Correlation between d Electrons and the Sweet Spot for the Hydrogen Evolution Reaction: Is Platinum Always the Best Electrocatalyst?

Herein, two Laves intermetallic series, ZrCo1.75M0.25 and NbCo1.75M0.25 (M = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt), were synthesized, and their hydrogen evolution reaction (HER) activities were examined to reveal the influence of d electrons to the corresponding HER activities. Owing to the different electronegativity between Zr and Nb (χZr = 1.33; χNb = 1.60), Co and/or M elements receive more electrons in ZrCo1.75M0.25 than that of the Nb one. This leads to the overall weak H adsorption energy (ΔGHad) of ZrCo1.75M0.25 series compared to that of NbCo1.75M0.25 and rationalizes well the superior HER activity of the Rh member compared to that of the Pt one in the ZrCo1.75M0.25 series. Under industrial conditions (333 K, 6.0 M KOH), ZrCo1.75Rh0.25 only requires an overpotential of 110 mV to reach the current density of 500 mA/cm2 and can be operated at high current density over 400 h. This work demonstrates that with a proper combination between elements in intermetallic phases, one can manipulate d electrons of the active metal to be closer to the sweet spot (ΔGHad = 0). The Pt member may no longer exhibit the best HER activity in series, and all elements exhibit the potential to outperform the Pt member in the HER with careful control of the d electron population.

Direct Electrosynthesis of Metal Nanoparticles on Ti3C2Tx-Mxene during Hydrogen Evolution

Herein, we propose a simple yet effective method to deposit metal nanoparticles on Ti3C2Tx-MXene via direct electrosynthesis. Without using any reducing reagent or annealing under reducing atmosphere, it allows the conversion of metal salts (e.g., PtCl4, RuCl3·yH2O, IrCl3·zH2O, AgNO3, and CuCl2·2H2O) to metal nanoparticles with a small particle size (ca. 2 nm). Under these circumstances, it was realized that the support effect from Ti3C2Tx-MXene (electron pushing) is quite profound, in which the Ti3C2Tx-MXene support will act as an electron donor to push the electron to Pt nanoparticles and increase the electron density of Pt nanoparticles. It populates the antibonding state of Pt-Pt bonds as well as the adsorbate level that leads to a "weakening" of the ΔGH* in the optimal position. This rationalizes the outstanding activity of Pt/Ti3C2Tx-MXene (5 wt %, η10 = 16 mV) for the hydrogen evolution reaction (HER). In addition, this direct electrosynthesis method grants the growth of two or multiple types of metal nanoparticles on the Ti3C2Tx-MXene substrate that can perform dual or multiple functions as desired. For instance, one can prepare an electrocatalyst with Pt (2.5 wt %) and Ru nanoparticles (2.5 wt %) on the Ti3C2Tx-MXene support from the same synthetic method. This electrocatalyst (Pt_Ru/Ti3C2Tx-MXene) can display good electrocatalytic HER performance in both acid (0.5 M H2SO4) and alkaline electrolytes (1.0 M KOH).

Regulating Intramolecular Electron Transfer of Nickel-Based Coordinations through Ligand Engineering for Aqueous Batteries

The integration of electronic effects into complexes for the construction of novel materials has not yet attracted significant attention in the field of energy storage. In the current study, eight one-dimensional (1D) nickel-based salicylic acid  complexes (Ni-XSAs, X = pH, pMe, pMeO, mMe, pBr, pCl, pF, and pCF3 ), are prepared by ligand engineering. The coordination environments in the Ni-XSAs are explored using X-ray absorption fine structure spectroscopy. The charge transfer of the complexes is modulated according to the difference in the electron-donating ability of the substituents, in combination with frontier orbital theory. Furthermore, density functional theory is used to investigate the effect of the substituent position on the electronic properties of the complexes. Ni-mMeSA exhibits better electrical conductivity than Ni-pMeSA. The electrochemical performance of Ni-mMeSA as an aqueous battery cathode is remarkably improved with a maximum energy density of 0.30 mWh cm-2 (125 Wh kg-1 ) and a peak power density of 33.72 mW cm-2 (14.03 kW kg-1 ). This study provides ideas for the application of new coordination chemistry in the field of energy materials science.

