The analytical characterization of corrosion for advanced water splitting materials employs different tools depending upon the nature of the technology (photoelectrochemical, advanced electrolysis) under consideration so the following information is broken into the relevant subheadings. The corrosion analysis node at NREL has different readiness levels with respect to the different water splitting technologies. Corrosion analysis of photoelectrochemical materials is fully developed and is readily used at NREL. Corrosion analysis of low temperature electrolysis have been used for evaluating fuel cell bipolar plate materials is readily available. Also available is a scanning electrochemical microscope (SECM), an electroanalytical scanning probe tool that can be used to image substrate topography and local reactivity with high resolution. The SECM can be operated in several modes that make it a useful tool in a variety of applications. Some examples include:

1. Determining values for kinetic rate constants
2. Mapping and quantifying differences in H2 (or O2) production at surfaces
3. Corrosion detection/monitoring
4. Studying adsorbed surface species on an electrode.

Category 1: Photoelectrochemical Materials

This capability involves testing water-splitting semiconductors under simulated operational conditions and performing electrochemical and physical diagnostic testing. The capability has been funded by the FCTO for >5 years. Promising photocathodes and/or photoanodes can be tested under constant current or voltage bias, with an appropriate reference electrode, in a wide range of aqueous electrolytes. Diagnostic current-potential (J-V) or impedance-based (EIS) electrochemical testing can be performed at any point during the testing to evaluate degradation. Upon the completion of testing, the electrolyte can be analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to quantify trace amounts (ppb) of relevant elements (Figure 1). ICP-MS can also be used to measure the loading of surface treatments, coatings, or co-catalysts by digesting the modified semiconductors in aqua regia and analyzing the resulting aqueous solutions for the elements of interest. Etching of tested electrodes can be evaluated using stylus or optical interference profilometry (Figure 2). If necessary, electrode surfaces can be analyzed by a suitable spectroscopic characterization tool (XPS, AES), scanning probe (AFM), scanning electron microscopy (SEM), or scanning transmission electron microscopy (STEM), under a different capability node, to better understand the chemical features and microstructure of degraded or stabilized electrode surfaces. A glove box is available for air-excluded sample testing, extraction and packaging prior to analysis.

Advanced Electrolytic (AE) Hydrogen Production. Low-and high-temperature electrolysis (including alternative chemistries):

Category 1: Low-Temperature Electrolysis Materials

NREL has extensive expertise on materials selection based on operating conditions and required working environment. To evaluate materials performance, and thus determine ways to optimize it, we perform a variety of materials characterization before and after material exposure to in-situ and ex-situ conditions. This characterization includes microstructural, chemical, mechanical, thermal, physical, and/or electrical evaluations. NREL has a variety of customized apparatuses for hydrogen production from biofuels, electrolysis for water splitting, photoelectrochemistry, and thermochemical processing.

Ex-situ materials degradation evaluations can be performed by exposing the materials to different environments (e.g., at selected temperatures under different controlled atmospheres), emulating the real conditions expected under real operation. Conventional long-term immersion techniques and electrochemical techniques are commonly employed to evaluate materials degradation. Electrochemical techniques are very fast responding and can determine mechanisms occurring at the material's surface. Changes associated with mass loss, chemical, and microstructural modifications can be determined. For water splitting, the most important material changes that we need to keep track of are those that can affect electrical, chemical, and mechanical properties.

Common electrochemical techniques used to evaluate materials performance in conductive liquid media such as aqueous solutions or molten salts are linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), among others. EIS is a powerful technique used to determine fundamental mechanisms occurring near the surface and is used to characterize thin films, surface reactions and occurring physicochemical events.

Figure 1 shows the electrochemical setup used to carry out a study of the passivation behavior of the anodic sintered titanium (Ti) porous transfer layer (PTL) up to very high overpotential (3 V), with and without platinum coatings. The results in Figure 2 showed that anodic Pt-coated Ti PTL should be used in PEM electrolysis because no passivation at the surface of platinum coatings was formed to hinder electronic transfer.

For low-temperature advanced electrolysis of water, we have access to several electrochemical setups at low and elevated temperatures, thermogravimetric analyzer (TGA), gas chromatographs (GC), inductively coupled plasma with mass spectrometer (ICP-MS), X-ray diffractometer (XRD); several optical and electron microscopes (TEM, SEM), among other state-of-the-art characterization tools.

Category 3: High-Temperature Electrolysis Materials

Challenges associated with next-generation membranes, that need high conductivity and be resistant to corrosive electrolytes at high temperatures, can be addressed using current expertise and infrastructure at NREL. NREL has dedicated laboratory facilities for the evaluation of material systems under extreme conditions to help inform materials selection. Corrosion evaluations can be performed at selected temperatures under controlled atmospheres using long-term corrosion tests and electrochemical corrosion evaluations. Electrochemical impedance spectroscopy (EIS) is a powerful technique used to determine fundamental corrosion mechanism and is used to characterize thin films, surface reactions and physicochemical events occurring at the material’s interface. Additional electrochemical techniques are available to perform the complete corrosion evaluation of materials. Same characterization techniques as explained before are utilized here to understand corrosion mechanisms and mitigation effectiveness.

Chemical degradation of materials can be assessed under controlled conditions, including temperature, pressure, atmosphere (vacuum, inert, non-reactive, and reactive), gas flow control, fluids (organic and inorganic) and material chemistry fluids (organic and inorganic), time of exposure, thermal cycling, and rates of heating/cooling rates (up to ~100°C/min), among others.

Novel materials can be synthesized using electrodeposition of compounds dissolved into electrolytes. The morphology of the product can be controlled by controlling the temperature, voltage and current density of the system. Several instruments are also available to evaluate thermo-physical properties. Differential scanning calorimeters (DSCs), thermogravimetric analyzers (TGAs) with mass spectroscopy can be utilized under different atmospheres to determine chemical and thermal behavior of systems. High temperature wettability of surfaces with different fluids can be determined up to 850°C and rheological properties can be evaluated up to 550°C.