage from a photovoltaic solar cell. It has been surprisingly observed that the voltage required to drive Reaction (2) (~550-600mV) can be supplied by one single PV solar cell. By way of comparison, a conventional alkaline electrolysis process for direct water splitting requires a voltage of 2.0-2.2V to promote Reaction (4). Since the voltage delivered by a single PV cell is typically in the range of 0.6-0.8V, in the past, to use solar energy, a minimum of three PV cells were required to be interconnected in succession to drive the alkaline electrolyzers. As a result, the solar energy efficiency per unit area of a solar panel for direct water electrolysis was reduced by at least a factor of three, hi addition, if one cell failed, all three in the series ceased to function. Furthermore, in a conventional grid connected PV module, further losses occurred when transforming the photovoltaic DC current to AC current, and then back to DC again, to feed the electrolyzer. In the method and system disclosed herein a solar module composed of single solar cells arranged in a parallel configuration was found to be simple to design and less prone to failure. Moreover, at low light intensity (eg. a cloudy day), when the output voltage dropped, two (or more) single solar cell modules could, for a period, easily be switched to be interconnected in series to supply a required output voltage. Li this regard, Figure 4 shows a schematic process flow diagram for an adaptation of the Hybrid Sulfur process in accordance with the present method and system. - S -

In Figure 4, a solar photovoltaic (PV) panel 10 is shown comprising a number of single solar cells arranged in a parallel configuration. The panel 10 is electrically connected to an electrolyzer 12 (ie. electrolysis cell) comprising catalyst-activated electrodes 14. The electrolyzer makes use of the output voltage (approximately 0.6V) of the panel 10. The electrolyzer 12 is operated to generate hydrogen gas at the cathode 14A and to oxidise an aqueous sulfite solution at the anode 14B to produce sulfuric acid.

In a recycle mode of operation, the resulting sulfuric acid solution, together with any unreacted sulfur dioxide, can be passed to a cracking reactor 16 in which the solution is subjected to concentrated solar radiation from eg. a heliostat field of a solar "power tower" system 18. In reactor 16, the thermal energy is used to dissociate SO₃ (2SO₃ → 2SO₂ + O₂ at approximately 950°C). This then enables the reaction of SO₂ with water to form sulfurous acid (H₂O + SO₂ → H₂SO₃), which is recycled for further oxidation in the electrolyzer 12. The process can also make use of a sulfur dioxide containing waste gas, such as the off-gases from metal smelting operations and/or power station emissions, whereby the cracking reactor 16 could be eliminated. The process would produce both hydrogen gas and sulfuric acid in this mode of operation.

Operating the photovoltaic solar cell and the electrolyzer together

Reference will now be made to Figures IA and IB to explain how power from a photovoltaic solar cell can drive the electrochemical step (Reaction (2)) of the Hybrid Sulfur process. Figures IA and IB will also be referred to, to describe the iV characteristics of an electrolyzer and solar cell respectively, and to analyse how the two work together. In Figure IB, at the so-called working point, the electrolyzer is driven by a current /_w at a potential of F_w. In addition, V₀₀ is the open circuit voltage, and ι_sc the short circuit current; (note that for ease of interpretation of the Figures, the current density of the solar cell is multiplied by the scaling factor/so as to appear the same size in the drawings IA and IB). Then, by referring to Figures 2 and 3, a more detailed explanation will be provided of how power from a single photovoltaic solar cell can drive the electrochemical step (Reaction (2)) of the Hybrid Sulfur process.

The diagrams employed in each of Figures 1 to 3 have the same basic structure, ^■ and show the current-voltage (iV) characteristics of the electrolyzer in the upper part, A, and the i V characteristics of the photovoltaic solar cell in the lower section, B.