In situ X-ray spectroscopies beyond conventional X-ray absorption spectroscopy on deciphering dynamic configuration of electrocatalysts

Realizing viable electrocatalytic processes for energy conversion/storage strongly relies on an atomic-level understanding of dynamic configurations on catalyst-electrolyte interface. X-ray absorption spectroscopy (XAS) has become an indispensable tool to in situ investigate dynamic natures of electrocatalysts but still suffers from limited energy resolution, leading to significant electronic transitions poorly resolved. Herein, we highlight advanced X-ray spectroscopies beyond conventional XAS, with emphasis on their unprecedented capabilities of deciphering key configurations of electrocatalysts. The profound complementarities of X-ray spectroscopies from various aspects are established in a probing energy-dependent "in situ spectroscopy map" for comprehensively understanding the solid-liquid interface. This perspective establishes an indispensable in situ research model for future studies and offers exciting research prospects for scientists and spectroscopists.

A step-by-step strategy to design active and stable quaternary intermetallic compounds for the hydrogen evolution reaction

Multinary intermetallic compounds with rich chemical compositions enable one to achieve a logical design for desired materials based on the required function. In this work, we have demonstrated a step-by-step strategy to design a quaternary intermetallic compound that exhibits highly active and stable performance for the hydrogen evolution reaction (HER). With binary intermetallic TaCo2 as the starting point, the minor inclusion of a ductile Cu element in TaCo2 to form ternary TaCu0.25Co1.75 can substantially lower the degradation rate from ca. 20% to 5% after sintering treatment (i.e., enhance connectivity between particles). However, the overpotential at a current density of 10 mA cm-2 (η10) increases by ca. 20 mV from TaCo2 to TaCu0.25Co1.75. Further incorporation of a HER active Ru element to cast quaternary TaCu0.125Ru0.125Co1.75 can decrease ca. 70 mV of η10 while maintaining long-term stability. This proves that one can design functional intermetallic compounds intentionally, which may be extended to different fields of application.

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.

Balance between Activity and Stability of Single Metal and Intermetallic Compounds for Electrocatalytic Hydrogen Evolution Reaction

The higher population of the antibonding state around the Fermi level will result in better activity yet lower stability of HER (Re vs Ru metal). There seems to be a limitation or balance for using a single metal since the bonding scheme of a single metal is relatively simple. Combining Re (strong bonding), Ru (HER active), and Zr metal (corrosion-resistant) grants ternary intermetallic compound ZrRe_1.75Ru₀₂₅, exhibiting excellent HER activity and stability in acidic and alkaline electrolytes. The overpotential at a current density of 10 mA/cm² (η₁₀) for ZrRe_1.75Ru₀₂₅ is much lower compared to that of ZrRe₂. Although the HER activity of ZrRe_1.75Ru₀₂₅ is not comparable to that of ZrRu₂, it demonstrates outstanding HER stability, while the current density of ZrRu₂ is over ca. 16% after 6 h. This suggests that intermetallic compounds can break the constraint between activity and stability in a single metal for HER, which may be applied in other fields as well.

Spatially and temporally understanding dynamic solid-electrolyte interfaces in carbon dioxide electroreduction

The ubiquity of solid-liquid interfaces in nature and the significant role of their atomic-scale structure in determining interfacial properties have led to intensive research. Particularly in electrocatalysis, however, a molecular-level picture that clearly describes the dynamic interfacial structures and organizations with their correlation to preferred reaction pathways in electrochemical reactions remains poorly understood. In this review, CO₂ electroreduction reaction (CO₂RR) is spatially and temporally understood as a result of intricate interactions at the interface, in which the interfacial features are highly relevant. We start with the discussion of current understandings and model development associated with the charged electrochemical interface as well as its dynamic landscape. We further highlight the interactive dynamics from the interfacial field, catalyst surface charges and various gradients in electrolyte and interfacial water structures at interfaces under CO₂RR working conditions, with emphasis on the interfacial-structure dependence of catalytic reactivity/selectivity. Significantly, a probing energy-dependent "in situ characterization map" for dynamic interfaces based on various complementary in situ/operando techniques is proposed, aiming to present a comprehensive picture of interfacial electrocatalysis and to provide a more unified research framework. Moreover, recent milestones in both experimental and theoretical aspects to establish the correct profile of electrochemical interfaces are stressed. Finally, we present key scientific challenges with related perspectives toward future opportunities for this exciting frontier.

