FIELD OF THE INVENTION

The present invention relates to catalyst and a method to produce the catalyst, such as a bifunctional electrocatalyst, for use in an anode of a Proton Exchange Membrane (PEM) electrolyzer.

The present invention also relates to a PEM electrolyzer comprising the bifunctional electrocatalyst, which shows high activity for oxygen evolution reaction (OER) and hydrogen oxidation reaction (HOR).

BACKGROUND OF THE INVENTION

Electrochemical water splitting under acidic conditions is a clean way towards producing hydrogen fuels.

PEM water electrolysis provides a cost-efficient way to produce hydrogen gas.

PEM electrolysers are characterized by a cathode where hydrogen ions are reduced to hydrogen gas and an anode where water is split in hydrogen ions and oxygen gas separated by a thin proton exchange (PE) membrane.

A way to improve efficiency in PEM electrolysis is the reduction of proton transport resistance, i.e. the reduction of the thickness of the PE membrane.

However, the use of thin PE membranes has the disadvantage of allowing for permeation of hydrogen gas, produced at the cathode, towards and to the anode. This is highly undesirable as the permeation may trigger explosive reactions at the anode.

Indeed safety requirements demand very low levels of hydrogen gas at the anode, such as in a safety range under 4 vol%.

Hence, an improved PEM electrolyzer allowing for the reduction of the content of hydrogen permeation to the anode would be advantageous.

In particular, an efficient thin membrane PEM electrolyzer capable of avoiding or reducing hydrogen permeation to the anode would be advantageous. Current state-of-the-art membrane material cannot deliver such property of both high proton conductivity and low gas permeation. OBJECT OF THE INVENTION

An object of the present invention is to solve the problem of permeation of hydrogen gas to the anode in PEM electrolyzers having thin membranes.

A further object of the present invention is to provide PEM electrolyzers able to reduce or remove permeation or presence of hydrogen gas to the anode under the safety range of 4 vol%.

Another object of the present invention is to provide a catalyst, such as a bifunctional catalyst, for use in an anode of a PEM electrolyzer, which is able to reduce the presence of hydrogen gas and to catalyze the dissociation of water to provide oxygen gas and hydrogen ion.

A further object of the invention is to provide a method of producing a catalyst for use in an anode of a PEM electrolyzer, which is able to reduce the content of hydrogen gas at the anode by active recombation of hydrogen, wherein the active hydrogen recombination is to convert the hydrogen back to protons instead of combining it with oxygen gas to form water.

An even further object of the invention may also be seen as to provide an alternative to the prior art.

In particular, it may be seen as an object of the present invention to provide an efficient PEM electrolyzer which is able to reduce the hydrogen gas concentration at the anode by providing anodic catalyst sites promoting active hydrogen recombination and, at the same time, oxygen evolution.

SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by a catalyst for use in an anode of a PEM electrolyzer, the catalyst comprising a nanoporous bimetallic alloy.

The catalyst may be also referred herein as bifunctional catalyst has having both the function of promoting active recombination of hydrogen as well as oxygen evolution when used in an anode of a PEM electrolyzer. The catalyst may be referred herein as electrocatalyst as to its specific use within the anode of a PEM electrolyzer.

In some embodiments, the catalyst is a bimetallic catalyst, i.e. a catalyst comprising a nanoporous bimetallic alloy.

A bimetallic catalyst is defined as a catalyst being bimetallic, i.e. a metal alloy consisting of at least two metals, at least at the surface, e.g. the most external surface, i.e. within the first three atomic layers.

The bimetallic catalyst may also be referred to as a bimetallic electrocatalyst, i.e. a catalyst for use in electrochemical water splitting, e.g. for use in a PEM electrolyzer.

In some embodiments, the bimetallic nanoporous alloy is an Ir-Pd nanoporous alloy having a surface Ir to Pd atomic ratio between 0.3 and 3.

In some further embodiments, the surface atomic ratio is between 0.8 and 1.2.

