DOI: 10.21950/ltopcy
title: Processing and surface modification of titanium open cell porous structures for PEM fuel cell bipolar plates

description: This dataset includes the data required to validate the results published in the article with title "Processing and surface modification of titanium open cell porous structures for PEM fuel cell bipolar plates", accepted at the International Journal of Hydrogen Energy. Includes the original microstructures taken in an optical microscope, three point bending (3PB) test files, fracture surface images, x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) files, corrosion testing files and Interfacial Contact Resistance (ICR) measurements.

To make the dataset interoperable and usable in the long term, the data is included preferentially in file formats like .txt for 3PB, XRD, XPS, corrosion and ICR data, .tiff for images.

The name of each file indicates how it was processed and then the type of data that this represents. The underscore (_) separates the different identifiers of the data, and this is the structure followed: material_sintering_postprocessing_test_samplenumber
Dashes will be used within the identifier to separate different condition of the material or test.

In some cases, the postprocessing will be omitted when there is no postprocessing performed, in this case, the name of the file is: material_sintering_test_samplenumber
When the number of results per test and condition is only one, the suffix _samplenumber will be ommited as well.
description: For a complete description of the methodology, including raw materials, their characteristics and the specific equipment, refer to the related publication.

Processing of the materials:

All the specimens were processed using the powder metallurgy approach, with titanium hydride (TiH2) as the precursor of Ti.
In the case of bulk specimens, the TiH2 powders were compacted in a cylindrical die applying 200 MPa of compacting pressure. Then, these were sintered in different atmospheres (vacuum, Ar or a sequence of Ar and N2). The sintering temperatures ranged between 900 and 1250 ºC, and the dwell time at the sintering temperature was kept to 2 h for vacuum and Ar, and 3 for the sequence of Ar and N2. To ensure hydride decomposition, the heating ramp consisted of three ranges: heating at 5 ºC/min from room temperature to 450 ºC, at 2 ºC/min between 450 and 650 ºC, and at 5 ºC/min from 650 ºC to the sintering temperature. Cooling was set to 5 ºC/min to room temperature.
The data regarding these specimens were labelled as Ti_[atmosphere]-[sint.temperature in ºC]-[sint. time in h]. 
For the materials sintered in a sequence of Ar and N2, the label was Ti_Ar-N2-[sint.temperature], the sintering time was kept constant and it was of 3 h, 1 h in the presence of Ar and 2 h in the presence of N2.

In the case of porous materials, the powders were mixed with Ammonium bicarbonate (NH4HCO3), which acted as space holder.This space holder was sieved prior to mixing to different particle size ranges (&lt;150 microns, between 150 and 250 microns, 250 to 500 microns and &lt;500 microns. The mixtures were defined by the volume fraction of NH4HCO3 in the mixture. The green compacts were prepared by pressing in a cylindrical die at 200 MPa and the space holder was removed afterwards by placing the compacts in a furnace at 55 ºC overnight. Then the materials were sintered with the same procedure as the bulk materials.
The data regarding these specimens is labelled as Ti-[volume fraction of space holder in percentage]Bic-[particle size range]_[atmosphere]-[sint.temperature]-[sint. time]. The particle size range was identified with 150 for the range &lt;150 microns, 150-250 and 250-500 for the two ranges in between and 500 for &lt;500 microns. The sintering labelling remains the same as for bulk specimens.


Post processing of the materials:

Some of the bulk and porous titanium was postprocessed using gas nitriding. The specimens were placed in a furnace with a constant flow of N2 gas where they were heated to 950-1000 ºC (the nitriding temperature) at 5 ºC/min and kept the nitriding temperature for 2 h before cooling to room temperature at 5 ºC/min.
These samples were labelled using the following structure: [material]_[sintering]_N-[nitriding temperature in ºC].
 

Characterization:

Optical microscopy:
The structure of the porous materials, to determine the porosity, was studied using optical microscopy. The specimens were prepared by grinding with SiC papers up to 2000 grit size, then polished with alumina suspensions of 1 and 0.3 microns. The samples were observed with 50x and 100X magnification. Each observation was saved as the original image and as the image with the scalebar burnt into it.
The labels for these tests are [material]_[sintering]_N-[postprocessing (not used)]_OM_[observation number]_[scale or empty if original image].

X-ray diffraction:
X-ray diffraction was done to bulk Ti without postprocessing using a XPERT MPD difractometer with a Cu anode, scanning in 2\theta between 20 and 100 º with step size of 0.02º and 1 s per step.
Postprocessed specimens were studied using grazing incidence XRD, to analyze the phases closer to the surface. These were performed in a XPERT PRO diffractometer with a Cu anode, using a grazing angle (\omega) of 1.5 º, which corresponds to around 750 nm of penetration in compounds like TiN and Ti. The scan range was restricted to 30 to 80 º in 2\theta, with step size of 0.02 º and 3 s per step.
The labels for the tested materials is [material]_[sintering]_[postprocessing (optional)]_XRD
The data was generated either as .csv formats, which include around 30 lines of information about the testing before the angle (2\theta) and the intensity are reported, or as .txt formats, which do not include additional information or even headers (the first column is the angle in degrees and the second the intensity).