It was further noted that the joining of a single solar cell and an electrolyzer resulted in a self-tuning system that could find a balance, or -working point, between the current and voltage produced by the solar cell, and that consumed by the electrolyzer. Similarly, in the electrolyzer itself, the cathodic current driving the reduction was equal to the anodic current driving the oxidation process. The working point of the solar cell was optimized for best efficiency of the electrolyzer, to thus maximize this current (as discussed with reference to Figures 2 and 3).

(i) iV Characteristics of the Electrolyzer (Figure IA)

The current- voltage (iV) characteristics of the electrolytic process are shown in Figure IA. All potentials are given with reference to the Standard Hydrogen Electrode (SHE). At the negative (-) electrode, the cathodic current reduced protons (H⁺) to hydrogen (H₂) by transferring electrons (e^~). At the positive (+) electrode, SO₂ was oxidized to sulfate by the anodic current and delivered electrons to the electrode. The cathodic current for the reduction equalled the anodic current driving the oxidation process so that there was no net accumulation of charge in the electrolyte. At low current densities, the current increased almost linearly with the applied voltage; however, this was not true at higher voltages, where mass transport and/or slow kinetics became limiting (as discussed below). However, for simplicity, the fall off of current with increasing voltage was only indicated for the oxidation process (Curve a), since this reaction was known to be current limiting at high applied potential. To accomplish Reaction (2) in an electrolyzer the net reaction consisted of two half reactions at the positive and negative electrodes as follows:

- pole: 2H⁺ + 2e → H₂ (5)

+ pole: H₂O + SO₃ ^2- → SO₄ ^2' + 2e + 2H⁺ (6) Net cell reaction SO₃ ^2" + H₂O → SO₄ ^2" + H₂ (2)

The electrode process associated with the reduction of protons (H⁺) to hydrogen (H₂) at the negative electrode (- pole), generated a cathodic current (that is, flow of electrons from the electrode into the electrolyte), as shown in the left section of Figure IA. The current relating to the oxidation of sulfite (SO₃ ^2") to sulfate (SO₄ ^2") at the positive electrode (+ pole) was represented by a anodic current (that is, flow of electrons from the electrolyte to the electrode), as illustrated by curve a, in the right section of Figure IA.

At given concentrations OfH⁺, H₂, H₂O, SO₃ ^2"and SO₄ ^2" in the solution there was a minimum potential difference Ϋ, as given by the Nernst Equation (from thermodynamics), to be applied between the electrodes to initiate Reaction (2) under reversible conditions. The electrolyzer was, by definition, balanced at V (ie. no current flowing through the cell, and the process taking infinite time). V for Reaction (2) under standard conditions is the reversible redox potential E⁰= 0.158V.

To generate hydrogen at a practical rate, a higher potential difference than Y was applied. The electrolyzer then ran under irreversible conditions, with the applied potential difference V_∞p being expressed by Equation I:

in which / is the current (in Amperes), η₊ and η, are the overpotentials at the positive and negative electrodes respectively, R_e (in Ohms) is the internal resistance in the electrolyzer, and R represents any other resistance in the cell. The overpotentials η₊ and η. were functions of interfacial kinetics (and may depend on the route of the electrochemical reactions). The size of R_e depended on mass and charge transport between the two electrodes, and thus on the conductivity of the electrolyte. It is apparent that R_e also depended on the design of the cell, such as the distance between electrodes, as well as any potential drop over membranes (when used).

It was seen that an increase in V_exp (due to any resistance R_e or R) scaled linearly with current as long as the interfacial kinetics were fast enough. However, at a certain applied potential, (depending, for example, on the choice of electrode material and the chemistry of the reactants) the rate of reaction at the electrode, rather than the transport of material to the interface, limited the current. At this point the increase in current with applied potential fell off.

In Figure IA, this phenomenon is illustrated on the upper right hand side of curve a. From the above discussion, the actual potential difference from the solar cell V _w (-Ve_xp) to drive Reaction (2) was a complex function of the design of the electrolyzer, current density and kinetics. V_eχ_P was thus expected to be considerably larger than Y.

