Stable Power Generation Research Articles

A New Pd Doped Proton Conducting Perovskite Oxide with Multiple Functionalities for Efficient and Stable Power Generation from Ammonia at Reduced Temperatures

AbstractThe combination of ammonia fuel and proton‐conducting fuel cells (PCFCs) technology may provide an ideal clean energy system for the future, considering matured NH3 synthesis technology and transportation and storage infrastructure, the high energy density of NH3, and the high efficiency of fuel cells. However, poor catalytic activity of the anode for NH3 decomposition, quick performance degradation due to the ammonia induced nickel coarsening, difficult sintering, and insufficient proton conductivity of electrolytes are the main challenges for stable and high‐power generation from ammonia‐fueled PCFCs. Herein, a new Ba(Zr0.1Ce0.7Y0.1Yb0.1)0.95Pd0.05O3−δ perovskite is reported as a key anode component and electrolyte, which demonstrates multifunctionalities and tackles most challenges of conventional PCFCs. The incorporation of a small amount of Pd boosts catalytic activity of the nickel‐perovskite cermet anode for NH3 decomposition and increases proton conductivity from the creation of B‐site cation deficiency and electrolyte sintering. The corresponding thin‐film electrolyte PCFC delivers a maximum power density of 724 mW cm–2 at 650 °C operated on NH3, much higher than the similar cell without Pd incorporation (450 mW cm–2). Furthermore, no apparent performance decay is observed for the cell operated at 550 °C in H2 and NH3 for 350 h, making it highly promising for practical applications.

The operation characters of biocathode microbial electrochemical system with microbial separator for domestic wastewater treatment: Power generation, long-term stability, and organic removal

Biocathode microbial electrochemical system (MES) is considered as a sustainable and eco-friendly technology for wastewater treatment. With domestic wastewater, the anode/cathode matching ratios of biocathode MES are optimized to match the anodic respiration and cathodic oxygen reduction rates. One anode module matched with two identical cathode modules obtains the highest power density. The long-term stability of MES is examined for one year with optimized architecture. Stable power generation and organic removal performance are achieved. A low effluent chemical oxygen demand (COD) is obtained at 36 ± 14 mg L−1 with a COD removal of 89 ± 4%. To meet the discharge standard in China (COD<50 mg L−1), the required hydraulic retention time (HRT) shows a linear relationship with influent COD concentration, and the COD removal rates fluctuant from 0.80 ± 0.05 to 1.13 ± 0.04 kg m−3 day−1. In the view of stable electrode function and current generation, the influent COD range of 200–800 mg L−1 is the stable range for biocathode MES. The bio-diversities of electrode and microbial separators are changed with influent COD. High influent COD reduces the relative abundance of exoelectrogens on the anode and introduces heterotrophic bacteria on the cathode.

Application of Climatic Characteristics Statistical Assessments and GIS Technologies for Solar Energy Systems Development

The problems of optimising the placement of photovoltaic panels to ensure the most stable power generation are considered. Method for assessing the risks of solar power plants operation instability in the based on taking into account the climatic characteristics of the variability of the insolation and snow cover using GIS technologies is presented. This approach is especially important for Russia, a country with a wide variety of natural and climatic conditions. To test this technique, we calculated the intra-annual and year-to year variation of the statistical characteristics of the total insolation incident for regions of Russia. The use of GIS technologies made it possible to assess the distribution of the features of the variability of solar radiation over the area. This paper presents the results of calculations and constructed maps for the Republic of Altai, the region of Russia with heterogeneous insolation conditions, where solar energy has been actively developing recent years. In these territories, zones with the lowest indicators of variability in the input of solar radiation have been identified where a high level of stability of electricity generation on photovoltaic power plants can be predicted.

Conceptual Structural Design and Comparative Power System Analysis of Ozone Dynamics Investigation Nano-satellite (ODIN)

The Ozone Dynamics Investigation Nano-Satellite (ODIN) is a CubeSat design proposed by Chungnam National University as contribution to the CubeSat Competition 2019 sponsored by the Korean Aerospace Research Institute (KARI). The main objectives of ODIN are (1) to observe the polar ozone column density (latitude range of 60° to 80° in both hemispheres) and (2) to investigate the chemical dynamics between stratospheric ozone and ozone depleting substances (ODSs) through spectroscopy of the terrestrial atmosphere. For the operation of ODIN, a highly efficient power system designed for the specific orbit is required. We present the conceptual structural design of ODIN and an analysis of power generation in a sun synchronous orbit (SSO) using two different configurations of 3U solar panels (a deployed model and a non-deployed model). The deployed solar panel model generates 189.7 W through one day which consists of 14 orbit cycles, while the non-deployed solar panel model generates 152.6 W. Both models generate enough power for ODIN and the calculation suggests that the deployed solar panel model can generate slightly more power than the non-deployed solar panel model in a single orbit cycle. We eventually selected the non-deployed solar panel model for our design because of its robustness against vibration during the launch sequence and the capability of stable power generation through a whole day cycle.

