NB/T 42082-2016Vanadium flow battery - Test method for electrode (English PDF)

1 Scope

NB/T 42082-2016 is the Chinese energy sector standard that fixes how the electrode of a vanadium flow battery is tested. It exists because the flow battery is the technology the Chinese grid is betting on for storage measured in hours rather than minutes, and because in that battery the electrode is not where the energy is kept but where the reaction happens. The energy sits in two tanks of vanadium electrolyte outside the stack, which is why capacity and power can be sized independently; what the electrode has to do is give the vanadium ions a large, wettable, conductive surface on which to change oxidation state, hold that performance for the twenty years the asset is financed over, and do it while electrolyte is pumped through it continuously. An electrode that is slightly less porous, slightly less conductive or slightly less uniform in thickness than the datasheet claims turns into a round-trip efficiency the project never reaches, and the loss is invisible until the plant is built. This standard is the measuring stick that makes electrode specifications comparable between suppliers. It sets the general requirements and the sample preparation, then defines the test method for each property that matters: thickness uniformity, resistivity, mechanical properties, electrochemical activity, apparent density, surface density and flow resistance, closing with the requirements for the test report. Three informative annexes carry the detail of the flow resistance test and of the test preparation. It was issued on 16 August 2016 by the National Energy Administration and took effect on 1 December 2016.

This standard specifies the general requirements and test methods for electrodes for vanadium flow batteries, mainly including thickness uniformity test, resistivity test, mechanical property test, electrochemical activity test, apparent density test, surface density test, flow resistance test. This standard applies to the test methods for vanadium flow battery electrodes.

2 Normative references

The following documents are essential for the application of this document. For all dated references, only the dated version applies to this document. For all undated references, the latest version (including all amendments) applies to this document.

GB/T 29840-2013 Vanadium flow battery - Terminology

3 Terms, definitions, symbols

3.1 Terms and definitions The terms and definitions defined in GB/T 29840-2013, as well as the following terms and definitions, apply to this document. For ease of use, some terms and definitions in GB/T 29840 are repeated below.

3.1.1 Electrode The place where electrochemical reaction occurs. [GB/T 29840-2013 Definition 2.6]

3.1.2 Electrochemical activity The ability of the electrode surface to undergo electrochemical reaction. 3.1.3 - Temperature: 25 °C ± 5 °C; - Air humidity: 5% ~ 95%. For tests with special requirements for environmental conditions, the test environment conditions shall be determined by negotiation between the supplier and the buyer.

5 Test preparation

5.1 Sample preparation Sample preparation is as follows: - Samples are randomly selected from the same batch or different batches; - Samples shall be flat and undamaged; - Soak the samples in acetone for at least 1 hour; then dry them in an oven at 120 °C for more than 2 hours and set aside.

5.2 Test instruments and apparatus The instruments and apparatus used in this standard and their accuracy requirements are as follows: - Thickness gauge: Used to measure the thickness of the sample, which has an accuracy of not less than 10 µm; - Caliper: Used to test the length and width of the sample, which has an accuracy of not less than

0.1 mm; - Electronic balance: Used to test the mass of the sample, which has an accuracy of not less than

0.1 mg; - Low resistance tester: Used to test the vertical resistance of the sample, which has an accuracy of not less than

0.01 mOmega; - Mechanical properties testing machine: Used to test the mechanical strength of the sample, which has an accuracy of ±0.5% of its full scale; - Electrochemical tester: Used to test the activity of the sample, which has an accuracy of not less than 1 mA and a voltage accuracy of not less than 1 mV; - Precision pressure controller: Pressure accuracy is ±2% of its full scale; - Stainless steel fixture; - Flat plate forming machine; Where: - The average thickness of the sample under a certain pressure, in millimeters (mm); di - The thickness measurement value of a certain point of the sample under a certain pressure, in millimeters (mm); n - The number of measurement data points.

