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This paper studies an extremely large-scale reconfigurable intelligent surface (XL-RIS) empowered covert communication system in the near-field region. Alice covertly transmits messages to Bob with the assistance of the XL-RIS, while evading detection by Willie. To enhance the covert communication performance, we maximize the achievable covert rate by jointly optimizing the hybrid analog and digital beamformers at Alice, as well as the reflection coefficient matrix at the XL-RIS. An alternating optimization algorithm is proposed to solve the joint beamforming design problem. For the hybrid beamformer design, a semi-closed-form solution for fully digital beamformer is first obtained by a weighted minimum mean-square error based algorithm, then the baseband digital and analog beamformers at Alice are designed by approximating the fully digital beamformer via manifold optimization. For the XL-RIS's reflection coefficient matrix design, a low-complexity alternating direction method of multipliers based algorithm is proposed to address the challenge of large-scale variables and unit-modulus constraints. Numerical results unveil that i) the near-field communications can achieve a higher covert rate than the far-field covert communications in general, and still realize covert transmission even if Willie is located at the same direction as Bob and closer to the XL-RIS; ii) the proposed algorithm can enhance the covert rate significantly compared to the benchmark schemes; iii) the proposed algorithm leads to a beam diffraction pattern that can bypass Willie and achieve high-rate covert transmission to Bob.

Single-atom catalysts have found considerable applications in the field of electrochemical CO 2 reduction reaction (CO 2 RR) due to their unique coordination environments. However, during the preparation of single-atom catalysts, some metal nanoparticles (NPs) are inevitably generated, which suffer from low selectivity in CO 2 RR. In this regard, complex postprocessing solution treatments are usually conducted to remove metal NPs using acid. Herein, we fabricated Ni(NC)-based catalysts composed of single Ni atoms and Ni NPs, both of which feature local Ni−N coordination via a simple Ni-metal organic framework (MOF)-assisted strategy. Based on X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) spectroscopy measurements, nitrogen species in N-doped carbon have been demonstrated to be coordinated with surface nickel species to form Ni−N motifs, which makes Ni at a low-valent state for efficient CO 2 RR. Consequently, the catalyst exhibited high performances toward CO 2 RR with CO Faradic efficiencies (FE CO ) maintained over 90% from −0.65 to −0.90 V vs reversible hydrogen electrode (RHE). More importantly, the FE CO of 99% could be obtained at a considerable current density (j) of −160 mA cm −2 in a flow cell configuration. These findings suggest that regulating the surface environment of Ni species can activate the original inert reaction sites into active reaction sites, providing a promising avenue to design highperformance electrocatalysts for CO 2 RR.

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Copper-based materials are efficient electrocatalysts for the conversion of CO 2 to C 2+ products,a nd most these materials are reconstructed in situ to regenerate active species. It is achallenge to precisely design precatalysts to obtain active sites for the CO 2 reduction reaction (CO 2 RR). Herein, we develop astrategy based on local sulfur doping of aCu-based metal-organic framework precatalyst, in whichthe stable CuÀ Smotif is dispersed in the framework of HKUST-1 (S-HKUST-1). The precatalyst exhibits ahigh ethylene selectivity in an Htype cell with am aximum faradaic efficiency (FE) of 60.0 %, and delivers acurrent density of 400 mA cm À2 with an ethylene FE up to 57.2 %i naflowc ell. Operando X-raya bsorption results demonstrate that Cu d+ species stabilized by the CuÀS motif exist in S-HKUST-1 during CO 2 RR. Density functional theory calculations indicate the partially oxidized Cu d+ at the Cu/Cu x S y interface is favorable for coupling of the *CO intermediate due to the modest distance between coupling sites and optimizedadsorption energy.

Surface modification has been proven to be an effective approach for ion exchange membranes to achieve separation of counterions with different valences by altering interfacial construction of membranes to improve ion transfer performance. In this work, we have fabricated a series of novel cation exchange membranes (CEMs) by modifying sulfonated polysulfone (SPSF) membranes via codeposition of mussel-inspired dopamine (DA) and 4′-aminobenzo-15-crown-5 (ACE), followed by glutaraldehyde cross-linking, aiming at achieving selective separation of specific cations. The as-prepared membranes before and after modification were systematically characterized in terms of their structural, physicochemical, electrochemical, and electrodialytic properties. In the electrodialysis process, the modified membranes exhibit distinct perm selectivity to K+ ions in binary (K+/Li+, K+/Na+, K+/Mg2+) and ternary (K+/Li+/Mg2+) systems. In particular, at a constant current density of 5.0 mA·cm–2, modified membrane M-co-0.50 shows significantly prominent perm selectivity in the K+/Mg2+ system and M-co-0.75 exhibits remarkable performance in the K+/Li+ system , superior to commercial monovalent-selective CEM (CIMS, , ). Besides, in the K+/Li+/Mg2+ ternary system, K+ flux reaches 30.8 nmol·cm–2·s–1 for M-co-0.50, while it reaches 25.8 nmol·cm–2·s–1 for CIMS. It possibly arises from the effects of pore-size sieving and the synergistic action of electric field driving and host–guest molecular recognition of ACE and K+ ions. This study can provide new insights into the separation of specific alkali metal ions, especially on reducing influence of coexisting cations K+ and Na+ on Li+ ion recovery from salt lake and seawater.

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