The role of high-resolution transmission electron microscopy and aberration corrected scanning transmission electron microscopy in unraveling the structure-property relationships of Pt-based fuel cells electrocatalysts

Platinum-based fuel cell electrocatalysts are structured on a nano level in order to extend their active surface area and maximize the utilization of precious and scarce platinum. Their performance is dictated by the atomic arrangement of their surface layers atoms via structure-property relationships. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are the preferred methods for characterizing these catalysts, due to their capacity to achieve local atomic-level resolutions. Size, morphology, strain and local composition are just some of the properties of Pt-based nanostructures that can be obtained by (S)TEM. Furthermore, advanced methods of (S)TEM are able to provide insights into the quasi-in situ, in situ or even operando stability of these nanostructures. In this review, we present state-of-the-art applications of (S)TEM in the investigation and interpretation of structure-activity and structure-stability relationships.

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.

Nonmetallic Active Sites on Nickel Phosphide in Oxygen Evolution Reaction

Efficient and durable catalysts are crucial for the oxygen evolution reaction (OER). The discovery of the high OER catalytic activity in Ni12P5 has attracted a great deal of attention recently. Herein, the microscopic mechanism of OER on the surface of Ni12P5 is studied using density functional theory calculations (DFT) and ab initio molecular dynamics simulation (AIMD). Our results demonstrate that the H2O molecule is preferentially adsorbed on the P atom instead of on the Ni atom, indicating that the nonmetallic P atom is the active site of the OER reaction. AIMD simulations show that the dissociation of H from the H2O molecule takes place in steps; the hydrogen bond changes from Oa-H⋯Ob to Oa⋯H-Ob, then the hydrogen bond breaks and an H+ is dissociated. In the OER reaction on nickel phosphides, the rate-determining step is the formation of the OOH group and the overpotential of Ni12P5 is the lowest, thus showing enhanced catalytic activity over other nickel phosphides. Moreover, we found that the charge of Ni and P sites has a linear relationship with the adsorption energy of OH and O, which can be utilized to optimize the OER catalyst.

Atomically dispersed Pd catalysts promote the oxygen evolution reaction in acidic media

A Pd-doped Pt₃Sn-based single atom alloy catalyst (Pd-Pt₃Sn) was synthesized via a hydrothermal method. The overpotential of Pd-Pt₃Sn is lower than that of commercial Pd/C and IrO₂ catalysts at 10 mA cm^-2. This is due to the synergistic effect between Pt, Sn and Pd and the influence of electronic effects on their catalytic performance.

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.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

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.