Dealloying-Induced Zeolite-like Metal Framework of AB2 Laves Phase Intermetallic Electrocatalysts.

Exploring an efficient and robust electrocatalyst for hydrogen evolution reaction (HER) at high pH and temperature holds the key to the industrial application of alkaline water electrolysis (AWE). Herein, we design an open tunnel structure by dealloying a series of Laves phase intermetallics, i.e., MCo2 and MRu0.25Co1.75 (M = Sc and Zr). The dealloying process can induce a zeolite-like metal framework for ScCo2 and ScRu0.25Co1.75 by stripping Sc metal from the center of a tunnel structure. This structural engineering significantly lowers their overpotentials at a current density of 500 mA/cm2 (η500) ca. 80 mV in 1.0 M KOH. Through a simple process, ScRu0.25Co1.75 can be easily decorated on a carbon cloth substrate and only requires 132 mV to reach 500 mA/cm2. More importantly it can maintain activity over 1000 h in industrial conditions (6.0 M KOH at 333 K), showing its potential for practical industrial applications.

Dual Function of Hypo-d-electronic Transition Metals in the Brewer Intermetallic Phase for the Highly Efficient Electrocatalytic Hydrogen Evolution Reaction in Alkaline Electrolytes.

Reported are the synthesis, material characterization, and electrocatalytic hydrogen evolution reaction (HER) in acid and alkaline electrolytes for the Brewer intermetallic phase, Nb6Co7 and Mo6Co7. It was realized that the overpotential at a current density of 10 mA/cm2 (η10) for Nb6Co7 (η10 = 62 mV) and Mo6Co7 (η10 = 143 mV) are both much lower than that of using a single Co metal (η10 = 253 mV) in alkaline electrolytes. The enhancement of electrocatalytic HER activity of Nb6Co7 and Mo6Co7 can be attributed to the hypo-hyper-d-electronic interaction between Nb/Mo and Co elements. Based on the result of density functional theory calculation, alloying between Nb/Mo and Co elements will increase the antibonding state population of the Co-Co bond near the Fermi level (EF), which induces the synergistic effect to influence the adsorption energy of the H atom (ΔGH) on the surface of Nb6Co7 and Mo6Co7. Moreover, the role of the Nb element is not only a simple electron donor but is also an anchor position for the OH molecule (i.e., dual function) due to the bonding character of the Nb-Co bond near EF. It can reduce the OH position effect as well as the activation energy for water dissociation, which rationalizes the high and robust HER performance of Nb6Co7 to that of commercial Pt/C (η10 = 67 mV) in alkaline electrolytes.

Function of Internal and External Fe in a Ni-Based Precatalyst System Toward Oxygen Evolution Reaction.

It is well known that the "iron" impurity will influence the oxygen evolution reaction (OER) in an alkaline electrolyte, especially for the Ni-based electrocatalyst. Many research studies have investigated the function of Fe in the OER active phase, such as M(OH)2/MOOH (M = Ni and/or Fe), while, surprisingly, very few studies have examined the function of Fe in the "precatalyst" system. Accordingly, in this work, the Ni3-xFexP (x = 0, 0.5, 1) series as an Ni-based precatalyst was employed to inspect the function of internal and external Fe in the Ni-based precatalyst system. It was realized that the sample with internal Fe (i.e., Ni2.5Fe0.5P and Ni2FeP) exhibits efficient OER activity compared to that of the Fe-free one (i.e., Ni3P) owing to the large amount of active M(OH)2/MOOH formed on the surface. This indicates that the internal Fe in the present system may have the ability to facilitate the phase transformation; it was later rationalized from electronic structural calculations that the d band center of the internal Fe (middle transition metal) and Ni (late transition metal) holds the key for this observation. Adding excessive ferrous chloride tetrahydrate (FeCl2·4H2O) as the external Fe in the electrolyte will greatly improve the OER performances for Ni3P; nevertheless, that the OER activity of Ni2FeP is still much superior than that of Ni3P corroborates the fact that the Fe impurity is not the only reason for the elevated OER activity of Ni2FeP and that internal Fe is also critical to the phase transformation as well as OER performance.

Exploring the synergistic effect of alloying toward hydrogen evolution reaction: a case study of Ni3M (M = Ti, Ge and Sn) series.

In this work, we have demonstrated that one can control the intrinsic activity of Ni metal toward the hydrogen evolution reaction (HER) by simply alloying Ni with different elements (i.e. Ti, Ge or Sn). The HER activities of Ni3M (M = Ti, Ge and Sn) series and Ni metal follow the order of Ni3Ti (η10 = 68 mV) > Ni3Sn (η10 = 122 mV) > Ni3Ge (η10 = 161 mV) > Ni (η10 = 273 mV). After normalizing their HER performances based on the roughness factor (RF), it was realized that Ni3Ti and Ni3Sn both exhibit higher intrinsic HER activities than that of Ni metal while Ni3Ge displays the worst HER performance. This trend was later rationalized by using density functional theory (DFT) calculation, which showed that blending Ni with Ti, Ge or Sn elements will alter the corresponding electronic structure and bonding scheme. Such a change in the bonding scheme (i.e. bonding state or antibonding state) will influence the adsorption energy of the H atom (ΔGHad) on an active site and is the main cause of the synergetic effect that results in the different HER efficiencies of Ni3M (M = Ti, Ge and Sn). Through the present case study, it was recognized that alloying is a simple yet effective strategy to promote the HER activity of an electrocatalyst. With a suitable combination between elements, it helps single metals (e.g. Co or Ni metal) exceed the limits on their intrinsic HER activities and has the potential to replace noble metals (e.g. Pt, Ir and Ru) in the future.