Every term to the right of Y in Equation I is related to energy losses. For an efficient utilization of solar energy, the ratio YlV_exp was kept as high as possible. As seen from Equation I and Figure IA, low currents generally increased this ratio. On the other hand, there was a practical limit to how slow the process could be driven. In a commercial system (producing hydrogen in accordance with Scheme I) this would require both scientific and engineering optimisation.

(ii) iV Characteristics of the Solar Cell (Figure IB)

The power output P from a solar cell is expressed by P=IV. To be able to compare the power output of solar cells the power is given per unit area. Thus the power per unit area/? = IV/ A = iV, where i is the current density, and A is the area of the solar cell (or module). A typical iV curve for a photovoltaic (PV) solar cell is given in Figure IB. V_oc is the open circuit voltage - that is, the maximum voltage (V) that the PV cell can deliver at a given light intensity. Correspondingly i_sc, the short circuit current, is the maximum current (/) that the PV cell can deliver at a given light intensity. The iV curve of a PV solar cell is characterized by its open circuit voltage, V_oc, short circuit current, i_sc, and fill factor ff, defined by Equation II:

where i_max and V_max are the current and voltage of the PV cell at p_max. p_max is the maximum power output per unit area of the PV cell at any given light intensity. Thus, the fill factor was a measure of how closely the /F curve resembled the rectangular shape of the product i_sc • V₀₀- It is to be noted that a PV solar cell in its conventional use is optimized for the highest possible power output per unit of solar energy.

On the other hand, the PV-solar ceH/electrolyzer in the present method and system was optimized for as high a yield of H₂ as possible per unit of solar energy. Thus, as long as the voltage was sufficient to drive the chemical reactions, a high current was favourable. In other words, as long as the voltage delivered by the PV cell was sufficient to drive the chemical reactions, in the present PV-solar cell/electrolyzer system and method it was desirable to provide as high a current density as possible from the solar cells. Thus, two cells in a series would, by necessity, reduce the current density delivered by the solar cells by at least a factor of two. Thus, a parallel cell configuration was generally optimal.

(iii) Working Point of the Self Tuning Integrated System

V_w and i_w denote the output current and potential at the working point of the solar cell, when powering the electrolyzer. A solar cell was chosen that could supply a voltage high enough to drive the electrolyzer. Li Figure 1, the voltage V_w can efficiently drive the electrolyzer at a current /„, if the i V characteristics of the electrochemical processes in the electrolyzer are in accordance with Curve a.

The working point of the solar cell was thus optimized for best efficiency of the combined system. An upper limiting factor to the voltage that could be applied was set by the shape of the iV curve of the electrolyzer. Note was made of the S-shape of the curve at the upper right of Curve a. Beyond the inflection point, the rate of increase of current with increasing voltage started to fall. Thus, further increase in applied voltage resulted in just a minor increase in current, and a decrease of efficiency of the whole system.

The fall-off in current was explained by a change in the charge transfer mechanism at the electrode interface. This change may, for example, be due to the fact that the reaction started to be kinetically limited rather than diffusion limited. The net result was that the turnover rate of Reaction (2) was hampered. It was thus apparent that this set a limit to the rate of cell reaction.

(iv) Scaling Factor In the late 1970s, Westinghouse constructed a pilot scale electrolyzer that reportedly was capable of producing hydrogen at potential F_w=568mV and a current density at the working point of /„,= 200 mAcm^"2. Assuming a realistic value of /„, = 10 mAcm^"2 for the solar cell, the scaling factor between the sizes of the electrolyzer and solar cell would be 20. That is, an electrolyzer with one square meter of electrodes would need a solar module of 20 m². The scaling factor/ was introduced in Figures 1-3 to enable expression of the performance of the electrolyzer and solar cell in terms of current density i, rather than current /.

