Steering the Selectivity of Carbon Dioxide Electroreduction from Single-Carbon to Multicarbon Products on Metal-Organic Frameworks via Facet Engineering.

Although metal-organic frameworks (MOFs) have attracted more attention for the electrocatalytic CO2 reduction reaction (CO2RR), obtaining multicarbon products with a high Faradaic efficiency (FE) remains challenging, especially under neutral conditions. Here, we report the controlled synthesis of stable Cu(I) 5-mercapto-1-methyltetrazole framework (Cu-MMT) nanostructures with different facets by rationally modulating the reaction solvents. Significantly, Cu-MMT nanostructures with (001) facets are acquired using isopropanol as a solvent, which favor multicarbon production with an FE of 73.75% and a multicarbon:single-carbon ratio of 3.93 for CO2RR in a neutral electrolyte. In sharp contrast, Cu-MMT nanostructures with (100) facets are obtained utilizing water, promoting single-carbon generation with an FE of 63.98% and a multicarbon: single-carbon ratio of only 0.18. Furthermore, this method can be extended to other Cu-MMT nanostructures with different facets in tuning the CO2 reduction selectivity. This work opens up new opportunities for the highly selective and efficient CO2 electroreduction to multicarbon products on MOFs via facet engineering.

Underwater quasi-omnidirectional wireless optical communication based on perovskite quantum dots.

In this paper, a quasi-omnidirectional transmitter is proposed and demonstrated for underwater wireless optical communication (UWOC) using the photoluminescence of perovskite quantum dots (QDs). The proposed transmitter, without complex driving circuits, is compact and reliable thanks to the lens-free design. The system performance is tested in a 50-m swimming pool with a water attenuation coefficient of 0.38 dB/m. The maximum data rates of on-off-keying (OOK) signals over 10-m and 20-m transmission distances can reach 60 Mbps and 40 Mbps, respectively. When four clients are adopted in a code division multiple access (CDMA) based UWOC network, the maximum data rates of each client can reach 10 Mbps and 7.5 Mbps over 10-m and 20-m underwater channels, respectively. The system can meet the requirements of the last meter end-user access in the Internet of underwater things (IoUT) and underwater optical cellular network systems.

200-m/500-Mbps underwater wireless optical communication system utilizing a sparse nonlinear equalizer with a variable step size generalized orthogonal matching pursuit.

Linear and nonlinear impairments in underwater wireless optical communication (UWOC) systems caused by the limited bandwidth and nonlinearity of devices severely degrade the system performance. In this paper, we propose a sparse Volterra series model-based nonlinear post equalizer with greedy algorithms to mitigate the nonlinear impairments and the inter-symbol interference (ISI) in a UWOC system. A variable step size generalized orthogonal matching pursuit (VSgOMP) algorithm that combines generalized orthogonal matching pursuit (gOMP) and adaptive step size method is proposed and employed to compress the Volterra equalizer with low computational cost. A maximum data rate of 500 Mbps is realized with the received optical power of -32.5 dBm in a 7-m water tank. In a 50-m swimming pool, a data rate of 500 Mbps over 200-m underwater transmission is achieved with a BER lower than the forward error correction (FEC) threshold of 3.8 × 10-3. The number of kernels of the sparse Volterra equalizer is reduced to 70% of that of the traditional Volterra equalizer without significant BER performance degradation. Compared with orthogonal matching pursuit (OMP) scheme and regularized orthogonal match pursuit (ROMP) scheme, the VSgOMP scheme reduces the running time by 68.6% and 29.2%, respectively. To the best of our knowledge, this is the first time that a sparse Volterra equalizer combined with VSgOMP algorithm is employed for the nonlinear equalization in a long-distance high-speed UWOC system.

Long-reach underwater wireless optical communication with relaxed link alignment enabled by optical combination and arrayed sensitive receivers.