6.3.2 The thickness standard deviation is calculated by formula (2). Where: sigma - The thickness standard deviation of the sample under a certain pressure, in millimeters (mm); di - The thickness measurement value of a certain point of the sample under a certain pressure, in millimeters (mm); - The average thickness of the sample under a certain pressure, in millimeters (mm); n - The number of measurement data points.

6.3.3 The thickness dispersion coefficient is calculated by formula (3). Where: delta - Coefficient of dispersion, reflecting the degree of dispersion on the unit mean; sigma - Standard deviation of the thickness of the sample under a certain pressure, in millimeters (mm); The sample shall not be larger than the cross-sectional size of the flat fixture of the mechanical properties tester. The recommended sample is round, which has a diameter of 50 mm. The number of samples shall not be less than 3 (to ensure that 3 valid values are obtained).

8 Mechanical properties test...

Appendix A

(Informative) Test of the total resistance of the two measuring electrodes and the contact resistance between the sample and the two measuring electrodes The total resistance of the two measuring electrodes and the contact resistance between the sample and the two electrodes can be calculated according to formula (A.1). Where: Rc - The sum of the body resistance of the two measuring electrodes and the contact resistance between the sample and the two measuring electrodes, in milliohms square centimeters (mOmega - cm2); Rm1 - The measured value of the sample resistance when the thickness is , that is, the sum of the vertical resistance of the sample per unit area, the body resistance of the two measuring electrodes, the contact resistance between the sample and the two measuring electrodes, in milliohms (mOmega); - The average thickness of sample 1, in centimeters (cm); Rm2 - The measured value of the sample resistance when the thickness is , that is, the sum of the vertical resistance of the sample per unit area, the body resistance of the two measuring electrodes, the contact resistance between the sample and the two measuring electrodes, in milliohms (mOmega); - The average thickness of sample 2, in centimeters (cm); S - The contact area between the sample and the two measuring electrodes, in square centimeters (cm2).

Note: Take at least 3 samples of different thicknesses for testing. For each sample thickness, 3 valid samples shall be taken as a group; the average value shall be calculated as the test result under the corresponding thickness.

Appendix B

(Informative) Flow resistance test B.1 Sample preparation B.1.1 The sample size is 100 cm2 (10 cm × 10 cm); the number of samples is not less than 3 (to ensure 3 valid values). The sample shape and size can also be determined by negotiation between the supplier and the buyer. B.1.2 Place the sample in two hollow frames of the same size; the size of the hollow part of the frame is 10 cm × 10 cm; make a test electrode with airtight edges. B.2 Test method B.2.1 Assemble the test electrode between a fixture with a sealant line and a liquid inlet and outlet on one side; apply a certain pressure on the two fixtures, to ensure that there is no leakage at the edge of the fixture, as shown in Figure B.1. B.2.2 Inject liquid into the side with liquid inlet and outlet; measure the inlet and outlet pressure difference of the liquid flowing through the fixture at a certain liquid inlet flow rate and inlet pressure, recorded as Deltap1. B.2.3 Take out the test electrode; repeat B.2.1 and B.2.2; measure the inlet and outlet pressure difference of the liquid flowing through the fixture, recorded as Deltap2. Under this condition, the difference between Deltap1 and Deltap2 is the net flow resistance of the liquid flowing through the sample. B.2.4 Gradually increase the applied inlet pressure; repeat B.2.2 and B.2.3 for each increase of

0.1 MPa; record the net flow resistance value under different pressures.

Note: The recommended inlet pressure difference range is

0.05 MPa ~

1.0 MPa. B.2.5 Stop the test, when the change rate of the current measured sample net flow resistance value and the net flow resistance value measured last time is no more than 5%.

Appendix D

(Informative) Test report D.1 Overview Based on the test conducted, the test report shall provide enough correct, clear, objective data for analysis and reference. There are three types of reports: summary, detailed, complete. Each type of report shall contain the same title page and table of contents. D.2 Report content D.2.1 Title page The title page shall introduce the following information:

a) National standard code;

b) Sample name, material composition, specifications;

c) Sample state conditioning and test standard environment;

d) Test machine model;

e) Results of each test and the average value of the results;