Porous carbons: Structure-oriented design and versatile applications

Carbon-based electrocatalysts for sustainable energy applications

Molybdenum disulfide (MoS2) is a promising nonprecious catalyst for the hydrogen evolution reaction (HER) that has been extensively studied due to its excellent performance, but the lack of understanding of the factors that impact its catalytic activity hinders further design and enhancement of MoS2-based electrocatalysts. Here, by using novel porous (holey) metallic 1T phase MoS2 nanosheets synthesized by a liquid-ammonia-assisted lithiation route, we systematically investigated the contributions of crystal structure (phase), edges, and sulfur vacancies (S-vacancies) to the catalytic activity toward HER from five representative MoS2 nanosheet samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated porous 2H phase. Superior HER catalytic activity was achieved in the porous 1T phase MoS2 nanosheets that have even more edges and S-vacancies than conventional 1T phase MoS2. A comparative study revealed that the phase serves as the key role in determining the HER performance, as 1T phase MoS2 always outperforms the corresponding 2H phase MoS2 samples, and that both edges and S-vacancies also contribute significantly to the catalytic activity in porous MoS2 samples. Then, using combined defect characterization techniques of electron spin resonance spectroscopy and positron annihilation lifetime spectroscopy to quantify the S-vacancies, the contributions of each factor were individually elucidated. This study presents new insights and opens up new avenues for designing electrocatalysts based on MoS2 or other layered materials with enhanced HER performance.

(Invited) Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets

Heterostructured Mo2 C-MoOx on carbon cloth (Mo2 C-MoOx /CC), as a model of easily oxidized electrocatalysts under ambient conditions, is investigated to uncover surface reconfiguration during the hydrogen evolution reaction (HER). Raman spectroscopy combined with electrochemical tests demonstrates that the MoVI oxides on the surface are in situ reduced to MoIV , accomplishing promoted HER in acidic condition. As indicated by density functional theoretical calculations, the in situ reduced surface with terminal Mo=O moieties can effectively bring the negative ΔGH* on bare Mo2 C close to a thermodynamic neutral value, addressing difficult H* desorption toward fast HER kinetics. The optimized Mo2 C-MoOx /CC only requires a low overpotential (η10 ) of 60 mV at -10 mA cm-2 in 1.0 m HClO4 , outperforming Mo2 C/CC and most non-precious electrocatalysts. In situ surface reconfiguration are shown on W2 C-WOx , highlighting the significance to boost various metal-carbides and to identify active sites.

Molybdenum Carbide-Oxide Heterostructures: In Situ Surface Reconfiguration toward Efficient Electrocatalytic Hydrogen Evolution.

Water electrolysis is a promising approach for large-scale and sustainable hydrogen production; however, its kinetics is slow and requires precious metal electrocatalysts to efficiently operate. Therefore, great efforts are being undertaken to design and prepare low-cost and highly efficient electrocatalysts to boost the hydrogen evolution reaction (HER). This is because traditional transition-metal electrocatalysts and corresponding hybrids with nonmetal atoms rely mainly on the interaction of metal–H bonds for the HER, which inevitably suffers from corrosion in extreme acidic and alkaline solutions. And as a result of all this effort, novel nanostructured electrocatalysts, such as carbon-encapsulated precious metals and non-precious metals including single metals or their alloys, transition-metal carbides, phosphides, oxides, sulfides, and selenides have all been recently reported to exhibit good catalytic activities and stabilities for hydrogen evolution. Here, the catalytic activity is thought to originate from the electron penetration effect of the inner metals to the surface carbon, which can alter the Gibbs free energy of hydrogen adsorption on the surface of materials. In this review, recent progresses of carbon-encapsulated materials for the HER are summarized, with a focus on the unique effects of carbon shells. In addition, perspectives on the future development of carbon-coated electrocatalysts for the HER are provided. Carbon-encapsulated electrocatalysts, such as carbon-encapsulated precious metals and non-precious metals (single metals or their alloys, metal carbides, phosphides, oxides, sulfides, and selenides), are emerging as promising candidates for water splitting. In this review, recent progresses in carbon-encapsulated electrocatalysts for hydrogen evolution are reviewed, especially the unique effects of carbon shells.

Jiahui Zhu

Papers

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