Mechanism model-based and data-driven approach for the diagnosis of solid oxide fuel cell stack leakage

Safety and reliability are key objectives for the efficient operation of solid oxide fuel cell (SOFC) power generation systems. Out of many possible faults, the gas leakage of SOFC stack remains a critical issue that leading to efficiency reduction or even degradation. Therefore, the real-time monitoring and diagnosis of gas leakage in the power generation systems are not only an important premise to improve the efficiency, but also can develop the corresponding fault-tolerant strategy for ensuring the system performance. Motivated by this fact, an on-line fault diagnosis scheme based on mechanism model and data-driven method is proposed to monitor and diagnose the gas leakage of the stack. Firstly, the two-state mechanism model of the SOFC stack is established, which can effectively describe the temperature of the fuel layer and air layer. Then, easily-measured stack inputs and outputs are selected, and a novel gas leakage state estimator combined with unscented Kalman filter (UFK) is developed to reconstruct the leakage state. Furthermore, an adaptive thresholds generator is designed to enhance the robustness of the diagnostic scheme. The performance of the fault diagnosis scheme under different leakage scenarios is evaluated, and the simulation results demonstrate the effectiveness of the proposed scheme. The sudden stack fuel leakage failure that occurred in the stable power generation experiment further illustrates the practicability of the scheme. The proposed fault diagnosis scheme has good practicability and can guide the next step compensates for leakage.

Influence of orifice in entropy generation and exergy analysis under transient condition for large diameter natural circulation loops

Stable power generation is of concern due to instability in NCL. The entropy generation analysis in a 38.89 mm diameter loop identified the flow reversals by a drop in its value after a peak. Orifi...

Cost Optimization Modeling of Renewable Energy Sources in Smart-Grid Using SCADA

In recent times, renewable energy production from renewable energy sources is an alternative way to fulfill the increased energy demands. However, the increasing energy demand rate places more pressure, leading to the termination of conventional energy resources. However, the cost of power generation from coal-fired plants is higher than the power generation’s price from renewable energy sources. This experiment is focused on cost optimization during power generation through pumped storage power plant and wind power plant. The entire modeling of cost optimization has been conducted in two parts. The mathematical modeling was done using MATLAB simulation while the hydro and wind power plant’s emulation was performed using SCADA (Supervisory control and data acquisition) designer implementation. The experiment was conducted using ranges of generated power from both power sources. The optimum combination of output power and cost from both generators is determined via MATLAB simulation within the assumed generated output power range. Secondly, the hydro-generator and wind generator’s emulation were executed individually through synchronizing the grid to determine each generator’s specification using SCADA designer, which provided the optimum power generation from both generators with the specific speed, aligning with results generated through MATLAB. Finally, the operational power cost (with no losses consideration) from MATLAB was compared with the local energy provider to determine the cost-efficiency. This experiment has provided the operational cost optimization of the hydro-wind combined power system with stable wind power generation using SCADA, which will ultimately assist in operations of large-scale power systems, remotely minimizing multi-area dynamic issues while maximizing the system efficiency.

A new strontium based reactive carbonate composite for thermochemical energy storage

A new reactive carbonate composite based on SrCO3 is presented as a material with high energy density for thermochemical energy storage, being an excellent material to meet the requirement for stable power generation from renewable sources.

Strategies for improvement of microbial fuel cell performance via stable power generation from real dairy wastewater

The microbial fuel cells (MFCs) are one of the emerging sustainable power sources, that are being effectively used for the simultaneous treatment of wastewater with concomitant generation of electricity. The present study develops a fed-batch process with an optimized level of two microorganisms for concurrent treatment of real dairy wastewater (RDW) and stable power generation using a single-chamber MFC. The MFC exhibits a maximum open circuit voltage of 652 mV at 264 h of fed-batch operation. The unstructured kinetic model suggests that the value of saturation constant (Ks) is 41.67 mg COD/L, indicating mixed culture has a higher affinity towards the utilization of RDW. The fed-batch mode achieves maximum COD removal efficiency of 95.31%, columbic efficiency of 31.58%, a maximum current density of 263 mA/m2, and power density (62.27 mW/m2). The optimum cell voltage and internal resistance of MFC are 0.48 V and 340 Ω, respectively. The study suggests that a fed-batch operation with mixed culture is suitable for the treatment of RDW and able to generate stable power from MFC.

