Surface Microstructural Controls on Electrochemical Hydrogen Absorption at Polycrystalline Palladium

Abstract The ease by which hydrogen is absorbed into a metal can be either advantageous or deleterious, depending on the material and application in question. For instance, in metals such as palladium (Pd), rapid absorption kinetics are seen as a beneficial property for hydrogen purification and storage applications, whereas the contrary is true for structural metals such as steel, which are susceptible to mechanical degradation in a process known as hydrogen embrittlement. It follows that understanding how the microstructure of metals (i.e., grains and grain boundaries) influences adsorption and absorption kinetics would be extremely powerful to rationally design materials (e.g., alloys) with either a high affinity for hydrogen or resistance to hydrogen embrittlement. To this end, scanning electrochemical cell microscopy (SECCM) is deployed herein to study surface structure-dependent electrochemical hydrogen absorption across the surface of flame annealed polycrystalline Pd in aqueous sulfuric acid (considered to be a model system for the study of hydrogen absorption). Correlating spatially-resolved cyclic voltammetric data from SECCM with co-located structural information from electron backscatter diffraction (EBSD) reveals a clear relationship between the crystal orientation and the rate of hydrogen adsorption-absorption. Grains that are closest to the low-index orientations [i.e., the {100}, {101}, and {111} facets, face-centered cubic (fcc) system] facilitate the lowest rates of hydrogen absorption, whereas grains of high-index orientation (e.g., {411}) promote higher rates. Apparently enhanced kinetics are also seen at grain boundaries, which are thought to arise from physical deformation of the Pd surface adjacent to the boundary, resulting from the flame annealing and quenching process. As voltammetric measurements are made across a wide potential range, these studies also reveal palladium oxide formation and stripping to be surface structure-dependent processes, and further highlight the power of combined SECCM-EBSD for structure-activity measurements in electrochemical science.

Abstract. The ease by which hydrogen is absorbed into a metal can be either advantageous or deleterious, depending on the material and application in question. For instance, in metals such as palladium (Pd), rapid absorption kinetics are seen as a beneficial property for hydrogen purification and storage applications, whereas the contrary is true for structural metals such as steel, which are susceptible to mechanical degradation in a process known as hydrogen embrittlement. It follows that understanding how the microstructure of metals (i.e., grains and grain boundaries) influences adsorption and absorption kinetics would be extremely powerful to rationally design materials (e.g., alloys) with either a high affinity for hydrogen or resistance to hydrogen embrittlement. To this end, scanning electrochemical cell microscopy (SECCM) is deployed herein to study surface structure-dependent electrochemical hydrogen absorption across the surface of flame annealed polycrystalline Pd in aqueous sulfuric acid (considered the model system for the study of hydrogen absorption). Correlating spatiallyresolved cyclic voltammetric data from SECCM with co-located structural information from electron backscatter diffraction (EBSD) reveals a clear relationship between the crystal orientation and the rate of hydrogen adsorption-absorption. Grains that are closest to the low-index orientations [i.e., the {100}, {101}, and {111} facets, face-centered cubic (fcc) system] facilitate the lowest rates of hydrogen absorption, whereas grains of high-index orientation (e.g., {411}) promoted higher rates. Apparently enhanced kinetics are also seen at grain boundaries, which is thought to arise from physical deformation of the Pd surface adjacent to the boundary, resulting from the flame annealing and quenching process.
As voltammetric measurements are made across a wide potential range, these studies also reveal palladium oxide formation and stripping to be surface structure-dependent processes, and further highlight the power of combined SECCM-EBSD for structure-activity measurements in electrochemical science.
Hydrogen absorption into metals is of wide interest, whether for hydrogen storage in metals such as palladium (Pd), or where it adversely affects the mechanical properties of structural materials such as steel [1]. During the absorption process, surface-adsorbed hydrogen atoms, originating from dissociative chemisorption of H2 gas or electro-reduction of H + from solution (i.e., the cathodic process during corrosion), diffuse into the bulk metal, occupying interstitial sites within the crystal lattice [2].