Surface atomic ratio is the ratio between the two metal atoms, i.e. Ir and Pd at the surface of the nanoporous bimetallic alloy.

For example, surface atomic ratio may refer to the surface atomic ratio measured by X-ray photoelectron spectroscopy (XPS).

In search for solutions, the inventors have identified that some surface atomic ratio show optimal performance when the catalyst is used at the anode of a PEM electrolyzer.

The nanoporous Ir-Pd alloy may have a layered structure.

Layered structure is defined as a structure formed by two or more layers having different composition.

An example of layered structure of the catalyst according to the invention may be a core-shell structure.

The nanoporous Ir-Pd alloy may comprise nanoparticles having a core-shell structure. Core-shell nanoparticles are composed by a core having a core composition encapsulated by a shell having a shell composition that is different from the one at the core.

The nanoparticles are advantageous, when employed in a PEM electrolyzer, as acting as active recombination catalyst, thus reducing the hydrogen gas content at the anode.

In some embodiments, the Ir-Pd nanoporous alloy consists of or comprises nanoparticles having an average size between 2 and 10 nm, such as between 4 and 5 nm.

In some embodiments, the nanoporous Ir-Pd alloy has an average composition and is characterized by having a Pd-rich-core having a core composition and an Ir-rich shell having a shell composition.

The average composition is the composition in average in the core and in the shell combined. The average composition is therefore referred herein as the average composition of the alloy in the bulk, i.e. both in the core and in the shell.

Pd-rich core is referred herein so as to define a core in which Pd is dominant, i.e. in which the content of Pd is higher than the average composition.

Ir-rich shell is referred herein so as to define a shell in which Ir is dominant, i.e. in which the content of Ir is higher than the average composition.

Dominant is referred herein with the meaning of higher content of the dominant atomic species in respect to the other atomic species.

The external surface of the catalyst, i.e. the shell, is characterized by having a bimetallic nature and a thickness between 1 to 3 atomic layers.

Thus, in some embodiments, the shell of the core-shell structure has a thickness between 1 and 3 atomic layers. In search for solutions to the problem of permeation of hydrogen gas in PEM electrolyzers having thin membranes, the inventors observed that bimetallic alloy nanoparticles comprising Ir and Pd with a preferred atomic ratio and characterized by a layer structure wherein Ir is dominant at the external surface showed an unexpected and surprising effect in improving hydrogen reduction.

In some embodiments, the shell composition has a higher content of Ir than the average composition or the core composition.

In some further embodiments, the core composition has a higher content of Pd than the average composition or than the shell composition.

The shell comprises higher amounts of Ir than the average composition, while the core comprises higher amounts of Pd than the average composition.

The content or amounts may be also referred herein as atomic fraction.

The Ir atomic fraction at the shell of the core-shell structure is higher than the average composition, while the Pd atomic fraction at the core is higher than the average composition.

In some embodiments, the Ir to Pd atomic ratio between Ir and Pd at the external layer, external surface or shell is between 3 to 1 and 2 to 1.

In a second aspect, the invention relates to a PEM electrolyzer comprising a cathode; a membrane; an anode; wherein said anode comprises the catalyst according to the first aspect of the invention, thereby promoting active or passive hydrogen recombination and oxygen evolution.

Data presented by the inventors show that bimetallic alloy nanoparticles having an Ir-rich external shell and a Pd-rich core, when used at the anode of a PEM electrolyzer, have a high oxygen evolution reaction (OER) activity and stability and, at the same time, catalyze hydrogen electrooxidation.

In some embodiments of the second aspect of the invention, the PEM electrolyzer comprises an anode comprising a catalyst comprising or consisting of an Ir-Pd nanoporous alloy having a surface atomic ratio between 0.8 and 2.8, such as between 0.8 and 1.2, for example 0.88.

This preferred surface atomic ratio has shown good hydrogen absorption and high OER activity and stability as well as high oxygen evolution.

