WO2017064279A1

WO2017064279A1 - Porphyrins as electrocatalysts and electrodes modified with same

Info

Publication number: WO2017064279A1
Application number: PCT/EP2016/074762
Authority: WO
Inventors: Mary Teresa PRYCE; Jennifer Catherine MANTON; Johannes Gerardus VOS
Original assignee: Dublin City University
Priority date: 2015-10-14
Filing date: 2016-10-14
Publication date: 2017-04-20
Also published as: GB201518190D0

Abstract

The invention relates to the novel use of metal-free porphyrins for electrocatalytic hydrogen production and for electrocatalytic reduction of carbon dioxide. The present invention also relates to electrodes modified with metal-free porphyrins for electrocatalytic hydrogen production or CO₂ reduction and to methods of preparing same.

Description

Porphyrins as electrocatalysts and electrodes modified with same

The present invention relates to the novel use of metal-free porphyrins for

electro catalytic hydrogen production. The present invention also relates to the novel use of metal-free porphyrins for electrocatalytic reduction of carbon dioxide. The present invention also relates to electrodes modified with metal-free porphyrins for electrocatalytic hydrogen production or CO2 reduction and to methods of preparing same. In embodiments, the invention relates to methods for electrocatalytic hydrogen production using metal-free porphyrin catalysts. Further embodiments relate to methods for electrocatalytic reduction of CO2 using metal-free porphyrin catalysts.

The use of hydrogen gas as an energy source is particularly desirable. In theory, hydrogen gas is a versatile and environmentally-friendly fuel insofar as it is a high energy density gas which can be produced from the world's most abundant resource, water. In addition, once produced, hydrogen acts as an energy carrier, i.e. it can be used to store energy initially generated by alternative means. Hydrogen gas could therefore allow energy to be harnessed from intermittent sources such as wind, wave and sunlight, and then to be released on demand, for instance to power fuel cells, to generate electricity etc.

At present, however, the majority of the world's supply of hydrogen gas is generated from reformation of fossil fuels, with only approximately 4% being generated from electrolysis of water. It is clear from future energy projections that the current harnessing of energy from fossil fuels will not sustain the future needs of a rising world population. In addition, rising greenhouse gas emissions associated with fossil fuel combustion have lead to an unprecedented rise in global warming, with its associated environmental impacts. As regulations tighten on the use of fossil fuels and emissions, and the introduction of further limits on their use seem likely, there is a rapidly growing need for improved methods for the generation of hydrogen from so-called 'clean' sources. In 1972, Fujishima and Honda (Fujishima A., Honda K. Electrochemical photolysis of water at a semiconductor electrode, Nature 1972: 238, pp. 37-38} reported the generation of hydrogen and oxygen in a photoeletrochemical cell illuminated with near UV light, using a titanium dioxide electrode. Since then, there is been extensive research into catalysts for use in electrocatalytic hydrogen production from water.

A major limitation to the increased use of hydrogen fuel is the lack of economically viable methods for its production from clean sources. At present, efficient methods for industrial hydrogen generation rely heavily on the use of noble metals, such as platinum, palladium and ruthenium. These rare and, consequently, expensive earth metals drive up the cost of hydrogen production, making them uncompetitive in comparison to methods utilising fossil fuels. Although methods for hydrogen generation which rely on inexpensive solid materials are known (Kost K.M., Bartak D. E. Electrodeposition of Platinum Microparticles into Polyaniline Films with

Electrocatalytic Applications, Analytical Chemistry (1988}: 60(20}: 2379-2384; Ma C, Sheng J., Brandon N., Zhang C, Li G. Preparation of tungsten carbide-supported nano Platinum catalyst and its electrocatalytic activity for hydrogen evolution. International Journal of Hydrogen Energy. (2007} 32 : 2824 - 2829; Norskov J. K., Christensen C. H. Toward Efficient Hydrogen Production at Surfaces. Science (2006} 312: 1322;

Greeley J., Jaramillo T. F., Bonde J., Chorkendorff I., Norskov J. K., Computational high- throughput screening of electrocatalytic materials for hydrogen evolution. Nature. Materials. (2006} 5: 909}, hydrogen evolution is generally not observed at potentials near equilibrium, and these methods therefore require the application of significant overpotential. This reduces the efficiency of these methods, often making them unsuitable for scale-up to levels required for industrial application.

Metallated complexes are known as catalysts for hydrogen generation. Early research in this area found the most efficient catalysts to comprise precious transition metals but of late, research in the area has been focused on organic frameworks such as porphyrin or phthalocyanine compounds with incorporated first or second row transition metal centres ( . D'Souza, A. Villard, E. Van Caemeibecke, M. Franzen, T. Boschi, P. Tagiiatesta, K. M. Kadish, Inorg. Chem., 1993, 32, 4042 "Studies of the electrochemical behaviour of cobalt porphyrins bearing electron withdrawing groups"; T. Abe, H. Imaya, S. Tokita, D. Wohrle, M. Kaneko, Journal of Porphyrins and Phthalocyanines, 1997, 1 , 215

"Photoelectrochemical Proton Reduction with Coated Zinc Tetraphenylporphin Dispersed into Poly(4-vinylpyridine}"; T. Abe, F. Taguchi, H. Imaya, F. Zhao, J. Zhang, M. Kaneko, Polymers for Advanced Technologies, 1998, 9, 559 "Cobalt porphyrin

incorporated into a polymer for proton reduction"; F. Taguchi, T. Abe, M. Kaneko, Journal of Molecular Catalysis A., 1999, 140, 41 "Metalloporphyrins incorporated into Nafion polymers for electrochemical proton reduction";/ Knoll, S. Swavey, Inorganica Chimica Acta., 2009, 362, 2989 "Cobalt and platinum porphyrins utilised in

electrochemical hydrogen production"; F. Zhaoa . Zhanga, T. Abea, D. Wohrleb, M. Kaneko, Journal of Molecular Catalysis A: Chemical, 1999, 145, 245 "Cobalt

