**2.2 OER electrochemical performance of metal-intercalated and surface adsorbed ZrP systems**

To determine the activity of our metal-modified ZrP products towards the OER, cyclic voltammetry experiments were conducted using a Rotating Disk Electrode (RDE) assembly in alkaline electrolyte (0.1 M KOH). The methodology employed was according to the benchmarking protocols suggested for OER electrocatalysts [10–12]. The primary figure of merit from this data is the overpotential necessary to achieve 10 mA/cm2 (*η<sup>j</sup>* = 10 mA/cm2). The overpotential measured at 10 mA/ cm<sup>2</sup> is the potential difference between the potential to achieve 10 mA/cm<sup>2</sup> and the thermodynamic potential of water oxidation (1.23 V vs. RHE). All samples were active for the OER, requiring between 0.5 and 0.7 V of overpotential to reach 10 mA/cm2 , depending on the choice of metal cation, the M:ZrP molar ratio used during synthesis, and whether the metal was intercalated into or adsorbed onto ZrP. In general, lower overpotentials are observed for the higher M:ZrP molar ratios, ascribed to higher metal loadings. Also, OER activities for the metal-adsorbed ZrP catalysts are greater than or equal to those of their metal-intercalated counterparts at the same loading, as seen by their lower overpotentials (**Figure 7**). This is

**209**

(10:1 M:ZrP).

**Figure 7.**

*10 mA/cm2*

*Water Splitting Electrocatalysis within Layered Inorganic Nanomaterials*

somewhat surprising as XPS showed that higher metal loadings were achieved in the intercalated systems. This suggests that the OER is dominated by catalysis on the outer surface of the ZrP supported metal-based systems rather than within the layers, which may be limited by mass transport. These results serve as a basis for

*Electrochemical performance comparison of all four metal systems for adsorbed and intercalated species at* 

Our previous finding suggests that ZrP can serve as a support for transition metal-based OER catalysts and that the reaction occurs preferentially on the surface of the layered ZrP nanoparticles rather than the interlayer space [69]. Based on these results, we expected that exposing surface sites through exfoliation of ZrP could improve these catalytic systems. With the goal of developing more active materials, we prepared exfoliated ZrP nanosheets and modified these exfoliated nanoparticles with Co2+ and Ni2+ [72]. These systems underwent reaction at the same molar ratio than that of the best performing metal-adsorbed ZrP system

followed by an acid wash with HCl. To modify the exfoliated ZrP with the transition metals, an aqueous suspension of the nanosheets is put in contact with an aqueous solution of the metal salt precursor. The XRPD pattern of exfoliated ZrP shows the extreme broadening characteristic of successful exfoliation (**Figure 8**). The diffractograms of Co and Ni-modified ZrP nanosheets are very similar to that of exfoliated ZrP confirming that no further restacking occurs after metal modification (**Figure 8**). Transmission electron microscopy (TEM) also confirms this as the ZrP nanosheets show a fainter contrast when compared with *α*-ZrP nanoparticles which is consistent with its thinner nature, since in TEM areas that contain heavy atoms or are thick appear darker (**Figure 9a**–**d**). After exfoliation, the nanosheets retain the hexagonal shape of *α*-ZrP and no hydrated zirconia nanoparticles are observed decorating the edges of the sheets, indicating that the hydrolysis prone

OH<sup>−</sup> in an ice bath

*. Solid and dashed lines represent intercalated* 

ZrP exfoliation was carried out by adding an excess of TBA<sup>+</sup>

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

developing improved OER catalyst systems.

 *except for Ni(II) which was compared at 3 mA/cm2*

*and adsorbed metal ZrP systems, respectively. Taken from reference [69].*

**2.3 Metal-modified exfoliated ZrP**

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

#### **Figure 7.**

*Water Chemistry*

~1600 cm<sup>−</sup><sup>1</sup>

no intercalation is observed.

**adsorbed ZrP systems**

sary to achieve 10 mA/cm2

with lattice water molecules. These bands appear at ~3600, ~3500, ~3140, and

place 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

samples show a diminished relative intensity of the shoulder at the left part

**2.2 OER electrochemical performance of metal-intercalated and surface** 

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

is the potential difference between the potential to achieve 10 mA/cm<sup>2</sup>

the thermodynamic potential of water oxidation (1.23 V vs. RHE). All samples were active for the OER, requiring between 0.5 and 0.7 V of overpotential to reach

during synthesis, and whether the metal was intercalated into or adsorbed onto ZrP. In general, lower overpotentials are observed for the higher M:ZrP molar ratios, ascribed to higher metal loadings. Also, OER activities for the metal-adsorbed ZrP catalysts are greater than or equal to those of their metal-intercalated counterparts at the same loading, as seen by their lower overpotentials (**Figure 7**). This is

To determine the activity of our metal-modified ZrP products towards the OER, cyclic voltammetry experiments were conducted using a Rotating Disk Electrode (RDE) assembly in alkaline electrolyte (0.1 M KOH). The methodology employed was according to the benchmarking protocols suggested for OER electrocatalysts [10–12]. The primary figure of merit from this data is the overpotential neces-

