Hollow waxberry-like cobalt–nickel oxide/S,N-codoped carbon nanospheres as a trifunctional electrocatalyst for OER, ORR, and HER

Transition metal oxides with perovskite and spinel structures for electrochemical energy production applications

Transition metal oxide-based materials are an interesting alternative to substitute noble-metal based catalyst in energy conversion devices designed for oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution reactions (HER). Perovskite (ABO₃) and spinel (AB₂O₄) oxides stand out against other structures due to the possibility of tailoring their chemical composition and, consequently, their properties. Particularly, the electrocatalytic performance of these materials depends on features such as chemical composition, crystal structure, nanostructure, cation substitution level, e_g orbital filling or oxygen vacancies. However, they suffer from low electrical conductivity and surface area, which affects the catalytic response. To mitigate these drawbacks, they have been combined with carbon materials (e.g. carbon black, carbon nanotubes, activated carbon, and graphene) that positively influence the overall catalytic activity. This review provides an overview on tunable perovskites (mainly lanthanum-based) and spinels featuring 3d metal cations such as Mn, Fe, Co, Ni and Cu on octahedral sites, which are known to be active for the electrochemical energy conversion.

Hydrogen-etched CoS2 to produce a Co9S8@CoS2 heterostructure electrocatalyst for highly efficient oxygen evolution reaction

There is a pressing requirement for developing high-efficiency non-noble metal electrocatalysts in oxygen evolution reactions (OER), where transition metal sulfides are considered to be promising electrocatalysts for the OER in alkaline medium. Herein, we report the outstanding OER performance of Co₉S₈@CoS₂ heterojunctions synthesized by hydrogen etched CoS₂, where the optimized heterojunction shows a low η₅₀ of 396 mV and a small Tafel slope of 181.61 mV dec⁻¹. The excellent electrocatalytic performance of this heterostructure is attributed to the interface electronic effect. Importantly, the post-stage characterization results indicate that the Co₉S₈@CoS₂ heterostructure exhibits a dynamic reconfiguration during the OER with the formation of CoOOH in situ, and thus exhibits a superior electrocatalytic performance.

Herein, we report the outstanding OER performance of Co₉S₈@CoS₂ heterojunctions synthesized by hydrogen etched CoS₂, where the optimized heterojunction shows a low η₅₀ of 396 mV and a small Tafel slope of 181.61 mV dec⁻¹.

Boosting the hydrogen evolution reaction activity of Ru in alkaline and neutral media by accelerating water dissociation

Electrochemical water splitting via a cathodic hydrogen evolution reaction (HER) is an advanced technology for clean H2 generation. Ru nanoparticle is a promising candidate for the state-of-the-art Pt catalyst; however, they still lack the competitiveness of Pt in alkaline and neutral media. Herein, a ternary HER electrocatalyst involving nano Ru and Cr2O3 as well as N-doped graphene (NG) that can work in alkaline and neutral media is proposed. Cr2O3 and NG feature strong binding energies for hydroxyl and hydrogen, respectively, which can accelerate the dissociation of water, whereas Ru has weak hydrogen binding energy to stimulate hydrogen coupling. The HER activity of Ru is greatly enhanced by the promoted water-dissociation effect of NG and Cr2O3. To achieve a current density of 10 mA cm-2, the as-obtained Ru-Cr2O3/NG only needs a very low overpotential of 47 mV, which outperforms the activity of Pt/C in alkaline media. The strategy proposed here, multi-site acceleration of water dissociation, provides new guidance on the design of a highly efficient, inexpensive, and biocompatible HER catalyst in nonacidic condition.

Hybrid OER Electrocatalyst Combining Mesoporous Hollow Spheres of N, P-Doped Carbon with Ultrafine Co2NiOx

A heterostructure combining mesoporous hollow spheres of N, P-doped carbon (meso-NPC) with ultrafine Co₂NiO_x nanoparticles has been synthesized as a highly active electrocatalyst, named meso-NPC/Co₂NiO_x. The meso-NPC hollow spheres were first fabricated via a novel nanocasting method using mesoporous SiO₂ as the "mold" and the atomic ratio of P in the meso-NPC can be tuned by controlling the amount of one of the filling organics, etidronic acid. Because of the large surface area and abundant surface hydroxyls, the meso-NPC formed strong bonding with the Co₂NiO_x nanoparticles (<2 nm in size) loaded on it. Serving as an OER electrocatalyst, the heterogeneous meso-NPC/Co₂NiO_x shows great working enhancement compared to its single-component counterparts. Through further mechanism study by X-ray photoelectron spectroscopy, a strong effect of electron transfer is found from the Co₂NiO_x to the meso-NPC, which leads to increases in the oxidation state of transition metals (TMs). Most importantly, we also reveal that the increase in the amount of the P dopant in the meso-NPC/Co₂NiO_x system can efficiently facilitate this metal-to-support charge transfer, which also implies that the biphasic interaction between the Co₂NiO_x nanoparticles and the meso-NPC hollow spheres generates active catalytic sites of both TM-N-C and TM-P-C surface species.