Potential of Tantalum Membrane as High Efficiency Hydrogen Production

Hydrogen is considered as a very clean and high energy density fuel. As one of the most promising energy carriers, it can replace the fossil fuel currently used as the main energy source.

In our era, the use of sustainable electrocatalysts to produce green hydrogen is very necessary for the energy conversion and storage of renewable energy and the future of non-carbon energy. Metal Ta has attracted great interest because of its excellent mechanical and chemical properties with high melting point, good biocompatibility and high corrosion resistance, and is considered as a favorable material. Therefore, it can be used in different technical applications, including the synthesis of electronic equipment, chemical processing equipment, medical equipment, and high-temperature applications. Ta has recently been developed as an alloying element of titanium or niobium, forming a highly protective passive film with excellent corrosion resistance. Ta is the most stable and anticorrosive material, which has excellent corrosion resistance. In concentrated and hot acidic electrolyte, it can reach 200°C in a wide temperature range (but not in HF). This makes metal one of the most sustainable cathodes in concentrated sulfuric acid electrolyte. It is a typical rare metal, and it is easy to form stable Ta oxide on its surface. It is a stable and highly protective film, which can effectively protect the metal substrate from its corrosive environment.

The electronic structure on the surface of Ta can be changed by forming metal oxide films, thus improving the electrocatalytic activity of Ta to HER. Therefore, the Ta cathode with these interesting properties can be used as an efficient electrocatalyst to generate hydrogen fuel.

Here, we have studied the applicability of using tantalum as a promising electrocatalytic material to produce hydrogen from acidic solution.

Morphological structure, lattice system and composition

Fig. 1 shows SEM images of tantalum electrode surface morphology before (a) and after (b) heat treatment and their corresponding EDX spectra. The scanning electron microscope image figure Fig. 1b confirmed that Ta was slightly affected by immersion in concentrated H2SO4 (3.0 M) solution. The EDX analysis of tantalum samples supports the high-purity tantalum metal used in this study, and the EDX analysis of tantalum samples heated in 3.0 M H2SO4 solution verifies the existence of very thin TaOx layer.

Fig. 1. SEM micrograph and EDX spectrum correspond to: (a) newly worn Ta electrode in 3.0 M H2 SO4 electrolyte (b).

Fig. 2 shows the recorded XRD pattern of crysTalline ta. XRD data show typical diffraction peaks related to a good crystal body-centered cubic (bcc) cell, and their 2θ values are 38.4, 55.5, 69.4, 82.4 and 94.9. These diffraction lines respectively correspond to the following diffraction planes in the crystal lattice (110), (200), (211), (220) and (310).

A complete XPS survey scanned the binding energy of Ta between 0 and 800 eV after HER (as shown in Figure 3a), which confirmed the different states of Ta and O formed on the surface of Ta. XPS data of ta show that the contribution of Ta2 O5 4f5/2 and Ta2 O5 4f7/2 lies in the binding energy of 28.88 and 27.06 eV, respectively (fig. 3 for Ta/TaOx structure, the other two main peaks of the binding energy of Ta 4f5/2 and Ta 4f7/2 lie in 24.22 and 22.13 eV, respectively. XPS analysis confirmed the existence of Taox film combined with Ta meTal, where x < 2.5, and determined the interaction between ta and o in TaOx within ta grains.

Fig. 2. XRD pattern of tantalum surface.

Fig. 3. (a) XPS measurement spectrum of Ta in H2SO4 electrolyte for 6 hours, and (b) high-resolution XPS scanning of Ta4f and (c) O 1s.

Electroanalysis of hydrogen on tantalum electrode

Cathodic polarization linear sweep voltammetry (LSV) was used to study the electrochemical hydrogen evolution of tantalum in different concentrations of H2SO4 electrolyte, and the scanning rate was 5 mV S1. In addition, the apparent activation energy of HER electrocatalyst on tantalum was obtained under different cathodic polarization potentials in 0.5 M H2SO4 electrolyte.

l Effect of electrolyte concentration

In H2SO4 electrolytes with different concentrations, the electrocatalytic activity of Ta electrode for HER was estimated by scanning the cathode LS voltammogram at 25°C in the potential range of 0.0 to 500 mV, as shown in Figure 4. The results show that the cathode current density (ic) of the heat exchanger (that is, the rate of H2 generation in cm2) is significantly dependent on the concentration of H2 SO4 electrolyte, and the tantalum cathode has a high ic value for the heat exchanger. With the increase of H2SO4 electrolyte concentration, the cathode current density of HER on tantalum increases.

Figure 4. Its performance: (a) Cathodic polarization curves of tantalum electrodes immersed in H2SO4 solutions with different concentrations at 25 C. Illustration, Cathodic thermal decomposition of platinum and tantalum in 0.5 M H2SO4 at 25 C. (b) Cathode current density (1500 mV) and acid concentration.

The inset of fig. 4 shows that the ca- cathode LSV scans tantalum and platinum electrodes in 0.5 M H2SO4 electrolyte at a scanning rate of 5mV S1. The steady-state open-circuit potentials (OCP) of Ta and Pt electrodes are 663 and +439 mV, respectively (see Figure 5). In 0.5 M H2SO4 electrolyte, we can see that in order to achieve the same current density, for example, IC = 100ma cm-2, the HER (ɳH) overpotentials on Ta and Pt cathodes are 520 and 940 mV, respectively. This proves that Ta is a promising candidate cathode for HER in H2SO4 electrolyte.