To extend the transmission distance and relax the strict alignment requirement of underwater wireless optical communication ((UWOC), we design and implement a UWOC system using a 3×1 fiber combiner and a high-sensitive multi-pixel photon counter (MPPC). The 50-m and 100-m transmission distances (corresponding to 24 attenuation lengths) are experimentally achieved with the data rates of 16.78 Mbps and 8.39 Mbps, respectively, in a 50-m standard swimming pool. Moreover, we also investigate and optimize the performance of misalignment tolerance of this system using two MPPCs as the detectors together with different diversity reception technologies. At the 50-m transmission distance, the maximum offset between the MPPC array and the light spot center can be extended to 9 m using the maximal ratio combining (MRC), while the maximum offset is 6 m when using single MPPC.

56-m/3.31-Gbps underwater wireless optical communication employing Nyquist single carrier frequency domain equalization with noise prediction.

We propose and experimentally demonstrate an underwater wireless optical communication (UWOC) system using a 520-nm laser diode (LD) and 32-quadrature amplitude modulation (32-QAM) single carrier signals. To mitigate the inter-symbol interference (ISI), a frequency domain equalizer combined with a time-domain decision feedback noise predictor is employed at the receiver. However, this structure cannot apply channel coding conjunctively. Therefore, an interleaver/deinterleaver pair is applied to handle the decoding delay, and thus systematic Reed-Solomon (RS) code can provide reliable feedback signals. With a 3-dB bandwidth of 200 MHz, the proposed system with the frequency domain equalization and noise prediction (FDE-NP) scheme can achieve a maximal net data rate of 3.48 Gbps, which is 17.2% higher than that of orthogonal frequency division multiplexing (OFDM) scheme. At a net data rate of 3.31 Gbps, we have successfully achieved a transmission distance up to 56 m. To the best of our knowledge, this is the first time to employ FDE-NP in UWOC where OFDM conventionally plays a prevailing role for high-speed transmission.

Experimental demonstration of an underwater wireless optical communication employing spread spectrum technology.

For some industrial underwater wireless optical communication (UWOC) applications, the transmission distance matters more than the communication rate. Attenuation length (AL) is an important distance indicator of UWOC system. In this paper, to the best of our knowledge, the spread spectrum (SS) technology is firstly applied in a UWOC system and the capability to extend transmission distance or AL is demonstrated. A 42-m UWOC is experimentally demonstrated with 6.68 ALs. Compared with the conventional not-return-to-zero on-off-keying (NRZ-OOK) modulation scheme, the proposed SS scheme with a spread spectrum gain (SSG) of 5 achieves an AL extension by 0.51 and 0.81, respectively, with the same data rate and bandwidth. And the minimum required signal-to-noise ratio (SNR) is reduced by 9 dB to as low as -0.8 dB. Besides, the feature of the SS scheme that could work in a bandwidth-limited long-reach underwater channel without the equalization process is experimentally demonstrated.

Diversity-reception UWOC system using solar panel array and maximum ratio combining.

We demonstrated a diversity-reception lens-free underwater wireless optical communication system employing a 2×2 solar panel array as detectors. The respective relationships between solar panel sizes and photocurrents, output voltages, system bandwidths were studied theoretically and experimentally. The signals output from the array were combined via maximum ratio combining in order to improve the signal quality. A 450-nm blue laser was used in the transmitter and a light spot with a size of 20mm×35mm was formed on the receiving plane after a 7-m transmission in tap water. With the solar panel array, a data rate of 84 Mbps was achieved with a bit error rate of 2.17 × 10-3 using 16-QAM OFDM signal. Meanwhile, a single solar panel with the same size as the array only achieved 60 Mbps. Solar panel array can bring about 40% data rate improvement. With the same detection area, the horizontal detection range of detectors increased from 37 mm for single solar panel to 55 mm for solar panel array thanks to the diversity reception. The results show that solar panel array using maximum ratio combining can enhance the transmission data rate as well as the detection range. The impacts of air bubbles, water fluctuation and microscopic particulates suspension on the proposed solar panel array-based system were also investigated. The results show that the received power sensitivity of solar panel array is 5.22 dB higher than single solar panel with the same detection area with a data rate of 60 Mbps and a BER of 10-3.