2D MOF-assisted Pyrolysis-displacement-alloying Synthesis of High-entropy Alloy Nanoparticles Library for Efficient Electrocatalytic Hydrogen Oxidation

Multimetallic alloy nanoparticles (NPs) have received considerable attention in various applications due to their compositional variability and exceptional properties. However, the complexity of both the general synthesis and structure-activity relationships remain the long-standing challenges in this field. Herein, we report a versatile 2D MOF-assisted pyrolysis-displacement-alloying route to successfully synthesize a series of binary, ternary and even high-entropy NPs that are uniformly dispersed on porous nitrogen-doped carbon nanosheets (PNC NSs). As a proof of utility, the obtained Co0.2Ru0.7Pt0.1/PNC NSs exhibits apparent hydrogen oxidation activity and durability with a record-high mass specific kinetic current of 1.84 A mg-1 at the overpotential of 50 mV, which is approximately 11.5 times higher than that of the Pt benchmark. Both experimental and theoretical studies reveal that the addition of Pt engenders a phase transition in CoRu alloys from hexagonal close-packed (hcp) to face-centered cubic (fcc) structure. The elevated reactivity of the resulted ternary alloy can be attributed to the optimized ad-sorption of hydrogen intermediate and the decreased reaction barrier for water formation. This study opens a new avenue for the development of highly efficient alloy NPs with various compositions and functions.

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 (E_F), which induces the synergistic effect to influence the adsorption energy of the H atom (ΔG_H) 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 E_F. 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 Ni_3-xFe_xP (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., Ni_2.5Fe_0.5P 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 (ΔG_Had) 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.

Alkali metal partial substitution-induced improved second-harmonic generation and enhanced laser-induced damage threshold for Ag-based sulfides

Two new pentanary sulfides LiAgIn2GeS6 and NaAgIn2GeS6 were derived from parent Ag2In2GeS6via Li and Na partial substitution of Ag element, and their SHG efficiencies and LIDT intensities enhanced concurrently.

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 RERu_0.25Ni_1.75 (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 (PrRu_0.25Ni_1.75) 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 ScRu_0.25M_1.75 (M = Co and Ni) suggests that doping Ru element in ScCo₂ will significantly enhance antibonding character around the Fermi level (E_F) and weaken hydrogen adsorption energy. On the other hand, the antibonding population for ScNi₂ and ScRu_0.25Ni_1.75 is similar at E_F, which accounts for their close intrinsic HER activities.

Crystal and Electronic Structure Modification of Synthetic Perryite Minerals: A Facile Phase Transformation Strategy to Boost the Oxygen Evolution Reaction

Geometry effect and electronic effect are both essential for the rational design of a highly efficient electrocatalyst. In order to untangle the relationship between these effects and electrocatalytic activity, the perryite phase with a versatile chemical composition, (Ni_xFe_1-x)₈(T_yP_1-y)₃ (T = Si and Ge; 1 ≥ x, y ≥ 0), was selected as a platform to demonstrate the influence of geometry (e.g., atomic size and bond length) and electronic (e.g., bond strength and bonding scheme) factors toward the oxygen evolution reaction (OER). It was realized that the large Ge atom in the perryite phase can expand the unit cell parameters and interatomic distances (i.e., weaken bond strengths), which facilitates the phase transformation into active metal oxyhydroxide during OER. The quaternary perryite phase, Ni₇FeGeP₂, displays excellent OER activity and achieves current densities of 20 and 100 mA/cm² at overpotentials of 239 and 273 mV, respectively. The oxidation state of Ni and Fe in the perryite phase before/after OER was analyzed and discussed. The result suggests that incorporating the Fe element in the system may increase the rate constant of OER (K_OER) and therefore keeps the Ni element in a low valance state (i.e., Ni²⁺). This work indicates that the manipulation of geometry and electronic factors can promote phase transformation as well as OER activity, which exemplifies a strategy to design a promising "precatalyst" for OER.

