The intensifying energy crisis and detrimental environmental impacts have stimulated enormous interest in developing clean and renewable energy sources to decrease the dependence on traditional fossil energy.(1−4) Electrochemical water splitting is considered to be an important supplement of hydrogen energy, which contains two half reactions, namely, hydrogen evolution reaction (HER) at the cathode(5) and oxygen evolution reaction (OER) at the anode.(6) Operation of fuel cells that use hydrogen as the fuel involves hydrogen oxidation reaction (HOR) at the anode(7) and oxygen reduction reaction (ORR) at the cathode.(8) For the above-mentioned reactions, electrocatalysts are of great significance to effectively promote the reaction efficiency. In most cases, overpotential (η), the difference between the applied potential to deliver a certain current density and the thermodynamic potential value, is selected as the primary parameter to evaluate the activity of an electrocatalyst.(9) In general, electrochemical measurements are carried out in a three-electrode system, where a suitable reference electrode (RE) is applied according to the nature of the electrolyte solution. η is conventionally reported with reference to the reversible hydrogen electrode (RHE). The RHE conversion from the practically applied RE in most published literature is based on the simplified theoretical Nernst equation.(19,20) However, it should be noted that this correction method will lead to errors in the assessment of electrocatalytic activity. As shown in Table 1, the potential of one certain RE obtained through theoretical Nernst equation calculation and standard RE calibration using cyclic voltammetry (CV) can be varied by at most 30 mV, a value that can be used to show researchers so-called “great enhancement” in electrocatalytic activity, as the difference of η of one catalyst prepared from different methodologies but with same composition is typically in the range of tens of millivolts (mV). Moreover, with a similar experimental calibration process, the obtained calibrated value also differs case by case. Therefore, it is highly important to report the electrocatalytic activity, especially the η value, through a unified and reliable method. The value is obtained from experimental calibration process. The value is obtained from Nernst equation calculation. Herein, we have conducted standard experimental calibrations of distinct REs in a three-electrode system (Figure 1) in order to understand the difference between the measured and theoretical value and how the electrolyte conditions such as pH value and temperature will influence the measured overpotential. Pt foil (Aldrich) was selected as both the working electrode (WE) and counter electrode (CE). The electrolyte was saturated with high-purity hydrogen for at least 30 min before performing the electrode calibration. CV was carried out at a scan rate of 1 mV/s, and the average of the two interconversion point values was taken as the thermodynamic potential.