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Background With the rapid increase in global energy demand, the burning of fossil fuels has caused a series of environmental problems. Researchers at home and abroad are committed to exploring clean energy and environmentally friendly and efficient energy storage and conversion devices. With the advantages of abundant resources, clean and efficient, high energy density, and environmental friendliness, hydrogen energy is an ideal renewable energy source. However, the sourcing and storage of hydrogen is one of the key factors restricting its development. The current hydrogen production methods include fossil fuel hydrogen production, biomass as raw material hydrogen production, and water splitting. Among them, hydrogen production by water splitting is increasingly attracting people's attention due to the advantages of green environmental protection, sustainability and ease of industrialization, etc. Water splitting involves oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). These electrocatalytic reactions, especially the OER, have slow kinetic rate, leading to high overpotential and low efficiency which severely restricts the development and practical application of energy conversion devices. The use of electrocatalyst can effectively reduce the energy barrier of the electrocatalytic reaction, accelerate the reaction rate, and reduce the overpotential so that the OER can be effectively completed, thereby improving the working efficiency of the conversion device. Therefore, exploring OER electrocatalysts with high performance has become one of the key factors to improve the performance of energy conversion devices. Theory The OER is an important half reaction of electrochemical energy conversion devices such as water splitting and metal-air batteries. Under acidic and alkaline conditions, the OER is a four-electron process with a slow kinetic rate, which restricts the performance of electrochemical energy conversion devices. Overall performance. According to density functional theory calculation, the OER under acidic and alkaline conditions both involves the adsorption of OOH*, O* and OH* intermediates. The difference is that the first step of OER under acidic conditions is the dissociation of water, and the final product are H+ and O2, while the first step of OER under alkaline conditions is the adsorption of OH-, and the final products are H2O and O2, as is shown in the following formula. Acid environment: Overall reaction: 2H2O → 4H+ + O2 + 4e- * + H2O ⇌ OH* + H+ + e- OH* ⇌ O* + H+ + e- O* + H2O ⇌ OOH* + H+ + e- OOH* ⇌ * + O2 + H+ + e- Alkaline environment: Overall reaction: 4OH- → 2H2O + O2 + 4e- * + OH- ⇌ OH* + e- OH* + OH- ⇌ O* + H2O + e- O* + OH- ⇌ OOH* + e- OOH* + OH- ⇌ * + O2 + H2O + e- Where, * means the active site on the catalyst surface, and OOH*, O* and OH* indicate adsorption intermediates. According to the four-step electronic reaction mechanism of OER, the important factors for improving the catalytic performance of OER can be analyzed from a theoretical perspective: (1) Good conductivity. Since the OER reaction process is a four-electron transfer reaction, good conductivity determines the rapid electron transfer, which helps the progress of each elementary reaction. (2) The catalyst has strong adsorption for OH-. The greater the amount of OH- adsorbed, the easier it is for the subsequent three-step electronic reactions to proceed. (3) Strong oxygen chemical desorption capacity and weak oxygen physical adsorption capacity. If the oxygen chemical desorption ability is strong, the O2 molecules produced during the catalytic process are more easily desorbed from the active site of the catalyst; if the oxygen physical adsorption capacity is weak, the O2 molecules are more likely to be precipitated from the electrode surface, and the OER reaction rate can be promoted. This has important guiding significance for the synthesis and preparation of OER catalysts. Performance evaluation of OER catalyst Initial potential and overpotential The initial potential is an important indicator for the catalytic activity of an electrocatalyst. But for the OER process, the initial potential is difficult to observe. Many OER electrocatalysts contain transition metal elements such as Fe, Co, Ni, etc. They will undergo oxidation reactions during the OER process and generate oxidation peaks, which is a great obstacle for the observation of the initial potential. Therefore, in the OER process, it is more scientific and reliable to observe the corresponding overpotential when the current density is 10 10 mA cm-2 or higher. The overpotential is obtained by linear sweep voltammetry (LSV). Overpotential refers to the difference between the electrode potential (vs. RHE) at a specific current density (usually 10 mA cm-2) and the equilibrium potential of the electrode reaction of 1.23 V, generally in mV. As shown in Figure 1, according to the difference in the overpotential of the OER electrocatalyst at a current density of 10 mA cm-2, the evaluation criteria for its catalytic effect are also different. The smaller the overpotential is, the less energy is required for the reaction, and the better the catalyst activity is. The overpotential of an OER catalyst with ideal catalytic activity is generally between 200~300 mV. Fig 1. Evaluation criteria of catalytic