Large Voltage Loss Research Articles

Synthesis of the solution-processed wide band-gap chalcopyrite CuGa(S,Se)2 thin film and its application to photovoltaics

A wide band-gap chalcopyrite CuGa(S,Se)2 (CGSSe) thin film was synthesized via the simple solution based method of precursor solution coating and subsequent heat treatment processes. Various characterizations such as X-ray diffraction, UV–vis–NIR spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Hall measurement, and dynamic secondary ion mass spectroscopy revealed a successful formation of the wide band-gap polycrystalline chalcopyrite film with nearly the same amounts of S and Se elements. The synthesized CGSSe film was applied as an absorber layer to the conventional thin film solar cell construction, which yielded an open circuit voltage (Voc) of 0.52V, a short circuit current density (Jsc) of 3.5mA·cm−2, and an overall power conversion efficiency (PCE) of 1.0%. The photovoltage was lower than the expected value implying that there is a large voltage loss relative to the wide band-gap (~2.11eV). It would be attributable to improper combination of the CGSSe absorber film and the CdS buffer layer in the conventional cell architecture, which may promote interface recombination due to an absence of a “spike” band alignment (i.e. conduction band offsets are slight positive values, 0.1eV<ΔEc<0.4eV) with the CGSSe absorber layer. Moreover, improvements in morphologies, such as enlarged grains and reduced pore sizes, need to be achieved for higher solar cell efficiency.

Cu–Zn disorder and band gap fluctuations in Cu2ZnSn(S,Se)4: Theoretical and experimental investigations

Cu2ZnSn(S,Se)4 (CZTS(e)) solar cells suffer from low‐open‐circuit voltages that have been blamed on the existence of band gap fluctuations, with different possible origins. In this paper, we show from both theoretical and experimental standpoints that disorder of Cu and Zn atoms is in all probability the primary cause of these fluctuations. First, quantification of Cu–Zn disorder in CZTS thin films is presented. The results indicate that disorder is prevalent in the majority of practical samples used for solar cells. Then, ab initio calculations for different arrangements and densities of disorder‐induced [CuZn + ZnCu] defect pairs are presented and it is shown that spatial variations in band gap of the order of 200 meV can easily be caused by Cu–Zn disorder, which would cause large voltage losses in solar cells. Experiments using Raman spectroscopy and room temperature photoluminescence combined with in situ heat‐treatments show that a shift in the energy of the dominant band‐to‐band recombination pathway correlates perfectly to the order‐disorder transition, which clearly implicates Cu–Zn disorder as the cause of band gap fluctuations in CZTS. Our results suggest that elimination or passivation of Cu–Zn disorder could be very important for future improvements in the efficiency of CZTS(e)‐based solar cells.

A Current-Independent Constant Anode Voltage Loss Using Sm-Doped Ceria Electrolytes in SOFCs

Numerous approaches have been used to solve the transport equation that describes a solid-oxide fuel cell with mixed ionic and electronic conductors, such as Sm-doped ceria. Typically, the open circuit voltage is calculated using Wagner’s equation. By accounting for the polarization voltage losses, the open circuit voltage can be calculated. However, electrodes degrade over time, and the open circuit voltage cannot be calculated during electrode degradation because the open circuit voltage remains unchanged. The current work proposes a solution to this problem using a current-independent constant anode voltage loss. The denial of large leakage currents does not deny the Wagner-Schmalzried method when a large current-independent constant anode voltage loss occurs. Furthermore, the importance of this voltage loss is discussed.

Direct methanol fuel cells with streamline graded structure under ultra-low fuel stoichiometry condition

Operating a direct methanol fuel cell (DMFC) with ultra-low fuel stoichiometry is desirable to build a compact portable power system. Through 3D computational modeling with statistical analysis, severe non-uniformity of fuel concentration leading to high methanol crossover in the inlet region and poor fuel supply in the outlet region is predicted under ultra-low fuel stoichiometry condition, which causes large voltage loss or even cell shut-down. After identifying controlling parameters of methanol transport, this paper proposes streamline-graded structures (SGS) of the anode channel to mitigate anode fuel distribution non-uniformity and to boost cell performance and fuel efficiency together. Computational results show that streamline-graded structures achieve about 10% of voltage gain and 3% of fuel efficiency improvement compared to the conventional DMFC design by mitigating anode non-uniformity.