The invention solves the problem of permeation of hydrogen gas in PEM electrolyzers having thin membranes as the bimetallic alloy nanoparticles act as an active recombination catalyst within the anode showing high hydrogen gas absorption.

In some embodiments, according to the second aspect of the invention, the membrane has a thickness between 1 and 500 micrometers, such as between 100 and 250 |_im.

PEM electrolyzers using thin membrane have shown hydrogen gas permeation during operation.

Membrane thicknesses of 125 and 175 micrometer, such as Nation 115 and 117, have shown undesirable sieving of hydrogen gas which has been mitigated by the presence of the catalyst according to the first aspect of the invention.

In a third aspect, the invention relates to a method of producing a catalyst for use in an anode of a PEM electrolyzer according to the second aspect of the invention, the method comprising: processing a solution containing Ir and Pd ions in an atomic ratio between 1: 1 and 1:6.

In some embodiments, the method of producing a catalyst according to the third aspect comprises: processing a solution containing Ir and Pd ions in an atomic ratio 1: 3.

Synthesis conditions with an atomic ratio Ir: Pd of 1: 3 have shown to produce an Ir/Pd shell/core structure providing the optimal combination of high hydrogen adsorption and high OER activity and stability.

The first, second, third and other aspects and embodiments of the present invention may each be combined with any of the other aspects and embodiments. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

The catalyst, the method to produce the catalyst and the PEM electrolyzer comprising the catalyst according to the invention will now be described in more details with regard to the accompanying figures. The figures show one way of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 shows a PEM electrolyzer schematic view according to some embodiments of the invention.

Figure 2 shows a flow-chart of a method to produce the catalyst according to some embodiments of the invention.

Figure 3 shows XPS survey spectra of different compositions of IrPd catalysts according to some embodiments of the invention.

Figure 4 shows a schematic representation of IrPd alloy formation with different compositions according to some embodiments of the invention.

Figure 5 shows mass activity of different catalysts by normalizing current with respect to mass loading of Ir according to some embodiments of the invention.

Figure 6 shows OER polarization curves performed for 2000 cycles at 100 mV s' ¹ scan rate and recorded the OER polarization curve at 10 mV s' ¹ after some intervals according to some embodiments of the invention.

Figure 7 shows OER performance retention trend for different catalysts, according to some embodiments of the invention.

Figure 8 shows hydrogen desorption/adsorption performance for different catalysts, according to some embodiments of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

Figure 1 shows a PEM electrolyzer schematic view according to some embodiments of the invention. The PEM electrolyzer 1 comprises a cathode 2, an anode 4 containing the IrPd alloy of the invention separated by a thin membrane 3.

The overall cell reaction occurring during PEM electrolysis is the following

H ₂ O -> 2H+ + ¹ / ₂ O2 + 2e-

At the cathode hydrogen cations get reduced to oxygen gas according to the reaction below

2 H+ + 2e- -> H ₂ (5)

At the anode water get oxidized according to the reaction below

2 H2O -> H ₂ + ¹ /2 O2 (6)

The use of thin membrane allows for permeation of hydrogen gas produced at the cathode into the anode, potentially triggering explosions.

Figure 2 shows a flow-chart of a method to produce the catalyst according to some embodiments of the invention.

The method of producing a catalyst for use in an anode of a PEM electrolyzer according to some embodiments of the invention comprises the step SI of processing a solution containing Ir and Pd ions in an Ir to Pd atomic ratio between 1: 1 and 1:6, such as 1:3.

The catalyst may be produced by a microwave assisted chemical reduction route from iridium trichloride hydrate (IrCh-nh O) and potassium tetrachloropalladate (K ₂ PdCI ₄ ) precursors in presence of sodium borohydride (NaBH ₄ ).

Figure 3 shows XPS survey spectra of different compositions of IrPd catalysts according to some embodiments of the invention. The spectra are collected at a pass energy of 50 eV over the binding energy range of 0 -1160 eV according to some embodiments of the invention.

