Lithium-Mediated Ammonia Electrosynthesis with Ether-Based Electrolytes.

Ammonia is of great importance in fertilizer production and chemical synthesis. It can also potentially serve as a carbon-free energy carrier for a future hydrogen economy. Motivated by a worldwide effort to lower carbon emissions, ammonia synthesis by lithium-mediated electrochemical nitrogen reduction (LiNR) has been considered as a promising alternative to the Haber-Bosch process. A significant performance improvement in LiNR has been achieved in recent years by exploration of favorable lithium salt and proton donor for the electrolyte recipe, but the solvent study is still in its infancy. In this work, a systematic investigation on ether-based solvents toward LiNR is conducted. The assessments of solvent candidates are built on their conductivity, parasitic reactions, product distribution, and faradaic efficiency. Notably, dimethoxyethane gives the lowest potential loss among the investigated systems, while tetrahydrofuran achieves an outstanding faradaic efficiency of 58.5 ± 6.1% at an ambient pressure. We found that solvent molecules impact the above characteristics by dictating the solvation configurations of conductive ions and inducing the formation of solid electrolyte interphase with different compositions. This study highlights the importance of solvents in the LiNR process and advances the electrolyte optimization for better performance.

Ultrafine Core@Shell Cu1Au1@Cu1Pd3 Nanodots Synergized with 3D Porous N-Doped Graphene Nanosheets as a High-Performance Multifunctional Electrocatalyst.

Rationally combining designed supports and metal-based nanomaterials is effective to synergize their respective physicochemical and electrochemical properties for developing highly active and stable/durable electrocatalysts. Accordingly, in this work, sub-5 nm and monodispersed nanodots (NDs) with the special nanostructure of an ultrafine Cu1Au1 core and a 2-3-atomic-layer Cu1Pd3 shell are synthesized by a facile solvothermal method, which are further evenly and firmly anchored onto 3D porous N-doped graphene nanosheets (NGS) via a simple annealing (A) process. The as-obtained Cu1Au1@Cu1Pd3 NDs/NGS-A exhibits exceptional electrocatalytic activity and noble-metal utilization toward the alkaline oxygen reduction, methanol oxidation, and ethanol oxidation reactions, showing dozens-fold enhancements compared with commercial Pd/C and Pt/C. Besides, it also has excellent long-term electrochemical stability and electrocatalytic durability. Advanced and comprehensive experimental and theoretical analyses unveil the synthetic mechanism of the special core@shell nanostructure and further reveal the origins of the significantly enhanced electrocatalytic performance: (1) the prominent structural properties of NGS, (2) the ultrasmall and monodispersed size as well as the highly uniform morphology of the NDs-A, (3) the special Cu-Au-Pd alloy nanostructure with an ultrafine core and a subnanometer shell, and (4) the strong metal-support interaction. This work not only develops a facile method for fabricating the special metal-based ultrafine-core@ultrathin-shell nanostructure but also proposes an effective and practical design paradigm of comprehensively and rationally considering both supports and metal-based nanomaterials for realizing high-performance multifunctional electrocatalysts, which can be further expanded to other supports and metal-based nanomaterials for other energy-conversion or environmental (electro)catalytic applications.

Electronic and Potential Synergistic Effects of Surface-Doped P-O Species on Uniform Pd Nanospheres: Breaking the Linear Scaling Relationship toward Electrochemical Oxygen Reduction.

Developing efficient oxygen reduction reaction (ORR) electrocatalysts is critical to fuel cells and metal-oxygen batteries, but also greatly hindered by the limited Pt resources and the long-standing linear scaling relationship (LSR). In this study, ∼6 nm and highly uniform Pd nanospheres (NSs) having surface-doped (SD) P-O species are synthesized and evenly anchored onto carbon blacks, which are further simply heat-treated (HT). Under alkaline conditions, Pd/SDP-O NSs/C-HT exhibits respective 8.7 (4.3)- and 5.0 (5.5)-fold enhancements in noble-metal-mass- and area-specific activity (NM-MSA and ASA) compared with the commercial Pd/C (Pt/C). It also possesses an improved electrochemical stability. Besides, its acidic ASA and NM-MSA are 2.9 and 5.1 times those of the commercial Pd/C, respectively, and reach 65.4 and 51.5% of those of the commercial Pt/C. Moreover, it also shows nearly ideal 4-electron ORR pathways under both alkaline and acidic conditions. The detailed experimental and theoretical analyses reveal the following: (1) The electronic effect induced by the P-O species can downshift the surface d-band center to weaken the intermediate adsorptions, thus preserving more surface active sites. (2) More importantly, the potential hydrogen bond between the O atom in the P-O species and the H atom in the hydrogen-containing intermediates can in turn stabilize their adsorptions, thus breaking the ORR LSR toward more efficient ORRs and 4-electron pathways. This study develops a low-cost and high-performance ORR electrocatalyst and proposes a promising strategy for breaking the ORR LSR, which may be further applied in other electrocatalysis.

