Electrochemical measurements

respectively(Table 9.4). As detailed in Section 2.2.1, the (100), (111) and the (110) surfaces can provide theoretically 0.11, 0.13 and 0.08 cationic site/A⁻², respectively. This calculation indicates that the cationic sites support 1.21, 3.75 and 4.07 charges on the (100), (111) and the (110) sample, respectively (Table 9.4). Ideally, if the reactions happen on the surface, there should be 1 to 1.66 charge per cationic site (depending on the phase, see Figure 9.1). The (110) and the (111) displays substantial higher charges per cationic site experimentally than what should be obtained forplanar electrochemical reactions. Therefore, the only reasonable possibility is that the electrochemical reactions occur in a volume and not on a surface. Thus, it is assumed, that the electrochemical reactions are realized in a volume which is supported by the nickel hydroxide layer growing on top the nickel oxide surfaces during the OER. This could be rendered possible thanks to the weak OH-OH interaction in the nickel hydroxide, which would accommodate reactions in the Ni(OH)₂ structure.

Table 9.5: Main characteristics of the oxidation and reduction wave of the hydroxide/oxy-hydroxide reaction before OER. Raw data taken from cyclic voltammetry realized at (100) mV/s. The estimated hydroxide thickness is determined either for a βor aγ phase transformation.

Orientation NiO (100) NiO (111) NiO (110)

FW peak (µC/cm²) 231 808 538

Ni in NiO (e⁻/cat) (FW) 1.21 3.75 4.07

Estimated Ni(OH)2 thickness (nm)β/γ 0.54/0.32 2.11/1.27 1.27 / 0.76 BW peak (µC/cm²) -202 (87%) -692 (85%) -632 (115%) Taking an average number of cationic site by volume of 0.026 atom.A⁻³ for Ni(OH)2, the reaction depth into the hydroxide is estimated to vary from 0.3 to 1.9 nm from the top of the nickel hydroxide surface (Table 9.5). The reaction depth range correlate relatively well with the XPS measurements, which provides a hydroxide thickness of 1.4 - 2.1 nm. It proves that Ni(OH)₂ would provide the catalytic material in which electrochemical reactions happen in a small volume of few nanometre depth. Then it can be assumed that electrochemical reaction occurs at the interface between the nickel hydroxide and the electrolyte but not on the NiO surface.

Figure 9.9: IV curves determined from the quasi-static measurements in linear scale (left) and logarithmic scale (right). The potential is iR corrected for each curves.

than 1.55 V vs RHE, is characterized by a high Tafel slope, meaning that the increase of current is low for a given increment of potential. The origin of the two regions will be discussed later but it can be said that, on a rotating disk electrode, mass transport limitation can be described by the Levich equation which is:

j_LA= 0.620n F D^2/3v^−1/6C_rω^1/2 (9.2) with jLA being the anodic limiting current, n the number of electrons exchanged (= 4), F the Faradic constant (= 96500 C/mol), D the diffusion coefficient of the hydroxides in water (∼10⁻⁵cm²/s), Cr the concentration of active species (= 0.001 mol/cm³),vthe kinematic viscosity of water (∼9.10⁻³ cm²/s) andω the angular rotation speed (210 rad/sec at 2000 rpm). For our setup, the calculations give an anodic limiting current jLA of 3.53 A/cm². This value is far from the current range in which the setup is working (<(100) mA/cm²). The anodic current limitation is only calculted for a theoritical diffusion layer at the electrode surface but other factors as surface reduction because of micro-oxygen bubbles fixed on the rotating electrode or mass transport limitation within the nickel hydroxide layer should be considered as well.

Indeed, the Levich equation allows to assess limiting current for bulk mass transportation from the electrolyte, but diffusion of species through the hydrous hydroxide layer has to be considered. Diffusion of species can be assessed both by EIS or QS. Being not enough accurate because of the large current at high overpotential EIS does not provide satisfactory results, but QS data can provide an estimation of the diffusion of the chemical species in the hydroxide layer in applying the so-called Cottrell equation in the relevant transition part of each measured QS plot [220]. Cottrell equation describes

transitional current when a potential stair is applied to an electrode:

I(t) =n F A C_OH−

rD_OH−

π t (9.3)

with C_OH⁻ the concentration of hydroxide, D_OH⁻ the diffusion coefficient of the hydroxide and t the time. One can extract an estimation of the diffusion coefficient of the active species through the hydroxide layer by identifying the linear part of the I(t) = f(t^−1/2) plot (Figure 9.10, left). The interfacial concentration in hydroxide can be considered equal to bulk concentration as the experimental current density is much lower than the limiting current as determined by the Levich equation.