activity Tafel slope Tafel plot is the relationship curve between electrode potential and polarization current. It can reflect the reaction kinetics of the OER process and speculate the mechanism reaction of the OER process. The equation formula is: η = a + b·log|j| Where η represents the overpotential, b represents the Tafel slope, j is the current density, and a is the constant. The Tafel slope obtained according to the equation can be used to clarify the kinetics and speed-determining steps in the reaction process. Generally, the smaller the Tafel slope, the less the electron transfer barriers of the catalyst during the catalysis process, and the better the catalytic activity. Stability The stability of the catalyst in the catalysis process directly determines whether it can be applied on a large scale in actual production and is one of the important indicators of catalyst performance. For the OER, there are many factors that affect the activity of the OER electrocatalyst. For example, the acidity and basicity of the solution will affect the stability of the catalyst. Many OER electrocatalysts are stable under alkaline conditions, but not good under acidic conditions. In addition, the contact way of the electrocatalyst and the working electrode also has a great influence on the stability. In general, the direct growth of the catalyst in situ on the working electrode will be more stable than the organic adhesion agent on the working electrode. Currently, there are two electrochemical tests to judge the stability of the catalyst. One is chronopotentiometry (i.e, Galvanostatic). A constant current is applied on the electrode, and then the stability of the electrocatalyst is judged by observing the change of the potential over time. Similarly, the i-t curve (i.e, potentiostatic) is also applicable to test the catalyst. By applying a constant potential on the electrode, observing the change of the current with time, we can determine the stability of the catalyst. The other is to perform thousands or even tens of thousands of cyclic voltammetry (CV) tests on the electrocatalyst in a certain potential sweep range, and judge the stability of the electrocatalyst by comparing the polarization curves of the electrocatalyst before and after the cyclic voltammetry test. In addition to electrochemical testing, the use of some phase characterization tests such as XRD, XPS, SEM, TEM, etc. to compare the phase changes of the electrocatalyst before and after catalysis can also be used to judge the stability of the electrocatalyst. Experiment Setup Instrument: Corrtest potentiostat WE: Glassy carbon working electrode with catalyst evenly applied on the surface RE: Ag/AgCl reference electrode CE: Graphite rod Solution: 0.1 M KOH Electrochemical test Electrocatalyst activity Technique: Cyclic Voltammetry (CV) Potential range: 0~1 V (vs. Ag/AgCl) Scan rate: 50 mV s-1 Technique – Linear Sweep Voltammetry (LSV): Potential range: 0~1V (vs. Ag/AgCl), scan rate 5 mV s-1 Fig 2. CV parameters setting   Fig 3. LSV parameters setting The electrochemical impedance spectroscopy (EIS) is used to study the electrocatalytic oxygen evolution kinetics of the catalyst, and the impedance spectrum is fitted by establishing an equivalent circuit. The circuit includes Rs (solution resistance), Rct (charge transfer resistance) and CPE (constant phase angle element). The electrochemical impedance (EIS) test conditions are 0.5 V (vs. Ag/AgCl), the frequency test range is 1 Hz ~100 kHz, and the disturbance voltage is 5 mV. Fig. 4. EIS parameters setting Electrocatalyst Stability The techniques of potentiostatic, galvanostatic and cyclic voltammetry tests are used to evaluate the catalyst stability. The galvanostatic test is to use the corresponding current under a certain current density (usually 10 mA cm-2) as a constant current output, observe the voltage change during the test time (10 h), and then evaluate the stability. The potentiostatic method is to use the corresponding potential under a certain current density (usually 10 mA cm-2) as a constant voltage output, observe the current change during the test time (10 h), and then evaluate the stability. In the cyclic voltammetry test, the voltage range is 0~1V (vs. Ag/AgCl) and the CV is cyclically scanned 1000 cycles. The stability of the catalyst is illustrated by comparing curves before and after the stability test and analyzing the changes. Fig 5. parameters setting Notices: RE: Ag/AgCl electrode should be reserved in darkness without light, and please don’t use in alkaline solution for a long time. Saturated calomel electrode should not be used in alkaline solution for a long time. Hg/HgO electrode is suitable for alkaline solution. CE- in long time CV and LSV test, the Pt wire or Pt plate will deposit on the surface of the cathode material. You’d better not use it in the test of non-precious metal materials in monolithic electrolysis cell. There are two problems in the glass electrolytic cell: the corrosion of the glass in the alkaline solution and the influence of the Fe impurity of the glass on the OER activity. If the experiment is not particularly accurate, a glass electrolytic cell is OK; but if you want to study the influence of Fe content, it is recommended to use polytetrafluoroethylene.