phthalocyanine incorporated into a polymer membrane for electrochemical proton reduction "; A. Kocaa, M.K. §enerb, M.B. Kogakb, A. Gulb, International Journal of Hydrogen Energy, 2006, 31, 2211 "Cobalt phthalocyanine for electrochemical proton reduction"; O.A. Osmanbasa, A. Kocaa, M. Kandazb, F. Karacaa, International Journal of Hydrogen Energy, 2008, 33, 3281 "Thiophene cobalt phthalocyanines used in hydrogen generation". In these systems it is generally accepted that the first step in catalysis is the protonation of the metal centre. The overall mechanism is then thought to proceed via either a mono or bimolecular process; the protonated metal centre becomes further protonated following a second reduction, followed by H2 release in the monomolecular approach, or, in the bimolecular mechanism, two singly protonated catalytic centres produce H2 when they are in close proximity. While some of these materials have been shown to be efficient hydrogen evolving catalysts, the high cost of the co-ordinated metal complexes represents a barrier to commercialisation. Manton et al., Dalton Transactions, 2014, 43, 3576-3582 "Porphyrin-cobaloxime complexes for hydrogen production, a photo- and electrochemical study, coupled with quantum chemical calculations" discloses the use of a free-base and a zinc-metallated monopyridyl triphenylporphyrin which was attached to a cobalt metal centre through the nitrogen on the pyridine group.

In view of the above, there is an increasing global need for cost-effective and efficient means of storing energy from renewable sources. In particular, there is a need for novel catalysts for hydrogen generation and, in particular, for novel catalysts for hydrogen generation which mitigate some of the disadvantages associated with the prior art, such as high cost, difficulty of preparation and requirement to co-ordinate the metal centre, inefficiency, etc.

In addition to the need for novel catalysts for hydrogen generation, there is a need for novel catalysts for CO2 reduction. While a variety of catalysts have been studied for CO2 reduction, a commercially viable route to CO or other chemicals from CO2 is not available. Selectivity is also an important concern for CO2 reduction catalysts due to the number of products that can be formed. Selective catalysts for CO2 reduction would therefore be highly advantageous. Selective catalysts for CO2 reduction that mitigate some of the advantages associated with known CO2 reduction catalysts, such as high cost, difficulty of preparation, inefficiency, lack of selectivity etc. would be particularly beneficial.

The inventors have advantageously developed a novel system utilising metal-free catalysts for producing hydrogen from aqueous sources. Surprisingly, when used to modify carbon-based electrodes, the metal-free catalyst lowers the overpotential needed to drive the water splitting reaction which produces hydrogen when compared to the bare electrodes. Results therefore indicate that that a metal centre is not necessary for hydrogen production, and that the metal-free porphyrin systems exhibit reduction processes which are capable of yielding a hydrogen-generating catalytic response. This has significant advantages in terms of fabrication and cost. BRIEF DESCRIPTION OF THE DRAWINGS:

FIGURE 1: Basic porphyrin structure.

According to a first aspect of the invention there is provided use of a metal-free porphyrin as a catalyst in the electrocatalytic generation of hydrogen from water. According to a second aspect of the invention there is provided use of a metal-free porphyrin as a catalyst in the electrocatalytic reduction of carbon dioxide.

Porphyrins are heterocyclic macrocyclic organic compounds comprising four modified pyrrole rings connected at their a carbons via methine bridges. The structure of porphine, the simplest porphyrin is shown in FIG. 1.

Throughout this specification, by "metal-free porphyrin" it is meant a porphyrin compound which does not comprise any metal atoms, or any substituents comprising metal atoms, i.e. it does not comprise a co-ordinated metal ion. In embodiments, it is intended to mean a porphyrin compound which does not include a metal at the centre of the macrocycle or on the periphery, i.e. it does not include a metal centre.

Advantageously, the inventors have determined that metal-free porphyrins are efficient catalysts for the generation of hydrogen from aqueous media. This means that the use of expensive, and often rare, metals can be avoided, representing a cost-efficient means of hydrogen generation. The metal-free porphyrins can be prepared by a facile process with minimal purification required, thereby allowing for further cost-efficiencies to be made when compared with prior art electrocatalysts for hydrogen production.

The metal-free porphyrins are also shown to be efficient, selective catalysts for CO2 reduction.

The metal-free porphyrins can be used as electrocatalysts for electrolysers.

The metal-free porphyrins can be used as electrocatalysts for fuel cells. The fuel cell may be a proton exchange membrane (PEM] fuel cell.

The electrocatalysts can be used in a tandem electrolyser-fuel cell, advantageously allowing for hydrogen to be produced and directly used to supply a fuel cell. In an embodiment, the metal-free porphyrin for electrocatalytic hydrogen production is a substituted or unsubstituted phenylporphyrin or a substituted or unsubstituted pyridylporphyrin. In an embodiment, the substituted or unsubstituted phenylporphyrin is a substituted or unsubstituted tetraphenyl porphyrin (TePP] or a substituted or unsubstituted triphenylporphyrin (TrPP}.

In an embodiment, the substituted or unsubstituted triphenylporphyrin is a substituted or unsubstituted monopyridyl-triphenylporphyrin.

In an embodiment, the substituted or unsubstituted phenylporphyrin or substituted or unsubstituted pyridylporphyrin is substituted with one or more electron withdrawing groups.

In an embodiment, the one or more electron withdrawing groups is selected from CN, F, Br, I, CI, a C1-C3 alkyl group, OH, COOH, OCH₃, S0₃H, N0₂, HN₃, OR¹, COOR¹ and NR½, wherein R¹ is a C1-C3 alkyl group. In an embodiment, the one or more electron withdrawing groups is selected from CN, F, Br, I and a C1-C3 alkyl group.

In an embodiment, the one or more electron withdrawing groups is selected from CN and Br.