This can reduce the ductility of a metal, making it brittle and at risk from cracking when subjected to stress, in a process known as hydrogen embrittlement [3,4]. Due to the societal and industrial implications of hydrogen embrittlement, the absorption of hydrogen into steels has attracted significant research attention [5][6][7]. One particular area of interest has involved modifying the microstructure of steels to reduce the affinity to hydrogen, which has involved the engineering of grain boundaries and/or manipulation of texture (i.e., grain structure). For instance, steels with higher grain boundary densities have been shown to promote the diffusion of hydrogen [8], whilst varying levels of hydrogen permeation have been measured between different crystal phases [9]. Palladium (Pd) is considered to be a model metal to study hydrogen absorption, due to high intrinsic hydrogen solubility and rapid entry kinetics [10,11]. It also possesses desirable properties for hydrogen storage, as Pd forms hydrides, a relatively safe form of stored hydrogen, under ambient conditions [12]. As such, there has been considerable effort to understand and engineer the composition and structure of Pd (nano)materials for fast hydrogen uptake and release [13]. For instance, numerous studies have focused on the shape (facet) dependent hydrogen-storage properties of Pd nanoparticles (PdNPs) [14][15][16]. Despite being the subject of many investigations, electrochemical hydrogen absorption into Pd has largely been studied at the macroscopic level with electrochemical techniques, predominantly on polycrystalline electrodes (e.g., thin films and bulk metal) [10,11,17], making it difficult to unambiguously resolve the (micro)structural controls on surface activity [18]. Furthermore, the limited number of studies on structurally well-defined electrodes such as single-crystals [19] have focused exclusively on the low-index facets [i.e., the {100}, {101}, and {111} facets of the facecentered cubic (fcc) system], and thus do not consider the important roles of high-energy surfaces, such as the high-index facets and grain boundaries [20,21].
Scanning electrochemical cell microscopy (SECCM) [22,23], a recent addition to the electrochemical droplet cell (EDC) family, is proving to be a very powerful technique for probing the spatially-dependent electrochemistry of complex electrode surfaces [18,24]. In SECCM, electrochemical measurements are performed in a statistically large number (typically hundreds to thousands) of small areas of a surface, defined by a droplet (meniscus) cell created between a nanopipet probe filled with electrolyte solution and substrate (working electrode) surface. When applied to polycrystalline electrodes, SECCM electrochemically interrogates the individual grains and grain boundaries that constitute the surface, which is correlated to co-located structural information from electron backscatter diffraction (EBSD) to resolve nanoscale structure−activity directly and unambiguously. Indeed, this pseudo single-crystal approach [25] has previously been employed to investigate the grain (and grain boundary) dependent electrochemistry of polycrystalline platinum [25][26][27][28], gold [29], boron-doped diamond [30] and low carbon steel [31][32][33]. Building on this body of work, herein we employ SECCM in tandem with co-located EBSD to investigate grain-dependent electrochemical proton reduction coupled to hydrogen adsorption-absorption into polycrystalline Pd in aqueous sulfuric acid. Apparently enhanced electrochemical kinetics are identified at grains of highindex orientation (e.g., {411}) and grain boundaries relative to the grains of low-index orientation (i.e., In order to visualize the surface structure-dependent electrochemical (adsorption-absorption) kinetics, plots of grain-surface current (isurf) versus crystallographic orientation relative to the low index orientations (i.e., {100}, {101}, and {111}) were created. To achieve this, the Euler angles (φ1, Φ, φ2) that define the orientation of a plane (measured with EBSD), were first used to calculate the Miller indices {hkl} for each grain: Using the calculated Miller indices, the deviation (θ) of the orientation of each plane {hkl} from any other plane {h * k * l * } was calculated as follows: cos( ) = ℎℎ * + * + * √ℎ 2 + 2 + 2 · √ℎ * 2 + * 2 + * 2

(4)
In the present context, {h * k * l * } refers to each of the low-index orientations, {100}, {101} and {111}, allowing each grain to be plotted in a three dimensional (3D) space, where each axis (x, y, z) corresponds to the deviation angle (θ) from each low-index plane, i.e., (θ001, θ011, θ111). When plotted in 3D space, all points (orientations) lay on a hyperbolic plane, due to the fact that the x, y and z coordinates are not linearly independent (i.e., θ001, θ011 and θ111 were calculated from the same two variables, Φ and φ2).
When projected onto a two-dimensional (2D) plane (represented by two arbitrary Cartesian coordinates, x and y), all grains lay in a section delimited by the functions representing the following three families of planes: {0 n 1}//ND, {n 1 1}//ND and {n n 1}//ND, with 0 ≤ n ≤ 1, as shown in Scheme 1, below.
Finally, each point representing a grain was colored based on average isurf measured on that grain (i.e., from SECCM scanning) adopting an independent color bar scale, allowing the electrochemical hydrogen absorption activity as a function of grain structure to be readily visualized. Scheme 1. Two-dimensional projection of grain orientations in a fcc cubic crystal system, represented in arbitrary coordinates. The black lines delineate the space that contains all possible grain orientations (given the symmetry of the cubic system). In the above projection, n is any real number between 0 and 1. previously reported [39].

Electrochemical systems and measurements.