XPS:
XPS was done to bulk specimens, with and without postprocessing. Each file, in .txt format, include several scans that are sequenced in the two columns. The number of scans is indicated in the second line: TotalNumOfRegions. Then, each scan begins with Region = [nº of scan], followed in the following line by the label of the scan and in the next line the number of points in the scan. The following lines represent the data in terms of binding energy in eV (first column) and the counts of the detector (second column).
The different regions or scans are labelled as: "general", which indicates that is the preliminary scan to detect elements, "O 1s", "Ti 2p", etc., which indicate the scan that studies a specific element.
The data has the following label: [material]_[sintering]_[postprocessing (optional)]_XPS
If the specimen was analyzed after corrosion testing (in the cathode conditions, as explained below), these are labelled as [material]_[sintering]_[postprocessing (optional)]_XPS-post-corrosion

Transverse rupture strength:
TRS was performed for both bulk and porous specimes without postprocessing. The specimens had dimensions of roughly 4x11x28 mm (thickness, width and length), the distance between supports was 23 mm. The testing was performed in a MicroTest universal testing machine, with a load cell of 15 kN coupled with an extensometer. Before testing samples were ground up to grit size 1000 using SiC paper. The tests were performed at a constant deformation rate of around 1 mm per minute.
The files are presented in a .txt format, using the label [material]_[sintering]_TRS_[specimen number]. The data is presented as columns separated by a semicolon working as delimiter. The header row has the labels of each column in Spanish, and the rest of the rows are the data recorded at each time interval. The columns are: Force (in kN), displacement of the LVDT probe (in mm), position of the cross-head (mm), the deflection at the center of the beam (%), the flexural stress (MPa) and the time (s). The equipment did not calculate the deflection or the stress, and the data in these columns are zeroes. Both force and displacements are negative.

Corrosion testing:
Corrosion tests were done following the standard procedure of the DoE Fuel Cell Office. A 1L electrochemical cell was used, with a double wall for heating with hot water and entrance of gases. The reference electrode was Ag/AgCl and the counter electrode was platinized Ti. The electrolyte of these tests was a solution of H2SO4 with pH=3 and it was heated to 70 ºC. For the anode environment, the test consisted on a linear sweep voltammetry from -0.4 V to 0.6 V vs Ag/AgCl, with steps of 0.1 mV/s, with Ar purge. For the cathode environment, the test was a chronoamperometry at constant voltage of 0.6 V vs Ag/AgCl during 24 h with air bubbling. The specimens were immersed in a holder that allowed the entire surface to be in contact with the electrolyte, which is especially useful for porous materials.
The files are presented with .txt formats. In the case of the anode conditions, the name of the data is [material]_[sintering]_[postprocessing (optional)]_corrosion-anode and the data is separated by tabulation. The first row has the headers of each column (time, potential applied, the current of the working electrode and its potential, as well as their units). The following rows are the data taken at each time interval.
In the case of the cathode condition, the name of the file is [material]_[sintering]_[postprocessing (optional)]_corrosion-cathode, and the data is arranges as well with tabulation and it has the headers of each column in the first row. In this case, the columns are the time (s), the corrected time (s) so that the beginning of the chronoamperometry is t=0s; the potential applied to the working electrode in V and the current going through it.
In order to calculate the current density, the current must be divided by the total surface of the specimens, which were disks of roughly 14-15 mm in diameter and 2 mm in height.


ICR:
ICR was measured with a micro-mechanical testing device, Au-coated copper plates, a DC power source, and a precision multimeter, using the four point probe method to measure voltage and current intensity.
The procedure was the following: a GDL (or carbon paper) was cut in a circle, whose area determines de contact area used in the ICR calculation. Then, the GDL was placed in between the copper plates, that are connected to the DC source, and as the force was increased stepwise, the voltage was recorded. In this setup, the current intensity is fixed. For each value of force, a voltage V1 is recorded.
Afterwards, the specimen to measure was sandwiched between two GDLs with the same area, placed between the copper plates, and force and current are applied simultaneously. The current is fixed, the force increases stepwise to similar values as in the previous case, and for each step the voltage V2 is recorded.
Finally, the ICR is calculated for each force value using the formula (V2-V1)/I*A, where I is the intensity and A the contact area. The applied pressure is calculated dividing the force applied in each step by the contact area.
The data is reported as the raw measurements in a .txt file where the first line indicates the intensity in A, the second line the contact area in cm2, and then there are two blocks of data: the first block has the measurements of V1, with the first line being the label GDL, the second one are headers of the two columns (F and V1, with the units used) and then they are followed by tabulated data in two columns with the first one being the force applied and the second the voltage registered. After one or more empty lines, the second block of data is reported, which belongs to the specimen sandwiched between two GDLs, and has the same structure as the previous block.
The data of these tests is labelled as [material]_[sintering]_[postprocessing (optional)]_ICR. If the specimen was analyzed after corrosion testing (in the cathode conditions, as explained below), these are labelled as [material]_[sintering]_[postprocessing (optional)]_ICR-post-corrosion