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 (E_M-H). They, however, have been rarely used in the hydrogen evolution reaction (HER) because of the same reason (i.e. moderate E_M-H), which results in poor HER efficiency. In this study, by applying lanthanide contraction and ligand effect, we have successfully lowered the E_M-H and substantially boosted the HER activity of Laves intermetallics (RECo₂ and RERu_0.5Co_1.5 (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 PrRu_0.5Co_1.5 (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 E_M-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 (Ni_xFe_1-x)₈(T_yP_1-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 (E_f), 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 (GH_ad) 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.

Electronic structure inspired a highly robust electrocatalyst for the oxygen-evolution reaction

We demonstrated that the electronic-band structure holds the key to electrocatalytic durability towards the oxygen-evolution reaction (OER).

Intermetallic compounds with high hydrogen evolution reaction performance: a case study of a MCo2 (M = Ti, Zr, Hf and Sc) series

Noble metals (e.g., Ru, Ir and Pt) or their derivatives exhibit very appealing activity toward the hydrogen evolution reaction (HER), but their high price and low reserves impede their wide use. Herein, we propose a strategy in which, through the manipulation of crystal and electronic structure, one can convert a common metal to have a Pt-like performance for HER. To achieve this goal, a series of MCo2 (M = Ti, Zr, Hf and Sc) has been synthesized by using a rapid arc-melting method. TiCo2 exhibits comparable HER activity to that of Pt/C, for which it requires only -70 mV (V vs. RHE) to reach 10 mA cm-2 with a Tafel slope of 33 mV decade-1 in 1.0 M KOH. Moreover, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) indicate that the lower adsorption energy (ΔGH*) of H on the Co atom in TiCo2, due to the change in Co electronic state, is another key factor to account for its high HER activity. This case study offers a good illustration of how to transform a non-noble metal so it behaves like a noble metal toward HER and can potentially be applied under other conditions.

Partial substitution induced centrosymmetric to noncentrosymmetric structure transformation and promising second-order nonlinear optical properties of (K0.38Ba0.81)Ga2Se4

Novel noncentrosymmetric (K_0.38Ba_0.81)Ga₂Se₄ can be obtained from centrosymmetric BaGa₂Se₄via partial substitution, and it demonstrates promising balanced NLO properties.

Electrochemical Hydrogen Evolution Reaction Efficiently Catalyzed by Ru2 P Nanoparticles

Developing alternatives to Pt catalysts is a prerequisite to cost-effectively produce hydrogen. Herein, we demonstrate Ru₂ P nanoparticles (without any doping and modifications) as a highly efficient Pt-like catalyst for the hydrogen evolution reaction (HER) in different pH electrolytes. On transferring the hexagonal close-packed crystal structure of Ru to the orthorhombic structure of Ru₂ P, a greatly improved catalytic activity and stability toward HER is found owing to Ru-P coordination. The electronic state change originates from the P-Ru bonding structures, which accounts for the HER activity improvement compared with Ru nanoparticles. Specifically, Ru₂ P nanoparticles can drive 10 mA cm^-2 at a very low overpotential of 55 mV, only 8 mV more than Pt/C in an acidic solution; and an extremely low overpotential of approximately 50 mV is needed in alkaline solution, about 20 mV less than the Pt/C catalyst. The Volmer-Tafel mechanism is indicated on Ru₂ P nanoparticles with the typical Tafel slope of 30 mV dec^-1 of Pt metal indicating a Pt-like catalytic ability. Ru₂ P is more active in the Ru-P family as H atoms prefer to adsorb on Ru atoms rather than on the P element according to theoretical calculations. Considering the low price of Ru (20 % of Pt), anti-corrosion ability in the electrolyte, and the safe and reliable fabrication approach, the powder Ru₂ P nanoparticles make an excellent HER catalyst with great promise for large-scale water electrolysis applications.