Imaging and Quantification of Water inside Membrane-Electrode Assembly of Polymer Electrolyte Fuel Cell during Power Generation By Use of Neutron Beam

The polymer electrolyte fuel cells (PEFCs) have attracted attentions as energy-conversion devices of high power density with no emission of carbon dioxide. For the efficient and stable power generation of a PEFC, the water management inside the membrane-electrode assembly (MEA) is very important. A proper water management can adequately moisten the electrolyte membrane and improve the performance of a PEFC, while an excessive water can block the flow channels and the pores in gas diffusion layers (GDLs), as well as the catalyst layers; the mass transports are inhibited from the flow channels to the Pt catalysts resulting in a poor power generation. In case of large liquid water accumulations, the performance of the power generation is temporary decreased.1 Neutron imaging techniques have been used for understanding the distribution of liquid water inside PEFCs, and water droplets within the pores of the GDL and/or on the flow-channel walls have been mainly reported.2–4 However, the distribution of smaller amounts of water contained in catalyst layers or an electrolyte membrane has not been reported, which is very important because of the large effect on the mass transport inside the MEA.The visualization of water in an MEA was carried out using a pulse neutron beam at BL22 (RADEN) of MLF, J-PARC, Japan 5. The neutrons with the wavelength of 5-13.5 Å was separated and used to increase the attenuation of the neutron beam by water. A PEFC with 10 straight channels, 30 mm in length and 1 mm in width, was used. The endplates and current collectors were made of aluminum alloy (A7075) for the neutron penetration. The transmitted neutrons were converted to photons by a scintillator (ZnS(Li)) for imaging with a CCD camera. The neutron beam size was 102.4 mm ×102.4 mm (2048×2048 pixels), while a plate containing boron carbide (B4C) powders was used for a window of 50 mm × 50 mm to reduce the unpreferable radio activation of the cell. The cell temperature was kept at 80 oC. To obtain the background image, a dry N2 gas was supplied both to the anode and the cathode. Figures 1(a) and 1(b) show the background image under the dry condition and the image of water in the MEA at 100% RH (obtained by the division by the background image using an “ImageJ” software), respectively. The values at pixels aligned in the perpendicular direction in the area surrounded by yellow solid lines in Fig. 1(b) were integrated to be shown in Fig. 1(c). The neutron transmittance at the MEA shown by a red rectangle in Fig. 1(b) was lower than that in the other area. In this way, water in the MEA was detected by neutron imaging for the first time. The imaging of water during the power generation is now in progress, along with the qualification of water.Reference Hirakata, S. et al. Investigation of the effect of pore diameter of gas diffusion layers on cold start behavior and cell performance of polymer electrolyte membrane fuel cells. Electrochim. Acta 108, 304–312 (2013).Kramer, D. et al. In situ diagnostic of two-phase flow phenomena in polymer electrolyte fuel cells by neutron imaging: Part A. Experimental, data treatment, and quantification. Electrochim. Acta 50, 2603–2614 (2005).Turhan, A. et al. Quantification of liquid water accumulation and distribution in a polymer electrolyte fuel cell using neutron imaging. J. Power Sources 160, 1195–1203 (2006).Park, J. et al. Neutron imaging investigation of liquid water distribution in and the performance of a PEM fuel cell. Int. J. Hydrogen Energy 33, 3373–3384 (2008).Shinohara, T. et al. The energy-resolved neutron imaging system , RADEN. Rev. Sci. Instrum. 91, 043302 (2020). AcknowledgmentThis study was supported by Japan Science and Technology Agency (JST) and “SPer-FC Project” of New Energy and Industrial Technology Development Organization (NEDO). The neutron imaging was performed at BL22 (RADEN) of J-PARC (Proposal No. 2019A0217). Figure 1

Multimodal fault-tolerant control for single-phase cascaded off-grid PV-storage system with PV failure using hybrid modulation

A fault-tolerant control for single-phase cascade off-grid photovoltaic-storage system with PV failure is proposed in this paper. Depending on states of PVs, different operating modes are proposed to enhance reliability of the system under PV failure, including normal mode, partial PV fault mode, and full PV fault mode. Hybrid modulation is employed to reduce switching losses. Subsequently, the proposed fault-tolerant control achieves stable power generation by regulating different kinds of converter cells independently. A downscaled prototype rated at 220 V is established on hard-in-loop platform, and the experimental results demonstrate the validation of the proposed control strategy.

