**2. Metal-modified ZrP based electrocatalysts for the OER**

To facilitate the economic viability of water splitting, the efficiency of electrolyzers must be improved by addressing the overpotential losses associated with the sluggish OER kinetics. To this end, recent studies have focused on developing catalysts materials using earth-abundant transition metals [64]. Significant research has been devoted to improving OER electrocatalysts by using a wide variety of strategies that either increase the intrinsic activity of active sites or by increasing the number of them [65]. One general strategy that has been effective is to support active materials onto supports that engender improved performance [65–68]. ZrP properties make it a potential candidate as a support for active OER catalysts. Its ability to confine catalysts, high thermal stability, stability under a wide range of pH values, and its overall robustness are all desired for an ideal support. In our work, we intercalated the earth-abundant transition metal cations Fe2+, Fe3+, Co2+, and Ni2+ into ZrP and assessed these composite materials as OER electrocatalysts [69].

## **2.1 Metal-intercalated and surface adsorbed ZrP systems**

To intercalate the desired transition metals, a suspension of θ-ZrP must be mixed with a solution of the metal salt precursor and left stirring for 5 days so that ion-exchange reaches equilibrium. To optimize metal loading for improved catalysis performance, we synthetized these composite materials with several synthesis metal salt:ZrP molar ratios (10:1, 5:1, 3:1, 1:1, 1:3, 1:5, 1:10, and 1:20 M:ZrP). A stepwise process is expected as a function of intercalant solution molarity; the intercalation reaction initiates from the edges of the particle and proceeds by diffusion of the metal cations towards the interior of the interlayer sheets [70]. The XRPD patterns (**Figure 5a**) for all four metal samples show that the first diffraction peak of ZrP is shifted to lower 2θ angles, indicating larger interlayer distances and successful intercalation. Increasing the M:ZrP molar ratios results in peak broadening and shifting in all samples indicating a more mixed phase is present and that the

**207**

**Figure 5.**

*Water Splitting Electrocatalysis within Layered Inorganic Nanomaterials*

layered structure has not achieved its maximum cation loading within the interlayer. However, at the highest loadings (i.e., 1:1–10:1 molar ratios), the original peak at 2θ = 11.6° disappeared, and a new peak emerged at significantly lower values of 2θ, reaching a final value indicative of the maximum interlayer distance for that particular metal cation intercalated within ZrP. As expected, +2 cations produced intercalated products with the first diffraction peak at lower angles than those produced by +3 cations. Compared to *α*-ZrP, the maximum interlayer distance increase observed for +2 cations was 2 Å, while for +3 cations it was 0.6 Å (**Figure 5b**). This difference in the increase in interlayer distance between the divalent and trivalent metal cations can be attributed to the difference electrostatic forces within the layers, consistent with Coulomb's Law. Trivalent cations produced a smaller increase because of a stronger electrostatic attraction between the metal cation and the

Ion-exchange in ZrP occurs at the Brönsted acid groups (P-OH) which are also present at the surface of the nanoparticles. Hence, there is no way of preventing that the metal cations get adsorbed to the surface. To obtain more insights into the nature of the activity of the samples, a metal-modified ZrP system in which the metals are only adsorbed onto the surface of the nanoparticles was also prepared. To prepare these samples, *α*-ZrP must be used as the ZrP source as the metal cations are large enough to not intercalate into the interlayer. XRPD data shows that the interlayer distance of these dry sample remains that of *α*-ZrP, 7.6 Å (**Figure 5b**). The presence of the metals in these systems was confirmed by high resolution X-ray photoelectron spectroscopy (XPS). XPS was also used to determine the atomic concentration on both metal-modified ZrP systems, intercalated and adsorbed [69]. Due to the uptake of metal cations within the much larger area of the interlayers of ZrP rather than solely on the surface in the adsorbed case, XPS high resolution scans show that intercalated ZrP systems have higher atomic metal content when

Another useful tool to characterize ZrP systems is Fourier transform infrared spectroscopy (FT-IR). *α*-ZrP has four characteristic bands associated

*(a) XRPD patterns for Fe(II), Fe(III), Co(II), and Ni(II)-intercalated ZrP at (from top to bottom) 10:1, 5:1, 3:1, 1:1, 1:3, 1:5, 1:10, and 1:20 M:ZrP molar ratios. The bottom diffraction pattern in all frames is that of pure α-ZrP; (b) interlayer distance as a function of M:ZrP molar ratio for the various metal-intercalated ZrP materials. Metal-adsorbed systems are represented as dashed lines which have an exact interlayer spacing as* 

*pure α-ZrP indicating that metal intercalation did not occur. Taken from reference [69].*

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

negatively charged ZrP layers.

compared to adsorbed systems at similar M:ZrP ratios.

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

*Water Chemistry*

corresponding to TBA<sup>+</sup>

**Figure 4.**

an acid can be performed [60].

*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

intercalated in ZrP [63]. The TBA+

*.*

To facilitate the economic viability of water splitting, the efficiency of electrolyzers must be improved by addressing the overpotential losses associated with the sluggish OER kinetics. To this end, recent studies have focused on developing catalysts materials using earth-abundant transition metals [64]. Significant research has been devoted to improving OER electrocatalysts by using a wide variety of strategies that either increase the intrinsic activity of active sites or by increasing the number of them [65]. One general strategy that has been effective is to support active materials onto supports that engender improved performance [65–68]. ZrP properties make it a potential candidate as a support for active OER catalysts. Its ability to confine catalysts, high thermal stability, stability under a wide range of pH values, and its overall robustness are all desired for an ideal support. In our work, we intercalated the earth-abundant transition metal cations Fe2+, Fe3+, Co2+, and Ni2+ into ZrP and assessed these composite materials as OER electrocatalysts [69].