The XPS spectra (a), (b), (c), (d), (e) and (f) correspond respectively to samples having Ir surface atomic ratio of or close to 0%, 14%, 21%, 47%, 73%, and 100%, while that of (g) corresponds to state-of-the-art commercial IrC electrocatalyst for OER.

For the XPS spectra the XPS atomic ratio has been calculated and reported in Table 1 below. Table 1 XPS survey spectrum for prepared IrPd composition

The XPS data confirms an Ir-rich surface for samples produced with an Ir to Pd ratio of 1: 1 and 1: 3, corresponding to an Ir to Pd surface atomic ratio of 2.70 and 0.89 respectively.

Figure 4 shows a schematic representation of IrPd alloy formation with different compositions according to some embodiments of the invention.

Labels (a), (b), (c), (d), (e) and (f) correspond respectively to samples having Ir surface atomic ratio of or close to 100%, 73%, 47%, 21%, 14% and 0%.

The schematic representation depicts a possible alloy formation showing an Ir-rich surface for samples produced with an Ir to Pd ratio of 1: 1 and 1: 3 based on the XPS surface atomic ratio data. Figure 5 shows mass activity of different catalysts by normalizing current with respect to mass loading of Ir according to some embodiments of the invention. The labels (a), (b), (c), (d), (e) and (f) correspond respectively to samples having Ir surface atomic ratio of or close to 100%, 73%, 47%, 21%, 14% and 0%, while that of (g) corresponds to state-of-the-art commercial IrO2 electrocatalyst for OER.

Figure 6 shows OER polarization curves performed for 2000 cycles at 100 mV s' ¹ scan rate. The OER polarization curve were recorded at 10 mV s' ¹ after some intervals according to some embodiments of the invention.

Figure 7 shows performance retention trend for different catalysts, according to some embodiments of the invention.

The labels (a), (b), (c), (d), (e) and (f) correspond respectively to samples having Ir surface atomic ratio of or close to 100%, 73%, 47%, 21%, 14% and 0.

It can be noticed from figures 5, 6 and 7 that with decreasing Ir content, the catalysts show increasing mass specific activity towards OER. However, leaching of Pd under corrosive conditions makes the catalysts with higher Pd content less stable. Hence, an optimal Pd content is obtained based on the catalytic performance for OER in terms of high mass activity and high stability. The IrPd- 1: 3 catalyst, i.e. the one having Ir atomic fraction of or close to 0.25 exhibits reasonably high mass activity (twice of the commercial OER catalyst) along with the highest stability during electrochemical stress cycling (90% retention compared to 83% for commercial electrocatalyst for OER).

Figure 8 shows CV curves performed at 10 mV s' ¹ scan rate in 0.1 M HCIO4 electrolyte for IrPd-0: 1, IrPd-l:9, IrPd-l:6, IrPd-l: 3, IrPd-l: l, IrPd-l:0 and IrO2 commercial catalyst, according to some embodiments of the invention.

Figure 8 compares the cyclic voltammograms of the catalysts in acidic media. The labels (a), (b), (c), (d), (e) and (f) correspond respectively to samples having Ir surface atomic ratio of or close to 100%, 73%, 47%, 21%, 14% and 0%, while that of (g) corresponds to state-of-the-art commercial IrOz electrocatalyst for OER.

Presence of redox peaks for potentials below 0.4 V corresponds to the activity of the catalysts for hydrogen adsorption/desorption process. High intensity of the hydrogen adsorption/desorption peaks corresponds to high activity of the catalyst for electrochemical reactions involving hydrogen (i.e., hydrogen oxidation, hydrogen reduction, etc.). The electrochemical surface area (ECSA) values of the catalysts as obtained from their hydrogen adsorption peaks are shown in Table 2. The high ECSA values, equivalent to the physical surface area (PSA) of the catalysts shows their high activity towards HOR. This, combined with the OER characteristics, confirms the bifunctional character of the catalyst for HOR and OER.

Table 2 Estimate of parameters to evaluate PSA and ECSA of IrPd compositions

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is possible and advantageous.