Figure 9.10: Top: Cotrell plot of the transitional regime during the quasi-static experiment (see Figure 9.2). The Cottrell region correspond to the linear part between 9 and 34 seconds. Bottom: extracted diffusion coefficient for the different sample for a concentration of reactant equal to 1 mol/L. The coefficient diffusion in the hydrous hydroxide layer is extracted for high over-potential (area 3). The area 1 and 2 has been excluded for the evaluation of the diffusion coefficient.

Thus, for the right plot of Figure 9.10 the extracted diffusion coefficient shows that hydroxide diffusion in the nickel oxy-hydroxide is about one order of magnitude higher for the (110) oriented NiO thin film than for both the (100) and (111) oriented thin films.

Tafel slope

In addition, it is observed that for the IV curve plotted in logarithmic scale that the curves obtained for the (111) and the (100) oriented sample are similar where the (111) curve resembles to a translation of the (100) curve to higher current (Figure 9.9, right). The IV curve in logarithmic scale obtained on the (110) oriented surface differs from the two other curves, especially at high overpotential where the Tafel slope is larger.

The Tafel slope determined from Figure 9.9 (right) at low (< 1.55 V vs RHE) and high (>1.55 V vs RHE) overpotential are displayed in Table 9.6.

At low overpotential, the Tafel slope can be considered as roughly similar for the three samples. On the contrary, the Tafel slope is the lowest for the (110) oriented sample (210 mV/dec), while it is higher for the (111) and the (100) oriented sample with 348 mV/dec and 373 mV/dec, respectively.

Table 9.6: Tafel slopes, expressed in mV/dec, obtained with the (100), (110) and the (111) oriented samples at low (<1.55 V vs RHE) and high (>1.55 V vs RHE) overpotential. The data are extracted from Figure 9.9 (right).

Potential (V) vs RHE NiO (110) NiO (111) NiO (100)

<1.55 V 46 57 41

>1.55 V 210 348 373

9.3.4 110 vs 111

The (110) sample shows a higher catalytic activity for the OER particularly at high overpotential (>1.55 V vs RHE) composed to the (111) sample (Figure 9.9 and Table 9.6). Moreover, the comparison of the frontward and backward coulombic charge of the nickel hydroxide/oxy-hydroxide phase transformation gives a ratio equal to 85 % and 115 % for the (111) and the (110) sample, respectively. It can therefore be assumed that different nickel hydroxide species are produced for each orientation.

However, the equivalent capacitance of the Helmholtz double layer has a similar value for both samples (Figure 9.11) and the CPE α factor is above 0.9 (Table 9.7). These two observations could mean that the hydroxide layer homogeneity is similar on both surfaces withsomewhat an equivalent number of adsorbption sites in contact with the electrolyte.

Figure 9.11: Equivalent capacitance values of the Helmholtz layer and charge transfer resistance values for the (100), (110) and (111) samples, as determined by EIS at different potentials.

Interestingly, the nickel hydroxide oxidation pre-step produces an equivalent coulombic charge higher on the (111) sample than on the (110) sample (×

1.5, Table 9.5) but the nickel hydroxide is thicker with the (110) sample than on the (111) sample with 2 nm against 1.7 nm, respectively (Table 9.4). This suggests that the (111) sample produces a thinner hydroxide layer for which, when oxidized, the cationic sites gain a higher oxidation number than the cationic sites from the hydroxide growing on top of the (111) oriented sample.

Table 9.7: αfactor values of the Helmholtz double layer for the three oriented sample as determined after fitting of the EIS measurements.

Potential vs RHE (V), non iR corrected

NiO (110) NiO (111) NiO (100)

1.45 0.91 0.92 0.86

1.55 0.92 0.92 0.89

1.65 0.92 0.94 0.88

1.75 0.92 0.89 0.85

The estimated hydroxide thicknesses from the electrochemical pre-oxidation waves comply with the XPS measurements in assuming that the α-NiO(OH)₂ phase is present on top of the (111) oriented sample. Indeed, it has been found that the Ni oxidation state is 3.5-3.67 in a γ-NiOOH phase (oxidation of the α-NiO(OH)2 phase) while the Ni oxidation state is 2.7-3.044 in the β-NiOOH phase (oxidation of theβ-NiO(OH)2 phase). Thus, the hydroxide thickness is 1.25 nm on top of the (111) oriented sample in assuming a α-NiO(OH)2 → γ-NiOOH phase transformation but for the β-NiO(OH)2 → β-NiOOH phase transformation the hydroxide thickness would be 2.1 nm (Table 9.5). However,

the XPS measurements suggest that the hydroxide thickness is about 1.7 nm on top of the (111) oriented sample (Table 9.4). In consequence, it discards the possibility to have the β-NiO(OH)2 phase growing on top the (111) oriented sample. On the contrary, for the (110) oriented sample, the XPS and the electrochemical measurements do not discriminate between both α and β hydroxide phase.