Effect of ammonia as a complexing agent on electrokinetic remediation of copper-contaminated soil

Ammonium acetate was used as anolytes in the electrokinetic removal of copper from soil. The electrokinetic experiments were conducted using a rectangular reactor 7.5 cm in width, 10 cm in height, and 11 cm in length. Granular graphite was used as the electrode material. All experiments were conducted under constant current conditions (10 mA). The results show that a high concentration of ammonium acetate (>0.1 M) was needed to sustain electroosmotic flow. The apparent electrical conductivity of the system was dominated by a thin layer of soil (1 cm) close to the cathode. When the ammonium acetate concentration was less than 0.1 M, the apparent electrical conductivity of this soil layer decreased to less than 0.001 S/m and caused large voltage loss. The electroosmotic conductivity of the soil was increased and sustained at 1 × 10 −4 cm 2/V-s by a high concentration of ammonium acetate, whereas it decreased to less than 1 × 10 −6 cm 2/V-s when the concentration of ammonium acetate was low. The efficiency of copper removal increased with the concentration of ammonium acetate, reaching 36% after 1 wk of treatment. Most of the un-removed copper was concentrated in the soil close to the cathode. For the 0.5 M ammonium acetate experiment, the volume of copper containing soil was less than 10% of the original soil after treatment. Therefore, a remediation strategy can be developed to first treat the contaminated soil by electrokinetics and then, focus on removing copper from the soil close to the cathode.

Model of cathode catalyst layers for polymer electrolyte fuel cells: The role of porous structure and water accumulation

The cathode catalyst layer (CCL) is the major competitive ground for reactant transport, electrochemical reaction, and water management in a polymer electrolyte fuel cell (PEFC). Our model, presented here, accounts for the full coupling of random porous morphology, transport properties, and electrochemical conversion in CCLs. It relates spatial distributions of water, oxygen, electrostatic potential, and reaction rates to the effectiveness of catalyst utilization, water handling capabilities, and voltage efficiency. A feedback mechanism, involving the non-linear coupling between liquid water accumulation and oxygen depletion is responsible for the transition from a state of low partial saturation with high voltage efficiency to a state with excessive water accumulation that corresponds to highly non-uniform reaction rate distributions and large voltage losses. The transition between these states could be monotonous or it could involve bistability in the transition region. We introduce stability diagrams as a convenient tool for assessing CCL performance in dependence of composition, porous structure, wetting properties, and operating conditions.

Semi-analytical 1D + 1D model of a polymer electrolyte fuel cell

Semi-analytical model of polymer electrolyte fuel cell (PEFC), which takes into account oxygen and water transport across the cell and along the channel is developed. Analytical formulas for along-the-channel profiles of local current density at large voltage loss are obtained. These expressions show that the cell may operate in one of the three regimes: (i) oxygen-limiting (local current density j exponentially decreases with the distance z along the channel); (ii) water-limiting ( j exponentially increases with z); (iii) mixed regime: water-limiting close to the channel inlet and oxygen-limiting in the rest part of the cell. Recently measured and model profiles of local current density are in good agreement. In the mixed regime a large parasitic along-the-channel proton current may arise. The expressions for the limiting current density of the cell, for optimal feed composition and for optimal oxygen stoichiometry are derived.

A feasibility analysis for alkaline membrane direct methanol fuel cell: thermodynamic disadvantages versus kinetic advantages

As the proton exchange membrane direct methanol fuel cell (PEMDMFC) faces sustaining obstacles, alkaline membrane direct methanol fuel cell (AMDMFC) is attracting increasing attention. Although some advantages may be expected, the feasibility of AMDMFC does not seem well verified. In this paper, thermodynamic disadvantages and kinetic advantages of AMDMFC are elucidated. In thermodynamic aspect, a large voltage loss due to the pH difference across the membrane is predicted by theoretical calculation; in kinetic aspect, besides the well-known superiority of alkaline media for oxygen reduction, experimental data show much higher anodic performance in carbonate/bicarbonate than in acid. In-situ FTIR measurements indicate that methanol can be fully oxidized to carbon dioxide in carbonate/bicarbonate as in sulfuric acid. Taking into account all the foreseeable advantageous and disadvantageous factors, AMDMFC is worth study, and an alkaline membrane stable at elevated temperatures is the prerequisite for a successful AMDMFC.