(21) The commonly used Hg/HgO, Hg/Hg₂Cl₂, and Ag/AgCl REs were selected for alkaline, acidic, and neutral systems, respectively. Three parallel experiments were conducted for each electrode to eliminate accidental error. (The specific correction values for each experiment are listed in Table S1.) Figure 1. Experimental setup of the sealed standard three-electrode cell for reference electrode calibration. Pt foil was used as both the working electrode and counter electrode, and the electrolyte was saturated with high-purity hydrogen. Hg/HgO Electrode: Hg | HgO(s) | OH^–. Hg/HgO RE is usually used in alkaline solutions.(22) In most literature contributions, the experimental electrode potential versus Hg/HgO was converted to the potential versus RHE according to the following Nernst equation:(15,23)(1)However, the results in Table 1 suggest the possible systematic error of the Nernst equation for Hg/HgO electrode, especially in 0.1 M KOH dilute electrolyte. Thus, we performed RHE calibration to investigate the effect of alkaline concentration on the correction value at 25 °C. As shown in Figures 2 and S1, the calibrated value for Hg/HgO electrode in 0.1 M KOH at 25 °C is 0.894 V, which is about 30 mV greater than that calculated from the Nernst equation. This comes from a liquid junction potential (LJP) between the electrolyte inside the Hg/HgO electrode (1 M KOH) and the operating electrolyte (0.1 M KOH).(24) For the calibration in 1 M KOH (Figure S2), the average measured value is 0.927 V, which seems very close to the theoretical value (0.924 V). However, it does not mean that the equation is applicable here. In fact, the error is occasionally eliminated by the difference between the theoretical (14) and practical (13.65) pH value of 1 M KOH. When the measured pH value (Table S2) is adopted for the calculation, which is obviously more rigorous, the calculated value is about 20 mV less than the measured value. This discrepancy is even obvious in 0.1 M KOH. These results indicate that the errors resulting from the use of the conventional theoretical value are inevitable but non-negligible. Thus, it is highly essential to perform RHE calibration of the Hg/HgO electrode under experimental conditions. Figure 2. RHE calibrated potential of Hg/HgO electrode with different methodologies: measured at 25 °C (experimental calibration) and theoretically calculated values based on Nernst equation with pH value in theory and the practical pH value. Hg/Hg₂Cl₂ Electrode (SCE): Hg | Hg₂Cl₂(s) | KCl. The Hg/Hg₂Cl₂ electrode is more suitable for acidic systems. The potentials referred to RHE are generally calibrated according to the following equation:(25)(2) The RHE calibration of Hg/Hg₂Cl₂ electrode at 25 °C with different H₂SO₄ concentration conditions was performed (Figures 3, S3, and S4). In 0.1 M H₂SO₄, the experimentally and theoretically corrected values are 0.299 and 0.283 V, respectively, which are 0.260 and 0.241 V in 0.5 M H₂SO₄. The differences between the experimental value and the theoretical value are mainly due to the reference electrode itself,(24) such as the purity of the mercury, the existence of oxygen inside the reference electrode, and the