The entire SEPM set up was situated on a vibration isolation platform (BM-8, Minus K, U.S.A.), located within an aluminum faraday cage equipped with heat sinks and acoustic foam. This configuration has previously been shown to minimize electrical noise, thermal drift and mechanical vibration [38,40]. The QRCE potential was controlled, with respect to ground, and the current flowing at the Pd foil working electrode (i.e., isurf), held at a common ground, was measured using a home-built electrometer. isurf was measured every 4 µs, which was averaged 256 times to give a data acquisition rate of 4 × (256 + 1) = 1028 µs (note that one extra iteration is used to transfer the data to the host computer  Ag/AgCl at a voltammetric scan rate (υ) of 2 V s −1 , for a total of 6 potential cycles at each meniscus contact.  Figure 1a is consistent with previous studies on the bulk material [11,17], and contrasts with studies performed on nanomaterials [42] and/or thin films [10,11], where due to the finite volume of Pd, the hydrogen adsorption (Hads) and hydrogen absorption (Habs) processes can be distinguished voltammetrically (i.e., where the Pd substrate becomes fully saturated with Habs on the voltammetric timescale).

Results and Discussion
For comparative purposes, the voltammetric response of Pd was subsequently investigated at the nanoscale in the SECCM format; the CV is shown in Figure 2b. The CV obtained at the nanoscale is superficially similar to that obtained at the macroscale, with processes corresponding to PdOx formation, PdOx stripping and hydrogen adsorption-absorption occurring at potentials above ca. 0.6 V, around ca. 0.2 V and below ca. −0.15 V vs Ag/AgCl, respectively. It is interesting to note at the nanoscale, PdOx formation occurs in a single process (i.e., single peak at ca. 0.7 V vs Ag/AgCl, Figure   2b), compared to multiple processes at the macroscale (i.e., multiple peaks at ca. 0.35, 0.5 and 0.65 V vs Ag/AgCl, Figure 2a). This can be understood by considering that the grain size of annealed Pd (10s to 100s μm scale, vide infra) is orders-of-magnitude larger than the SECCM probe size (< 1 μm diameter), meaning Figure 2b is effectively a single crystal measurement [25], whereas a multitude of grains of different crystallographic orientation are probed simultaneously during the macroscopic measurement in Figure 2a. Considering the hydrogen adsorption-absorption process, as the working electrode area (defined by the meniscus cell) is very small compared to the total volume of the electrode, the Pd foil effectively acts as an 'infinite' sink for Habs in this configuration. Also, as Pd possesses high intrinsic hydrogen solubility and rapid entry kinetics [10,11,42], any structural influence on the rate of electrochemical hydrogen absorption will be the result of grain-dependent electron-transfer kinetics for the hydrogen adsorption process (vide infra). processes. Note that although the current associated with the PdOx formation/stripping processes increases with cycling (Figure 2b), the relative trends between the individual grains do not, evident from Movie S1. Although not the focus of this study, it is also interesting to note that grains that possess only subtly different crystallographic orientations (e.g., grains 1 and 11 in Figure 3b) can have very different PdOx formation/stripping behaviors, as alluded to above. Similar trends are also seen in the case of the hydrogen absorption reaction, explored in greater detail below. is close in orientation to grains 1 and 7, the former facilitates significantly higher hydrogen absorption rates than the latter two, demonstrating that even small changes in orientation can have a major influence on the affinity to hydrogen of a given surface.
Additional scans were carried out on other areas of the annealed polycrystalline Pd surface; examples are shown in Figures 4 and 5. Note that due to differences in the SECCM probe size, the raw isurf values vary significantly from scan-to-scan. Nonetheless, as the area wetted by the meniscus cell (i.e., working electrode area) was measured accurately by SEM imaging (example shown in Figure 3a), comparisons between different scan areas have been made in terms of current density (i.e., jsurf). Figure  4a comprises 7 unique grains, each of which possess different electrochemical behavior (i.e., PdOx formation/stripping and hydrogen absorption), as shown in the Supporting Information, Movie S2. A spatially-resolved equipotential image taken from Movie S2 at a potential of − 0.45 V vs Ag/AgCl is shown in Figure 4b, from which data were used to construct the grain-average jsurf versus crystallographic orientation plot in Figure 4c. Grain 4, which possesses an orientation close to the {411} high-index plane, is by far the most active (i.e., gave rise to the most negative hydrogen absorption current at − 0.45 V vs Ag/AgCl). Incidentally, grain 4 is similar in orientation to grain 8 in Figure 3f, which was also relatively active for hydrogen absorption. Grain 5 is the least active in Figure 4, which has an orientation close to the low-index {101} plane, a deviation of 3.1º. This is consistent with Figure   3f, where the least active grains were also those closest to the low-index orientations (i.e., {100} in Figure 3 and {101} in Figure 4). The hydrogen absorption properties of the different crystal facets on Pd is most frequently studied on shape-engineered PdNPs [44]. While it is generally accepted that the shape of PdNPs does influence the hydrogen uptake rate, the precise relationship between crystallographic structure and function (i.e., absorption kinetics) is not well established [15,16]. Our results indicate that electrochemical hydrogen absorption is a strongly grain-dependent process, occurring most readily on grains of high-index orientation (e.g., grains close to the {411} plane) compared to those of low-index orientation (e.g., grains close to the {100}, {101} or {111} planes). Limited macroscopic electrochemical studies on Pd single-crystal electrodes also report strongly structure-dependent voltammetric behavior (i.e., PdOx formation/stripping and/or hydrogen absorption), although such studies only consider the low-index planes [19]. Indeed, two major advantages of the SECCM pseudo single-crystal approach [25] are that it avoids the difficult preparation process required for singlecrystal electrodes, and additionally allows high-energy surfaces such as high-index grains and grain boundaries (vide infra) to be interrogated in a single scanning experiment.  Hydrogen absorption at grain boundaries. Alluded to above, in addition to presenting a collection of grains of different orientation, polycrystalline metals also possess grain boundaries (i.e., the interface between neighboring crystallites), which have been shown to be electrochemical (electrocatalytic) hotspots for some electrochemical reactions in previous SECCM studies [25,29,32]. at the grain boundaries of metals such as Pd [45], Ni [46] and Al [47].