In an embodiment, the substituted or unsubstituted phenylporphyrin is a

tetraphenylporphyrin compound of formula (I}:

Formula (I] wherein each R is independently selected from H, CN, F, Br, I, CI, a C₁-C3 alkyl group, OH, COOH, OCH3, SO3H, N0₂, HN3, OR¹, COOR¹ and NR½, wherein R¹ is a C1-C3 alkyl group.

In an embodiment, each R of formula (I] is independently selected from H, CN, F, Br, I and a C1-C3 alkyl group.

In an embodiment, the substituted or unsubstituted tetraphenylporphyrin is a compound of formula (la} :

Formula (la] wherein each R is independently selected from H, CN, F, Br, I, CI, a C₁-C3 alkyl group, OH, COOH, OCH3, SO3H, N0₂, HN3, OR¹, COOR¹ and NR½, wherein R¹ is a C1-C3 alkyl group.

In an embodiment, each R of formula (la] is independently selected from H, CN, F, Br, I and a C1-C3 alkyl group.

In an embodiment, each R of formula (la] is independently H, CN or Br.

In an embodiment, the substituted or unsubstituted tetraphenylporphyrin is selected from Formula (Ia} (f) and(Ia](ii} :

CN

Formula I(a](i]

Formula I (a] (if) In an embodiment, the metal-free porphyrin is a substituted or unsubstituted pyridylporphyrin.

The substituted or unsubstituted pyridylporphyrin may be a mono-, di, tri- or tetrapyridylporphyrin.

In an embodiment, the substituted or unsubstituted pyridylporphyrin is a pyridyl phenylporphyrin, which may be monopyridyl-triphenylporphyrin (MPyTrPP}.

Alternatively, the pyridyl phenylporphyrin may be dipyridyl-diphenylporphyrin or tripyridyl-monophenylporphyrin.

In an embodiment, the pyridylporphyrin is substituted with one or more electron withdrawing groups.

In an embodiment, the one or more electron withdrawing groups is selected from CN, F, Br, I, CI, a C1-C3 alkyl group, OH, COOH, OCH₃, S0₃H, N0₂, HN₃, OR¹, COOR¹ and NR½, wherein R¹ is a C1-C3 alkyl group.

In an embodiment, the one or more electron withdrawing groups is selected from CN, F, Br, I and a C1-C3 alkyl group.

In an embodiment, the monopyridyl-triphenylporphyrin is a compound of formula (II}:

Formula (II};

wherein each R is independently selected from H, CN, F, Br, I, CI, a C₁-C3 alkyl group, OH, COOH, OCH3, SO3H, NO2, HN₃, OR¹, COOR¹ and NR½, wherein R¹ is a C1-C3 alkyl group, with the proviso that one of R' is N, and the other R' are selected from CH and CR, wherein R is as defined previously.

In an embodiment, each R of formula (II] is independently selected from H, CN, F, Br, I and a C1-C3 alkyl group. In an embodiment, each R of formula (II] is independently H, CN or Br.

In an embodiment, the metal-free porphyrin for electrocatalytic CO2 reduction is a substituted phenylporphyrin or a substituted pyridylporphyrin.

The substituted phenylporphyrin may be a substituted tetraphenylporphyrin or a substituted triphenylporphyrin. The substituted triphenylporphyrin may be a substituted monopyridyl- triphenylporphyrin.

In an embodiment, the substituted phenylporphyrin or substituted pyridylporphyrin is substituted with one or more electron withdrawing groups.

The one or more electron withdrawing groups may be selected from CN, F, Br, I, CI, a Ci- C₃ alkyl group, OH, COOH, OCH₃, S0₃H, N0₂, HN₃, OR¹, COOR¹ and NR½, wherein R¹ is a Ci-C₃ alkyl group.

In an embodiment, the one or more electron withdrawing groups is selected from CN, F, Br, I and a Ci-C₃ alkyl group.

The substituted tetraphenylporphyrin may be a compound of formula (III}:

Formula (III] wherein each R is independently selected from H, CN, F, Br, I, CI, a C1-C3 alkyl group, OH, COOH, OCH3, SO3H, N0₂, HN3, OR¹, COOR¹ and NR½, wherein R¹ is a C1-C3 alkyl group, with the proviso that at least one R is not H.

The substituted tetraphenylporphyrin may be a compound of formula (Ilia}:

Formula (Ilia}.

Each R of formula (III] or of formula (Ilia] may be independently selected from H, CN, F, Br, I and a C1-C3 alkyl group, with the proviso that at least one R is not H.

Each R of formula (III] or of formula (Ilia] may be independently selected from H, CN or Br, with the proviso that at least one R is not H.

The substituted tetraphenylporphyrin may be selected from a compound of Formula (IIIa](i] and a compound of Formula (IIIa](if):

CN

Formula (IIIa](i]

Formula (IIIa](ii] In an embodiment, the metal-free porphyrin is a substituted pyridylporphyrin.

The substituted pyridylporphyrin may be a mono-, di, tri- or tetrapyridylporphyrin. In an embodiment, the substituted pyridylporphyrin is a pyridyl phenylporphyrin, which may be monopyridyl-triphenylporphyrin (MPyTrPP}.

In an embodiment, the pyridyl-phenylporphyrin is substituted with one or more electron withdrawing groups.

In an embodiment, the one or more electron withdrawing groups is selected from CN, F, Br, I, CI, a C 1- C3 alkyl group, OH, COOH, OCH₃, S0₃H, N0₂, HN₃, OR¹, COOR¹ and NR½, wherein R¹ is a Ci-C₃ alkyl group.

The substituted pyridylporphyrin may be a compound of Formula (IV}:

Formula (IV] wherein each R is independently selected from H, CN, F, Br, I, CI, a C₁-C3 alkyl group, OH, COOH, OCH3, SO3H, NO2, HN₃, OR¹, COOR¹ and NR½, wherein R¹ is a C1-C3 alkyl group, with the proviso that one of R' is N, and the other R' are selected from CH and CR, wherein R is as defined above and at least one R is not H.