, depending on the choice of metal cation, the M:ZrP molar ratio used

(*η<sup>j</sup>* = 10 mA/cm2). The overpotential measured at 10 mA/

and

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

of ZrP are observed in the region of ~1100–950 cm<sup>−</sup><sup>1</sup>

of the orthophosphate group vibrations at ~1050 cm<sup>−</sup><sup>1</sup>

[71]. When intercalation occurs, the intercalant species will dis-

(**Figure 6a**). Intercalated

that is attributed to the

**208**

cm<sup>2</sup>

**Figure 6.**

*Taken from Ref. [69].*

10 mA/cm2

*Electrochemical performance comparison of all four metal systems for adsorbed and intercalated species at 10 mA/cm2 except for Ni(II) which was compared at 3 mA/cm2 . Solid and dashed lines represent intercalated and adsorbed metal ZrP systems, respectively. Taken from reference [69].*

somewhat surprising as XPS showed that higher metal loadings were achieved in the intercalated systems. This suggests that the OER is dominated by catalysis on the outer surface of the ZrP supported metal-based systems rather than within the layers, which may be limited by mass transport. These results serve as a basis for developing improved OER catalyst systems.

#### **2.3 Metal-modified exfoliated ZrP**

Our previous finding suggests that ZrP can serve as a support for transition metal-based OER catalysts and that the reaction occurs preferentially on the surface of the layered ZrP nanoparticles rather than the interlayer space [69]. Based on these results, we expected that exposing surface sites through exfoliation of ZrP could improve these catalytic systems. With the goal of developing more active materials, we prepared exfoliated ZrP nanosheets and modified these exfoliated nanoparticles with Co2+ and Ni2+ [72]. These systems underwent reaction at the same molar ratio than that of the best performing metal-adsorbed ZrP system (10:1 M:ZrP).

ZrP exfoliation was carried out by adding an excess of TBA<sup>+</sup> OH<sup>−</sup> in an ice bath followed by an acid wash with HCl. To modify the exfoliated ZrP with the transition metals, an aqueous suspension of the nanosheets is put in contact with an aqueous solution of the metal salt precursor. The XRPD pattern of exfoliated ZrP shows the extreme broadening characteristic of successful exfoliation (**Figure 8**). The diffractograms of Co and Ni-modified ZrP nanosheets are very similar to that of exfoliated ZrP confirming that no further restacking occurs after metal modification (**Figure 8**). Transmission electron microscopy (TEM) also confirms this as the ZrP nanosheets show a fainter contrast when compared with *α*-ZrP nanoparticles which is consistent with its thinner nature, since in TEM areas that contain heavy atoms or are thick appear darker (**Figure 9a**–**d**). After exfoliation, the nanosheets retain the hexagonal shape of *α*-ZrP and no hydrated zirconia nanoparticles are observed decorating the edges of the sheets, indicating that the hydrolysis prone

edges were preserved by temperature control during the exfoliation reaction and that the structure of the layers did not change [58]. This was also confirmed by XPS as the P/Zr ratio after exfoliation remains constant at ~2.

#### **Figure 8.**

*XRPD patterns of α-ZrP, exfoliated ZrP, and metal-modified exfoliated ZrP samples. Reprinted with permission from [72].*

#### **Figure 9.**

*(a, b) TEM images of α-ZrP nanoparticles. Scale bar: 0.5 and 100 nm, respectively. (c, d) TEM images of exfoliated ZrP. Scale bar: 0.5 μm and 100 nm, respectively. Reprinted with permission from [72].*

**211**

**Figure 11.**

*permission from Ref. [72].*

**Figure 10.**

*permission from Ref. [72].*

*Water Splitting Electrocatalysis within Layered Inorganic Nanomaterials*

**2.4 OER electrochemical performance of metal-modified exfoliated ZrP** 

parts. The overpotential necessary to reach a current density of 10 mA/cm2

*Linear sweep voltammograms of (a) Ni(II)/ZrP systems and (b) Co(II)/ZrP systems. Reprinted with* 

*Mass normalized catalytic currents for (a) Ni(II)/ZrP systems and (b) Co(II)/ZrP systems. Reprinted with* 

Linear sweep voltammetry (LSV) was used to assess the activity of these exfoliated materials (**Figure 10**) [72]. OER catalytic currents for the exfoliated materials were shifted to lower potentials when compared to their surface adsorbed counter-

Co-modified exfoliated nanosheets was 0.450 V, an improvement of 41 mV over the surface adsorbed Co material. For the Ni-modified the overpotential necessary to

To elucidate the nature of the increased activity of the exfoliated materials, we determined the intrinsic activity of each catalytic site in both types of systems [72]. To construct a mass normalized current plot, we performed inductively plasmamass spectrometry (ICP-MS) measurements on our samples to quantify the amount of nickel and cobalt metal content in the exfoliated and bulk materials. ICP-MS measurements show that the exfoliated samples are substantially better at adsorbing Co and Ni cations, leading to higher loadings than non-exfoliated ZrP. For our mass normalized plots, we assumed that all metal content quantified by ICP-MS

is 0.410 V, an improvement of 181 mV over the

for the

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

reach a current density of 3 mA/cm2

surface adsorbed Ni material.

**electrocatalysts**