(v) Effect of Varying Light Intensity and PV type (Figures 2 & 3) Figure 2B shows one PV cell at different light intensities, while Figure 3B shows three different PV cells, each with a different bandgap. Based on the concept introduced in Figure 1, Figure 2 can be used to explain the interaction of a single photovoltaic solar cell and an electrolyzer under different operational conditions.

Referring to Figure 2A three i V curves (a, b, c) are shown for different anode [(+) electrode] materials in the electrolyzer. The different materials had different overpotentials and thus different abilities to facilitate the oxidation process. It was noted that both the potential for onset of electrolysis and "the slope" of the curves could vary, as illustrated by Curves a, b and c. The cathodic current as a function of potential was the same in all three examples, as shown in the lower left of the diagram in Figure 2 A. Figure 2B shows the i V characteristics of a single PV solar cell at three different light intensities, showing how both V_oc and i_sc increase with increasing light intensity. AU three curves allowed electrolysis to be driven in an electrolyzer with the properties of Curve a, in Figure 2A. As the light intensity increased, the current /,_sC and potential K_<,_calso increased. For a given combination of a solar cell and electrolyzer, at any given light flux on the PV cell, the system adjusted to a working point, so that /,„ was the same at the (+) and (-) poles. For optimum operation of the combined solar- cell/electrolyzer system, fluctuations of solar intensity leading to variation in the working point, fell on the steep slopes of the Curve a (as much as possible) as illustrated by 4,_/, i_w2 and i_W3.

The working voltage V_w required from the PV cell to drive the electrolyzer needed to be less than V_oc (the potential range of the solar cell) for the current to flow. When the potential range V_oc was too small to meet the demands of the electrolyzer, i_w was low, and the system was inefficient. This is illustrated by the Curve b.

The projection of the voltage of Curve b onto the i V curves for the PV solar cell revealed that the solar cell was just able to drive the electrolyzer. At the highest light intensity the working point is very low (far down the right hand side of the solar cell iV curve) so that i_w became very small. The onset of electrolysis for Curve c was more unfavourable, due to the slow increase in current with increasing potential, hi contrast, the low overpotential of the oxidation process in Curve a made it possible to drive the electrolyzer, even at low light intensity. With varying light intensity the output currents (i_wi, i_W2, i_W3) still had favourable working points on the upper flat part of the PV cell iV curves for this scenario. Increasing the current beyond /_W3 drove both the electrolyzer and solar cell into an unfavourable position.

Referring now to Figure 3 A three i V curves for the electrolyzer are shown. All three had the same cathodic current behaviour, but had different anodic current behaviour with increasing overpotential in the order a, b and c. For simplicity, the shapes of the curves were kept the same. To drive the electrolyzers at a, b and c, the output voltage V_w of the PV cell at the working point was correspondingly increased, as illustrated in Figure 3B.

Solar emission is polychromatic. For this reason there was an intimate relationship between the size V_w and /„, depending on the bandgap of the semiconductor used in the PV solar cells. To deliver a large V_w the bandgap was large. However, a large bandgap decreased the number of photons absorbed from the solar spectrum and thus the photocurrent i_w. Thus, if V_W3> V_W2> V_wi, then i_W3 < i_W2 < i_wi- The most favourable electrolyzer to drive with the output current from one solar cell was the one represented by /F curve a. The lower the voltage V_w, needed to drive the electrolyzer, the lower the bandgap of the semiconductor. A commercial system with a currently available electrolyzer would require around 0.568V. Knowing that E⁰ = 0.158V for Reaction (2), and assuming that the electrolyzer was able to be driven at a voltage of 0.568V, it was possible to calculate the efficiency η of the electrolytic step in the in the Hybrid Sulfur process: η = 100*0.158/0.568 = 27.8 %. Optimising the electrolyzer to achieve the lowest possible overvoltage and internal resistance (see Equation I) provided high efficiency of the PV-solar cell/electrolyzer system. The present system and method allowed solar cells to be connected in parallel form in a module, greatly increasing the amount of current produced by a given area of solar collector. Tailoring the solar cell to the requirements of the electrolysis cell in this manner was expected to enable utilization of a larger proportion of the solar spectrum for H₂ generation, compared to a conventional PV solar panel with the solar cells interconnected in series.