In an embodiment, each R of formula (II] is independently selected from H, CN, F, Br, I and a C₁-C3 alkyl group, with the proviso that at least one R is not H.

In an embodiment, each R of formula (II] is independently H, CN or Br, with the proviso that at least one R is not H.

According to an aspect of the invention there is a provided an electrode modified with metal-free porphyrin.

The metal-free porphyrin may be that as outlined in detail above. The modified electrode may be used in the electrocatalytic production of hydrogen or in the electrocatalytic reduction of CO2. The term 'modified' in the context of an electrode as used herein would be well understood by a person skilled in the art. Specifically, the term 'modified' is intended to mean that the electrode has been coated on at least one surface by the metal-free porphyrin. The coating or modification can be performed by any known method, such as drop casting, spin coating, dip-coating, spray casting, printed electrodes and incorporation into a conductive polymer such as, but not limited to,

polyvinylpyrrolidone (PVP], Nafion, polypyrrole, polyanaline etc. In an embodiment, the electrode is a carbon-based electrode.

Carbon-based electrodes are cheap and readily available making them suitable for use in the present invention. Carbon electrodes don't efficiently produce hydrogen, so they are a suitable basis for experiments to assess hydrogen generation catalytic activity. Advantageously, modification of carbon-based electrodes with metal-free porphyrins according to the invention was shown to lower the overpotential which needed to be applied to produce hydrogen.

In an embodiment, the carbon-based electrode is selected from the group consisting of glassy carbon, graphite plate, carbon monolith, carbon paper, carbon fibre and carbon paste.

According to an aspect of the invention there is provided a method for the

electro catalytic reduction of water to hydrogen, wherein the reduction is catalysed by a metal-free porphyrin. There is also provided a method for the electrocatalytic reduction of CO2, wherein the reduction is catalysed by a metal-free porphyrin. The metal-free porphyrin may be used to modify an electrode, preferably a carbon-based electrode. The aqueous media maybe agitated during the reduction reaction.

Agitation of the aqueous media is beneficially shown to increase H2 production. Agitation of the aqueous media is also shown to increase CO2 reduction.

According to an aspect of the invention there is provided a method of preparing a modified carbon-based electrode, the method comprising immobilising a metal-free porphyrin catalyst on at least one surface of a carbon-based electrode.

In an embodiment, the metal-free porphyrin catalyst is immobilised on a surface of a carbon-based electrode by:

preparing a solution of the metal-free porphyrin catalyst in a solvent;

applying the solution on at least a surface of a carbon-based electrode; and

drying the electrode.

According to an aspect of the invention there is provided an electrolyser, wherein the electrolyser comprises a metal-free porphyrin catalyst as described above.

According to an aspect of the invention there is provided a fuel cell, wherein the fuel cell comprises a metal-free porphyrin catalyst as described above. In an embodiment, the fuel cell is a proton exchange membrane (PEM] fuel cell.

Suitable fuel cells are, for example, those described in WO2014/037494, the content of which is incorporated by reference in its entirety . The invention will now be described by way of illustration only in the following examples:

Example 1: Preparation of Tetraphenylporphyrin (TePP) 50 mmol of freshly distilled pyrrole (Sigma Aldrich] and 50 mmol of benzaldehyde (Sigma Aldrich] were added to 99% propionic acid (175 ml] (Sigma Aldrich], and brought to reflux temperature. The acidic solution was allowed to reflux for 2 hours. A black solution resulted. The mixture was allowed to cool to room temperature and was stored in the fridge (2 - 8°C] overnight. The black solution was filtered under vacuum. The purple crystals formed were collected and washed several times with cold methanol to give a crystalline solid.

Example 2: Preparation of CN-Tetraphenylporphyrin (CN-TePP)

50 mmol of freshly distilled pyrrole (Sigma Aldrich] and 50 mmol of CN-substituted benzaldehyde (Sigma Aldrich] were added to 99% propionic acid (175 ml] (Sigma Aldrich], and brought to reflux temperature. The acidic solution was allowed to reflux for 2 hours. A black solution resulted. The mixture was allowed to cool to room temperature and was stored in the fridge (2 - 8°C] overnight. The black solution was filtered under vacuum. The purple crystals formed were collected and washed several times with cold methanol to give a crystalline solid. Purification was carried out using column chromatography on a silica gel using chloroform:ethanol (98:2] as the mobile phase.

Example 3 : Preparation of Br-Tetraphenylporphyrin (Br-TePP) Br-TePP was prepared using the same methodology as set out in Example 2, but in which the CN-benzaldehyde was replaced with Br-benzaldehyde (Sigma Aldrich].

Example 4: Preparation of Monopyridyl-triphenylporphyrin (MPyTrPP) 50 mmol of freshly distilled pyrrole, 37.5 mmol of benzaldehyde and 12.5 mmol of 4- pyridine carboxaldehyde were added to 99% propionic acid (175 ml], and brought to reflux temperature. The acidic solution was allowed to reflux for 2 hours. A black solution results. The mixture was allowed to cool to room temperature and was stored in the fridge (2 - 8°C] overnight. The black solution was filtered under vacuum. The purple crystals formed were collected and washed several times with cold methanol to give a bright purple crystalline solid. Purification was carried out using column chromatography on a silica gel. The initial mobile phase used was chloroform : ethanol (98 : 2], moving to 97 : 3 after the first fraction eluted.

Example 5: Preparation of Modified Electrodes

1 x 10-⁴ M solutions of the TePP, CN-TePP, Br-TePP and MPyTrPP of Examples 1 to 4 were prepared in DMF and 1.5 μί, of each solution was drop-cast using a Gilson

Pipetman® P20 pipette onto the surface of a glassy carbon electrode (0.07 cm³} and left to evaporate overnight in darkness in a 22°C oven.

Example 6: Electrocatalytic Hydrogen Generation

Electrocatalytic hydrogen generation experiments were carried out at room

temperature in a sealed two-compartment H-shaped cell.