Performance analysis of 200 MW solar coal hybrid power generation system for transitioning to a low carbon energy future

Many researchers related to the power engineering have studied different innovative approaches to increase the operating performance of the power plants with regard to coal consumption and environmental impacts reduction. Solar coal hybrid power generation (SCHPG) system is one of the good approaches for improving operating performance and ecological indices in the short and midterm. The operating performance and ecological indices of 200 MW coal-fired thermal power plant could be improved by integrating a parabolic trough collector (PTC) solar field. The numerical simulation is performed using IPSEpro software based on the heat balance method in the design and off-design mode. In this paper, the first high-pressure heater (HPH-1) is replaced by PTC solar field with an aperture area of 35970 m2 in the aim of preheating the feedwater and achieving continuous and stable power generation. Simulated results show that the operating performance of SCHPG system achieved a design-point value in the summer season. Due to the DNI value and incidence angle, the low value of the operating performance occurred in the winter season. In the design-point condition, a decline in fuel consumption of 4.15 tons per hour and an increase in power output of 7.4 MW per hour are caused by fuel-saving and power-boosting operating modes. Moreover, specific equivalent coal consumption was reduced by 2.91% compared to the reference power plant. The annual reduction of coal consumption and CO2 emission was estimated at 9910 and 11,000 tons, respectively. This paper revealed that the SCHPG system can significantly contribute to the protection of the environment and the reduction of the mitigation of climate change.

Research on Generator Side PWM Control of Synchronous Permanent Magnet Wind Power Generation System

This paper mainly studies the generator side converter of permanent magnet synchronous wind power generation system using dual PWM converter as the main circuit. This paper mainly studies and analyzes the control principle and control mode of generator side PWM in wind power generation system, and models and simulates the motor side converter based on rotor field oriented control. According to the control characteristics of the machine side converter, the specific idea of adopting the PWM vector control strategy is analyzed. The vector control strategy based on feedforward decoupling is proposed. The results of MATLAB / Simulink simulation show that: the PMSM Wind power generation system with vector control strategy can achieve stable power generation operation with variable speed and constant frequency, and can effectively adjust the output power of the power generation system, and the output current has good sinusoidal performance and small harmonic.

Stable soliton pulse generation from a SnSe2-based mode-locked fiber laser

Based on an as-prepared high-quality tin diselenide-polyvinyl alcohol (SnSe2-PVA) film, a stable passively erbium-doped (Er-doped) soliton mode-locked fiber laser at 1561.38 nm was realized. The output performances were investigated experimentally. At the pump power of 310 mW, the shortest pulse duration of 1.197 ps was obtained with a fundamental frequency of 3.45 MHz, corresponding to the peak power of 317 W, which is much higher than previous reports of SnSe2 mode-locked fiber lasers. The results show that SnSe2 can offer an exciting platform for stable high peak power and ultrashort pulse generation.

Large scale roll-to-roll production of polyurethane-acrylate-based hydrophobic film: a next-generation protection layer for solar devices

In the present study, we fabricate a hydrophobic film using a roll-to-roll technique capable of large-area continuous production and examine the film’s applicability to solar panels. The hydrophobic film was fabricated using a polyurethane-acrylate (PUA)-based UV-curable material, which has high optical transmittance and high releasability. The roll-to-roll system used in this study was custom manufactured to transfer micro-patterns of 20 μm in size onto the PUA-based film. In addition, a UV irradiation device was attached to the system to enable production of the hydrophobic film at a rate of 150 mm min-1. Through experiments, we confirmed that attaching the fabricated hydrophobic film not only enabled surface self-cleaning on the solar panels, but also ensured stable solar power generation. Thus, the hydrophobic film can be applied to installations that require external periodic cleaning and management, such as the exterior walls of buildings and solar panels.

