**1. Introduction**

Global energy consumption is projected to increase drastically in the coming decades [1]. To meet this demand, it is estimated that there are 1000–2000 years of fossil fuel resources [2]. Nonetheless, while fossil fuels could meet this huge demand of energy, CO2 emissions from these resources would contribute to the recognized danger of climate change by increasing anthropogenic carbon emissions to the atmosphere. This motivates the development of sustainable energy production technologies, including fuel production, using solar energy in a process that has been called artificial photosynthesis. However, there are large scientific and technical challenges involved in these schemes.

One promising scheme for this purpose is the use of hydrogen as a fuel. Hydrogen has the largest energy density over any other fuel and it is the most abundant molecule in the universe. Hydrogen's energy density is 120 MJ/kg, more than twice than that of natural gas and almost three times higher than petroleum [3]. The problem with hydrogen is that even though it is very abundant, it is hard to obtain in pure form since it readily reacts with other substances and it is mostly found in compounds. Currently, ~96% of hydrogen is produced from fossil fuels with the steam methane reforming process [4]. Thus, methods for producing hydrogen from hydrogen-containing resources like biomass and water need to be more environmentally friendly and economical in order to substitute current methods of hydrogen production [4]. The high interest of hydrogen as a fuel arises because this gas is highly flammable, burns cleanly, and the cost of solar-based electricity is falling rapidly, including that used for hydrogen production [5]. The product of hydrogen combustion is water and energy, making this process extremely clean:

2H2(g) + O2(g) → 2H2O(g) ∆H = −286 kJ/mol

Out of all energy resources, solar energy is the most abundant, but it is an intermittent resource [6]. Therefore, to effectively use solar energy, we must convert and store it. One way to store this energy is in the form of chemical fuels, such as hydrogen. The idea is to split water in its components (hydrogen and oxygen) with the help of solar energy since 4.92 eV is stored when two water molecules are split [7]. This approach to store energy in the form of chemical bonds (a process that mimics the natural photosynthetic process) is called artificial photosynthesis. An example of artificial photosynthesis is the process occurring in a solar fuel cell. In such cells water is split using sunlight as the energy source. This reaction involves two separate redox reactions, one being the oxidation of water to produce oxygen and protons (a 4-electron process) and the other one is the reduction of protons to form dihydrogen:

 Water oxidation: 2H2O → O2 + 4H<sup>+</sup> + 4e<sup>−</sup> (Oxygen evolution reaction,OER).

 Proton reduction: 2H<sup>+</sup> + 2e<sup>−</sup> → H2 (Hydrogen evolution reaction,HER).

Electrochemical water splitting can be achieved by using devices that can harvest the sun's energy. The two main configurations of these devices consist of (1) a photovoltaic (PV) device connected to a separate electrolyzer with catalysts that drive the necessary half reactions (PV/electrolysis) and (2) a fully integrated system where the catalysts are deposited on top of the light absorbing materials (photoelectrochemical, PEC devices) [8]. The efficiency of these devices is calculated based on the solar-to-hydrogen (STH) or solar-to-fuel (STF) efficiency, which is defined as the amount of chemical energy produced in the form of fuel divided by the solar energy input, with no external bias applied [9]. High STH efficiencies are desired as it has been proved that it is the factor with the biggest impact on the final cost of the fuel produced on any of these systems [8]. Although, other factors such as stability and material cost are also important for the final cost of the fuel.

Theoretical efficiencies calculated using combinations of published catalysts for the OER and the HER in a PEC device show that the STH efficiencies are far lower than the maximum thermodynamically achievable efficiency of 41% [8]. This highlights the need to develop more active catalysts, especially for the OER

