CHAPTER 1
Borohydride electro-oxidation on metal electrodes: structure, composition and solvent effects from DFT
Mary Clare Sison Escaño DOI: 10.1039/9781782622727-00001
University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan E-mail: mcescano@u-fukui.ac.jp
1 Introduction
An efficient borohydride (BH4-) electro-oxidation in alkaline media facilitates rapid development of direct borohydride fuel cells (DBFC) as power sources for portable devices and other applications. While there have been reviews on the experimental and theoretical studies of BH4- electro-oxidation, there has been no reports that consolidate, compare systematically and update electro-oxidation of borohydride on vast structures and composition of metals and metal alloys. Such focused review is very insightful in terms of formation of new ideas for design and development of better catalysts. Theoretical investigations are mostly conducted using density functional theory (DFT). Beyond-DFT has also been used and will also be discussed.
One of the parameters in evaluating the effectiveness of an anode catalyst is the resulting initial oxidative adsorption structure of BH4-. The theoretical anodic reaction of direct BH4- oxidation in a DBFC can be described as:
[MATHEMATICAL EXPRESSION OMITTED] (1)
Furthermore, depending on the anode catalyst, pH, temperature and residence time on the anode, a thermo-catalytic hydrolysis reaction can also occur, generating H:
BH4- + 4H2O -> B(OH)4- + 4H (2)
The in situ H2(g) evolution complicates the anode operation by generating a mixed potential and gas shielding the anode surface, thereby hampering the BH4- mass transfer and adsorption and lowering the effective ionic conductivity in the anode compartment. Ideally, an effective electro-catalyst should promote reaction (1) over reaction (2). Promoting (1) implies a more controlled interaction of BH4- and H2O species with the anode catalyst. To discuss this in more detail, the initial step of oxidation is considered without solvation:
BH4- + * (metal site) -> BHy* + (4 - y)H* + e- (3) and with solvation effects:
BH4aq- + H2O* -> BHy* + (4 - y)H* + H2Oaq + e- (4)
Eqn (3) indicates that a borohydride ion "sees" a metal site, adsorbs and releases electron due to the potential difference. There are four ways by which the ion adsorbs depending on the catalyst: as a molecule, y = 4 (BH4*) or as a dissociated structure, y=1–3 (i.e.BH*+3H*, BH*+2H*, BH*+H*). Same is the case for eqn (4), however the formation of either of these structures depends on the catalyst and the interaction with water (or solvent). It can be noted that a molecular adsorption structure (BH4*) can inhibit fast formation of H2 gas making the catalyst selective for direct pathway (1), on the other hand, an almost completely dissociated BH4- (BH*+3H*) can promote H2 gas evolution and hence leading to reaction pathway (2), and the partially dissociated structures (BH2*+2H*, BH3*+H*) can be somewhere in between. Moreover, the strength of the adsorption of the borohydride especially for the molecularly adsorbed and partially adsorbed ones can impact shifts in the overpotentials (e.g. weak adsorption?can contribute to higher overpotential; strong adsorption -> can contribute to lower overpotential). In this article, the initial oxidative adsorption structures and energies of borohydride with and without solvation effects obtained by DFT across many structures of metal anode catalysts are systematically compared.
2 Pure metal surfaces
A Same metal facet: (111)
A borohydride is a trigonal molecule (Fig. 1 inset) with a calculated B–H bond distance of 1.25 Å and H–B–H angle of 109°, in excellent agreement with experiment.
Early theoretical studies on borohydride electro-oxidation tackled the adsorption of borohydride via eqn (3) on the (111) facet of 3d, 4d and 5d metals. The (111) surface of the metal is shown in Fig. 1, along with the adsorption site (*) labelled as bridge, hcp-hollow, fcc-hollow or top. The adsorption site is defined as the location of the boron atom, which is the center of the trigonal molecule. Table 1 lists the adsorption structure and energies depending on the transition metal.
The adsorption energies are referenced to isolated metal surface and the gas phase borohydride ion. Generally, there is an increasing trend on the adsorption energies as one goes from left to right within the same period of the periodic table. For the 3d transition metals, stronger adsorption leads to partially dissociated structures. However, the adsorption structures for 4d and 5d metals do not really correlate well with the adsorption energies. That is, a strong adsorption does not imply dissociated structures. For instance, in osmium (Os), the adsorption energy is strong but a molecular adsorption structure is noted. The adsorption structures for 4d and 5d transition metals are depicted in Fig. 2 and for the 3d metals, the structures are shown in Fig. 3.