coarse crystalline of calomel. Moreover, one can see that the calculated value with the measured practical pH value for SCE is higher than the theoretical value, giving a different trend in comparison with the Hg/HgO system. Figure 3. RHE calibrated potential of Hg/Hg₂Cl₂ electrode with different methodologies: measured at 25 °C (experimental calibration) and theoretically calculated values based on Nernst equation with pH value in theory and the practical pH value. Ag/AgCl Electrode: Ag | AgCl(s) | KCl. In electrochemical measurements, Ag/AgCl is usually used as the RE to evaluate the performance of the catalysts in the neutral system. The conversion of Ag/AgCl (saturated KCl) reference electrode to RHE is expressed as the following equation:(26)(3) We carried out the RHE calibration of the Ag/AgCl electrode in 0.1 M Na₂SO₄ solution at 25 °C (Figure S5), and the calibrated value is 0.706 V, which is around 90 mV greater than the value calculated from the Nernst equation (Figure 4). With the measured pH value (Table S2), the difference is even greater (180 mV). Such a big difference is mainly induced by the dissolution of AgCl, as the electrolyte inside the Ag/AgCl electrode is saturated KCl, making AgCl facilely soluble.(27) It further leads to the fluctuation of the reference electrode potential. Moreover, the dissolved AgCl will clog the reference electrode. If the exudation of KCl solution from inside the Ag/AgCl electrode is blocked, the LJP will be unstable and changed.(28) As a consequence, compared with the other two REs, the difference between the measured and calculated values is much greater for the Ag/AgCl electrode. Figure 4. RHE calibrated potential of the Ag/AgCl electrode with different methodologies: measured at 25 °C (experimental calibration) and theoretically calculated values based on Nernst equation with pH value in theory and the practical pH value. The effect of temperature on the calibration and also the tested value of the overpotential are investigated (Figure S6). The calibration experiments were performed at 20, 25, and 30 °C. The correction values decrease with the increase in temperature. Notably, the trend is against the one calculated from the Nernst equation, because the change in temperature might directly affect the standard electrode potential of the REs.(24) Of course, the difference between the tested values is within 10 mV for all the systems, making it reasonable to neglect the influence of temperature in most situations except for the electrocatalysts with extremely low overpotential (e.g., noble-metal-based HER catalysts).(29−31) In addition, electrocatalytic studies involving thermal or photothermal effect should be careful about the difference in calibration at different temperatures. Otherwise, much greater errors may be introduced. It should be noted that the cell geometry and electrode position can also affect the measured potential because of the electrolyte contribution, especially when the RE is far from the WE.