The recent advances and new insights resulting thereof in applying defect engineering to improving the thermoelectric performance and mechanical properties of inorganic materials are reviewed.

The rational design and synthesis of nonprecious, efficient, and stable electrocatalysts to replace precious noble metals are crucial to the future of hydrogen economy. Herein, a partial sulfurization/phosphorization strategy is proposed to synthesize a nonstoichiometric pyrrhotite-type cobalt monophosphosulfide material (CoSP) with a hexagonal close-packed phase for electrocatalytic water splitting. By regulating the degree of sulfurization, the P/S atomic ratio in the cobalt monophosphosulfide can be tuned to activate the Co/Co couples. The synergy between the nonstoichiometric nature and the tunable P/S ratio results in the strengthened Co/Co couples and tunable electronic structure and thus efficiently promotes the oxygen/hydrogen evolution reaction (OER/HER) processes toward overall water splitting. Especially for OER, the CoSP material, featured with a uniform yolk-shell spherical morphology, shows a low overpotential of 266 mV at 10 mA cm (η) with a low Tafel slope of 48 mV dec as well as high stability, which is comparable to that of the reported promising OER electrocatalysts. Coupled with the high HER activity of CoSP, the overall water splitting is demonstrated with a low η at 1.59 V and good stability. This study shows that phase engineering and composition control can be the elegant strategy to realize the Co/Co couple activation and electronic structure tuning to promote the electrocatalytic process. The proposed strategy and approaches allow the rational design and synthesis of transition metal monophosphosulfides toward advanced electrochemical applications.

Uniform Ni C nanodots dispersed in ultrathin N-doped carbon nanosheets were successfully prepared by carburization of the two dimensional (2D) nickel cyanide coordination polymer precursors. The Ni C based nanosheets have lateral length of about 200 nm and thickness of 10 nm. When doped with Fe, the Ni C based nanosheets exhibited outstanding electrocatalytic properties for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). For example, 2 at % Fe (atomic percent) doped Ni C nanosheets depict a low overpotential (292 mV) and a small Tafel slope (41.3 mV dec ) for HER in KOH solution. An outstanding OER catalytic property is also achieved with a low overpotential of 275 mV and a small Tafel slope of 62 mV dec in KOH solution. Such nanodot-incorporated 2D hybrid structures can serve as an efficient bifunctional electrocatalyst for overall water splitting.

In addition, it is of importance to note that a high power factor ( S 2 / ρ ) is also indispensable for a given TE material to maximize its output power. [ 3 ] Therefore, by taking account of the practical application, a simultaneously optimization in the electrical and thermal transport properties of TE materials is imperative to maximize the output power and conversion efficiency concurrently. Recently, some strategies have been proved to be effective and even high ZT values greater than 2.0 have been achieved in the Pb-based TE materials, [ 4,5 ] such as the band convergence, [ 6 ] electronic density of states (DOS) distortion, [ 7 ] carrier energy barrier fi ltering, [ 8 ] and the conduction (valence) band modifi cation [ 9 ] to enhance Seebeck coefficient for high power factor; while defect engineering, [ 10 ] nanostructuring, [ 11 ] and multiscale hierarchical architecturing [ 12 ] to enhance the scattering of phonons for low thermal conductivity. However, the environmentally hazardous Pb element prevents them from widespread application. Therefore, it is of signifi cance to develop eco-friendly TE materials for middle temperature application, such as Mg 2 Si, [ 13 ] Half-Heusler, [ 14 ] and BiCuSeO [ 15 ] based TE materials.More recently, chalcopyrite CuInTe 2 is being considered as a promising p-type TE material because of the merits of environmentally friendly chemical component, intrinsically high electrical conductivity, and high Seebeck coeffi cient owing to the degenerate energy bands near the valence band maximum (VBM). [ 16 ] Thus many efforts have been made theoretically and experimentally [17][18][19][20][21][22] to enhance its TE performance already, such as Cu defi ciency and cation substitution. However, since there are two cation sites (Cu and In) in CuInTe 2 , cation substitution often lacks specifi city and may generate both electron and hole simultaneously, making it diffi cult to enhance the electrical transport properties substantially. Besides, the less phonon scattering, mainly by point defects originated from chemical component regulation, rendering its thermal conductivity is still relatively high. It is well known that refi ning the microstructure into nanoscale is often effective in optimizing thermal transport properties, yet the electrical transport properties are usually deteriorated due to the inevitably scattering of carriers, as typical shown in the CuInTe 2 /graphene [ 23 ] and our