A 0.1 M solution of sodium hydrogen phosphate (Na PC ] was prepared and brought to pH 2.0 using ortfto-phosphoric acid. 15 ml of this buffer solution was added to each compartment of the H-shaped cell. All solutions were purged with argon for 20 minutes prior to each experiment, and were then kept under an argon atmosphere throughout the length of the experiment.

The electrodes of Example 5 were used as the working electrode, a platinum wire was used as the counter electrode and Ag/AgCl (filled with 3 M KC1] was used as reference electrode.

Each catalyst-coated electrode was held at potentiostatic electrolysis at -1.2 V (vs. Ag/AgCl] for 1 hour. After this time a 1 ml sample of the headspace was injected onto a Gas Chromatography -Flame Ionization Detector/Thermal Conductivity Detector (GC- FID/TCD] to quantify the amount of ¾ produced over the course of the hour long experiment. This ¾ peak was measured and referenced with regard to a 1000 ppm standard of ¾ gas to ascertain the number of moles of ¾ produced. The results of these experiments are shown in Table 1. TPP CNTPP BrTPP

Hydrogen produced (1 hr] 1 x lO ⁵ moles 4 x lO ⁵ moles 5 x lO ⁶ moles

Current density 13 mA/cm² 22 mA/cm² 5 mA/cm²

Current efficiency 80-90% 80-90% 60-70%

Table 1: Electrocatalytic hydrogen generation.

Example 7: Effect of selection of carbon-based electrode

The effect of the selection of the carbon-based electrode on H2 generation was evaluated for glassy carbon and graphite plates modified with TePP catalyst, as follows:

Electrocatalytic hydrogen generation experiments were carried out at room

temperature in a sealed two-compartment H-shaped cell. A 0.1 M solution of sodium hydrogen phosphate (NaHzPC ] was prepared and brought to pH 2.0 using ortfto-phosphoric acid. 15 ml of this buffer solution was added to each compartment of the H-shaped cell. All solutions were purged with argon for 20 minutes prior to each experiment, and were then kept under an argon atmosphere throughout the length of the experiment.

Graphite plates (1.15 cm²} were allowed to sit in a 1 x 10 ⁴ M solution of the TePP catalyst prepared in example 1 for 10 minutes to allow the catalyst to adhere to the surface. They were then placed in a 22°C oven in the dark overnight to dry. For comparison, either the TePP-coated glassy carbon electrodes of Example 5 or the the modified graphite plates prepared above were used as the working electrode. In each case a platinum wire was used as the counter electrode and Ag/AgCl (filled with 3 M KC1] was used as reference electrode. Each catalyst-coated electrode was held at potentiostatic electrolysis at -1.2 V (vs. Ag/AgCl} for 1 hour. After this time a 1 ml sample of the headspace was injected onto a GC-FID/TCD to quantify the amount of ¾ produced over the course of the hour long experiment. This ¾ peak was measured and referenced with regard to a 1000 ppm standard of ¾ gas to ascertain the number of moles of ¾ produced. The results of these experiments are shown in Table 2.

The results of these experiments are shown in Table 2.

Table 2: Comparison of graphite v glassy carbon as electrode for ¾ generation.

From these results we can conclude that, due to the greater surface area, the modified graphite plates allow for more ¾ to be produced. However the results indicate that the scaling up of the electrode surface does not linearly equate to the moles of hydrogen produced. This may be attributed to the lower current density observed when utilising the modified graphite plates when compared to the glassy carbon. Example 8: Effect of process conditions

The effect of variations of the process conditions on the efficacy of the system for ¾ production was investigated, as follows:

Example 8(i): Agitation Electrocatalytic hydrogen generation experiments were carried out at room

temperature in a sealed two-compartment H-shaped cell.

A 0.1 M solution of sodium hydrogen phosphate (Na PC ] was prepared and brought to pH 2.0 using ortfto-phosphoric acid. 15 ml of this buffer solution was added to each compartment of the H-shaped cell. For comparison the electrolyte solution in each cell was stirred or not, to investigate the effect of agitation of the solution of electrocatalytic performance. All solutions were purged with argon for 20 minutes prior to each experiment, and were then kept under an argon atmosphere throughout the length of the experiment.

The TePP-coated electrodes of Example 5 were used as the working electrode, a platinum wire was used as the counter electrode and Ag/AgCl (filled with 3 M KC1] was used as reference electrode.

Each catalyst-coated electrode was held at potentiostatic electrolysis at -1.2 V (vs. Ag/AgCl] for 1 hour. After this time a 1 ml sample of the headspace was injected onto a GC-FID/TCD to quantify the amount of ¾ produced over the course of the hour long experiment. This ¾ peak was measured and referenced with regard to a 1000 ppm standard of ¾ gas to ascertain the number of moles of ¾ produced.

The results of these experiments are shown in Table 3.

Table 3: Effect of agitation on ¾ generation. The results included in Table 3 demonstrate that agitation of the aqueous media led to a significant increase in ¾ production. This may be because agitation ensures that bubbles don't adhere to the electrode surface, reducing its activity.

Example 8(ii) : Temperature Electrocatalytic hydrogen generation experiments were carried out at room

temperature in a sealed two-compartment H-shaped cell.

A 0.1 M solution of sodium hydrogen phosphate (NahhPC ] was prepared and brought to pH 2.0 using ortfto-phosphoric acid. 15 ml of this buffer solution was added to each compartment of the H-shaped cell. For comparison the temperature at which the potentiostatic electrolysis was carried out was varied from 0 °C to 25 °C to 40 °C. All solutions were purged with argon for 20 minutes prior to each experiment, and were then kept under an argon atmosphere throughout the length of the experiment. The TePP-coated electrodes of Example 5 were used as the working electrode, a platinum wire was used as the counter electrode and Ag/AgCl (filled with 3 M KC1] was used as reference electrode.