Fig. 5. Changes of open-circuit potentials (OCP, vs SCE) of Pt (●) and Ta (□) electrodes in 0.5 M H2SO4 electrolyte at 25 C with time.

Fig. 6. Tafel diagram of tantalum electrode in different concentrations of H2 SO4 at 25 C.

Fig. 6 shows the cathode Tafel diagram of HER recorded on the ta electrode in H2SO4 electrolyte with different concentrations at 25 c. Tafel slope (b) can be calculated according to the best linear fitting of the measured potentiodynamic polarization curve. According to the relation b = 2.303RT/(1- α)F, the Tafel slope, which is inversely proportional to the cathode transfer Coecient (1-α), is considered as an important parameter to define the rate-determining step (rds) of HER, the dynamics of the electrode process and the inherent properties of the cathode.

According to the data listed in Table 1, the cathodic transfer coefficient (1- α) decreases with the increase of H2SO4 concentration in the electrolyte. When tantalum cathode is used, the value of exchange current density (io) increases with the increase of acid concentration.

The data collected in Table 2 confirmed that our tantalum electrode had good electrocatalytic activity for HER in H2SO4 electrolyte.

l HER activation energy

The influence of electrolyte temperature on the H2 precipitation rate of Ta catalyst cathode was investigated by measuring the cathodic polarization curve in sulfuric acid electrolyte. In fact, it is very important to study the catalytic activation of HER by electrocatalysts at different temperatures for determining the apparent activation energy (Ea) needed to predict HER mechanism.

Therefore, 0.5 M H2SO4 solution was selected as the test electrolyte, in which the rate of H2 production on the Ta electrode developed at a considerable rate. Fig. 7a shows a set of HER polarization curves recorded on tantalum substrate at various electrolyte temperatures in the range of 293–343 K.. The corresponding Tafel curve is shown in Figure 7a and its estimated Tafel slope value is given in Table 3 as a function of electrolyte temperature.

Fig. 7.(a) In a static naturally aerated 0.5 M H2SO4 solution, the cathode on the Ta electrode evolved hydrogen at different temperatures, and the scanning rate was 10mv S1. Illustration: Cathode Tafel line on tantalum electrode immersed in static and naturally ventilated 0.5 M H2 SO4 solution at a scanning rate of 25°C and 5mV S1. Arrhenius diagram For the 0.5 M H2SO4 solution of cathodic hydrogen evolution on the Ta electrode in stagnant natural gas charging, the scanning rate is 5mV s-1, with different polarization potentials, 1100 mV (□), 1250 mV (●) and 1500 mV (▲).

Table 3 with the increase of electrolyte temperature, bc value decreased from 104.2mV dec-1 at 293 K to 72.2mV dec-1 at 343 k. This revealed that HER on tantalum cathode changed from Volmer mechanism to Heyrovsky mechanism, which enhanced the electrocatalytic activity of tantalum on HER.

Study on electrochemical impedance spectroscopy

In order to obtain additional evidence of electrocatalytic activity of tantalum cathode for helium, electrochemical impedance spectroscopy (EIS) was studied.

Fig. 8 clearly shows that the properties of tantalum cathodic impedance diagram are similar for all solutions.

Table 4 shows the fitted impedance pa- Ta electrocatalyst parameters at different H2SO4 concentrations at a fixed operating cathode potential of 1100 mV (relative to SCE).

It may not be necessary to use constant phase element (CPE) to illustrate the electrode/electrolyte interface as a substitute for an ideal capacitor. For comparison, EIS data of ta electrode in 0.5 M H2SO4 electrolyte (OCP) and 1000 mV cathode potential (relative to SCE) were also recorded, as shown in Figure 9.

Fig. 9. The impedance spectrum is (a) Nyquist diagram and (b) Bode diagram of tantalum electrode OCP and 0.5 M H2SO4 electrolyte at 1000 mV (relative to SCE) cathode potential.

conclusion

Tantalum is considered as a promising electrocatalyst cathode in sulfuric acid electrolyte. The results of polarization and impedance show that, compared with other eCient-Hull catalysts, hydrogen generation occurs at a reasonable overpotential (H). Among them, the H of helium on tantalum required to reach a current density of 100ma cm-2 is estimated to be 520 mV, while that on platinum is 940 mV, which proves that tantalum can be used as the cathode for the production of green H2 in H2SO4 electrolyte, which is related to the most active electrocatalysis of helium, precious metal Pt. In addition, the activation energy of electrocatalytic HER on the Ta cathode is low (11.5 kJ mol-1 at 1500 mV in 0.5 M H2 SO4 electrolyte), and the evolution of H2 develops on the Ta surface through the single electron transfer step. The ideal combination of lower charge transfer resistance and higher electric double layer capacitance obtained from EIS data makes crystalline ta the best candidate among other electrocatalysts reported so far. HER on tantalum electrode is carried out by Volmer-Heyrovsky reaction mechanism. In the production of green hydrogen fuel, metallic tantalum is considered as the best material for safety, stability and sustainability.