Potential-Dependent Mechanistic Study of Ethanol Electro-oxidation on Palladium.

We herein used the density functional theory (DFT) method and the implicit continuum solvation model to study the potential-dependent mechanism of ethanol oxidation reaction (EOR) on palladium (Pd). Energy evolutions of the EOR on low-index Pd surfaces, including (111), (110), and (100), were obtained as a function of the electrode potential. Moreover, the onset potentials for key intermediates and products were calculated. In addition, the potential range for adsorbed ethanol as the most stable adsorption state for proceeding the EOR was determined to be between 0.15 and 0.78 V via the calculated Pourbaix diagrams when considering hydrogen underpotential deposition and Pd(II) oxide formation as competing reactions. Specifically, the behavior of Pd(111) as the dominating facet decided the overall activity of the EOR with onset potentials to acidic acid/acetate at 0.40 V, to carbon dioxide at 0.71 V, and to oxide formation at 0.78 V. Pd(110) was predicted to exhibit the optimal activity toward the EOR with the lowest onset potentials to both the first dehydrogenation process and carbon dioxide at 0.08 and 0.60 V, respectively. A computational potential-dependent mechanism of the EOR was proposed, which agrees well with the experimental curve of linear sweeping voltammetry on the commercial Pd/C electrocatalyst. Our study suggests that targeted control of products can be tuned with proper overpotential and thus provides a foundation for the future development of EOR electrocatalysts.

Promoting Effects of Au Submonolayer Shells on Structure-Designed Cu-Pd/Ir Nanospheres: Greatly Enhanced Activity and Durability for Alkaline Ethanol Electro-Oxidation.

Rationally engineering the surface physicochemical properties of nanomaterials can improve their activity and durability for various electrocatalytic and energy conversion applications. Cu-Pd/Ir (CPI) nanospheres (NSs) anchored on N-doped porous graphene (NPG) [(CPI NSs/NPG)] have been recently demonstrated as a promising electrocatalyst for the alkaline ethanol oxidation reaction (EOR); to further enhance their electrocatalytic performance, the NPG-supported CPI NSs are coated with Au submonolayer (SML) shells (SMSs), through which their surface physicochemical properties can be tuned. CPI NSs/NPG is prepared by our previously developed method and possesses the special structures of composition-graded Cu1Pd1 and surface-doped Ir0.03. The Au SMSs with designed surface coverages are formed via an electrochemical technology involving incomplete Cu underpotential deposition (UPD) and Au3+ galvanic replacement. A distinctive volcano-type relation between the EOR electrocatalytic activity and the Au-SMS surface coverage for CPI@AuSML NSs/NPG is revealed, and the optimal CPI@Au1/6ML NSs/NPG greatly surpasses commercial Pd/C and CPI NSs/NPG in electrocatalytic activity and noble metal utilization. More importantly, its electrocatalytic durability in 1 h chronoamperometric and 500-cycle potential cycling degradation tests is also significantly improved. According to detailed physicochemical characterizations, electrochemical analyses, and density functional theory calculations, the promoting effects of the Au SMS for enhancing the EOR electrocatalytic activity and durability of CPI NSs/NPG can be mainly attributed to the greatly weakened carbonaceous intermediate bonding and properly increased surface oxidation potential. This work also proposes a versatile and effective strategy to tune the surface physicochemical properties of metal-based nanomaterials via incomplete UPD and metal-cation galvanic replacement for advancing their electrocatalytic and energy conversion performance.