The efficient operation of vanadium redox flow batteries requires the half cell specific monitoring of the state of charge (SOC). Monitoring of the SOC is affected by temperature fluctuations, which need to be distinguished from the SOC signal. Several SOC monitoring approaches have been proposed in the literature and either been evaluated individually or compared to one other approach. The aim of this study is to contribute to SOC sensor development for VRFB by providing a first correlated assessment of several established and new SOC monitoring methods. We perform multi observable measurements of: half cell electrolyte potentials, electrolyte densities, electrolyte volumes, electrolyte viscosity related pressure drops, pH related potentials and UV/Vis absorbances. We determine SOC correlations of these observables in full charge/discharge cycles at constant temperature and derive temperature corrections in additional measurements over a range of 12°C to 32°C at 25%, 50% and 75% SOC. We compare the SOC and temperature sensitivity as well as the accuracy of these monitoring methods at constant and varied temperature. We find that the established half cell electrolyte potentials and UV/Vis absorbances can estimate the SOC at constant temperature with measurement errors of 1.7% and 4%, respectively and with slightly higher errors at varying temperatures. The viscosity related pressure drop and electrolyte density are both suitable for SOC estimation if temperature corrections are included. The pH related potentials for the positive half cell and both half cell electrolyte volumes could be used for a rough SOC estimate, but need accurate temperature corrections and further long term stability evaluation.

中文翻译：

---

钒氧化还原液流电池充电状态估计的相关多可观测评估——经验相关性和温度依赖性

全钒氧化还原液流电池的高效运行需要对半电池的充电状态 (SOC) 进行特定监测。 SOC的监测会受到温度波动的影响，需要与SOC信号区分开来。文献中提出了几种 SOC 监测方法，并单独评估或与其他方法进行比较。本研究的目的是通过对几种已建立的和新的 SOC 监测方法进行首次相关评估，为 VRFB 的 SOC 传感器开发做出贡献。我们对以下内容进行多项可观察测量：半电池电解质电势、电解质密度、电解质体积、电解质粘度相关压降、pH 相关电势和紫外/可见光吸光度。我们确定了恒温下完整充电/放电循环中这些可观测值的 SOC 相关性，并在 12°C 至 32°C 范围内、25%、50% 和 75% SOC 下的额外测量中得出温度校正。我们比较了恒定和变化温度下这些监测方法的 SOC 和温度灵敏度以及准确性。我们发现，建立的半电池电解质电势和 UV/Vis 吸光度可以估计恒温下的 SOC，测量误差分别为 1.7% 和 4%，并且在不同温度下误差稍高。如果包括温度校正，则与粘度相关的压降和电解质密度都适合 SOC 估计。正极半电池和两个半电池电解质体积的 pH 相关电位可用于粗略的 SOC 估计，但需要准确的温度校正和进一步的长期稳定性评估。