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

Interfacial Design of Ti3C2Tx MXene/Graphene Heterostructures Boosted Ru Nanoclusters with High Activity Toward Hydrogen Evolution Reaction

The development of a cost-competitive and efficient electrocatalyst is both attractive and challenging for hydrogen production by hydrogen evolution reaction (HER). Herein, a facile glycol reduction method to construct Ru nanoclusters coupled with hierarchical exfoliated-MXene/reduced graphene oxide architectures (Ru-E-MXene/rGA) is reported. The hierarchical structure, formed by the self-assembly of graphene oxides, can effectively prohibit the self-stacking of MXene nanosheets. Meanwhile, the formation of the MXene/rGA interface can strongly trap the Ru3+ ions, resulting in the uniform distribution of Ru nanoclusters within Ru-E-MXene/rGA. The boosted catalytic activity and underlying catalytic mechanism during the HER process are proved by density functional theory. Ru-E-MXene/rGA exhibits overpotentials of 42 and 62 mV at 10 mA cm-2 in alkaline and acidic electrolytes, respectively. The small Tafel slope and charge transfer resistance (Rct) values elucidate its fast dynamic behavior. The cyclic voltammetry (CV) curves and chronoamperometry test confirm the high stability of Ru-E-MXene/rGA. These results demonstrate that coupling Ru nanoclusters with the MXene/rGA heterostructure represents an efficient strategy for constructing MXene-based catalysts with enhanced HER activity.

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).

Platinum-palladium-on-reduced graphene oxide as bifunctional electrocatalysts for highly active and stable hydrogen evolution and methanol oxidation reaction

In the context of the gradual depletion of global fossil fuel resources, it is increasingly necessary to explore new alternative energy. Hydrogen energy has attracted great interest from researchers because of its green and pollution-free characteristics. Moreover, the methanol oxidation reaction (MOR) can combine the hydrogen evolution reaction (HER), replacing the anode reaction (oxygen evolution reaction-OER) in overall water splitting and efficiently producing hydrogen. In this study, platinum-palladium nanoparticles on reduced graphene oxide (PtPd/rGO) were successfully synthesized as HER and MOR bifunctional electrocatalysts under alkaline conditions by the stepwise loading of Pt and Pd bimetallic nanoparticles on rGO using a simple liquid-phase reduction method. PtPd/rGO-2 with 0.99 wt% Pt and 2.86 wt% Pd in the HER has the lowest overpotential (87.16 mV at 100 mA cm-2), with the smallest Tafel slope (18.9 mV dec-1). The exceptional mass activity of PtPd/rGO-2 in the MOR reaches 10.75 A mg-1PtPd, which is 18.22 and 53.75 times greater than that of commercial Pt/C (Pt/C) and commercial Pd/C (Pd/C), respectively. PtPd/rGO-2 is 0.935 V lower in the coupling reaction of HER and MOR (MOR ∥ HER) compared to the overall water splitting (OER ∥ HER) without methanol (10 mA cm-2). This is probably because appropriate Pt and Pd loading exposes many more catalytic sites, and the synergistic interaction between Pt, Pd, and Pt-Pd enhances the catalytic performance. This strategy can be used for the synthesis of novel bifunctional electrocatalysts.

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.

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.

Fe Engineering on Ru Nanosheets for Enhanced Hydrogen Evolution in pH-Universal Media

Fe-modified Ru nanosheets are achieved via preintercalated Al species serving as the self-sacrificial template. Benefiting from the amphoteric feature of Al and strong corrosion of Fe³⁺ ions, Fe is effectively incorporated into pristine Ru nanosheets. Correspondingly, the surface oxophilicity is improved, promoting the Volmer step. The charge density redistribution weakens hydrogen combination on Ru and thus accelerates the desorption kinetics (Heyrovsky step). Meanwhile, more defective sites are exposed, leading to an enhanced hydrogen production in pH-universal electrolytes.