Eventually, as the electrochemical reactions occur in the nickel hydroxide layer, the nickel hydroxide can be oxidized into two oxy-hydroxide phases (either the β or theγ phase). Also, the electrochemical performance of the oriented samples suggest that the nickel hydroxide layer on top of the (110) and the (111) sample are different, and the comparison with the XPS measurements, it can be estimated that the nickel hydroxide growing on top of the (111) sample provides an oxy-hydroxide counterpart with a higher oxidation state.

Therefore, to comply with the Bode scheme (Figure 9.1), it can be consistent to assume that the (111) sample would produce an hydroxide mainly made of α-Ni(OH)2, while the (110) sample would be likely producing the more stable β-Ni(OH)2 phase.

9.3.5 100 vs 111

Taking a look at the fast CV scans at (100) mV/s with respect to the current wave typical of the nickel hydroxide/oxy-hydroxide phase transformation situated right before the OER (Figure 9.8 and Table 9.5), it is possible to evaluate the pseudo-capacitive properties of the nickel hydroxide material. It can be observed a clear difference between the three samples. The backward coulombic charges represent around 85-87 % of the measured forward coulombic charge for the (111) and (100) surfaces, while it reaches up to 115

% for the (110) sample (Table 9.5). Thus, the (100) and (111) sample produce a hydroxide with similar pseudo-capacitive properties.

Also, looking back at the IV curve in log scale of the (100) and (111) samples, one can notice similar shapes but the shape of the curve obtained with the (111) sample is shifted about 0.25 decades upwards in comparison to the curve obtained with the (100) oriented sample. The observed vertical translation in Figure 9.9 between the log(I) vs.V curves of (100) and (111) sample might be indicative for the fact that close catalytic activity is obtained for both surface orientations. In other words, the same active sites are present on both surfaces, but the amount of the active site on the surface might be higher on the (111) sample than on the (100) sample. Indeed, a translation in the Tafel plots can be interpreted by a higher exchange current density for the (111) sample, which can be either related to an increase of the geometrical surface or an increase of the number of active site density. Since AFM measurements before electrochemical experiments² do not highlight a clear increase of the geometrical surface for the (111) sample over the (100) sample

2Unfortunately AFM measurements after the electrochemical measurements have not been performed.

(only 4.7%, Figure 9.6 and Table 9.3), the translation could be attributed to an increase of the number of active sites on the (111) surface compared to the (100) surface. This increase is qualitatively in line with both the higher coulombic charge in the NiOOH → NiO(OH)2 region of the (111) sample compared to the (100) (× 3.5, Table 9.5) and with the XPS measurements, which confirm a higher nickel hydroxide thickness on the (111) surface than on the NiO (100) (×1.4, Table 9.5). Moreover, the equivalent capacitance of the Helmholtz double layer is higher on (111) sample than on the (100) (×

2.2, Figure 9.11).

These observations lead to the assumption that similar hydroxides grow on the (100) and (111) surfaces (same pseudo-capacitive properties and same catalytic activities) but the quantity of active material is lower on the (100) surface than on the (111) one (lower active site density, thinner hydroxide layer on the (100) surface).

In addition, on the (100) sample, the hydroxide layer might be inhomogeneous as suggested by the lower value taken by the α factor of the Helmholtz layer determined by EIS in comparison to the αfactor for the (111) oriented sample (Table 9.7). The assumption that the (111) sample develops a more homogeneous hydroxide coverage than the (100) sample is consistent with the work of Cappus et al. which was realized in an UHV chamber [63]. His work suggested that hydroxyl groups do not bond to regular sites on a (100) surface, but rather on defective ones, while a full hydroxyl coverage is obtained with the (111) surface. Therefore, it would mean the hydroxide on the (100) sample could be inhomogeneous and can be found in higher quantity nearby crystallographic defective sites (e.g.: at grain boundaries) than on regular site.

Instead hydroxides might bond on defective site and on regular sites for the (111) sample, providing a more homogeneous hydroxide layer.

Finally, it can be assumed that comparable hydroxides grow on both (111) and (100) oriented thin films but the (100) oriented sample is less covered than the (111) counterpart. As in the previous part, it has been assumed that the (111) oriented surface produces the α-Ni(OH)2 hydroxide, by extension it is assumed that also the (100) oriented thin film develops the α-Ni(OH)2phase during the OER experiments.