Each catalyst coated electrode was held at potentiostatic electrolysis at -1.2 V (vs. Ag/AgCl] for 1 hour. After this time a 1 ml sample of the headspace was injected onto a GC-FID/TCD to quantify the amount of ¾ produced over the course of the hour long experiment. This ¾ peak was measured and referenced with regard to a 1000 ppm standard of ¾ gas to ascertain the number of moles of ¾ produced.

The results of these experiments are shown in Table 4.

Charge # moles

Temp (°C) passed after TON H₂ Efficiency H₂(%)

1 hr (C) H₂

0 1.58 6.39 x lO^"6 42,500 78

25 3.25 1.56 x l0-⁵ 100,000 90

40 0.27 1.02 x l0-⁶ 6,800 73

Table 4: Effect of temperature on ¾ generation The results included in Table 4 demonstrate that temperature does not play a significant part in the ¾ generation. However, it is likely that the kinetics of the catalysis may be slowed at lower temperatures (i.e. below ~ 8°C] while higher temperatures (i.e. above ~ 40°C] may cause decomposition of the catalyst. Therefore, ambient temperatures, i.e. in the range of ~16°C to 32°C are likely to be the most effective for the H₂ reduction reaction which is commercially beneficial.

Example 8(iii): Light

Electrocatalytic hydrogen generation experiments were carried out at room

temperature in a sealed two-compartment H-shaped cell.

A 0.1 M solution of sodium hydrogen phosphate (Na PO^ was prepared and brought to pH 2.0 using ortfto-phosphoric acid. 15 ml of this buffer solution was added to each compartment of the H-shaped cell. For comparison experiments were carried out in darkness, in daylight and with visible white light irradiation. All solutions were purged with argon for 20 minutes prior to each experiment, and were then kept under an argon atmosphere throughout the length of the experiment. The TePP-coated electrodes of Example 5 were used as the working electrode, a platinum wire was used as the counter electrode and Ag/AgCl (filled with 3 M KC1] was used as reference electrode.

The results of these experiments are shown in Table 5.

Table 5: Effect of light on H2 generation.

The results included in Table 5 do not demonstrate any significant difference in ¾ production when light is applied, showing that light does not play a role in the catalysis. Again, this is of benefit commercially as it simplifies the reaction requirements. Example 9: Effect of applied potential using monopyridyl-triphenyl porphyrin (mPyTrPP)

Electrocatalytic hydrogen generation experiments were carried out at room

temperature in a sealed two-compartment H-shaped cell.

A 0.1 M solution of sodium hydrogen phosphate (Nah PC ] was prepared and brought to pH 2.0 using ortfto-phosphoric acid. 15 ml of this buffer solution was added to each compartment of the H-shaped cell. For comparison the bulk electrolysis potential applied was varied to ascertain the optimum potential for ¾ generation with MPyTPP. These results were compared with a bare, unmodified glassy carbon electrode at the same bulk electrolysis potentials. All solutions were purged with argon for 20 minutes prior to each experiment, and were then kept under an argon atmosphere throughout the length of the experiment.

The results of these experiments are shown in Table 6.

Charge

Potential TON Charge passed after 1 hr TON * passed after 1

Applied (V) (CHx lO ⁱ)

hr (C) (x 10 ^!) H₂ H₂

MPyTPP Bare Electrode

-0.8 0.63 - 0.001 -

-1.0 0.31 380 0.19 -

-1.1 0.70 15,000 0.20 -

-1.2 3.01 37,000 0.26 -

-1.3 8.42 165,000 8.97 -

Table 6: Effect of applied potential for mPyTrPP catalysts. The results included in Table 6 demonstrate that there is a significant difference between the bare electrode and the mPyTrPP-modified electrode at -1.2V, however, at - 1.3 V there is a similar charge passed. Based on these results the bulk electrolysis potential selected for further study was -1.2 V (vs. Ag/AgCl}. Example 10: Electrocatalytic CO2 reduction

Electrocatalytic CO2 reduction experiments were carried out at room temperature in a sealed V-shaped cell. A 0.1 M solution of tetrabutylammonium hexafluorophosphate (TBAPFe] was prepared in dry acetonitrile (ACN}. 15 ml of this solution was added to the V-shaped cell. 3% (v/v] deionised water was added and all solutions were purged with argon followed by CO2 for 20 minutes prior to each experiment, and were then kept under a CO2

atmosphere throughout the length of the experiment. The CN-modified and Br-modified electrodes of Example 5 were used as the working electrode, a platinum wire was used as the counter electrode and Ag/AgCl (filled with 0.1 M TBAPF6 and 1 mmol AgNOs] was used as reference electrode. Each catalyst-coated electrode was held at potentiostatic electrolysis at -2.6 V (vs.

Ag/AgCl] for 1 hour. After this time a 1 ml sample of the headspace was injected onto a GC-FID/TCD to quantify the amount of CO/ methane produced over the course of the hour long experiment. This CO/ methane peak was measured and referenced with regard to a 1,000 ppm standard of CO/ methane gas to ascertain the number of moles of CO/ methane produced.

The results of these experiments are shown in Table 7.

Table 7: Electrocatalytic CO2 reduction.

From the results shown in Table 7, it can be seen that the addition of an electron withdrawing group has allowed the reduction of CO2 selectively to CO. With CO2 reduction a number of products can be formed during the catalytic process, so it is important to have selectivity. From these results it can be seen that CN-TePP is a far superior catalyst to Br-TePP. While not wishing to be bound by theory, the inventors consider that this is probably due to the stronger electron withdrawing nature of the CN group. Example 11: Effect of Process Conditions

The effect of variations of the process conditions on the efficacy of CN-TePP for CO2 reduction was investigated, as follows:

Example ll(i): Effect of Temperature

Electrocatalytic CO2 reduction experiments were carried out at room temperature in a sealed V-shaped cell.