Intricate Li-Sn Disorder in Rare-Earth Metal-Lithium Stannides. Crystal Chemistry of RE3Li4- xSn4+ x (RE = La-Nd, Sm; x < 0.3) and Eu7Li8- xSn10+ x ( x ≈ 2.0)

Reported are the syntheses, crystal structures, and electronic structures of six rare-earth metal-lithium stannides with the general formulas RE₃Li_{4- x}Sn_{4+ x} (RE = La-Nd, Sm) and Eu₇Li_{8- x}Sn_{10+ x}. These new ternary compounds have been synthesized by high-temperature reactions of the corresponding elements. Their crystal structures have been established using single-crystal X-ray diffraction methods. The RE₃Li_{4- x}Sn_{4+ x} phases crystallize in the orthorhombic body-centered space group Immm (No. 71) with the Zr₃Cu₄Si₄ structure type (Pearson code oI22), and the Eu₇Li_{8- x}Sn_{10+ x} phase crystallizes in the orthorhombic base-centered space group Cmmm (No. 65) with the Ce₇Li₈Ge₁₀ structure type (Pearson code oC50). Both structures can be consdered as part of the [RESn₂] _n[RELi₂Sn] _m homologous series, wherein the structures are intergrowths of imaginary RESn₂ (AlB₂-like structure type) and RELi₂Sn (MgAl₂Cu-like structure type) fragments. Close examination the structures indicates complex occupational Li-Sn disorder, apparently governed by the drive of the structure to achieve an optimal number of valence electrons. This conclusion based on experimental results is supported by detailed electronic structure calculations, carried out using the tight-binding linear muffin-tin orbital method.

Valence- and element-dependent water oxidation behaviors: in situ X-ray diffraction, absorption and electrochemical impedance spectroscopies

Metal oxides of the spinel family have shown great potential towards the oxygen evolution reaction (OER), but the fundamental OER mechanism of spinel oxides is still far from being completely understood, especially for the role of the metal ions. Owing to various coordinated sites of divalent/trivalent metals ions and surface conditions (morphology and defects), it is a great challenge to have a fair assessment of the electrocatalytic performance of spinel systems. Herein, we demonstrated a series of MFe₂O₄ (M = Fe, Co, Ni, Zn) with a well-controlled morphology to achieve a comprehensive study of electrocatalytic activity toward OER. By utilizing several in situ analyses, we could conclude a universal rule that the activities for OER in the metal oxide systems were determined by the occurrence of a phase transformation, and this structural transformation could work well in both crystallographic sites (T_d and O_h sites). Additionally, the divalent metal ion significantly dominated the formation of oxyhydroxide through an epitaxial relationship, which depended on the atomic arrangement at the interface of spinel and metal oxyhydroxide, while trivalent metal ions remained unchanged as a host lattice. The metal oxyhydroxide was formed during a redox reaction rather than being formed during OER. The occurrence of the redox reaction seems to accompany a remarkable increase in resistance and capacitance might result from the structural transformation from spinel to metal oxyhydroxide. We believe that the approaching strategies and information obtained in the present study can offer a guide to designing a promising electrocatalytic system towards the oxygen evolution reaction and other fields.

An Unusual Triple-Decker Variant of the Tetragonal BaAl4-Structure Type: Synthesis, Structural Characterization, and Chemical Bonding of Sr3Cd8Ge4 and Eu3Cd8Ge4

Reported are the synthesis and the crystal structures of the new ternary phases Sr₃Cd₈Ge₄ and Eu₃Cd₈Ge₄. The structures of both compounds have been established by single-crystal and powder X-ray diffraction methods. They crystallize in the tetragonal space group I4/mmm (No. 139, own structure type, Pearson symbol tI30) with Z = 2, and lattice parameters as follows: a = 4.4941(14) Å; c = 35.577(7) Å for Sr₃Cd₈Ge₄, and a = 4.4643(12) Å; c = 35.537(9) Å for Eu₃Cd₈Ge₄, respectively. The most prominent feature of the structure is the complex [Cd₂Ge] polyanionic framework, derived by unique ordering of the Cd and Ge atoms in fragments that bear resemblance to the BaAl₄ structure type. Temperature dependent DC magnetization measurements indicate that Eu₃Cd₈Ge₄ displays Curie-Weiss paramagnetic behavior with no sign of magnetic ordering in the measured range. Theoretical considerations of the electronic structure on the basis of the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method are also presented and discussed.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

We review the fundamental aspects of metal oxides, metal chalcogenides and metal pnictides as effective electrocatalysts for the oxygen evolution reaction.

