ABSTRACT
Electricity generation by natural water evaporation generators (NWEGs) using porous materials shows great potential for energy harvesting, but mechanistic investigations of NWEGs have mostly been limited to streaming potential studies. In this study, we propose the coexistence of an evaporation potential and streaming potential in a NWEG using ZSM-5 as the generation material. The iron probe method, salt concentration regulation, solution regulation, and side evaporation area regulation were used to analyze the NWEG mechanism. Our findings revealed that a streaming potential formed as water flowed inside the ZSM-5 nanochannels, driven by electrodynamic effects that increased from the bottom to the top of the generator. In addition, an evaporation potential existed at the surface interface between ZSM-5 and water, which decreased from the bottom to the top as the evaporation height of the generator increased. The resulting open-circuit voltage (Voc) depended on the balance between the evaporation and streaming potentials, both of which were influenced by the evaporation enthalpy (Ee) or vapor pressure. Generally, a higher Ee or lower vapor pressure led to a lower evaporation potential and subsequently a lower Voc. A dual mechanism involving synergistic evaporation potential and streaming potential is proposed to explain the mechanism of NWEGs.
ABSTRACT
Regulating the selectivity between CO and CH4 during CO2 hydrogenation is a challenging research topic. Previous research has indicated that potassium (K) modification can adjust the product selectivity by regulating the adsorption strength of formate/CO* intermediates. Going beyond the regulation mechanism described above, this study proposes a K-guided selectivity control method based on the regulation of key intermediates HCO*/H3CO* for Ni catalysts supported on reducible carrier CeO2. By incorporating K, the CO selectivity of CO2 hydrogenation shifts from around 25.4% for Ni/CeO2 to approximately 93.8% for Ni/CeO2-K. This can be attributed to K modification causes electron aggregation in the bonding regions of HCO* and H3CO* intermediates, thus enhancing their adsorption strength. Consequently, the reaction pathway from HCO*/H3CO* to CH4 is limited, favoring the decomposition of formates to CO products. Moreover, the addition of K leads to a moderate decrease in CO2 conversion from 55.2% to 48.6%, which still surpasses values reported in most other studies. This reduction is associated with a decline in reducible Ni species and oxygen vacancy concentration in Ni/CeO2-K. As a result, the adsorption capacity for CO2 and H2 reduces, ultimately reducing CO2 hydrogenation activity.
ABSTRACT
The improvement of electricity production for water evaporation-driven generators (WEGs) remains a challenge. Herein, two types of WEGs were designed to study the resistance matching for improving the electricity production using the method of nanoarchitectonics. One type of reduced graphene oxide/carbon nanotube (RGO/CNT) WEG was constructed using RGO with adjustable resistances as working material and CNTs with fixed resistance as electrode material. The other type of graphene oxide (GO)/RGO WEG was constructed using RGO with adjustable resistance as electrode material and GO with fixed resistance was used as working material. The open circuit voltage of RGO/CNT increased from 15 to 56 mV and then decreased to 22 mV with increasing RGO resistance. The short circuit current of RGO/CNT also first increased and then decreased. The performance of GO/RGO was similar with that of RGO/CNT. Typically, the RGO/CNT and GO/RGO WEG showed the highest performance when the working material to electrode material resistance ratio was 2272 and 2365, respectively. It showed that the best resistance ratio of working material to electrode material was in the range of 2000-2500, which helped to improve about 2-5 times of electricity efficiency in the WEG. The present work provides a new direction for optimizing performance of WEGs.