Although the data presented in Movie S4, Movie S5 and Figure 6 suggest enhanced hydrogen absorption at grain boundaries with specific geometries, it is important to consider the morphology (topology) of the Pd surface, adjacent to the boundary. Figure 7 presents a detailed examination of a grain boundary where elevated hydrogen absorption currents were detected. A section of the isurf map (originally presented in Figure 6b-i), accompanied by co-located SEM images of the "active sites" is shown in Figure 7a. The SEM images reveal that the flame annealing process has induced a degree of deformation at the grain boundary surface termination. While the formation of a more defective surface could certainly influence electrochemical (electrocatalytic) activity (vide infra), surface deformation is also expected to increase the surface roughness, locally increasing the electrochemically active surface area (ECSA). Indeed, CVs extracted from the active sites (numbered in Figure 7a), presented in Figure   7b, demonstrate that the isurf arising from PdOx formation/stripping, as well as from double layer capacitance is also larger at the "active" pixels, suggesting an increased ECSA contributes to the elevated hydrogen absorption current at the "active" grain boundaries.
It is also worth noting that atoms with low-lattice coordination numbers present at surface defects such as grain boundaries are postulated to serve as the active sites for (electro)catalytic processes, due to an enhanced ability to bind reactants, and break/form covalent bonds [18,48,49]. For instance, it has previously been reported that the rate of hydrogen absorption is higher for shapecontrolled PdNPs containing a higher number of vertices, promoted by palladium-hydride phase nucleation at these undercoordinated sites [15]. Due to the surface deformation shown in Figure 7, it is plausible that there is an increased density of surface defects in the regions neighboring the "active" grain boundary. For example, many of the scanning points (pixels) that are shown to be active in Figure   7a (e.g., points 1 and 2), do not lie on the boundary itself, but rather are situated in deformed areas adjacent to the boundary, consistent with a previous SECCM study on electrochemical CO2 reduction at polycrystalline Au [29]. Thus, while SECCM alone cannot unambiguously distinguish between increased ECSA versus enhanced activity as the source of the local enhancement in isurf, this study highlights that careful analysis of the droplet "footprints" is vital to avoid misleading conclusions about the intrinsic activity of microscopic surface features such as grain boundaries.

Conclusions
In this study, using annealed polycrystalline Pd as a model system, it was shown that the rate of electrochemical hydrogen absorption (in 0.5 M H2SO4) is strongly influenced by surface structure (e.g., grains and grain boundaries). With the use of SECCM, multi-cycle CVs were collected at different locations on individual grains, which were subsequently identified (i.e., structurally characterized) with EBSD. Through this correlative electrochemical microscopy approach, grains close to the low-index orientations (i.e. {100}, {101}, and {111} planes of fcc Pd) were shown to exhibit the lowest rates of hydrogen absorption, whereas grains with high-index orientations (e.g., {411}) facilitated the highest rates. Enhanced hydrogen absorption currents were measured at some grain boundaries across the surface of polycrystalline Pd, which was attributed to surface deformation giving rise to an increased ECSA and/or high defect density, leading to enhanced hydrogen absorption activity. Overall, these data improve our understanding of how surface structure can influence the ingress of hydrogen into metals, which is particularly important in the areas of corrosion (e.g., hydrogen embrittlement) and energy conversion/storage (e.g., hydrogen generation, purification and storage).