B Different metal surface facets
The relationship between the adsorption structure and energies with the different facets of metal surfaces is investigated in Os. The facets considered, the adsorption structures and the adsorption energies are given in Table 2. Figures 4, 5 and 6 depict the (0001), (1010) and (1100) facets of Os and the adsorption structures, respectively. (0001) is the basal plane of the Os hcp bulk lattice, while the (1010) and (1100) are the prism planes. It is well known that (0001) (Fig. 4) is akin to the fcc (111) surface except for the stacking of the atomic layers. On the other hand, (1010) and (1100) have characteristic rows and trenches (Figs. 5–6). These two surfaces differ mainly in the position of the trench atoms. For instance, in the (1010), a trench atom is directly under the fourfold hollow site (h1) while in the (1100), it is under the bridge site (b2).
Based on the Table 2, it can be noted that the adsorption energy increases depending on how "open" the facets are. The (1010) and (1100) facets contain rows and trenches that expose the second layer atoms (hence "open"), thereby facilitating greater conformation and interaction with the borohydride than the (0001), which does not have these structures. The "b" distance, which is the distance between the parallel rows, is greater in (1100) than in the (1010), making the former more "open".
Thus, the adsorption energy follows this order: (0001)<(1010)<(1100). In terms of the adsorption structure, it is observed that within the same kind of metal, the structure can be correlated with the order of the adsorption energies: BH*
3 Metal alloy surfaces
A Same composition but different types of alloying metal
Because of the development of anode catalysts that are both selective and cost-effective, the metal alloys have been studied theoretically. In ref. 15, metal alloys based on Au, which are of AuM type (M is a 3d transition metal=Cr, Mn, Fe, Co and Ni), are investigated. Table 3 lists the adsorption structure and the adsorption energies on Au3M (111) surfaces. The initial oxidative adsorption structures are depicted in Fig. 7.
It can be noted that for Au, alloying does not change the adsorption structure of borohydride. It remains molecular with a slight elongation of three B–H bonds. The molecule prefers the M site for the adsorption and the adsorption energy increases in the following order: Au3Ni3Mn3Cr3Fe3Co. The mechanism of the alloying effect is explained using the density of states (DOS) of the metal and the molecule as shown in Fig. 8. Without the M atom, in this case Fe, the DOS of the Au is fully occupied (Fig. 8(a)). Upon alloying with Fe, a spin-down DOS forms at and above the Fermi level (EF). The local DOS shown in Fig. 8(b) indicates that these states originate from the Fe, explaining the preference of the borohydride on the M site. Figure 8(c)further clarifies the bonding interactions via partial charge density plots corresponding to DOS peaks near the EF.
B Same alloying metal but different compositions
Other metal alloys have also been considered for instance, Pd–Ir. In this case, the alloying component is not a 3d transition metal but a 5d. Pd is a 4d metal. Two types of composition are studied: Pd2Ir1 (111) and Pd3Ir3(111). The adsorption structure and energies are shown in Table 4. Figure 9 depicts the preferred adsorption structures.
Alloying the Pd with Ir, increases the free energy of the adsorption (i.e. more positive shift in the free energy). Such energy also increases upon increase in the composition of the alloying metal. The structures on the Pd–Ir alloys are shown in Fig. 9. Although both structures are molecular, the orientation of the B–H bonds and the position of the center B atom are different. For PdIr(111), the boron is at the bridge site with two H atoms spanning towards Pd top sites and the other two H atoms towards the Ir top sites. Thus, the molecule conforms with the surface via two H atoms. In the Pd3Ir3 (111) (greater Ir composition), the molecule conforms with the surface via three H atoms spanning towards two Pd top sites and one Ir top site. This is the typical conformation of the borohydride on some 4d and 5d metals.
4 Pure metal surfaces – solvated
A Effect of water in Au(111) and Pt(111)
To consider the effect of the water as the solvent, the Gibbs free energy, ?G, for the initial oxidative adsorption of the BH based on eqn (4) is derived to draw the thermodynamics of the reaction. There are two ways by which this can be done, the vacuum slab method and the double-reference method. Mainly, the difference between the two methods is the inclusion of water molecules in the model (or supercell). In the former, as the name suggests, there are no water molecules that interact with the borohydride within the supercell and the effect of solvent is integrated via the free energies of H2O adsorption (H2O*) and the aqueous H2O (please see eqn (4)) alone, which are computed in separate supercells. The latter on the otherhand, includes water molecules within the supercell and so the effect of solvent is integrated via the usual free energies of HO adsorption (H2O*) and the aqueous HO as in the above method plus the borohydride–water interactions. The reader is suggested to refer to the given sources above for details. Figure 10 best captures the solvent effects using the double reference method on Au and Pt.
