Lower Open-circuit Voltage Values Research Articles

Layered iron dichalcogenides with high ion mobility and capacity as promising anode materials for alkali metal-ion batteries: A first-principles study

Developing high-performance anode materials remains a key challenge for alkali metal-ion batteries (AIBs). Benefiting from a high surface-to-bulk ratio, layered materials are promising candidates for anodes due to their potential high specific capacity and low ion transport resistance derived from the numerous ion storage sites and inherent open channels. In this work, we predict that the recently synthesized layered iron dichalcogenides, namely FeX2 (X = S, Se, Te), are suitable anode materials for AIBs through first-principles calculations. The results show that both the pristine and alkali-metalized FeX2 monolayers are structurally stable and present a metallic nature. In addition, the alkali metal-ions exhibit low diffusion barriers (0.15 eV for Li-ions, 0.08 eV for Na-ions, and 0.05 eV for K-ions) on FeX2 monolayers, which indicates that they possess a high ionic conductivity as well as an excellent rate capability and cycling performance. Besides, the calculated low open-circuit voltage values (0.39, 0.29, and 0.19 V vs. Na/Na+ for FeS2, FeSe2, and FeTe2, respectively) reveal that layered FeX2 materials are suitable to serve as anodes for sodium-ion batteries (SIBs). As a result, FeS2 and FeSe2 monolayers exhibit a high theoretical specific capacity (893.6 and 501.4 mAh g−1, respectively) for SIBs, while the theoretical volume-specific capacity of FeTe2 monolayer (2343.3 mAh cm−3) is around two times larger than that of standard commercial cells. Our findings suggest that these currently unexploited layered iron dichalcogenides could be potential high-performance anode materials for AIBs.

Performance Analysis of Silicon Carrier Selective Contact Solar Cells With ALD MoOx as Hole Selective Layer

Carrier selective contact (CSC) architecture is one of several promising candidates for high-efficiency silicon solar cells operating close to the thermodynamic limit. However, an industrially feasible process is lacking for manufacturable CSC cells. Specifically, although evaporated molybdenum oxide (MoOx) has been actively researched as a hole selective layer, a more industry-compatible deposition process is critically needed. Atomic layer deposition (ALD) has been widely employed in the industry for material stacks requiring precise thickness control and uniformity. In this paper, we present a performance analysis of CSC solar cells fabricated using ALD MoOx as the hole selective layer. As a first step, the standard ALD recipe was modified to obtain sub-stoichiometric MoOx with a higher work function due to increased oxygen vacancy concentration. The modified MoOx recipe resulted in low open-circuit voltage values in fabricated not-passivated CSC solar cells. A 4 nm thick amorphous silicon layer inserted in the hole selective stack showed significant improvement in minority lifetime but worse solar cell performance due to increased series resistance. Besides indicating a strong trade-off between passivation and series resistance, the solar cell data highlights series resistance as a key performance limiter in CSC solar cells.

Exploring the Efficient Na/K Storage Mechanism and Vacancy Defect-Boosted Li+ Diffusion Based on VSe2/MoSe2 Heterostructure Engineering.

As typical 2D materials, VSe2 and MoSe2 both play a complementary role in Li/Na/K storage. Therefore, we designed and optimized the VSe2/MoSe2 heterostructure to gain highly efficient Li/Na/K-ion batteries. Most importantly, achieving fast Li/Na/K-ion diffusion kinetics in the interlayer of VSe2/MoSe2 is a key point. First of all, first-principles calculations were carried out to systematically investigate the packing structure, mechanical properties, band structure, and Li/Na/K storage mechanism. Our calculated results suggest that a large interlayer spacing (3.80 Å), robust structure, and metallic character pave the way for achieving excellent charge-discharge performance for the VSe2/MoSe2 heterostructure. Moreover, V and Mo ions both suffer a very mild redox reaction even if Li/Na/K ions fill the interlayer space. These structures were all further verified to show thermal stability (300 K) by means of the AIMD method. By analyzing the Li/Na/K diffusion behavior and the effect of vacancy defect on the structural stability and energy barrier for Li interlayer diffusion, it is found that the VSe2/MoSe2 heterostructure exhibits very low-energy barriers for Na/K interlayer diffusion (0.21 eV for Na and 0.11 eV for K). Compared with the VSe2/MoSe2 heterostructure, the V0.92Se1.84/MoSe2 heterostructure not only can still maintain a stable structure and metallic character but also has much lower energy barrier for Li interlayer diffusion (0.07 vs 0.48 eV). These discoveries also break new ground to eliminate the obstacles preventing Li+ diffusion in the interlayer of other heterostructure materials. Besides, both VSe2/MoSe2 and V0.92Se1.84/MoSe2 heterostructures have low average open-circuit voltage (OCV) values during Li/Na/K interlayer diffusion (1.07 V for V0.92Se1.84/MoSe2 vs Li+, 0.86 V for VSe2/MoSe2 vs Na+, and 0.54 V for VSe2/MoSe2 vs K+), such low OCV values are beneficial for anode materials with excellent electrochemical properties. The above findings offer a new route to design anode materials for Li/Na/K-ion batteries.