Metal Corrosion When the metal material is in contact with the surrounding medium, the material is destroyed due to chemical or electrochemical action. Metal corrosion is a thermodynamic spontaneous process, converting a metal of high-energy state into a metal compound of a low-energy state. Among them, the corrosion phenomenon in the petroleum and petrochemical industry is more complicated, including the electrochemical corrosion of brine, H2S and CO2. The nature of most corrosion processes is electrochemical. The electrical properties of the metal/electrolyte solution interface (electric double layer) are widely used in corrosion mechanism studies, corrosion measurement, and industrial corrosion monitoring. The electrochemical methods commonly used in metal corrosion research are: open circuit potential (OCP), polarization curve (Tafel plot), electrochemical impedance spectroscopy (EIS). 1.Techniques in Corrosion study 1.1OCP On an isolated metal electrode, one anode reaction and one cathode reaction is performed at the same speed at the same time, which is called the coupling of the electrode reaction. The reaction of mutual coupling is called the “conjugation reaction”, and the whole system is called the “conjugate system”. In the conjugated system, the two electrode reactions inter-couplings with each other, and when the electrode potentials are equal, the electrode potentials don’t vary with time. This state is called “stable state”, and the corresponding potential is called “stable potential”. In the corrosion system, this potential is also called “(self) corrosion potential Ecorr”, or “open circuit potential (OCP)”, and the corresponding current density is called “(self) corrosion current density icorr”. Generally speaking, the more positive the open circuit potential, the more difficult it is to lose electrons and be corroded, indicating that the corrosion resistance of the material is better. CS potentiostat/galvanostat electrochemical workstation can be used to monitor the real-time electrode potential of the metal material in the system for a long time. After the potential is stabilized, the open circuit potential of the material can be obtained. 1.2 Polarization curve (Tafel plot) Generally, the phenomenon that the electrode potential deviates from the equilibrium potential when there is a current passing through is called “polarization”. In electrochemical system, when polarization occurs, the negative shift of the electrode potential from the equilibrium potential is called “cathodic polarization”, and the positive shift of the electrode potential from the equilibrium potential is called “anodic polarization”. To express the polarization performance of an electrode process completely and intuitively, it is necessary to experimentally determine the over-potential or the electrode potential as a function of current density, which is called “polarization curve”. The icorr of the metal material can be calculated based on the Stern-Geary equation. B is the Stern-Geary coefficient of the material, Rp is the polarization resistance of the metal. Principle to obtain icorr through Tafel extrapolation method Corrtest CS studio software can automatically do fitting to the polarization curve. The tafel slop on anode segment and cathode segment, i.e., ba and bc can be calculated. icorr can also be obtained. Based on the Faraday law and in combining with the electrochemical equivalent of the material, we can convert it to metal corrosion rate (mm/a) . 1.3 EIS Electrochemical impedance technology, also known as AC impedance, measures the change of voltage (or current) of an electrochemical system as a function of time by controlling the current (or voltage) of the electrochemical system as a function of sinusoidal variation over time. The impedance of the electrochemical system is measured, and further, the reaction mechanism of the system (medium/coating film/metal) is studied, and the electrochemical parameters of the fitting measurement system are analyzed. The impedance spectrum is a curve drawn from the impedance data measured by a test circuit at different frequencies, and the impedance spectrum of the electrode process is called an electrochemical impedance spectrum. There are many types of EIS spectrum, but the most commonly used are the Nyquist plot and the Bode plot. 2.Experiment example Taking an article published by a user using the CS350 electrochemical workstation as an example, a concrete introduction to the method of the metal corrosion measurement system is introduced. The user studied the corrosion resistance of Ti-6Al-4V alloy stent prepared by conventional wrought method(specimen #1), selective laser melting method(specimen #2) and electron beam melting method(specimen #3). The stent is used for human implantation, so the corrosion medium is simulated body fluid (SBF). The temperature of the experimental system also needs to be controlled at 37℃. Instrument: CS350 Potentiostat/galvanostat Experimental device:CS936 jacketed flat corrosion cell, Constant temperature drying oven Experimental drugs: Acetone, SBF, Room temperature curing epoxy resin Experimental medium: Simulated body fluid (SBF)：NaCl-8.01,KCl-0.4,CaCl2-0.14,NaHCO3-0.35,KH2PO 4-0.06, glucose -0.34, unit is: g/L Specimen(WE) Ti-6Al-4V Alloy stent 20×20×2 mm, Exposed working area is 10×10 mm The non-test area is coated/sealed with room temperature curing epoxy resin. Reference electrode(RE): Saturated calomel