(24) The position of the RE and the separation of the compartments of the reference and working electrodes are crucial to obtain reliable data for the electrode potential or to control the potential of the WE. In order to avoid this issue, the ohmic potential drop (iR drop), which arises between the WE and RE, should be corrected in reporting the overpotential values. In addition, for electrochemical measurements that use a solid-state or quasi-solid-state RE, proper calibration might also be required when potential values need to be reported, because of the presence of liquid junction potential between the RE and sample solutions by the slow leaching of the solidified reference electrolyte.(32) This, however, is a much more difficult task as the electrolyte itself varies case by case, not to mention the geometrical influence of the RE. The next question is how often we need to calibrate the REs for obtaining reliable potential values. We performed RHE calibration on these three electrodes with different time intervals (Figure 5). The CV curves are provided in Figures S7–S9. Among the electrodes, Hg/HgO and Hg/Hg₂Cl₂ are relatively stable. As for the Ag/AgCl electrode, its electrode potential fluctuated by about 20 mV after 10 days of storage. It is worth noting that the stability of the REs is manufacture-related in our experience. Therefore, RHE calibration for all kinds of REs is not a once-and-for-all process. Moreover, we also encourage researchers to report the full cell data for water splitting, because the error caused by RE calibration can be eliminated this way. Figure 5. Time-dependent RHE calibrated potential values of the Hg/HgO, Ag/AgCl, and Hg/Hg₂Cl₂ reference electrodes. Therefore, it is necessary to regularly calibrate the reference electrodes during the evaluation of electrocatalyst activity (especially in cases where relative potentials are reported) because of the nonnegligible difference (can be more than 30 mV) between the experimentally calibrated value and conventional value based on the simplified Nernst equation. Such a potential difference can be utilized to demonstrate the so-called great enhancement in intrinsic activity (i.e., reduction in overpotential to deliver a certain current density). In addition, experimental conditions such as operating temperature and practical pH value can also impact the reference electrode calibration and therefore should be clearly presented. We hope this work will help the standardization of measuring and reporting the overpotential values in the field of electrocatalysis. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.0c00321.

• Experimental details and additional figures and tables (PDF)

Experimental details and additional figures and tables (PDF) Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest. Electronic Supporting Information files are available without a subscription to ACS Web Editions. The American Chemical Society holds a copyright ownership interest in any copyrightable Supporting Information. Files available from the ACS website may be downloaded for personal use only. Users are not otherwise permitted to reproduce, republish, redistribute, or sell any Supporting Information from