A 0.1 M solution of tetrabutylammonium hexafluorophosphate (TBAPFe] was prepared in dry acetonitrile (ACN}. 15 ml of this solution was added to the V-shaped cell. 3% (v/v] deionised water was added and all solutions were purged with argon followed by CO2 for 20 minutes prior to each experiment, and were then kept under a CO2

atmosphere throughout the length of the experiment. For comparison the temperature at which the potentiostatic electrolysis was carried out was varied from -20°Cto 0°C to 25°C to 40°C.

The CN-modified electrodes of Example 5 were used as the working electrode, a platinum wire was used as the counter electrode and Ag/AgCl (filled with 0.1 M TBAPF6 and 1 mmol AgNOs] was used as reference electrode.

Each catalyst-coated electrode was held at potentiostatic electrolysis at -2.6 V (vs. Ag/AgCl] for 1 hour. After this time a 1 ml sample of the headspace was injected onto a GC-FID/TCD to quantify the amount of CO/ methane produced over the course of the hour long experiment. This CO/ methane peak was measured and referenced with regard to a 1000 ppm standard of CO/ methane gas to ascertain the number of moles of CO/ methane produced.

The results of these experiments are shown in Table 8. Charge passed # moles TON Efficiency

Temp (°C)

after 1 hr (C) CO CO CO(%)

-20 0.72 8.9x 10-⁸ 600 2

0 0.86 8.5 x lO^"8 600 2

25 0.87 2.25 x lO^"6 15000 50

40 0.94 2.30 x lO ⁷ 1500 4

Table 8: Effect of temperature of CO2 reduction.

From these results it can be concluded that the reaction performs most effectively at ambient temperatures, i.e. from ~ 16°C to ~ 32°C, which is commercially advantageous.

Example 11 (ii): Effect of applied potential

A 0.1 M solution of tetrabutylammonium hexafluorophosphate (TBAPF6] was prepared in dry acetonitrile (ACN}. 15 ml of this solution was added to the V-shaped cell. 3% (v/v] deionised water was added and all solutions were purged with argon followed by CO2 for 20 minutes prior to each experiment, and were then kept under a CO2

atmosphere throughout the length of the experiment. For comparison the bulk electrolysis potential applied was varied to ascertain the optimum potential for H2 generation with CN-TePP.

The CN-modified electrodes of Example 5 were used as the working electrode, a platinum wire was used as the counter electrode and Ag/AgCl (filled with 0.1 M TBAPF6 and 1 mmol AgNOs] was used as reference electrode. Each catalyst-coated electrode was held at potentiostatic electrolysis at -2.6 V (vs. Ag/AgCl] for 1 hour. After this time a 1 ml sample of the headspace was injected onto GC-FID/TCD to quantify the amount of CO/ methane produced over the course of the hour long experiment. This CO/ methane peak was measured and referenced with regard to a 1,000 ppm standard of CO/ methane gas to ascertain the number of moles CO/ methane produced.

The results of these experiments are shown in Table 9.

Table 9: Effect of applied potential on CO2 reduction.

From the results included in Table 9 it can be seen that the number of moles of CO produced is increased with the more negative bulk electrolysis potentials. Also the more negative the potential which is applied, the more selective towards CO production the catalysis becomes. This is advantageous as it indicates that selectivity to CO can be controlled.

Example ll(iii): Effect of water content

atmosphere throughout the length of the experiment. For comparison the percentage of water added to the electro catalytic cell was varied.

Each catalyst-coated electrode was held at potentiostatic electrolysis at -2.6 V (vs.

The results of these experiments are shown in Table 10.

Table 10: Effect of water content.

From the results included in Table 10 it can be concluded that the percentage of water added to the electrochemical cell has an effect on the selectivity of the CO2 reduction products formed. Again, this is advantageous as it suggests that selectivity to CO can be controlled.

In summary, the utilisation of metal-free porphyrin systems as in the present invention represents a significant advance in hydrogen generation technology, primarily as it does not require the incorporation of an expensive metal centre for the hydrogen production to occur. Modified electrodes according to the invention are capable of generating hydrogen gas from aqueous media at low cost, through the use of inexpensive and easy to prepare catalysts, which comprise earth abundant elements. The catalysts advantageously operate at atmospheric temperature and pressure. Hydrogen gas generated through the use of the catalyst can be collected and used as a means of storing energy generated from renewable sources such as wind, wave or solar energy or can be used directly as a fuel in a fuel cell. The invention thus represents an important step forward in the development of new, sustainable energy sources.

The metal-free porphyrin systems can advantageously also be used for reduction of CO2 and in particular for selective reduction of CO2 to CO. As for hydrogen generation, the catalysts can be used at atmospheric temperatures and pressures, and can readily be prepared at low cost. Adjustment of the applied potential and the volume of water added to the cell can be used to control the selectivity of the reaction to CO.

Claims

Claims:

1. Use of a metal-free porphyrin as a catalyst in the electrocatalytic generation of hydrogen from water.

2. The use according to claim 1, wherein the metal-free porphyrin is a substituted or unsubstituted phenylporphyrin or a substituted or unsubstituted pyridyl porphyrin.

3. The use according to claim 2, wherein the substituted or unsubstituted

phenylporphyrin is a substituted or unsubstituted tetraphenylporphyrin or a substituted or unsubstituted triphenylporphyrin.

4. The use according to claim 4, wherein the substituted or unsubstituted

triphenylporphyrin is substituted or unsubstituted monopyridyl-triphenylporphyrin.

5. The use according to any of claims 2 to 4, wherein the substituted or unsubstituted phenylporphyrin or substituted or unsubstituted pyridylporphyrin is a substituted phenylporphyrin or substituted pyridylporphyrin which is substituted with one or more electron withdrawing groups.

6. The use according to claim 5, wherein the one or more electron withdrawing groups is selected from CN, F, Br, I, CI, a C C₃ alkyl group, OH, COOH, OCH₃, S0₃H, N0₂, HN₃, OR¹, COOR¹ and NR^X ₃, wherein R¹ is a Ci-C₃ alkyl group.

7. The use according to claim 6, wherein the one or more electron withdrawing groups is selected from CN, F, Br, I and a Ci-C₃ alkyl group.