It was also apparent that the energy efficiency depended on the performance of the electrolyzer. Further improvements of the electrolytic process in the electrolyzer can be foreseen. For example, with recent developments of membranes and electrode materials, even lower output potentials (V_w) of the solar cells required to drive Reaction (2) can be expected.

Alternative Cell Configurations

As mentioned above, more efficient utilisation of the solar cells can be achieved by connecting two cells in series to increase the voltage. There is a large ratio of solar cell area to electrolyzer area, but doubling the available voltage (and halving the current) can result in a working point close to the maximum power point of the solar cell. This can provide a more favourable configuration than using the voltage generated by single cells. However, instead of using two DSSCs in series to generate the necessary potential, a tandem cell configuration could be used. A tandem cell comprising two sandwiched cells with different dyes on top of each other is known that resuls in 10% solar energy conversion efficiency. Such a tandem cell can supply the electrolyzer with an appropriate voltage. Although the current density per unit area of tandem cell is somewhat smaller than for a single cell it nevertheless can be superior in efficiency in this application, since the area ratio of the PV module to the electrolyzer will decrease compared to two DSSCs in series.

Direct coupling of the electricity generation and electrolyzer as described herein is potentially more efficient than a grid connected approach, as it removes the need for power transforming and, if properly configured, can have higher power efficiency. This can reduce the capital cost associated with the solar field although the electrolyzer will be increased in size as it only operates when solar power is available. The system can be engineered to minimise the voltage drop in conductors, as the parallel connection of PV and electrolysis cells can result in high currents at low voltages. The PV and electrolyzer cells can therefore be located in close proximity. An application for such systems can be decentralised hydrogen production, using sulfur dioxide as a feedstock. The embodied energy in the SO₂ is accessed and "topped-up" using solar driven electrolysis, producing hydrogen and sulfuric acid. The sulfuric acid can be collected and then reprocessed at a central facility to produce SO₂ again. This can overcome issues associated with transporting hydrogen as an energy carrier, since SO₂ can easily be compressed to a liquid. This can enable independent energy production in off-grid situations.

Effect of Varying Insolation on a Direct Coupled System

Electrolyzer performance can be described by a relatively constant iV curve, however, the output from a solar module is not constant and is strongly influenced by insolation. Variations in insolation affect the current and, to a lesser extent, the voltage available from a PV cell, which can result in the operating point moving up and down the /F curve for the electrolyzer. Figure 5 shows how an SRNL electrolyzer behaves when directly coupled to a DSSC module under varying insolation intensity. Figure 5 confirms that the insolation has a significant impact on the operation of the electrolyzer in a direct coupled system. In this case the system has been sized so that the electrolyzer curve intersects the solar PV curve close to the maximum power point (MPP) at the maximum insolation. The cells are assumed to be connected in series pairs to provide a higher voltage. In this example, the ratio of PV area to electrolyzer area is 150. As the insolation drops, the working point shifts away from the MPP on the curves to the left of the knee, meaning that there is a loss in terms of the efficiency of power extraction from the solar module. The direct coupled scenario is therefore best suited to areas where the insolation is relatively high and consistent (e.g. deserts).

The effect of intermittent operation due to variations in insolation was also considered, as this was noted to cause transient effects on the operation of the process. It was noted that short term perturbations in the supply voltage or current to the electrolyzer will cause changes in the conversion rate of SO₂ to sulfuric acid. Depending on the configuration of the overall plant, this can be managed by reducing the feed-rate, if the same conversion efficiency is required, or accepting a decrease in conversion efficiency at the same feed-rate. It was noted that the potential required would increase slightly with the degree of conversion, in line with concentration effects in the Nernst equation, so a slightly greater power efficiency would be expected with a decrease in conversion. This can, however, be balanced by the need to recycle a higher proportion of unconverted SO₂.

