Vanadium redox flow battery (VRFB) deployment at elevated temperature is constrained by vanadium(V) [V(V)] precipitation, viscosity growth, and stack-level capacity fade. The objective was to test a composite acid ligand synergy (CALS) electrolyte design, in which sulfate–phosphate acid windows and multi-coordination additives are tuned jointly to stabilize VO2⁺ complexes and maintain performance between 40 and 55 °C. Three CALS formulations and one sulfuric acid baseline (all 1.80 mol L−1 vanadium, free acidity 2.20–2.30 mol L−1) were prepared in triplicate batches and screened at 40–55 °C. Physicochemical properties, solubility, and vanadium(IV)/V(V) speciation were quantified by rheometry, conductivity, density, ultraviolet–visible (UV–Vis) spectroscopy, and Raman spectroscopy. Electrochemical performance was evaluated in half-cells, 5 cm × 5 cm single cells, and a five-cell 100 cm2 stack (two stacks per condition) at 80–160 mA·cm−2 and 150–300 mL·min−1. Linear and mixed-effects regression related viscosity, conductivity, and VO2+ fraction to capacity fade and energy efficiency. CALS batches passed 55 °C clarity screening in 94–100% of cases versus 72% for baseline (χ2(1) = 4.21, φ = 0.37, p = 0.040). At 50 °C, viscosity fell from 6.2 ± 0.3 to 4.3 ± 0.2 mPa·s and conductivity rose from 0.35 ± 0.01 to 0.40 ± 0.01 S·cm−1 (F(1,16) = 9.84, η2p = 0.38, p = 0.006). Single-cell energy efficiency over cycles 20–100 improved by 3–7 percentage points across 40–55 °C (F(1,16) = 15.72, η2p = 0.50, p = 0.001), and stack capacity fade at 55 °C, 80 mA·cm⁻2 decreased from 0.065 to 0.038% per cycle (F(1,8) = 7.12, η2p = 0.47, p = 0.028). Maximum V solubility at 55 °C increased from 1.80 to 2.35–2.40 mol L⁻1, with precipitate-free vials at 240 h rising from 38 to 92–100% (H(3) = 8.74, ε2 = 0.73, p = 0.033). VO2+ complex fractions at 55 °C, cycle 300 stabilized at 66.5–68.2% for CALS versus 52.3% for baseline (t(4) = 3.24, d = 1.45, p = 0.031). Predictive models linking viscosity, conductivity, and VO2+ fraction to capacity fade and energy efficiency achieved R2 = 0.78–0.84 (all p ≤ 0.0004). CALS electrolytes couple sulfate–phosphate speciation control with multi-coordination additives to extend vanadium solubility, reduce capacity fade, and preserve high stack energy efficiency at 50–55 °C, providing quantitative design rules for warm-running VRFB systems.

Mechanism and stack-level verification of the synergistic effect of composite acid window and multi-coordination additive on the stability of vanadium electrolyte at high temperature

期刊论文
发表日期：

2026年3月12日

查看全文

Xianqi Hu

摘要

Vanadium redox flow battery (VRFB) deployment at elevated temperature is constrained by vanadium(V) [V(V)] precipitation, viscosity growth, and stack-level capacity fade. The objective was to test a composite acid ligand synergy (CALS) electrolyte design, in which sulfate–phosphate acid windows and multi-coordination additives are tuned jointly to stabilize VO₂⁺ complexes and maintain performance between 40 and 55 °C. Three CALS formulations and one sulfuric acid baseline (all 1.80 mol L⁻¹ vanadium, free acidity 2.20–2.30 mol L⁻¹) were prepared in triplicate batches and screened at 40–55 °C. Physicochemical properties, solubility, and vanadium(IV)/V(V) speciation were quantified by rheometry, conductivity, density, ultraviolet–visible (UV–Vis) spectroscopy, and Raman spectroscopy. Electrochemical performance was evaluated in half-cells, 5 cm × 5 cm single cells, and a five-cell 100 cm² stack (two stacks per condition) at 80–160 mA·cm⁻² and 150–300 mL·min⁻¹. Linear and mixed-effects regression related viscosity, conductivity, and VO₂⁺ fraction to capacity fade and energy efficiency. CALS batches passed 55 °C clarity screening in 94–100% of cases versus 72% for baseline (χ²(1) = 4.21, φ = 0.37, p = 0.040). At 50 °C, viscosity fell from 6.2 ± 0.3 to 4.3 ± 0.2 mPa·s and conductivity rose from 0.35 ± 0.01 to 0.40 ± 0.01 S·cm⁻¹ (F(1,16) = 9.84, η²_p = 0.38, p = 0.006). Single-cell energy efficiency over cycles 20–100 improved by 3–7 percentage points across 40–55 °C (F(1,16) = 15.72, η²_p = 0.50, p = 0.001), and stack capacity fade at 55 °C, 80 mA·cm⁻² decreased from 0.065 to 0.038% per cycle (F(1,8) = 7.12, η²_p = 0.47, p = 0.028). Maximum V solubility at 55 °C increased from 1.80 to 2.35–2.40 mol L⁻¹, with precipitate-free vials at 240 h rising from 38 to 92–100% (H(3) = 8.74, ε² = 0.73, p = 0.033). VO₂⁺ complex fractions at 55 °C, cycle 300 stabilized at 66.5–68.2% for CALS versus 52.3% for baseline (t(4) = 3.24, d = 1.45, p = 0.031). Predictive models linking viscosity, conductivity, and VO₂⁺ fraction to capacity fade and energy efficiency achieved R² = 0.78–0.84 (all p ≤ 0.0004). CALS electrolytes couple sulfate–phosphate speciation control with multi-coordination additives to extend vanadium solubility, reduce capacity fade, and preserve high stack energy efficiency at 50–55 °C, providing quantitative design rules for warm-running VRFB systems.