Sb 2 Si 2 Te 6 , a 2D material, exhibits an intrinsically high thermoelectric figure of merit ZT of 1.08 at 823 K. The thermoelectric performance can be further enhanced by a cellular nanostructure with ultrathin Si 2 Te 3 nanosheets covering the Sb 2 Si 2 Te 6 grains. The Si 2 Te 3 acts as a hole-transmitting electron-blocking filter and, at the same time, causes extra phonon scattering that leads to ultralow thermal conductivity and a high ZT value of 1.65 at 823 K.

We investigate the structural and physical properties of the AgSnmSbSem+2 system with m=1-20 (i.e. SnSe matrix and ~5-50 % AgSbSe2) from length scales ranging from atomic, nano and macro. We find the 50:50 composition, with m=1 (i.e. AgSnSbSe3), forms a stable cation disordered cubic rock-salt p-type semiconductor with a special multipeak electronic valence band structure. AgSnSbSe3 has an intrinsically low lattice thermal conductivity of ~0.47 Wm -1 K -1 at 673 K owing to the synergy of cation disorder, phonon anharmonicity, low phonon velocity, and low-frequency optical modes. Furthermore, Te alloying on the Se sites creates a quinary high entropy NaCl-type solid solution AgSnSbSe3-xTex with randomly disordered cations and anions. The extra point defects and lattice dislocations lead to glass-like lattice thermal conductivities of ~0.32 Wm -1 K -1 at 723 K and higher hole carrier concentration than AgSnSbSe3. Concurrently, the Te alloying promotes greater convergence of the multiple valence band maxima in AgSnSbSe1.5Te1.5, the composition with the highest configurational entropy. Facilitated by these favorable modifications, we achieve a high average power factor of ~9.54 μWcm -1 K -2 (400-773 K), a peak thermoelectric figure of merit ZT of 1.14 at 723 K and a high average ZT of ~1.0 over a wide temperature range of 400-773 K in AgSnSbSe1.5Te1.5.

Interfacing epitaxial dinickel phosphide to 2D nickel thiophosphate nanosheets for boosting electrocatalytic water splitting

Presented are the development of novel 0D-2D nanohybrids consist of nickel-based bimetal phosphorus trisulfides (Ni 1−x Fe x PS 3 ) nanomosaic that decorated on the surface of MXene nanosheets (denoted as NFPS@MXene). The nanohybrids are obtained through a facile self-assemble process of transition metal layered double hydroxide (TMLDH) on MXene surface; followed by the low temperature in-situ solid-state reaction step. By tuning the Ni:Fe ratio, the as-synthesized NFPS@MXene nanohybrids exhibit excellent activities when tested as electrocatalysts for overall water splitting. Particularly, with the initial Ni:Fe ratio of 7:3, the obtained Ni 0.7 Fe 0.3 PS 3 @MXene nanohybrid reveals low overpotential (282 mV) and Tafel slope (36.5 mV dec -1 ) for oxygen evolution reaction (OER) in 1 m KOH solution. Meanwhile, the Ni 0.9 Fe 0.1 PS 3 @MXene shows low overpotential (196 mV) for hydrogen evolution reaction (HER) in 1 m KOH solution. When integrated them for overall water splitting, the Ni 0.7 Fe 0.3 PS 3 @MXene || Ni 0.9 Fe 0.1 PS 3 @MXene couple shows a low onset potential of 1.42 V and needs only 1.65 V to reach a current density of 10 mA cm -2 , which is better than the all noble metal IrO 2 || Pt/C electrocatalyst (1.71 mV@10 mA cm −2 ). Given the chemical versatility of Ni 1−x Fe x PS 3 and the convenient self-assemble process, the nanohybrids demonstrated in this work are promising for energy conversion applications.