**203**

**Figure 1.**

*(a) The structure of α-ZrP. (b) Polyhedral model of the structure of α-ZrP.*

*Water Splitting Electrocatalysis within Layered Inorganic Nanomaterials*

as it is the main cause of energy loss in the form of kinetic overpotentials during fuel production. Furthermore, to bring these technologies towards economical implementation, it is of much importance to continually improve device performance. Besides, benchmarking studies have shown that catalyst stability is also a major issue as the reactions are mostly carried in harsh chemical conditions, especially in very high or low pH [10–12]. Recently, density functional theory (DFT) calculations have shown that performing the OER in a confined nanoscopic environment improves the electrochemistry of the reaction by lowering the overpotential and increasing the catalytic efficiency by 10% [13]. These theoretical results were modeled on a layered RuO2 system and attributed the improvement in activity to interactions of intermediates with the opposite surface of the metal oxide. There is evidence that encapsulation of catalysts can lead to improvements on selectivity and activity for a variety of reactions, including water oxidation [14–17]. This motivated us to use the layered compound zirconium phosphate (ZrP) as a support for active OER catalysts to mimic an environment that theoretical works have modeled. We want to target the issues presented by OER catalysts by developing catalytic systems based on ZrP nanomaterials with the goal of optimizing efficiencies of future solar water

Zirconium phosphates are part of the group of water-insoluble phosphates of tetravalent metals containing layered structures. Zirconium bis(monohydrogen orthophosphate) monohydrate (Zr(O3POH)2·H2O, *α*-ZrP) is the most extensively studied phase of ZrP. *α*-ZrP has an interlayer distance of 7.6 Å with a layer thickness of 6.6 Å (**Figure 1a** and **b**) [18]. *α*-ZrP has a structure in which the zirconium atoms in each layer align nearly to a plane with bridging phosphate groups located alternately above and below the metal atom plane [19]. Three oxygen atoms of the phosphate group bond to three different Zr4+ and each Zr4+ ion coordinates with oxygens from six different phosphate groups [19]. The fourth oxygen from the phosphate group, which has a proton, points towards the interlayer region and the surface of the nanoparticles. This proton can be exchanged with cations or molecules. The structure of *α*-ZrP contains a zeolitic cavity in the interlayer region with a diameter of 2.61 Å that is occupied by a

*DOI: http://dx.doi.org/10.5772/intechopen.88116*

splitting devices.

**1.1 Zirconium phosphates**

water molecule [20, 21].

*Water Splitting Electrocatalysis within Layered Inorganic Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.88116*

as it is the main cause of energy loss in the form of kinetic overpotentials during fuel production. Furthermore, to bring these technologies towards economical implementation, it is of much importance to continually improve device performance. Besides, benchmarking studies have shown that catalyst stability is also a major issue as the reactions are mostly carried in harsh chemical conditions, especially in very high or low pH [10–12]. Recently, density functional theory (DFT) calculations have shown that performing the OER in a confined nanoscopic environment improves the electrochemistry of the reaction by lowering the overpotential and increasing the catalytic efficiency by 10% [13]. These theoretical results were modeled on a layered RuO2 system and attributed the improvement in activity to interactions of intermediates with the opposite surface of the metal oxide. There is evidence that encapsulation of catalysts can lead to improvements on selectivity and activity for a variety of reactions, including water oxidation [14–17]. This motivated us to use the layered compound zirconium phosphate (ZrP) as a support for active OER catalysts to mimic an environment that theoretical works have modeled. We want to target the issues presented by OER catalysts by developing catalytic systems based on ZrP nanomaterials with the goal of optimizing efficiencies of future solar water splitting devices.

#### **1.1 Zirconium phosphates**

*Water Chemistry*

extremely clean:

form dihydrogen:

Water oxidation: 2H2O → O2 + 4H<sup>+</sup> + 4e<sup>−</sup>

Proton reduction: 2H<sup>+</sup> + 2e<sup>−</sup>

the final cost of the fuel.

One promising scheme for this purpose is the use of hydrogen as a fuel. Hydrogen has the largest energy density over any other fuel and it is the most abundant molecule in the universe. Hydrogen's energy density is 120 MJ/kg, more than twice than that of natural gas and almost three times higher than petroleum [3]. The problem with hydrogen is that even though it is very abundant, it is hard to obtain in pure form since it readily reacts with other substances and it is mostly found in compounds. Currently, ~96% of hydrogen is produced from fossil fuels with the steam methane reforming process [4]. Thus, methods for producing hydrogen from hydrogen-containing resources like biomass and water need to be more environmentally friendly and economical in order to substitute current methods of hydrogen production [4]. The high interest of hydrogen as a fuel arises because this gas is highly flammable, burns cleanly, and the cost of solar-based electricity is falling rapidly, including that used for hydrogen production [5]. The product of hydrogen combustion is water and energy, making this process