Integrated Solar cell/Electrolyzer

In a "stand alone self tuning system" the solar panel and electrolyzer are separate units. This requires an external circuit to interconnect the solar cells individually into modules and, in turn, the electrolyzer with the solar cell modules. As mentioned above the number of solar cell units in the modules will, by necessity, be large and thus wiring would be required to be dimensioned and optimized so that it can carry the high currents with minor loss in potential, adding some cost to the system. In an alternative configuration the solar cell was mounted (e.g.sandwiched) on top of the electrolyzer. In this way external circuiting was minimised. The area of the electrolyzer was then able to be of the same size as the solar cell, with the electrolyzer working at the same current densities as the solar cell (e.g. around 10-30mAcm^" ). As shown in Figure 5, the overpotential of the electrolyzer was a strong function of current density (i.e. the higher the current density the more the overpotential; overpotential increasing as potential increases), hi an integrated solar cell/electrolyzer the overvoltage dramatically drops. From Figure 5, it can be seen that the voltage even from one solar cell will be sufficient. Moreover, the solar heat generated in the system can thermodynamically favour the electrochemistry (as well as adjoining unit operations, operating at 80°C) and promote the oxidation and reduction processes. The circulating electrolyte can simultaneously be used as a coolant to protect the solar cells from overheating, hi addition, series connections are totally avoided (i.e. where a failure of a single PV or electrolysis cell takes the entire module or stack off-line). Engineering of the integrated solar-cell/electrolyzer" configuration needs to make the flow of electrolyte facile, and also allow hydrogen to be collected in a usual manner.

Stand Alone, Voltage Controlled Configuration

This configuration retained some of the advantages of the directly coupled system but had more flexibility in terms of the operation of the electrolyzer. The key difference was the incorporation of voltage control between the PV cells and the electrolyzer. This enabled series connection of the electrolysis and PV cells, to reduce the current load on interconnections by operating at higher voltages using stacks of electrolysis cells. It was noted that this can also be achieved to some extent by direct coupling multiple units of solar cells with an appropriate multiple of electrolyzer units. Further, in the stand alone, voltage controlled configuration the iV curve for the PV cells was effectively replaced by a power contour. Voltage control allowed a new range of possible operating points along this contour, although for a particular electrolyzer configuration only one operating point was possible at the intersection with the power contour. To avoid similar constraints to the directly coupled system the number of cells was made a control variable; that is, the stack of cells had a characteristic /Fprofile and thus current and voltage were not changed in isolation. Voltage control allowed some optimisation of the system in terms of the base operating point, unless the number of electrolysis cells fed by the PV cells was changed. This resulted in moderate gains over a system in which the number of PV cells in series is balanced against the number of electrolysis cells in the stack to arrive at the desired voltage and current working point. This system was more adaptable to changes in the electrolyzer and PV performance characteristics.

Figure 6 shows the increase in system efficiency by incorporation of voltage control to enable the PV module to operate at the maximum power point.

Figure 7 shows a voltage control strategy applied to two of the lower insolation cases from Figure 6. Figure 7 demonstrates that some gains in power efficiency are possible using this approach. It was noted that the overall gain possible in a given situation depends on the degree of variability in insolation at the particular site being considered. The lower the insolation, the greater the benefit that can be gained from voltage control: the lowest insolation case in Figure 7 shows that an extra 25% current density can be obtained by controlling the voltage and moving the PV cells back to the maximum power point (with a reduction in current and therefore hydrogen production compared to the peak value). It was noted that insolation on a PV panel inclined at the latitude angle tends to be relatively constant during a clear day, with reasonably rapid increases in the morning and decreases in the evening.

