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

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 counterparts. The overpotential necessary to reach a current density of 10 mA/cm2 for the 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 reach a current density of 3 mA/cm2 is 0.410 V, an improvement of 181 mV over the surface adsorbed Ni material.

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

#### **Figure 10.**

*Water Chemistry*

**Figure 8.**

*permission from [72].*

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

*XRPD patterns of α-ZrP, exfoliated ZrP, and metal-modified exfoliated ZrP samples. Reprinted with* 

*(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].*

as the P/Zr ratio after exfoliation remains constant at ~2.

**210**

**Figure 9.**

*Linear sweep voltammograms of (a) Ni(II)/ZrP systems and (b) Co(II)/ZrP systems. Reprinted with permission from Ref. [72].*

#### **Figure 11.**

*Mass normalized catalytic currents for (a) Ni(II)/ZrP systems and (b) Co(II)/ZrP systems. Reprinted with permission from Ref. [72].*

in the materials were active and accessible to perform OER. **Figure 11a** and **b** show the LSVs where the OER currents are normalized by the mass of the metal content. These exfoliated systems maintain reasonably high intrinsic activity values that, when coupled to a significant greater number of active sites leads to higher geometric activity. We concluded that the enhancement in activity is due to the fact that the inner layer surfaces are now more electrochemically accessible [72]. Through exfoliation the number of ion-exchange sites increases which increases the number of catalytic species that are distributed on the surface of zirconium phosphate, therefore giving rise to the improved performance.
