Highly Energy-Efficient Vanadium Redox Flow Batteries with Thermo-Chemically Activated Graphite Felt Electrodes
Bibliographic record
Abstract
The Vanadium Redox Flow Battery (VRFB) is a promising energy storage technology for large scale stationary applications that require a long cycle life of over 10 years. The specific feature of VRFB is the slower kinetics of redox reactions (V2+/V3+) on the negative electrode (NE) as compared to the kinetics of the redox reaction (VO2 + /VO2+) on the positive electrode (PE) and poor kinetic reversibility.. The commonly used electrodes for VRFBs are graphite felt electrodes (GFE). One of the main challenges facing these GFEs is their activation that enhances a rate of redox reactions due to the increase of GFE wettability and surface area. Another challenge is capability to maintain GFE activation over cycling. In this work, we report on thermal and thermo-chemical activation of GFEs which leads to the formation of functional groups (carboxyl C=O, phenolic C-O, and adsorbed O or H2O for the enhancement of the electrode wettability and active surface area. The physicochemical characterization of the activated GFEs was performed using X-ray photoelectron spectroscopy (XPS) and contact angle measurements. The electrochemical study was conducted by cyclic voltammetry (CV) measurements in 0.2M VOSO4/2M H2SO4 at 1 mV/s and cycling tests (50 cycles, three tests for every GFE) in a commercial 9cm2 VRFB ( Standard Energy Corp.) using a charge/discharge current density of 80 mA/cm2 at 20oC and a commercial vanadium electrolyte 1.6M [V]/2M H2SO4. The increase of temperature of GFE heat treatment of GFEs from 400 to 500oC resulted in the growth of the energy efficiency (EE) of VRFBs with such electrodes from 81.3 to 87.2%, respectively, due to the increase of adsorbed oxygen or water from 1.6 to 8.72% and C-O (C-OH) groups from 0.49 to 2.09%, respectively and wettability (contact angle (CA) decreases from 78.7 to 65.1o). Analysis of CVs of the redox reaction VO2 + /VO2+ shows the decrease of the peak of potential separation ΔEp (Epa-Epc) from 0.357 to 0.345 V and the peak of current ratio (Jpa/Jpc) from 1.406 to 1.26, indicating lower polarization due to the increase of the electrode surface area, acceleration and reversibility of these redox reaction, respectively (Table 1). The proposed additional treatment of the heat treated GFE at 400oC (GFE400) in strong oxidants H2O2, H2SO4 and HNO3 for the decomposition of C=O and the enhancement of the content C-O groups and GFE wettability didnt result in the performance improvement (VRFB with GFE400+10%H2O2 - energy efficiency of 78%). The thermal treatment of GFE at 500oC demonstrated improvement of VRFB cell performance, namely growth of energy efficiency to 87.2% and VE to 90.1%, % due to the increase of wettability and active surface area of GFEs. Figure 1
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".