the ACS website, either in whole or in part, in either machine-readable form or any other form without permission from the American Chemical Society. For permission to reproduce, republish and redistribute this material, requesters must process their own requests via the RightsLink permission system. Information about how to use the RightsLink permission system can be found at http://pubs.acs.org/page/copyright/permissions.html. We acknowledge the financial support from the National Natural Science Foundation of China (21471039, 21571043, 21671047, and 21871065), and Guangdong Provincial Key Laboratory of Energy Materials for Electric Power (No. 2018B030322001). This article references 32 other publications.

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如何可靠地报告电催化剂的超电势

日益加剧的能源危机和不利的环境影响激发了人们对开发清洁和可再生能源以减少对传统化石能源的依赖的巨大兴趣。（1-4）电化学水分解被认为是氢能的重要补充，其中包括两个半反应，即在阴极的氢放出反应（HER）和在阳极的氧逸出反应（OER）。（6）使用氢作为燃料的燃料电池的运行涉及氢的氧化反应（HOR）。阳极（7）和阴极的氧还原反应（ORR）。（8）对于上述反应，电催化剂对有效提高反应效率具有重要意义。在大多数情况下，超电势（η）选择提供一定电流密度所施加的电势与热力学电势值之间的差作为评估电催化剂活性的主要参数。（9）通常，电化学测量是在三电极系统中进行的，根据电解质溶液的性质使用合适的参比电极（RE）。常规上参考可逆氢电极（RHE）报道η。在大多数已发表的文献中，实际应用的RE的RHE转换基于简化的理论Nernst方程。（19,20）但是，应注意的是，这种校正方法将导致电催化活性评估中的错误。如表1所示，通过理论能斯特方程计算和使用循环伏安法（CV）进行的标准RE校准获得的某一RE的电位最大可变化30 mV，该值可用于显示研究人员所谓的“大幅度提高”的电催化活性因为由不同方法制备但具有相同组成的一种催化剂的η之差通常在数十毫伏（mV）的范围内。此外，在类似的实验校准过程中，所获得的校准值也会因情况而异。因此，通过统一而可靠的方法报告电催化活性，尤其是η值非常重要。该值是从实验校准过程中获得的。该值是从能斯特方程计算得出的。在这里 为了了解测量值与理论值之间的差异以及电解质条件（例如pH值和温度）如何影响测量到的超电势，我们已经在三电极系统（图1）中对不同的RE进行了标准实验校准。选择铂箔（Aldrich）作为工作电极（WE）和对电极（CE）。在执行电极校准之前，将电解质用高纯氢饱和至少30分钟。以1 mV / s的扫描速率进行CV，并将两个相互转换点值的平均值作为热力学势能。（21）常用的Hg / HgO，Hg / Hg 在执行电极校准之前，将电解质用高纯氢饱和至少30分钟。以1 mV / s的扫描速率进行CV，并将两个相互转换点值的平均值作为热力学势能。（21）常用的Hg / HgO，Hg / Hg 在执行电极校准之前，将电解质用高纯氢饱和至少30分钟。以1 mV / s的扫描速率进行CV，并将两个相互转换点值的平均值作为热力学势能。（21）常用的Hg / HgO，Hg / Hg分别为碱性，酸性和中性体系选择了₂ Cl ₂和Ag / AgCl REs。对每个电极进行了三个平行实验，以消除意外错误。（每个实验的具体校正值列在表S1中。）图1.用于参考电极校准的密封标准三电极电池的实验装置。铂箔既用作工作电极又用作对电极，并且电解质被高纯度氢饱和。Hg / HgO电极：Hg | 氧化汞| OH ^–。Hg / HgO RE通常用于碱性溶液中。（22）在大多数文献贡献中，根据以下能斯特方程将实验电极电势相对于Hg / HgO的电势转换为相对RHE的电势：（15,23）（1）但是，表1中的结果表明，Hg / HgO电极的能斯特方程可能存在系统误差，尤其是在0.1 M KOH稀电解液中。因此，我们进行了RHE校准，以研究碱性浓度对25°C下校正值的影响。如图2和S1所示，Hg / HgO电极在25°C下于0.1 M KOH中的校准值为0.894 V，这比从能斯特方程式计算出的校准值大约30 mV。这来自于Hg / HgO电极内部的电解质（1 M KOH）与工作电解质（0.1 M KOH）之间的液接电位（LJP）。（24）对于1 M KOH的校准（图S2），平均测量值为0.927 V，这似乎非常接近理论值（0.924 V）。但是，这并不意味着该公式在这里适用。事实上，该误差有时会因1 M KOH的理论（14）和实际（13.65）pH值之间的差异而消除。当采用测得的pH值（表S2）进行计算时（显然更为严格），计算值比测得值小约20 mV。在0.1 M KOH中，这种差异甚至很明显。这些结果表明，使用常规理论值导致的误差是不可避免的，但不可忽略。因此，在实验条件下对Hg / HgO电极进行RHE校准非常重要。图2.用不同的方法对Hg / HgO电极进行RHE校准的电势：在25°C下测量（实验校准），并基于Nernst方程在理论上根据pH值和实际pH值计算得出理论值。当采用测得的pH值（表S2）进行计算时（显然更为严格），计算值比测得值小约20 mV。在0.1 M KOH中，这种差异甚至很明显。这些结果表明，使用常规理论值导致的误差是不可避免的，但不可忽略。因此，在实验条件下对Hg / HgO电极进行RHE校准非常重要。图2.用不同方法对Hg / HgO电极进行RHE校准的电势：在25°C下进行测量（实验校准），并基于Nernst方程在理论上根据pH值和实际pH值计算得出理论值。当采用测得的pH值（表S2）进行计算时（显然更为严格），计算值比测得值小约20 mV。在0.1 M KOH中，这种差异甚至很明显。这些结果表明，使用常规理论值导致的误差是不可避免的，但不可忽略。因此，在实验条件下对Hg / HgO电极进行RHE校准非常重要。图2.用不同的方法对Hg / HgO电极进行RHE校准的电势：在25°C下测量（实验校准），并基于Nernst方程在理论上根据pH值和实际pH值计算得出理论值。在0.1 M KOH中，这种差异甚至很明显。这些结果表明，使用常规理论值导致的误差是不可避免的，但不可忽略。因此，在实验条件下对Hg / HgO电极进行RHE校准非常重要。图2.