8. The use according to claim 7, wherein the one or more electron withdrawing groups is selected from CN and Br.

9. The use according to any of claims 3, 5 or 6, wherein the substituted or

unsubstituted tetraphenylporphyrin is a compound of formula (I):

Formula (I) wherein each R is independently selected from H, CN, F, Br, I, CI, a C1-C3 alkyl group, OH, COOH, OCH3, SO3H, N0₂, HN₃, OR¹, COOR¹ and NR^, wherein R¹ is a d-C₃ alkyl group.

10. The use according to claim 9, wherein the substituted or unsubstituted tetraphenyl porphyrin is a compound of formula (la):

Formula (la).

11. The use according to claim 9 or claim 10, wherein each R of formula (I) or of formula (la) is independently selected from H, CN, F, Br, I and a C1-C3 alkyl group.

12. The use according to claim 11, wherein each R of formula (I) or of formula (la) is independently selected from H, CN or Br.

13. The use according to claim 12 wherein the substituted or unsubstituted

tetraphenylporphyrin is selected from a compound of Formula (Ia)(i) and a compound of Formula (Ia)(ii):

CN

Formula (Ia)(i)

Formula (la) (if)

14. The use according to any of claims 3, 4 or 6, wherein the substituted or unsubstituted triphenylporphyrin is a compound of Formula (II):

Formula (II) wherein each R is independently selected from H, CN, F, Br, I, CI, a C1-C3 alkyl group, OH, COOH, OCH3, SO3H, N0₂, HN₃, OR¹, COOR¹ and NR½, wherein R¹ is a C C₃ alkyl group, with the proviso that one of R' is N, and the other R' are selected from CH and CR, wherein R is as defined above.

15. A modified electrode, wherein the electrode has been modified with a metal-free porphyrin catalyst as recited in any of claims 1 to 14.

16. A modified electrode as in claim 15, wherein the electrode is a carbon-based electrode.

17. A modified electrode as in claim 16, wherein the carbon-based electrode is selected from the group consisting of glassy carbon, graphite plate, carbon monolith, carbon paper, carbon fibre and carbon paste.

18. A method for the electrocatalytic reduction of water to hydrogen, wherein the reduction is catalysed by a metal-free porphyrin as recited in any of claims 1 to 14.

19. An electrolyser comprising a metal-free porphyrin catalyst.

20. Use of a metal-free porphyrin as a catalyst in the electrocatalytic reduction of carbon dioxide.

21. The use according to claim 20, wherein the metal-free porphyrin is a substituted phenylporphyrin or a substituted pyridylporphyrin.

22. The use according to claim 21, wherein the substituted phenylporphyrin is a substituted tetraphenylporphyrin or a substituted triphenylporphyrin.

23. The use according to claim 22, wherein the substituted triphenylporphyrin is substituted monopyridyl-triphenylporphyrin.

24. The use according to any of claims 21 to 23, wherein the substituted

phenylporphyrin or substituted pyridylporphyrin is substituted with one or more electron withdrawing groups.

25. The use according to claim 24, wherein the one or more electron withdrawing groups is selected from CN, F, Br, I, CI, a C C₃ alkyl group, OH, COOH, OCH₃, S0₃H, N0₂, HN₃, OR¹, COOR¹ and NR^, wherein R¹ is a d-C₃ alkyl group.

26. The use according to claim 25, wherein the one or more electron withdrawing groups is selected from CN, F, Br, I and a Ci-C₃ alkyl group.

27. The use according to claim 26, wherein the one or more electron withdrawing groups is selected from CN and Br.

28. The use according to any of claims 22, 24 or 25, wherein the substituted

tetraphenylporphyrin is a compound of formula (III):

Formula (III) wherein each R is independently selected from H, CN, F, Br, I, CI, a C1-C3 alkyl group, OH, COOH, OCH3, SO3H, N0₂, HN₃, OR¹, COOR¹ and NR^, wherein R¹ is a C C₃ alkyl group, with the proviso that at least one R is not H.

29. The use according to claim 28, wherein the substituted tetraphenylporphyrin is a compound of formula (Ilia):

Formula (Ilia).

30. The use according to claim 28 or claim 29, wherein each R of formula (III) or of formula (Ilia) is independently selected from H, CN, F, Br, I and a C1-C3 alkyl group, with the proviso that at least one R is not H.

31. The use according to claim 30, wherein each R of formula (III) or of formula (Ilia) is independently selected from H, CN or Br, with the proviso that at least one R is not H.

32. The use according to claim 31 wherein the substituted tetraphenylporphyrin is selected from a compound of Formula (IIIa)(i) and a compound of Formula (IIIa)(ii):

CN

Formula (IIIa)(i)

Formula (Ilia) (if)

33. The use according to any of claims 32, 33 or 35, wherein the substituted triphenylporphyrin is a compound of Formula (II):

Formula (IV) wherein each R is independently selected from H, CN, F, Br, I, CI, a C1-C3 alkyl group, OH, COOH, OCH3, SO3H, N0₂, HN₃, OR¹, COOR¹ and NR½, wherein R¹ is a C C₃ alkyl group, with the proviso that one of R' is N, and the other R' are selected from CH and CR, wherein R is as defined above and at least one R is not H.

34. A modified electrode, wherein the electrode has been modified with a metal-free porphyrin catalyst as recited in any of claims 20 to 33.

35. A modified electrode as in claim 34, wherein the electrode is a carbon-based electrode.

36. A modified electrode as in claim 35, wherein the carbon-based electrode is selected from the group consisting of glassy carbon, graphite plate, carbon monolith, carbon paper, carbon fibre and carbon paste.

37. A method for the electrocatalytic reduction of carbon dioxide, wherein the reduction is catalysed by a metal-free porphyrin as recited in any of claims 20 to 33.

38. A fuel cell comprising a metal-free porphyrin catalyst.

39. Use of a metal-free porphyrin catalyst substantially as hereinbefore described and with reference to the description.