2H2(g) + O2(g) → 2H2O(g) ∆H = −286 kJ/mol

Electrochemical water splitting can be achieved by using devices that can harvest the sun's energy. The two main configurations of these devices consist of (1) a photovoltaic (PV) device connected to a separate electrolyzer with catalysts that drive the necessary half reactions (PV/electrolysis) and (2) a fully integrated system where the catalysts are deposited on top of the light absorbing materials (photoelectrochemical, PEC devices) [8]. The efficiency of these devices is calculated based on the solar-to-hydrogen (STH) or solar-to-fuel (STF) efficiency, which is defined as the amount of chemical energy produced in the form of fuel divided by the solar energy input, with no external bias applied [9]. High STH efficiencies are desired as it has been proved that it is the factor with the biggest impact on the final cost of the fuel produced on any of these systems [8]. Although, other factors such as stability and material cost are also important for

Theoretical efficiencies calculated using combinations of published catalysts for the OER and the HER in a PEC device show that the STH efficiencies are far lower than the maximum thermodynamically achievable efficiency of 41% [8]. This highlights the need to develop more active catalysts, especially for the OER

Out of all energy resources, solar energy is the most abundant, but it is an intermittent resource [6]. Therefore, to effectively use solar energy, we must convert and store it. One way to store this energy is in the form of chemical fuels, such as hydrogen. The idea is to split water in its components (hydrogen and oxygen) with the help of solar energy since 4.92 eV is stored when two water molecules are split [7]. This approach to store energy in the form of chemical bonds (a process that mimics the natural photosynthetic process) is called artificial photosynthesis. An example of artificial photosynthesis is the process occurring in a solar fuel cell. In such cells water is split using sunlight as the energy source. This reaction involves two separate redox reactions, one being the oxidation of water to produce oxygen and protons (a 4-electron process) and the other one is the reduction of protons to

(Oxygen evolution reaction,OER).

→ H2 (Hydrogen evolution reaction,HER).

**202**

Zirconium phosphates are part of the group of water-insoluble phosphates of tetravalent metals containing layered structures. Zirconium bis(monohydrogen orthophosphate) monohydrate (Zr(O3POH)2·H2O, *α*-ZrP) is the most extensively studied phase of ZrP. *α*-ZrP has an interlayer distance of 7.6 Å with a layer thickness of 6.6 Å (**Figure 1a** and **b**) [18]. *α*-ZrP has a structure in which the zirconium atoms in each layer align nearly to a plane with bridging phosphate groups located alternately above and below the metal atom plane [19]. Three oxygen atoms of the phosphate group bond to three different Zr4+ and each Zr4+ ion coordinates with oxygens from six different phosphate groups [19]. The fourth oxygen from the phosphate group, which has a proton, points towards the interlayer region and the surface of the nanoparticles. This proton can be exchanged with cations or molecules. The structure of *α*-ZrP contains a zeolitic cavity in the interlayer region with a diameter of 2.61 Å that is occupied by a water molecule [20, 21].

**Figure 1.** *(a) The structure of α-ZrP. (b) Polyhedral model of the structure of α-ZrP.*

### **1.2 Intercalation of guest species into ZrP**

Intercalation is defined as the reversible insertion of guest species into a lamellar host structure with maintenance of the structure features of the host [22]. For *α*-ZrP, the direct intercalation of small cations is possible if they are smaller than 2.61 Å, but for larger cations and molecules intercalation is not significant and/or these species are exchanged at very slow rates [23–26]. To circumvent this problem, *α*-ZrP pre-intercalated phases with sodium ions or n-butylammonium (both produce expanded phases) are commonly used as precursors to intercalate the intended guest species. One problem that arises with this method is that the pre-intercalated species do not necessarily exchange completely with the intended guest, thus becoming a contaminant in the intercalation product.