用不同的方法对Hg / HgO电极进行RHE校准的电势：在25°C下测量（实验校准），并基于Nernst方程在理论上根据pH值和实际pH值计算得出理论值。在0.1 M KOH中，这种差异甚至很明显。这些结果表明，使用常规理论值导致的误差是不可避免的，但不可忽略。因此，在实验条件下对Hg / HgO电极进行RHE校准非常重要。图2.用不同的方法对Hg / HgO电极进行RHE校准的电势：在25°C下测量（实验校准），并基于Nernst方程在理论上根据pH值和实际pH值计算得出理论值。Hg / Hg ₂ Cl ₂电极（SCE）：Hg | Hg ₂ Cl ₂（s）| 氯化钾。Hg / Hg ₂ Cl ₂电极更适合于酸性系统。RHE的电势通常根据以下等式进行校准：（25）（2）在25°C下使用不同的H ₂ SO ₄浓度条件对Hg / Hg ₂ Cl ₂电极进行RHE校准（图3，S3 ，和S4）。在0.1 MH ₂ SO _4中，实验和理论校正值分别为0.299 V和0.283 V，在0.5 MH中为0.260 V和0.241 V₂ SO ₄。实验值与理论值之间的差异主要归因于参比电极本身（24），例如汞的纯度，参比电极内氧的存在以及甘汞的粗晶。此外，可以看到，在实际SCE的实际pH值下测得的计算值高于理论值，与Hg / HgO系统相比，趋势有所不同。图3.用不同方法对Hg / Hg ₂ Cl ₂电极进行RHE校准的电势：在25°C下测量（实验校准），并基于Nernst方程在理论上根据pH值和实际pH值进行理论计算。Ag / AgCl电极：Ag | 氯化银| 氯化钾。在电化学测量中，通常使用Ag / AgCl作为RE来评估中性体系中催化剂的性能。Ag / AgCl（饱和KCl）参比电极到RHE的转换公式如下：（26）（3）我们在0.1 M Na ₂ SO ₄中对Ag / AgCl电极进行了RHE校准。溶液在25°C时（图S5），校准值为0.706 V，比能斯特方程计算得出的值（图4）大90 mV。使用测得的pH值（表S2），差异甚至更大（180 mV）。如此大的差异主要是由AgCl的溶解引起的，因为Ag / AgCl电极内部的电解质是饱和的KCl，使AgCl容易溶解。（27）进一步导致参比电极电位的波动。而且，溶解的AgCl会堵塞参比电极。如果阻止了从Ag / AgCl电极内部的KCl溶液的渗出，LJP将变得不稳定并发生变化。（28）因此，与其他两个RE相比，测量值和计算值之间的差异要大得多。 Ag / AgCl电极。图4。用不同的方法对Ag / AgCl电极进行RHE校准的电势：在25°C下进行测量（实验校准），并根据能斯特方程在理论上计算出pH值和实际pH值，从而得出理论计算值。研究了温度对校准的影响以及过电势的测试值（图S6）。校准实验在20、25和30°C下进行。校正值随着温度的升高而降低。值得注意的是，该趋势与能斯特方程计算得出的趋势相反，因为温度的变化可能会直接影响RE的标准电极电位。（24）当然，所有系统的测试值之间的差异在10 mV以内，因此在大多数情况下可以合理地忽略温度的影响，但过电位极低的电催化剂（例如贵金属基HER催化剂）除外。（29-31）此外，涉及热或光热效应的电催化研究应谨慎关于在不同温度下校准的差异。否则，可能会引入更大的错误。应该注意的是，由于电解质的作用，电池的几何形状和电极位置也会影响测量的电势，尤其是当RE远离WE时。（24）RE的位置和参比室的间隔工作电极对于获得可靠的电极电势数据或控制WE的电势至关重要。为了避免这个问题，欧姆电位降（（贵金属基HER催化剂）。（29-31）此外，涉及热或光热效应的电催化研究应注意不同温度下校准的差异。否则，可能会引入更大的错误。应该注意的是，由于电解质的作用，电池的几何形状和电极位置也会影响测量的电势，尤其是当RE远离WE时。（24）RE的位置和参比室的间隔工作电极对于获得可靠的电极电势数据或控制WE的电势至关重要。为了避免这个问题，欧姆电位降（（贵金属基HER催化剂）。（29-31）此外，涉及热或光热效应的电催化研究应注意不同温度下校准的差异。否则，可能会引入更大的错误。应该注意的是，由于电解质的作用，电池的几何形状和电极位置也会影响测量的电势，尤其是当RE远离WE时。（24）RE的位置和参比室的间隔工作电极对于获得可靠的电极电势数据或控制WE的电势至关重要。为了避免这个问题，欧姆电位降（涉及热或光热效应的电催化研究应注意不同温度下校准的差异。否则，可能会引入更大的错误。应该注意的是，由于电解质的作用，电池的几何形状和电极位置也会影响测量的电势，尤其是当RE远离WE时。（24）RE的位置和参比室的间隔工作电极对于获得可靠的电极电势数据或控制WE的电势至关重要。为了避免这个问题，欧姆电位降（涉及热或光热效应的电催化研究应注意不同温度下校准的差异。否则，可能会引入更大的错误。应该注意的是，由于电解质的作用，电池的几何形状和电极位置也会影响测量的电势，尤其是当RE远离WE时。（24）RE的位置和参比室的间隔工作电极对于获得可靠的电极电势数据或控制WE的电势至关重要。为了避免这个问题，欧姆电位降（应该注意的是，由于电解质的作用，电池的几何形状和电极位置也会影响测量的电势，尤其是当RE远离WE时。（24）RE的位置和参比室的间隔工作电极对于获得可靠的电极电势数据或控制WE的电势至关重要。为了避免这个问题，欧姆电位降（应该注意的是，由于电解质的作用，电池的几何形状和电极位置也会影响测量的电势，尤其是当RE远离WE时。（24）RE的位置和参比室的间隔工作电极对于获得可靠的电极电势数据或控制WE的电势至关重要。为了避免这个问题，欧姆电位降（RWE和RE之间出现的电压下降），应在报告超电势值时予以纠正。此外，对于使用固态或准固态RE的电化学测量，当需要报告电势值时，由于RE和样品溶液之间存在液体结电势，因此可能还需要适当校准。 （32）然而，这是一项艰巨的任务，因为电解质本身会因情况而异，更不用说RE的几何影响了。下一个问题是我们需要多久校准一次RE，以获得可靠的潜在值。我们在这三个电极上以不同的时间间隔进行了RHE校准（图5）。CV曲线在图S7–S9中提供。在电极之间，₂ Cl ₂相对稳定。对于Ag / AgCl电极，其电极电势在储存10天后波动约20mV。值得注意的是，根据我们的经验，可再生能源的稳定性与制造有关。因此，针对各种RE的RHE校准不是一劳永逸的过程。此外，我们还鼓励研究人员报告用于水分解的完整细胞数据，因为可以通过这种方式消除由RE校准引起的误差。图5. Hg / HgO，Ag / AgCl和Hg / Hg ₂ Cl ₂随时间的RHE校准电势值参比电极。因此，有必要在评估电催化剂活性期间定期校准参考电极（尤其是在报告相对电势的情况下），因为基于此值的实验校准值和常规值之间存在不可忽略的差异（可以大于30 mV）。简化的能斯特方程。这样的电位差可以用来证明内在活性的所谓的极大提高（即，降低传递特定电流密度的过电位）。此外，实验条件（例如工作温度和实际pH值）也会影响参比电极的校准，因此应清楚地说明。我们希望这项工作将有助于电催化领域中过电位值的测量和报告的标准化。可从https://pubs.acs.org/doi/10.1021/acsenergylett.0c00321免费获得支持信息。

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