To intercalate the desired transition metals, a suspension of θ-ZrP must be mixed with a solution of the metal salt precursor and left stirring for 5 days so that ion-exchange reaches equilibrium. To optimize metal loading for improved catalysis performance, we synthetized these composite materials with several synthesis metal salt:ZrP molar ratios (10:1, 5:1, 3:1, 1:1, 1:3, 1:5, 1:10, and 1:20 M:ZrP). A stepwise process is expected as a function of intercalant solution molarity; the intercalation reaction initiates from the edges of the particle and proceeds by diffusion of the metal cations towards the interior of the interlayer sheets [70]. The XRPD patterns (**Figure 5a**) for all four metal samples show that the first diffraction peak of ZrP is shifted to lower 2θ angles, indicating larger interlayer distances and successful intercalation. Increasing the M:ZrP molar ratios results in peak broadening and shifting in all samples indicating a more mixed phase is present and that the

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

**2. Metal-modified ZrP based electrocatalysts for the OER**

**2.1 Metal-intercalated and surface adsorbed ZrP systems**

cations can be displaced

**206**

layered structure has not achieved its maximum cation loading within the interlayer. However, at the highest loadings (i.e., 1:1–10:1 molar ratios), the original peak at 2θ = 11.6° disappeared, and a new peak emerged at significantly lower values of 2θ, reaching a final value indicative of the maximum interlayer distance for that particular metal cation intercalated within ZrP. As expected, +2 cations produced intercalated products with the first diffraction peak at lower angles than those produced by +3 cations. Compared to *α*-ZrP, the maximum interlayer distance increase observed for +2 cations was 2 Å, while for +3 cations it was 0.6 Å (**Figure 5b**). This difference in the increase in interlayer distance between the divalent and trivalent metal cations can be attributed to the difference electrostatic forces within the layers, consistent with Coulomb's Law. Trivalent cations produced a smaller increase because of a stronger electrostatic attraction between the metal cation and the negatively charged ZrP layers.

Ion-exchange in ZrP occurs at the Brönsted acid groups (P-OH) which are also present at the surface of the nanoparticles. Hence, there is no way of preventing that the metal cations get adsorbed to the surface. To obtain more insights into the nature of the activity of the samples, a metal-modified ZrP system in which the metals are only adsorbed onto the surface of the nanoparticles was also prepared. To prepare these samples, *α*-ZrP must be used as the ZrP source as the metal cations are large enough to not intercalate into the interlayer. XRPD data shows that the interlayer distance of these dry sample remains that of *α*-ZrP, 7.6 Å (**Figure 5b**). The presence of the metals in these systems was confirmed by high resolution X-ray photoelectron spectroscopy (XPS). XPS was also used to determine the atomic concentration on both metal-modified ZrP systems, intercalated and adsorbed [69]. Due to the uptake of metal cations within the much larger area of the interlayers of ZrP rather than solely on the surface in the adsorbed case, XPS high resolution scans show that intercalated ZrP systems have higher atomic metal content when compared to adsorbed systems at similar M:ZrP ratios.

Another useful tool to characterize ZrP systems is Fourier transform infrared spectroscopy (FT-IR). *α*-ZrP has four characteristic bands associated

#### **Figure 5.**

*(a) XRPD patterns for Fe(II), Fe(III), Co(II), and Ni(II)-intercalated ZrP at (from top to bottom) 10:1, 5:1, 3:1, 1:1, 1:3, 1:5, 1:10, and 1:20 M:ZrP molar ratios. The bottom diffraction pattern in all frames is that of pure α-ZrP; (b) interlayer distance as a function of M:ZrP molar ratio for the various metal-intercalated ZrP materials. Metal-adsorbed systems are represented as dashed lines which have an exact interlayer spacing as pure α-ZrP indicating that metal intercalation did not occur. Taken from reference [69].*

with lattice water molecules. These bands appear at ~3600, ~3500, ~3140, and ~1600 cm<sup>−</sup><sup>1</sup> [71]. When intercalation occurs, the intercalant species will displace interlayer water molecules. For this reason, bands associated with these water vibrational modes showed reduced relative intensity in the intercalated materials (**Figure 6a**). In contrast, metal-adsorbed samples showed very similar spectra to that of *α*-ZrP, with the water bands still present, indicating negligible intercalation (**Figure 6b**). The characteristic orthophosphate group vibrations of ZrP are observed in the region of ~1100–950 cm<sup>−</sup><sup>1</sup> (**Figure 6a**). Intercalated samples show a diminished relative intensity of the shoulder at the left part of the orthophosphate group vibrations at ~1050 cm<sup>−</sup><sup>1</sup> that is attributed to the vibration of the exchangeable proton of the phosphate group, which is lost when the proton is exchanged by intercalation via ion exchange with other species. For metal-adsorbed samples, this vibration is still present indicating once again that no intercalation is observed.

**Figure 6.**

*FTIR spectra of (a) intercalated and (b) adsorbed Fe(II), Fe(III), Co(II), and Ni(II) at 10:1 M:ZrP ratio. Taken from Ref. [69].*