Martí and Colón developed a new direct intercalation method that does not require a pre-intercalation step using a highly hydrated phase of zirconium phosphate, θ-ZrP [27]. θ-ZrP maintains the *α*-ZrP-type layered structure (**Figure 2**) but has an interlayer distance of 10.4 Å and has six water molecules per formula unit, in contrast with *α*-ZrP that only has one [28]. Zirconium bis(monohydrogen orthophosphate) hexahydrate (θ-ZrP) converts back to *α*-ZrP when it dehydrates. X-ray powder diffraction (XRPD) can be used to distinguished between both ZrP phases. When θ-ZrP is dried, producing *α*-ZrP, the first diffraction peak at 2θ = 8.6° which corresponds to the 002-plane reflection of ZrP and that of the interlayer distance, shifts towards 11.6°; the angle corresponding to an interlayer distance of 7.6 Å, characteristic of *α*-ZrP (**Figure 3**). For this reason, if a dry intercalation product is analyzed by XRPD and the first diffraction peak corresponds to a distance greater than 7.6 Å, this indicates that the intercalation reaction was successful [29]. One of three possible patterns can be observed by XRPD for intercalation products of ZrP; either (i) a pattern with a peak corresponding to a larger interlayer spacing at lower 2θ values than 11.6° indicates that the intercalant was introduced into the interlayer, (ii) a pattern with two distinct peaks, one at 2θ = 11.6° and one that appears at lower

**205**

not present at all.

**Figure 3.**

**1.3 Chemical exfoliation of ZrP nanoparticles**

*XRPD patterns of α-ZrP (black) and θ-ZrP (red).*

ammonium hydroxide (TBA+

exfoliation with TBA<sup>+</sup>

*Water Splitting Electrocatalysis within Layered Inorganic Nanomaterials*

2θ values than 11.6° indicates that a mixed phase is present [30], and (iii) a pattern with no change in the reference peak, indicating that the intercalant species did not intercalate and is adsorbed on the outer surface of the layered structure or that is

ZrP has been used for the intercalation of several photo-, bio- and redox-active compounds for a wide variety of applications including artificial photosynthesis, amperometric biosensors, and drug delivery [27, 30–42]. Even though ZrP has previously been studied for catalysis [43–47], membrane composites for proton exchange water electrolyzers [48–52], and as additive for catalytic layers for OER in order to protect metal oxide catalysts [53], our work is, to the best of our knowledge, the first time ZrP is used as an inorganic support for catalysts for the OER.

The process of separating the layers of a layered material is known as exfoliation. This process has been extensively studied for a myriad of layered materials and the two-dimensional materials (2D) that result have been shown to have several advantages over their bulk systems [54]. *α*-ZrP has been successfully exfoliated through a variety of methods [55–58], and its nanosheets used for different applications [59–62]. The main strategy for ZrP exfoliation consists on the intercalation of small amines with positive charges that can easily displace the protons from the phosphate groups in an acid-base reaction and enter the interlayer space. If a high enough concentration of these amines is used, an amine double layer will form in the interlayer

space, leading to exfoliation due to cation-cation repulsions (**Figure 4**) [56].

OH<sup>−</sup>). If TBA+

consist of single nanosheets of ZrP suspended with TBA<sup>+</sup>

One of the most highly used amines for the exfoliation of ZrP is tetra-*n*-butyl-

reaction is temperature sensitive as it has been found that that hydrolysis of the ZrP edges occurs due to the OH<sup>−</sup> ions. However, the rate of hydrolysis of ZrP during

OH<sup>−</sup> at 0 °C is essentially zero [58]. If the exfoliated material

OH<sup>−</sup> is used, the exfoliated material will

attached to them. This

*DOI: http://dx.doi.org/10.5772/intechopen.88116*

**Figure 2.** *The structure of θ-ZrP.*

*Water Splitting Electrocatalysis within Layered Inorganic Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.88116*

*Water Chemistry*

**1.2 Intercalation of guest species into ZrP**

becoming a contaminant in the intercalation product.