Non-limiting Examples of the method and system will now be provided. Example 1

Referring again to Figure 4, an electrolyzer was constructed to implement the electrolysis step in the Hybrid Sulfur process. The electrolyzer provided the main component of the electrochemical step and comprised two compartments separated by a membrane. The compartments each contained an electrode, with an electrolyte in each (ie. anode compartment with an anolyte, and cathode compartment with a catholyte). It was also noted that flow through cells can, in future, be constructed.

An external circuit supplied the electrolyzer with potential and current flows. The anolyte comprised small amounts of sulfuric acid with dissolved sulfur dioxide in water. The catholyte comprised sulfuric acid solution. When the electrolyzer was operated, hydrogen was evolved at the cathode and sulfuric acid was evolved at the anode. The hydrogen is collected and the sulfuric acid and unreacted sulfur dioxide was recycled through the system. As shown above, the Hybrid Sulfur process requires input of both electrical and thermal energy. It is noted that the thermal energy required (at around 950⁰C) could be supplied through solar applications (eg. as demonstrated for methane reforming). However, in the present method and system the electrical energy was supplied by solar power through the use of a single solar cell or cells in parallel.

The solar cell employed created an electric current using the photons emitted by the sun. Decoupling the electrochemical step from the photon capture enabled utilisation of more of the solar spectrum than existing photo-catalysts. A small applied voltage corresponding to a large current was preferable. Since the Sulfite/Sulfate reaction to produce hydrogen required approximately 60OmV, a solar cell was selected and configured to supply this voltage.

To test the viability of the cell set up, a solar cell was connected to two platinum electrodes immersed in a sodium sulfite solution. The set up was left in direct sunlight for approximately 5 hours. After this time, approximately 25mL of gas (ie. hydrogen) was produced.

The following further experimental work was proposed:

• Determining appropriate conditions under which the electrolyzer in a version of the Hybrid Sulfur process will operate optimally.

• Investigation of the mechanistic pathways of electrolytic oxidation of sulfur dioxide dissolved in water with sulfuric acid present (elementary reactions, intermediate species, reaction rate).

• Investigation and screening of optimal electrode materials (anode and cathode). • Determination of the effect of temperature on the oxidation of sulfur dioxide dissolved in water with sulfuric acid present at atmospheric pressure.

• Investigation of the input/output composition of anolyte/catholyte (mainly the effect of sulfuric acid concentration).

• Integration of results and recommendation of conditions with regards to electrode material, temperature, electrolyte composition (including SO₂ concentration) and incorporation of unit with connected unit operations.

Example 2

In a further variation of the method and system, an investigation was made into sourcing the SO₂ from non-renewable sources, such as the off-gases from metal smelting operations and/or power station emissions. It was noted that sourcing the SO₂ in this way could eliminate the difficult high temperature step of decomposing the sulphuric acid, whereby the following important effects could follow:

(1) a difficult processing step in the closed loop Hybrid Sulfur cycle would be eliminated;

(2) a major energy input could be removed from the overall energy balance of the process; (3) overall process economics could be further improved.

It was also noted that the hydrogen produced may actually have considerably more energy than the electrical energy required to produce it.

To investigate the feasibility of this process variation the following calculation was performed:

Solar Thermochemical Hydrogen

Calculation of H₂ production rate from hypothetical power station

SO₂ Emission rate 5 kg/sec

432 tonnes/day

SO₂ Molecular weight 64.054 g/mol

H₂ Molecular weight 2.016 g/mol

Stoichiometry 1 mol H₂/mol SO₂

Hydrogen production 13.6 tonnes/day

78 mol/sec

Power consumption:

Cell potential 0.6 Volts

Energy required (nFE) 118.2 kJ/mol

Power required 9,225 kW

Power required 9.23 MW

Embodied energy

Heat of combustion 285 kJ/mol HHV

Energy contained 22.2 MW (HHV)

Energy Payback (assume 60% conversion efficiency in, say, PEM fuel cell)

144% HHV

In this process variation, it was noted that the SO₂ would need to be separated from the other gases (especially O₂). However, the fact that the oxidation of SO₂ to SO₃ is thermodynamically favourable was noted to theoretically advantage this process variation.