Metal-Organic Powder Thermochemical Solid-Vapor Architectonics toward Gradient Hybrid Monolith with Combined Structure-Function Features

The conversion of inorganic powders or metal powders into gradient components is typically achieved with the help of a non-uniform electric or pressure field. However, the current technologies are ineffective in converting powder into gradient-structured hybrid components with combined structure-function features because of the lack of a non-uniform field to control evolution of the structures. The present work addresses this challenge by taking advantage of uniformly distributed metal nanoparticles and non-uniformly distributed reactive gases generated during thermopyrolysis of metal-organic powder to generate gradient metal-carbon monolith via combined chemical vapor deposition and solid-state welding. The obtained monolith shows good mechanical strength and high catalytic activity, allowing it to be used as a working electrode for a seawater battery. This method connecting gas-solid-state synthesis together with processing may lead to other gradient hybrid monoliths in the future.A gradient carbon-metal hybrid component can be manufactured by thermopyrolysis of metal-organic framework powder. The non-uniformly distributed carbonaceous vapor and uniformly dispersed Co nanoparticles are essential for generating the gradient carbon monolith. This monolith exhibits good mechanical strength and high catalytic activity, and thus can be directly utilized as an electrode for a Mg/HO battery to generate electricity from seawater. The assembled battery shows power density comparable with that of a battery with Pt as the electrode.Direct manufacturing from powder to final component is of great significance for industry. However, it remains a challenge to develop a one-pot “powder-to-product” strategy to produce gradient hybrid components with combined structural and functional advantages. In this work, we report metal-organic powder thermochemical solid-vapor architectonics to direct zeolitic imidazolate framework powder into gradient cobalt/carbon monolith by utilizing in situ generated reactive vapor (non-uniform distribution) and Co nanoparticles (uniformly distributed) to achieve a combination of chemical vapor deposition/growth and solid-state welding. The gradient hybrid monolith possesses good mechanical stability, allowing it to be directly used as a freestanding working electrode for hydrogen evolution reaction in a seawater battery. This catalyst shows a low overpotential of 84 mV at a current density of 10 mA cm. Furthermore, a stable power generation of over 168 h in seawater has also been realized.

Effect of regenerator on the direct steam generation solar power system characterized by prolonged thermal storage and stable power conversion

Abstract The direct steam generation (DSG) solar power system using two stage accumulators and cascade steam-organic Rankine cycle (RC-ORC) has remarkably enlarged storage capacity. It can facilitate stable power generation and address the challenges of conventional DSG systems. Regenerator is generally an issue worthy of discussion in organic Rankine cycle (ORC) systems. However, its influence on the newly proposed DSG system has not been investigated yet and is expected to be appreciable. Introducing a regenerator affects not only the ORC efficiency, RC-ORC efficiency, heat exchanger area, but also heat storage capacity, discharge duration, discharge efficiency, aperture area of collectors and the net profit ( Δ P ). Detailed performance comparison between the DSG systems without/with regenerator is carried out in this paper. The results indicate that at a given power output, aperture area is reduced by the regenerator especially for MM, R365mfc and pentane due to the increment in ORC, RC-ORC and discharge efficiencies, as well as the decrement in heat input. Discharge duration is shortened by 0.01–1.78 h depending on ORC fluids. R365mfc exhibits the maximum Δ P (4.19 ∼ 6.48 million USD), followed by MM and pentane. On the contrary, Δ P is negative for benzene (−5.61 ∼ -4.31 million USD).

Look-ahead bidding strategy for concentrating solar power plants with wind farms

The concentrating solar power (CSP) plant with the thermal energy storage (TES) is one of the most effective methods to solve the intermittent characteristics of solar energy. CSP plants combined with wind farms could provide continuous, stable power generation and reduce the uncertainty of the wind power. In this paper, a look-ahead technique is proposed to optimize the bidding strategy for a joint system of CSP plants with wind farms considering both the day-ahead and the following day. It is assumed that the CSP plant participates in the ancillary service (AS) market bidding while providing the reserve capacity for the wind farm to counteract the output fluctuations. As the price-taker, the joint system has different levels of attention to the market price and renewable energy output. Therefore, the scenario set and the chance-constrained programming (CCP) is used to describe its uncertainty, respectively. Besides, in order to further enhance the flexibility and maximize the economic revenues of the system, an electric heater (EH) is added to the model. The joint operator can adjust the final heat storage level to balance profit opportunities during the current market window against potential opportunities in the subsequent market window. Finally, we compare the results of different bidding models to demonstrate the effectiveness of the proposed model and the advantages of the joint system.