electrode Counter electrode(CE): CS910 Pt conductivity electrode The jacketed flat corrosion cell 2.1 Experiment steps and parameters setting 2.1.1 OCP Before testing. the working electrode needs to be polished from coarse to fine (360 mesh, 600 mesh, 800 mesh, 1000 mesh, 2000 mesh in order) till the surface is smooth. After polishing, rinse it with distilled water and then degrease it using the acetone, put it in a constant temperature drying oven and dry at 37℃ for use. Assemble the specimen onto the corrosion cell, introduce the simulated body fluid into the corrosion cell, and insert the saturated calomel electrode (SCE) with a salt bridge into the flat corrosion cell. Be sure that the tip of the Luggin capillary right face the working electrode surface. The temperature is controlled at 37℃ by water circulation. Connect the electrodes with the potentiostat by the cell cable. Experiment→stable polarization→OCP OCP You should enter a file name for the data, set the total time of the testing, and start the test. The OCP of metal material in the solution changes slowly, and it takes a relatively long period to keep stable. So it’s suggested to set time no shorter than 3000s. 2.1.2 Polarization curve Experiment→stable polarization→potentiodynamic Potentiodynamic scan Set the initial potential, final potential and scan rate, select the potential output mode as “vs. OCP”. The “Use” can be checked to choose the vertex E#1 and vertex E#2. If it’s not checked, then the scan will not go through the corresponding potential. There are up to 4 independent polarization potential set points. The scan starts from the initial potential, to “vertex E#1 ” and “vertex E#2”, and finally to the final potential. Click the "Enable" check box to turn on or off "Intermediate Potential 1" and "Intermediate Potential 2". If the check box is not selected, the scan will not pass this value and set the potential scan to the next one. It is noteworthy that the polarization curve measurement can only be conducted on the condition that the OCP is already stable. Usually after 10 minutes’ quiet time, we will open the OCP stable function by clicking the following: → The software will start the testing automatically after the potential fluctuation is lower than10mV/min In this experiment example, the user set the potential -0.5~1.5V (vs. OCP) You can set the condition to stop or reverse the scan. This mainly used in pitting potential measurement and Passivation curve measurement. 2.2 Results 2.2.1 OCP By open circuit potential test we can obtain the free corrosion potential Ecorr , from which we can judge the corrosion resistance of the metal material. Generally speaking, the more positive the Ecorr is, the harder the material is corroded. 1-OCP of Ti-6Al-4V alloy stent prepared by conventional wrought method 2- OCP of Ti-6Al-4V alloy stent prepared by selective laser melting method 3- OCP of Ti-6Al-4V alloy stent prepared by electron beam melting method From the graph we can conclude that the corrosion resistance of specimen #1&2 are better than #3. 2.2.2 Tafel plot analysis(corrosion rate measurement) The polarization of this experiment is as follows: As is shown, from the calculated corrosion rate value we can get the same conclusion as what we obtained by OCP measurement. The corrosion rate is calculated by Tafel plot. We can see the values of corrosion rate comply with the conclusion we obtained by OCP method. Based on the Tafel plot, we can obtain the corrosion current density icorr by the analysis fitting tool integrated in our CS studio software. Then according to other parameters such as working electrode area, material’s density, the equivalent weight, the corrosion rate is calculated. Steps are: Import the data file by clicking Data fitting Click cell info. , and enter the value accordingly. If you’ve already set the parameters in the cell &electrode setting before testing, then you don’t need to set cell info. here again. Click “Tafel” to the Tafel fitting. Choose the auto Tafel fitting or manually fitting for the data of anode segment/cathode segment, then the corrosion current density, free corrosion potential, corrosion rate can be obtained. You can drag the fitting result to the graph. 3. EIS measurement Experiments → Impedance → EIS vs. Frequency EIS vs. frequency EIS analysis EIS of Q235 carbon steel in 3.5% NaCl solution is as follows: Q235 carbon steel impedance plot- Nyquist The above Nyquist plot is composed of the capacitance arc (marked by the blue frame) and the Warburg impedance (marked by the red frame). Generally speaking, the bigger the capacitance arc, the better the corrosion resistance of the material. Equivalent circuit fitting for the Q235 carbon steel EIS results Steps are as follows: Draw the equivalent circuit of the capacitance arc - use the model in the “quick fit” to obtain R1, C1, R2. Draw the equivalent circuit of Warburg impedance part - use the model in the “quick fit” to obtain the specific value of Ws. Drag values to the complex circuit→ change all the elements type to be “Free+” →click Fit From the results, we see the error is less than 5%, indicating that the self-defined equivalent circuit we draw is in accordance with the impedance circuit of actual measurement. The Bode fitting plot is generally in accordance with the original plot.   Bode: Fitting plot vs. actual measurement result