Whilst a method and system for producing hydrogen has been described with reference to specific embodiments, it should be appreciated that they can be embodied in many other forms. For example, as mentioned above, the process conditions in the electrolysis cell can be further optimised - eg. by using a two compartment electrolysis system with hydrogen evolution and sulfurous acid oxidation that is separated by a proton permeable membrane. Investigation can also be made into fine tuning the PV cell to the electrolyzer, to further optimise efficiency, etc.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the method.

Symbols and Definitions

The following table sets forth the symbols used in the equations and Figures of this specification; with the units adopted in accordance with IUPAC recommendations.

Claims

1. A method for producing hydrogen comprising the step of electrolysing a solution comprising SO₂ and H₂SO₄ so as to produce hydrogen, using the photovoltage generated from one or more single solar cells.

2. A method as claimed in claim 1 wherein the one or more single solar cells are configured so as to produce as high as possible a photocurrent for electrolysis.

3. A method as claimed in claim 2 wherein two or more single solar cells are interconnected in parallel to produce the highest possible photocurrent.

4. A method as claimed in claim 3 comprising a further step of switching between the parallel and a series connection of the solar cells.

5. A method as claimed in claim 4 wherein the switch between the parallel and series connections depends on the photovoltage being produced by the cells, and is made so as to optimise the current density produced by the cells.

6. A method as claimed in any one of the preceding claims wherein, in the electrolysing step, the hydrogen is cathodically generated and collected from the cell, whereas sulfate ions are anodically produced from sulfite ions.

7. A method as claimed in claim 6 wherein the resultant sulfuric acid, together with unreacted sulfur dioxide, is recycled.

8. A method as claimed in any one of the preceding claims wherein the (or each) solar cell is operated so as to have a higher photon conversion and produce a higher current density.

9. A method as claimed in any one of the preceding claims wherein the (or each) solar cell is a dye-sensitized solar cell.

10. A method as claimed in any one of the preceding claims wherein the SO₂ is sourced from an industrial process off-gas that comprises SO₂.

11. A system for producing hydrogen comprising:

- one or more single solar cells arranged to generate a photovoltage; - an electrolysis cell in which a solution comprising SO₂ and H₂SO₄ is electrolysed to produce hydrogen, using the photovoltage generated from the one or more single solar cells.

12. A system as claimed in claim 11 wherein the one or more single solar cells are configured so as to produce as high as possible a photocurrent for the electrolysis cell.

13. A system as claimed in claim 12 wherein two or more single solar cells are interconnected in parallel to produce the highest possible photocurrent.

14. A system as claimed in claim 13 further comprising a switch arranged for switching between the parallel and a series connection of the solar cells.

15. A system as claimed in claim 14 wherein the switch is operable to optimise the current density produced by the cells.

16. A system as claimed in any one of claims 11 to 15 wherein the one or more single solar cells and the electrolysis cell are integrated.

17. A system as claimed in claim 16 wherein the one or more single solar cells are directly mounted a top of the electrolysis cell.

18. A system as claimed in any one of claims 11 to 17 wherein the (or each) solar cell is a dye-sensitized solar cell.

19. A system as claimed in any one of claims 11 to 18 wherein the electrolysis cell comprises one or more negative and positive electrodes, each located in a respective compartment separated by a membrane, whereby the hydrogen is generated at the (or each) negative electrode and is then removed from the cell, and sulfate ions are produced from sulfite ions at the (or each) positive electrode

20. Hydrogen or sulfate produced by the method or system as set forth in any one of the preceding claims.