Intercalation is defined as the reversible insertion of guest species into a lamellar host structure with maintenance of the structure features of the host [22]. For *α*-ZrP, the direct intercalation of small cations is possible if they are smaller than 2.61 Å, but for larger cations and molecules intercalation is not significant and/or these species are exchanged at very slow rates [23–26]. To circumvent this problem, *α*-ZrP pre-intercalated phases with sodium ions or n-butylammonium (both produce expanded phases) are commonly used as precursors to intercalate the intended guest species. One problem that arises with this method is that the pre-intercalated species do not necessarily exchange completely with the intended guest, thus

Martí and Colón developed a new direct intercalation method that does not require a pre-intercalation step using a highly hydrated phase of zirconium phosphate, θ-ZrP [27]. θ-ZrP maintains the *α*-ZrP-type layered structure (**Figure 2**) but has an interlayer distance of 10.4 Å and has six water molecules per formula unit, in contrast with *α*-ZrP that only has one [28]. Zirconium bis(monohydrogen orthophosphate) hexahydrate (θ-ZrP) converts back to *α*-ZrP when it dehydrates. X-ray powder diffraction (XRPD) can be used to distinguished between both ZrP phases. When θ-ZrP is dried, producing *α*-ZrP, the first diffraction peak at 2θ = 8.6° which corresponds to the 002-plane reflection of ZrP and that of the interlayer distance, shifts towards 11.6°; the angle corresponding to an interlayer distance of 7.6 Å, characteristic of *α*-ZrP (**Figure 3**). For this reason, if a dry intercalation product is analyzed by XRPD and the first diffraction peak corresponds to a distance greater than 7.6 Å, this indicates that the intercalation reaction was successful [29]. One of three possible patterns can be observed by XRPD for intercalation products of ZrP; either (i) a pattern with a peak corresponding to a larger interlayer spacing at lower 2θ values than 11.6° indicates that the intercalant was introduced into the interlayer, (ii) a pattern with two distinct peaks, one at 2θ = 11.6° and one that appears at lower

**204**

**Figure 2.**

*The structure of θ-ZrP.*

**Figure 3.** *XRPD patterns of α-ZrP (black) and θ-ZrP (red).*

2θ values than 11.6° indicates that a mixed phase is present [30], and (iii) a pattern with no change in the reference peak, indicating that the intercalant species did not intercalate and is adsorbed on the outer surface of the layered structure or that is not present at all.

ZrP has been used for the intercalation of several photo-, bio- and redox-active compounds for a wide variety of applications including artificial photosynthesis, amperometric biosensors, and drug delivery [27, 30–42]. Even though ZrP has previously been studied for catalysis [43–47], membrane composites for proton exchange water electrolyzers [48–52], and as additive for catalytic layers for OER in order to protect metal oxide catalysts [53], our work is, to the best of our knowledge, the first time ZrP is used as an inorganic support for catalysts for the OER.

#### **1.3 Chemical exfoliation of ZrP nanoparticles**

The process of separating the layers of a layered material is known as exfoliation. This process has been extensively studied for a myriad of layered materials and the two-dimensional materials (2D) that result have been shown to have several advantages over their bulk systems [54]. *α*-ZrP has been successfully exfoliated through a variety of methods [55–58], and its nanosheets used for different applications [59–62]. The main strategy for ZrP exfoliation consists on the intercalation of small amines with positive charges that can easily displace the protons from the phosphate groups in an acid-base reaction and enter the interlayer space. If a high enough concentration of these amines is used, an amine double layer will form in the interlayer space, leading to exfoliation due to cation-cation repulsions (**Figure 4**) [56].

One of the most highly used amines for the exfoliation of ZrP is tetra-*n*-butylammonium hydroxide (TBA+ OH<sup>−</sup>). If TBA+ OH<sup>−</sup> is used, the exfoliated material will consist of single nanosheets of ZrP suspended with TBA<sup>+</sup> attached to them. This reaction is temperature sensitive as it has been found that that hydrolysis of the ZrP edges occurs due to the OH<sup>−</sup> ions. However, the rate of hydrolysis of ZrP during exfoliation with TBA<sup>+</sup> OH<sup>−</sup> at 0 °C is essentially zero [58]. If the exfoliated material

**Figure 4.** *Schematic drawing of the ZrP exfoliation process with TBA+ .*

is dried, restacking of the layers occurs with a new expanded phase of 16.8 Å corresponding to TBA<sup>+</sup> intercalated in ZrP [63]. The TBA+ cations can be displaced with another cationic species if the latter is put in contact with a suspension of the exfoliated ZrP nanoparticles. Hence, if the desired material is the exfoliated nanosheets with their phosphate groups protonated, then a follow up reaction with an acid can be performed [60].
