Developing highly efficient oxygen evolution reaction (OER) catalysts and understanding their activity are pivotal for electrochemical conversion technologies. Here, we report NiFe Prussian blue analogue (PBA) as a promising electrocatalyst for OER in alkaline conditions. This material has an impressively low overpotential of 258 mV that reaches a current density of 10 mA cm. Post-mortem characterization showed that the as-prepared catalyst is entirely transformed into amorphous nickel hydroxide after the electrochemical treatment, and Ni(OH) acts as the active species. Operando X-ray spectroscopic studies further found that this in situ generated Ni(OH) displays an unique feature that allows deprotonation under applied potential creating NiOOH that contains Ni ions. The deprotonation reaction is reversible and potential-dependent, i.e., the amount of Ni increases with increasing applied potential. Theoretical calculations were used to show that the role of Ni is to trigger oxidized oxygen ions as electrophilic centers with the subsequent activation of anion redox reactions for OER.

Operando XAS combined with DFT calculations allows us to draw a phase diagram of the surface chemical state as a function of applied potential, showing hydroxyl filling process and potential-dependent deprotonation process.

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received widespread attention during the past decades. It enables direct conversion between heat and electricity which has been used in space exploration, microelectronics, automobiles and so on. [1] However, the widespread application of thermoelectric devices in daily life is severely limited by the low thermoelectric conversion efficiency, which is mainly related to the dimensionless figure of merit ZT = α 2 σT/κ, where α, σ, and T are the Seebeck coefficient, electrical conductivity, absolute temperature, respectively and thermal conductivity κ is composed of electronic (κ e ) and lattice (κ l ) contributions. [2] Among the developed TE materials with high ZT (≥1), the V 2 VI 3 (V represents Group V elements Sb and Bi, and VI is Group VI elements S, Se and Te) compounds and derivatives have been intensively investigated for a long history since 1950s, which are widely considered as the benchmark of high performance TE materials in low-to mid-temperature range. [3] The outstanding TE performance of V 2 VI 3 materials represented by Sb 2 Te 3 , Bi 2 Te 3 , and Bi 2 Se 3 can be ascribed to its high power factor (PF) due to the suitable carrier concentration and low κ derived from the layered structure weakly bonded by van der Waals interaction. [3] Optimized by isoelectronic extrinsic doping, much improved TE performance with the maximum ZT around 1 has been The (Bi,Sb) 2 Te 3 (BST) compounds have long been considered as the benchmark of thermoelectric (TE) materials near room temperature especially for refrigeration. However, their unsatisfactory TE performances in wide-temperature range severely restrict the large-scale applications for power generation. Here, using a self-assembly protocol to deliver a homogeneous dispersion of 2D inclusion in matrix, the first evidence is shown that incorporation of MXene (Ti 3 C 2 T x ) into BST can simultaneously achieve the improved power factor and greatly reduced thermal conductivity. The oxygen-terminated Ti 3 C 2 T x with proper work function leads to highly increased electrical conductivity via hole injection and retained Seebeck coefficient due to the energy barrier scattering. Meanwhile, the alignment of Ti 3 C 2 T x with the layered structure significantly suppresses the phonon transport, resulting in higher interfacial thermal resistance. Accordingly, a peak ZT of up to 1.3 and an average ZT value of 1.23 from 300 to 475 K are realized for the 1 vol% Ti 3 C 2 T x /BST composite. Combined with the high-performance composite and rational device design, a record-high thermoelectric conversion efficiency of up to 7.8% is obtained under a temperature gradient of 237 K. These findings provide a robust and scalable protocol to incorporate MXene as a versatile 2D inclusion for improving the overall performance of TE materials toward high energy-conversion efficiency.