The synergistic effect of a well-defined Au@Pt core-shell nanostructure toward photocatalytic hydrogen generation: interface engineering to improve the Schottky barrier and hydrogen-evolved kinetics

A well-defined co-catalyst system TiO2 nanotube-Au (core)-Pt (shell) was demonstrated to be the combination of the localized surface plasmon effect of gold and excellent proton reduction nature of platinum. Furthermore, surface engineering by the descending Fermi energies of gold and platinum was beneficial to electron transfer.

Cu3Ru6Sb8—a new ternary antimonide with a new structure type

Cu₃Ru₆Sb₈ is the first known ternary compound of the respective elements and crystallizes in its own structure type.

Heterojunction of Zinc Blende/Wurtzite in Zn1-xCdxS Solid Solution for Efficient Solar Hydrogen Generation: X-ray Absorption/Diffraction Approaches

In the past decade, inorganic semiconductors have been successfully demonstrated as light absorbers in efficient solar water splitting to generate chemical fuels. Pseudobinary semiconductors Zn1-xCdxS (0≤x≤1) have exhibited a superior photocatalytic reactivity of H2 production from splitting of water by artificial solar irradiation without any metal catalysts. However, most studies had revealed that the extremely high efficiency with an optimal content of Zn1-xCdxS solid solution was determined as a result of elevating the conduction band minimum (CBM) and the width of bandgap. In addition to corresponding band structure and bandgap, the local crystal structure should be taken into account as well to determine its photocatalytic performance. Herein, we demonstrated the correlations between the photocatalytic activity and structural properties that were first studied through synchrotron X-ray diffraction and X-ray absorption spectroscopy. The crystal structure transformed from zinc blende to coexisted phases of major zinc blende and minor wurtzite phases at a critical point. The heterojunction formed by coexistence of zinc blende and wurtzite phases in the Zn1-xCdxS solid solution can significantly improve the separation and migration of photoinduced electron-hole pairs. Besides, X-ray absorption spectra and UV-vis spectra revealed that the bandgap of the Zn0.45Cd0.55S sample extended into the region of visible light because of the incorporation of Cd element in the sample. These results provided a significant progress toward the realization of the photoelectrochemical mechanism in heterojunction between zinc blende and wurtzite phases, which can effectively separate the charge-carriers and further suppress their recombination to enhance the photocatalytic reactivity.

Synthesis and crystal structures ofRE7Zn21+xSi2–x[RE= Ce, Pr, and Nd; 0.09 (1) <x< 0.95 (1)]

The focus of this paper is on the synthesis and crystal structures of three Zn-rich compounds with the general formulaRE7Zn21+xSi2−x, whereRE= Ce [x= 0.95 (1); heptacerium docosazinc silicon], Pr [x= 0.09 (1); heptapraseodymium henicosazinc disilicon], and Nd [x= 0.53 (1); heptaneodymium docosazinc silicon]. The compounds were obtained by high-temperature reactions, using the respective elements as starting materials. The structures were determined by single-crystal X-ray diffraction. The title compounds crystalize in the orthorhombic space groupPbam(No. 55, Pearson symboloP60) and are isostructural with about a dozenRE7Zn21+xTt2−x(RE= La–Nd;Tt= Ge, Sn, and Pb) compounds previously reported by our group. The results from the present refinements confirm the previously published data onRE7Zn21+xSi2−x(RE= La and Ce;x≃ 1.45) [Maliket al.(2013).Intermetallics,36, 118–126]. Additionally, magnetic susceptibility measurements on the corresponding bulk samples show Curie–Weiss paramagnetic behavior from 5 to 300 K, consistent withRE3+ground states and local-moment magnetism due to the core 4felectrons.