Function of Doping Ru Element in the Hydrogen Evolution Reaction in Rare-Earth Transition-Metal Intermetallics.

Transition metal-based intermetallics are promising electrocatalysts for replacing the commercial Pt metal in the hydrogen evolution reaction (HER). In this work, RENi2 and RERu0.25Ni1.75 (RE = Pr, Tb, and Er) were synthesized and their electrocatalytic HER activities were explored. Among undoped compounds, PrNi2 exhibits the best performance and requires an overpotential of 55 mV, while partially replacing Ni with Ru element (PrRu0.25Ni1.75) can greatly reduce the overpotential to 20 mV at a current density of 10 mA/cm2. Such enhancement was recognized that belongs to their extrinsic property, and their intrinsic HER activities were similar after normalizing the electrocatalytic surface area. Further investigation on ScM2 and ScRu0.25M1.75 (M = Co and Ni) suggests that doping Ru element in ScCo2 will significantly enhance antibonding character around the Fermi level (EF) and weaken hydrogen adsorption energy. On the other hand, the antibonding population for ScNi2 and ScRu0.25Ni1.75 is similar at EF, which accounts for their close intrinsic HER activities.

Crystal and electronic structure manipulation of Laves intermetallics for boosting hydrogen evolution reaction.

Since the 1970s, Laves intermetallics (AB2) have been widely used in hydrogen storage technology (e.g., nickel-metal hydride batteries) due to the abundant interstitial sites and moderate metal-hydrogen bond strength (EM-H). They, however, have been rarely used in the hydrogen evolution reaction (HER) because of the same reason (i.e. moderate EM-H), which results in poor HER efficiency. In this study, by applying lanthanide contraction and ligand effect, we have successfully lowered the EM-H and substantially boosted the HER activity of Laves intermetallics (RECo2 and RERu0.5Co1.5 (RE = Pr, Tb, Y and Er)) to outperform those of commercial Pt/C catalyst. Hydrogen overpotential decreases from ErCo2 (η10 = 169 mV) to PrCo2 (η10 = 113 mV) and then to PrRu0.5Co1.5 (η10 = 29 mV). The expansion of lattice constants for PrCo2 may alleviate the obstacle of H atom diffusing through interstitial sites, while the inclusion of Ru element can raise the antibonding population of Co-Co/Ru bonds, which consequently lowers EM-H and thus elevates HER activity according to the Sabatier principle. This outcome indicates that the manipulation of the crystal structure and electronic structure factor is an efficient strategy to boost the HER activity of Laves intermetallics.

Synthesis, Crystal Structure, Electronic Structure, and Electrocatalytic Hydrogen Evolution Reaction of Synthetic Perryite Mineral.

Recently, it has been reported that the enstatite chondrite (EC) meteorite may contain enough hydrogen to provide a plausible explanation for water's initial existence on Earth. Perryite mineral is one of the key components of EC, but its detailed chemical composition and phase width remain elusive compared with other minerals found in EC. Therefore, we embark on a series of investigations of the synthesis, crystal structure, and electronic structure of the synthetic perryite mineral (NixFe1-x)8(TyP1-y)3 (T = Si and Ge; 1 ≥ x, y ≥ 0). Its crystal structures were established based on single-crystal and powder X-ray diffraction techniques. It is realized that its structural and phase stabilities are highly dependent on the nature of the doping element (i.e., Fe and Si). The inclusion of Si and Fe elements can greatly alter the bonding scheme near the Fermi level (Ef), which is vital to the phase stability and accounts for the chemical composition of the natural perryite mineral (quaternary compound) in EC meteorites. Furthermore, this phase exhibits good electrocatalytic activity toward the hydrogen evolution reaction (HER). The best and the worst HER performances are for the Ni8Ge2P and Ni8Si2P samples, respectively, which suggests that the long bond length and high polarity of the covalent bond are the preferred criteria to enhance the electrocatalytic HER in this series.

Lanthanide contraction regulates the HER activity of iron triad intermetallics in alkaline media.

In this work, we have systematically investigated the HER activity of the RE2Co17 (RE = Y, Pr, Gd, Tb, Ho and Er) series and revealed that their HER activities are highly correlated with the averaged Co-Co bond length of each compound. The HER performance follows the order of Gd2Co17 > Tb2Co17 > Pr2Co17 > Y2Co17 > Ho2Co17 > Er2Co17. This suggests that the unique feature of rare-earth metals, lanthanide contraction, can effectively alter the interatomic spacing and impact the corresponding HER activity. Additionally, Gd2Fe17 and Gd2Ni17 with different d electron density in the system were synthesized and comparison of their HER efficiencies is also discussed. Gd2Ni17 demonstrates the highest HER efficiency among all samples, and it only requires an overpotential (η) of 44 mV to acquire a current density of 10 mA cm-2. The theoretical calculation offers a clue that the H adsorption energy (GHad) for H atoms on Ni is lower than that on Co and Fe due to the high electron population in the antibonding state of the Ni atom. This well explains the origin of the synergistic effect for the high electrocatalytic HER of these iron triad intermetallics.