Shortcut Biological Nitrogen Removal (SBNR) in an MFC Anode Chamber under Microaerobic Conditions: The Effect of C/N Ratio and Kinetic Study

In this work, the feasibility of the Shortcut Biological Nitrogen Removal (SBNR) in the anodic chamber of a Microbial Fuel Cell (MFC) was investigated. Thirty day experiments were carried out using synthetic wastewaters with a Total Organic Carbon vs. nitrogen ratio (TOC/N) ranging from 0.1 to 1. Ammonium, nitrite, nitrate, pH, and TOC were daily monitored. Results showed that microaerobic conditions in the anodic chamber favored the development of nitritation reaction, due to oxygen transfer from the cathodic chamber through the membrane. Nitritation was found to depend on TOC/N ratio: at TOC/N equal to 0.1 an ammonium removal efficiency of up to 76% was observed. Once the oxygen supply to the cathodic chamber was stopped, denitritation occurred, favored by an increase of the TOC/N ratio: a nitrite removal of 80.3% was achieved at TOC/N equal to 0.75. The presence of nitrogen species strongly affected the potential of the electrochemical system: in the nitritation step, the Open Circuit Voltage (OCV) decreased from 180 mV to 21 mV with the decrease of the TOC/N ratio in the investigated range. Lower OCV values were observed in the denitritation steps since the organic carbon acted as the energy source for the conversion of nitrite to nitrogen gas. A kinetic analysis was also performed. Monod and Blackman models described the ammonium and the organic carbon removal processes well during the nitritation step, respectively, while Blackman-Blackman fitted experimental results of the denitritation step better.

Physical and electrical characterization of high-performance Cu2ZnSnSe4 based thin film solar cells

We report on the electrical, optical and physical properties of Cu2ZnSnSe4 solar cells using an absorber layer fabricated by selenization of sputtered Cu, Zn and Cu10Sn90 multilayers. A maximum active-area conversion efficiency of 10.4% under AM1.5G was measured with a maximum short circuit current density of 39.7mA/cm2, an open circuit voltage of 394mV and a fill factor of 66.4%. We perform electrical and optical characterization using photoluminescence spectroscopy, external quantum efficiency, current–voltage and admittance versus temperature measurements in order to derive information about possible causes for the low open circuit voltage values observed. The main defects derived from these measurements are strong potential fluctuations in the absorber layer as well as a potential barrier of the order of 133meV at the back side contact.

Physical and electrochemical characterization of catalysts for oxygen reduction in fuel cells

The cathode catalysts in low temperature fuel cells are associated with major cell efficiency losses, because of kinetic limitations of the oxygen reduction reaction. Additionally, methanol oxidation at the cathode leads to significant lowering of the efficiency in direct methanol fuel cells, which can be alleviated by use of methanol-tolerant catalysts. In this work, alternative carbon-supported platinum-alloy catalysts were investigated by physical methods. Second, methanol-tolerant ruthenium-selenide catalysts were characterized by physical and electrochemical methods. Besides V–i characteristics and electrochemical impedance spectroscopy as electrochemical methods, physical methods such as X-ray photoelectron spectroscopy, nitrogen adsorption, porosimetry by mercury intrusion and temperature programmed reduction are used to characterize the catalysts. The electrochemical characterization yields information about properties and behavior of the catalyst. In contrast to platinum a significantly different hydrophobic behavior of the RuSe/C catalysts is found. Low open circuit voltage values measured for RuSe/C indicate an effect on both electrodes. The anode reaction was also influenced by the different cathode catalysts. As a result of the formation of H2O2 at the cathode, which passes through the membrane from cathode to anode side, a mixed anode potential is formed. By comparing RuSe/C catalysts before and after electrochemical stressing, changes of the catalysts are determined. Postmortem surface analysis (by X-ray photoelectron spectroscopy) revealed that catalyst composition and MEA structure changed during electrochemical stressing. During fuel cell operation selenium oxide is removed from the surface of the catalysts to a large extent. Additionally, a segregation effect of selenium in RuSe to the surface is identified.

Comment: Analysis of efficiency limitations in high-low emitter back surface field silicon solar cells

Analytical and numerical models have been used to analyse the electrical behaviour of HLE-BSF n+-n-p-p+ silicon solar cells. It is shown that this structure should have lower open-circuit voltage values than the simple HLE solar cell due to the recombination current in the p+ region. Back surface passivation is proposed as a means to improve the Voc, that can be as high as 650 mV.

Erratum: Analysis of efficiency limitations in high-low emitter back surface field silicon solar cells

Analytical and numerical models have been used to analyse the electrical behaviour of HLE-BSF n+-n-p-p+ silicon solar cells. It is shown that this structure should have lower open-circuit voltage values than the simple HLE solar cell due to the recombination current in the p+ region. Back surface passivation is proposed as a means to improve the Voc, that can be as high as 650 mV.