Synthesis and structural characterization of RE7Zn21Tt2 (RE = La-Nd; Tt = Ge, Sn, and Pb): new structure type among the polar intermetallic phases

Reported are 11 new ternary phases with the general formula RE7Zn21Tt2 (RE = La-Nd; Tt = Ge, Sn, and Pb), synthesized from the respective elements by reactions at high temperature. Their structures, established on the basis of single-crystal and powder X-ray diffraction work, are shown to be a new structure type with the orthorhombic space group Pbam (No. 55, Pearson symbol oP60). This complex atomic arrangement features condensed polyhedra made up of Zn atoms, interspersed by Ge, Sn, or Pb atoms in trigonal-planar coordination. The structure bears resemblance with the La3Al11 and the LaRhSn2 structure types, which are compared and discussed. Temperature dependent dc magnetization measurements confirm RE(3+) ground states for all rare-earth elements, and the expected local-moment magnetism due to the partial filling of their 4f states for RE(3+) = Ce(3+), Pr(3+), and Nd(3+). Theoretical considerations of the electronic structure based on the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method are also presented: the calculations support the experimental observation of a small, but not negligible, homogeneity range in RE7Zn(21+x)Tt(2-x) (x < 0.5). The partial substitution of the tetrel atoms by the electron-poorer Zn appears to be an important attribute, leading to an optimal valence electron concentration and, thereby, to the overall electronic stability of the crystal structure of this family of polar intermetallics.

New polar intermetallic phases RE2Zn5Tt (RE = La-Nd; Tt = Sn and Pb): synthesis, structure, chemical bonding, and magnetic properties

Reported are the synthesis, crystal structure, electronic structure, and magnetic properties of a series of zinc-rich ternary phases with formulas RE2Zn5Tt (RE = La-Nd; Tt = Sn and Pb). The structures of these compounds have been established by single-crystal and powder X-ray diffraction. They crystallize in the orthorhombic space group Cmcm (No. 63, LaRhSn2 structure type, Pearson symbol oC32). The most prominent structural feature is the trigonal-planar coordination of the Sn(Pb) atoms; the latter interconnect layers of Zn atoms to comprise a complex [Zn5Tt] polyanionic framework. The structural relationships between the structure of the title compounds and the EuIn4, La3Al11, and YIrGe2 structure types are highlighted. Temperature-dependent DC magnetization measurements indicate Pauli-like paramagnetism for La2Zn5Sn, while Ce2Zn5Sn, Pr2Zn5Sn, and Nd2Zn5Sn display Curie-Weiss behavior in the high-temperature regime. At cryogenic temperatures, the magnetic responses of Ce2Zn5Sn, Pr2Zn5Sn, and Nd2Zn5Sn appear to deviate from the Curie-Weiss law; however, no magnetic orderings could be observed down to 5 K. Theoretical considerations of the electronic structure on the basis of the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method are also presented and discussed.

Synthesis, crystal structures and chemical bonding of RE5−xLixGe4(RE = Nd, Sm and Gd;x≃ 1) with the orthorhombic Gd5Si4type

The syntheses and single-crystal and electronic structures of three new ternary lithium rare earth germanides, RE5−xLixGe4(RE = Nd, Sm and Gd;x≃ 1), namely tetrasamarium lithium tetragermanide (Sm3.97Li1.03Ge4), tetraneodymium lithium tetragermanide (Nd3.97Li1.03Ge4) and tetragadolinium lithium tetragermanide (Gd3.96Li1.03Ge4), are reported. All three compounds crystallize in the orthorhombic space groupPnmaand adopt the Gd5Si4structure type (Pearson codeoP36). There are six atoms in the asymmetric unit: Li1 in Wyckoff site 4c, RE1 in 8d, RE2 in 8d, Ge1 in 8d, Ge2 in 4cand Ge3 in 4c. One of the RE sites,i.e.RE2, is statistically occupied by RE and Li atoms, accounting for the small deviation from ideal RE4LiGe4stoichiometry.