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.

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.

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.

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.

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 ZrRe1.75Ru025, exhibiting excellent HER activity and stability in acidic and alkaline electrolytes. The overpotential at a current density of 10 mA/cm2 (η10) for ZrRe1.75Ru025 is much lower compared to that of ZrRe2. Although the HER activity of ZrRe1.75Ru025 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.

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.

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 Co0.2 Ru0.7 Pt0.1 /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 adsorption 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 (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 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, (NixFe1-x)8(TyP1-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 (KOER) 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 (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.

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). Density functional theory (DFT) revealed the characteristic of Ni-Ni bonding interactions within Ni5P4, Ni5P2 and Ni3P were different and could influence their phase stabilities during the OER. Ni5P2 and Ni3P exhibited very robust OER performances at high current density (>350 mA cm-2) over 12 h whereas, for Ni5P4, obvious deterioration was observed. In situ/ex situ X-ray near-edge structure, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy indicated the phase stability of Ni5P4, Ni5P2 and Ni3P behaved differently during the OER. These materials transformed to Ni oxyhydroxide but the process for Ni5P2 and Ni3P was much slower even under high anodic potential 1.6 V (V vs. RHE). These results supported the theoretical prediction and provide a refreshing viewpoint for designing reliable electrocatalysts for the 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.

Partial substitution of Ba by K in the Ba sites of BaGa2Se4 creates a new selenide (K0.38Ba0.81)Ga2Se4 (1), which is crystallized in the noncentrosymmetric space group I4cm, different from the centric structures of all the ternary MI-MIII2-Q4 (MII = divalent Sr, Ba, Pb, Sn, Eu; MIII = trivalent Ga, In; Q = S and Se) chalcogenides. Its 1D structure features {[GaSe2]-}∞ chains, and K/Ba occupying the interchain cavities as the counter cations. Its powder sample demonstrates a second-harmonic generation intensity which is around 0.9 times that of AgGaS2 at 2.1 µm, and a laser-induced damage threshold which is 13.4 times that of AgGaS2. DFT calculations on its electronic structure and optical properties were also performed.