The re-orientation of the adsorbed borohydride occurs on Au. Two B–H bonds, instead of one, point away from the metal surface. There is negligible change in the orientation of the borohydride on Pt. It can be noted that for molecularly adsorbed structure (BH4*), the effect of the interaction with water molecules in the orientation of borohydride can be observed. In terms of energetics (?Gads), Fig. 11 shows that for Pt, the ?Gads as a function of electrode potential, V (SHE) is steeper when the vacuum slab method is used (dashed line) than when the double reference method is used (solid line). This is attributed to the dipole moment or polarizability effects. Basically, there is not much difference between the two methods for Au. Now, when the energies of the adsorption of borohydride in unsolvated (Table 1 of Section 1) and the solvated (Fig. 11) models of Au and Pt are compared, it can be noted that the trends are actually well-preserved.
B Water on different facets of metal surface
As discussed above the adsorption of water (H2O*) on the metal surface can bring about shifts in ?Gads. The study of the differences in the H2O* structure on various facets of metal surfaces is worthwhile. Due to its interesting interaction of borohydride with Os as discussed previously, the H2O* structure is obtained on this metal. The (0001), (1010) and (1100) surfaces of Os are used. Extensive adsorption configurations are evaluated, that is using the flat, up and down orientation of the molecule as well as its translation and rotation on the surface (please see Fig. 12). For instance, on (0001), for each water orientation (up and down), the molecule is rotated in-plane such that the O–H bond "parallel" to the surface can point towards different directions. Also, the O atom position is shifted to other high symmetric sites (i.e. translation). Thus, each adsorption configuration can be identified by X1–X2–X3, where X1 indicates the molecule orientation (flat (f), one H down (d) or one H up (u)), X2 is the position of the oxygen atom (hollow (h), top (t), bridge (b)) and X3 is the direction where the H expands as a result of planar rotation (towards the bridge (b) or hollow site (h)). Hence, f-tb means a flat orientation with O atom on top site and the O–H1 bond (see Fig. 12(a) for the H1 atom) pointing towards the bridge. A total of 24 adsorption configurations on (0001) facet are considered. The adsorption energy per configuration is calculated based on the following:
Eads = ET - (Eg + Es)
where ET, Eg and Es are the total energy of adsorbed system, isolated molecule and the metal surface (or slab), respectively. Table 5 lists the adsorption energies on (0001).
The most stable H2O* structure originated from the f-t-h initial configuration with an adsorption energy of -0.484 eV. Figure 13(a) depicts the final adsorption structure. The oxygen atom is on the top site and the O–H bonds are almost parallel to the surface. Both O–H bonds have the same lengths and the O–H–O angle is ~105.495°. The O–Os distance is 2.302 Å. A small change in the interlayer distance, d of ~0.029 Å of the surface upon H2O adsorption can be noted. Water monomer adsorption on metal surfaces has been studied in other metals previously. Although a direct comparison cannot be made due to the different metals used and the size of the supercell, in general, it can be noted that the O–Os, O–H distances and the H–O–H angle are comparable to those reported in the literature for the fcc (111) metals. For Au(111), the literature reports a slightly weaker binding of water on the surface.
Next, for the (1010), Fig. 12(b) shows the symmetric sites. Here, there are two bridge sites (b1, b2) corresponding to the a and b lateral distances. The same three orientations of H2O, in-plane rotations, and configuration notations in (0001) are also employed in (1010). Table 6 gives the adsorption energies. The most stable site originated from the d-b1-h initial adsorption configuration. The adsorption energy is -0.660 eV. Figure 13(b) depicts the final adsorption state. The molecule is in bent configuration with the O–Os axis forming a 23.53° angle with the vertical axis (see Fig. 13(b), bottom panel). One of the O–H bonds (O–H2) points towards the b1 site and is longer than the O–H1 bond. This is because the H2 atom is lower (or closer to the surface) than the H1. This bent structure is due to the "near top" adsorption, in contrast to (0001) where the molecule directly sits on the top site. The O–Os distance in (1010) is shorter by 0.093 Å as compared to that of the (0001). We note that this bending of water molecule towards the trench resulted in a stronger adsorption.