Metal-Organic Powder Thermochemical Solid-Vapor Architectonics Towards Gradient Hybrid Monolith with Combined Structure-Function Features

Direct manufacturing from powder to final component is of great significance for industry. However, it remains a challenge to develop a one-pot “powder to product” strategy to produce gradient hybrid component with combined structure-function advantages. In this work, we report a metal-organic powder thermochemical solid-vapor architectonics to direct zeolitic imidazolate framework powder into gradient cobalt/carbon monolith with atomically doped nitrogen, encapsulated cobalt nanoparticles, nanotubes arrays, and well-interconnected grains. The in situ generated H 2 vapor (non-uniform distribution) and Co nanoparticles (uniformly distributed) combines a chemical vapor deposition/growth and a solids-state welding together, leading to formation of the unique gradient monolith. The gradient carbon monolith is of good mechanical stability, therefore is directly used as a freestanding working electrode for hydrogen evolution reaction (HER) in a seawater battery. This catalyst shows a low overpotential of 84 mV at a current density of 10 mA cm-2 as well as good stability for HER at a constant overpotential of 200 mV for 5 h. Furthermore, a stable power generation of over 168 h in seawater has been realized.

Functional Design of the Gradient Carbon Fiber Electrode for Flow Batteries

To promote the universal application of renewable energy has become an important strategy for the energy security and sustainable economic development around the world. The proportion of renewable energy in the overall energy structure is greatly increasing in recent years, however, it is intermittent and unstable for the renewable energy generation, and the direct grid connection of the renewable energy generation will affect the stable operation of the power grid system. Supporting high-efficiency energy storage batteries to ensure the continuity and stability of power generation and power supply is an important way to achieve renewable energy development strategies. Thereinto, flow batteries (FBs) have a broad application prospect and huge market potential due to their safety, easy scale, flexible structure design and environmental friendliness.As the platform of the electrochemical reaction for FBs, the electrode is one of the key materials to determine its performance and cycle life. Currently the widely used electrode material of vanadium flow battery (VFB) is the polyacrylonitrile-based carbon felt (CF), and its low price and high stability make it the first choice of the electrode materials of FBs. However, the relatively lower electrochemical activity and specific surface area, in addition to higher electron resistance and transmission resistance of CF greatly limit the improvement of the battery performance and the reduction of the energy storage cost. For example, the power density of VFB is only 0.6 W cm-2, and its operating cost is about 650 $ kWh-1. In general, the basic requirements for the electrode materials of VFB as follows: i) high fluid permeability; ii) high specific surface area; iii) high electron conductivity; iv) low cost; v) high chemical and electrochemical stability. In order to achieve high electrolyte permeability, the conventional CF often possesses an average fiber diameter of up to 10 μm, resulting in a low electrochemical surface area. Therefore, only thicker electrodes can be used in VFB to ensure the electrochemical reaction interface, but this will result in a substantial increase in the ohmic resistance. In addition, the electrochemical activity of the electrode often depends on the oxygen-containing group on the surface of CF. However, the introduction of the oxygen-containing group also destroys the conductive network of the original graphite fiber simultaneously, thereby causing the loss of electronic conductivity of the electrode. The above several factors are coupled to each other, resulting in a very low power density of VFB.Fig. 1 Schematic diagram of the gradient carbon fiber electrode for FBs.Generally, the electron conductivity of the electrode is much larger than the ion conductivity of the electrolyte, and the ion migration path on the membrane side is short for FBs, so the electrochemical reaction tends to occur at the membrane side, which is often called the reaction zone; meanwhile the ion migration path at the plate side is long, which is not conducive to the reaction, so it is often caller the current collecting area. Thereinto, the former should has higher electrochemical activity, while the latter should possess higher conductivity. Therefore, the carbon fiber electrode for FBs should be constructed according to its specific structure characteristics. Based on this, a gradient carbon fiber electrode is proposed to resolve the contradiction among the conductivity, activity and transmission. Firstly, the fiber density exhibits a gradient distribution in the thickness direction of the electrode, the plate side with higher fiber density is used to reduce the contact resistance, while the membrane side with lower fiber density is used to reduce the mass transfer polarization. In addition, electrolyte will tend to flow through the membrane side due to its lower transmission resistance, which could ensure the supply of the active reactant. Secondly, the nano fibers are also gradiently distributed in the thickness direction of the electrode. The plate side of the electrode is mainly composed of graphite fibers with good conductivity, while the membrane side mainly consists of the composites of graphite fibers/nano fibers with good activity. And such gradient structure could result in an excellent electrochemical activity, a favorable conductivity and a preferable transmission performance simultaneously, successfully resolving the efficient coordination problem of charger transfer, electron conductivity and ion transport in FBs. Acknowledgement: This study was funded by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) and HKUST fund of Nanhai (Grant No.FSNH-18FYTRI01). Figure 1