Converting surface-oxidized cobalt phosphides into Co2(P2O7)-CoP heterostructures for efficient electrocatalytic hydrogen evolution.

Exploring noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) is a key issue in a hydrogen economy blueprint. As one of the promising candidates, transition metal phosphides unfortunately suffer from inevitable surface oxidation which obstructs active-site exposure. Herein, a facile reduction followed by a surface phosphorization is introduced to convert surface-oxidized cobalt phosphides to a Co2(P2O7)-CoP heterostructure embedded in N-doped carbon (Co2(P2O7)-CoP/NC), accomplishing an efficient HER in both acidic and alkaline electrolytes. It affords low overpotentials (η 10) of 88 and 97 mV to reach a current density of -10 mA cm-2, and small Tafel slopes of 51 and 61 mV dec-1 in 0.5 M H2SO4 and 1.0 M KOH, respectively, outperforming the parent surface-oxidized Co2P and most previously-reported Pt-free electrocatalysts. The remarkably improved electrocatalysis should be ascribed to the strong surface acidity of the Co2(P2O7) component and thereby the promoted HER kinetics on Co2(P2O7)-CoP interfaces. This work will encourage the development of cost-efficient electrocatalysts via surface engineering.

Revealing Facet Effects of Palladium Nanocrystals on Electrochemical Biosensing.

Noble-metal nanocrystals (NCs) are functional segments of biosensing platforms, but their sensitivity and facet effects are still challenging. Conventional synthesis using surfactants to direct crystal growth unfortunately causes adsorbate-surface hindrance, which not only reduces sensing responses but also leads to misunderstanding on facet-dependence. Herein, we utilize electrochemical CO displacement to remove residual surfactants from facet-engineered Pd NCs, and further investigate the structure-activity relationship on specific facets, for example, {100} in cubes, {111} in octahedrons, and {110} in rhombic dodecahedrons. Along with the remarkably boosted response, facet dependence is obvious for H2O2 sensing after surface cleaning. The Pd{100} shows high sensitivity, low detection limit, and wide applicable concentration range, superior to the {110} and {111}. This can be theoretically interpreted by the befitting *OH binding on {100} and thereby the facilitated H2O2 reduction kinetics. The outstanding selectivity to H2O2 ensures the high efficiency of Pd NCs to measure intracellular H2O2 and recognize different types of cancer cells. Moreover, facet effects are also evidenced in glucose detection, highlighting that this work can provide guidelines to design efficient sensing platforms.

Pd-Ag Alloy Electrocatalysts for CO2 Reduction: Composition Tuning to Break the Scaling Relationship.

Constructing solid-solution-alloy electrocatalysts with tunable surface electronic configurations is the key to optimize intermediate bindings and thereby to promote the activity and selectivity of the CO2 reduction reaction (CO2RR). Herein, Pd1-xAgx alloy electrocatalysts are investigated as a platform to uncover the electronic effects on the CO2RR. The optimal Pd0.75Ag0.25/C affords a superior CO Faradaic efficiency of 95.3% at -0.6 V (vs RHE) in 0.5 M KHCO3, performing at a high level among recently reported electrocatalysts. Experimental and theoretical analysis further evidence that varying the composition of Pd1-xAgx alloys can effectively alter the electronic configurations and consequently break the inherent scaling relationship of the binding energy of different intermediates (*COOH and *CO). Among Pd1-xAgx, Pd0.75Ag0.25 gains the obviously weakened *CO and *H bindings but retained well the binding with *COOH, contributing to the facilitated kinetics toward CO product. Elucidating a feasible way to break the scaling relationship and further uncover the underlying mechanism, this work will inspire new design strategies toward active and selective electrocatalysts.