Having smaller energy density than batteries, supercapacitors have exceptional power density and cyclability. Their energy density can be increased using ionic liquids and electrodes with sub-nanometer pores, but this tends to reduce their power density and compromise the key advantage of supercapacitors. To help address this issue through material optimization, here we unravel the mechanisms of charging sub-nanometer pores with ionic liquids using molecular simulations, navigated by a phenomenological model. We show that charging of ionophilic pores is a diffusive process, often accompanied by overfilling followed by de-filling. In sharp contrast to conventional expectations, charging is fast because ion diffusion during charging can be an order of magnitude faster than in bulk, and charging itself is accelerated by the onset of collective modes. Further acceleration can be achieved using ionophobic pores by eliminating overfilling/de-filling and thus leading to charging behavior qualitatively different from that in conventional, ionophilic pores.

Recent experiments have shown that the capacitance of subnanometer pores increases anomalously as the pore width decreases, thereby opening a new avenue for developing supercapacitors with enhanced energy density. However, this behavior is still subject to some controversy since its physical origins are not well understood. Using atomistic simulations, we show that the capacitance of slit-shaped nanopores in contact with room-temperature ionic liquids exhibits a U-shaped scaling behavior in pores with widths from 0.75 to 1.26 nm. The left branch of the capacitance scaling curve directly corresponds to the anomalous capacitance increase and thus reproduces the experimental observations. The right branch of the curve indirectly agrees with experimental findings that so far have received little attention. The overall U-shaped scaling behavior provides insights on the origins of the difficulty in experimentally observing the pore-width-dependent capacitance. We establish a theoretical framework for understanding the capacitance of electrical double layers in nanopores and provide mechanistic details into the origins of the observed scaling behavior. The framework highlights the critical role of "ion solvation" in controlling pore capacitance and the importance of choosing anion/cation couples carefully for optimal energy storage in a given pore system.

Using molecular dynamics simulations, we show that charge storage in subnanometer pores follows a distinct voltage-dependent behavior. Specifically, at lower voltages, charge storage is achieved by swapping co-ions in the pore with counterions in the bulk electrolyte. As voltage increases, further charge storage is due mainly to the removal of co-ions from the pore, leading to a capacitance increase. The capacitance eventually reaches a maximum when all co-ions are expelled from the pore. At even higher electrode voltages, additional charge storage is realized by counterion insertion into the pore, accompanied by a reduction of capacitance. The molecular mechanisms of these observations are elucidated and provide useful insight for optimizing energy storage based on supercapacitors.

A key concept in classical electrokinetic theories is that the viscosity of interfacial fluids is much higher than that of bulk fluids, and this concept is indirectly supported by experimental evidence and molecular dynamics simulations. However, a universal mechanism that encompasses the breadth of experimental evidence is still lacking. Here we show, using molecular dynamics simulations, that the "apparent" thickening of interfacial fluids in electrokinetic transport near molecularly smooth surface originates mainly from the fact that ion-wall interactions are not accounted for in the hydrodynamic model of classical electrokinetic theories. Specifically, strong ion-wall interactions cause intermittent adsorption of ions on charged walls, and this in turn leads to loss of driving force for flow and screening of fluid flow by the adsorbed ions. Although not considered in the classical electrokinetic theories, these effects can significantly suppress electrokinetic transport. Consequently, when the classical theories are used to interpret the electrokinetic data, the viscosity of interfacial fluids appears to be greatly enhanced even if their material viscosity is similar to that of the bulk fluids. This mechanism for the apparent thickening of interfacial fluids is applicable to electrokinetic transport near any type of charged surface.

In the present study, the NO and N2O formation characteristics during combustion of five biomass fuels (rice straw, wheat straw, corn stalk, sugarcane leaf, and eucalyptus bark) and one bituminous coal, as a comparison, were investigated in a horizontal fixed-bed reactor. It showed that there was still a considerable degree of N2O conversion for biomass fuels, although NO was formed in a much larger amount. Most of NO and N2O were formed during the devolatilization stage for biomass fuels; therefore, optimizing the supply of air and fuel during biomass combustion in actual boilers can be expected to achieve ultralow emissions of nitrogen oxides. There was no clear correlation between the NO and N2O yields and fuel N content, which indicated that the nitrogen functionality and other components, such as inherent mineral matter, present in biomass must have great influence on the nitrogen transformation during thermal processing. When rice straw was co-fired with eucalyptus bark, nonlinear behavior could be seen and the fuel N conversion to NO was largely enhanced. In the temperature range of 700–900 °C, the fuel N conversion to NO first increased and then decreased slightly, while the conversion to N2O showed a continuous decreasing trend. The fuel N conversion to NO and N2O showed a general increasing trend with the increase of the inlet oxygen concentration, and this phenomenon was more obvious at higher temperatures. The results presented in this study will help to gain some insight onto the fundamental mechanism of fuel N conversion during biomass combustion.