The use of stainless steel 254 to produce hydrogen

A. L. Gallina1, B. V. Dias1 and P. R. P. Rodrigues*,1 1GPEL – Grupo de Pesquisa em Eletroquímica. Department of Chemistry, Universidade Estadual do Centro Oeste -

UNICENTRO, 85040-080, Rua Simeão Camargo Varela de Sá, 03, Guarapuava, Brazil.

The search to produce alternative energy to the oil byproducts has been spread worldwide, mainly regarding fuel cells. In 2010 the Center of Management and Strategic Studies of the Science, Technology and Innovation Ministry in Brazil, published a technical document pointing to technological bottlenecks regarding the production of hydrogen via water electrolysis, as the application of low cost electrodes and minimization of energy necessary to generate hydrogen. The aim of this study is to employ stainless steel 254 electrodes in formic acid 3 mol L-1, aiming to minimize the costs of hydrogen production. The electrochemical techniques cathodic potentiodynamic polarization (CPP), chronoamperometry (CR) and electrochemical impedance spectroscopy (EIS) were employed. Results demonstrated that the carbon steel 254 can be used as an electrode for the production of hydrogen, as the results were about 45% more efficient than those observed for platinum, and the steel 254 represent lower cost.

Keywords: catalysis, energy, formic acid.

1. Introduction

The pursuit of humanity for comfort and mobility is tied to the ability to produce and use energy, however, this need involves significant environmental problems, due to the option of using fossil energy. Thus, governmental policies in Brazil and all over the world, have incentivized more effectively research projects which involve alternative energy to the fossil fuels [1-3]. Fuel cells are an actual alternative for the production of energy in a more environmentally friendly manner. These cells are supplied with several kinds of fuel such as methanol, ethanol, hydrogen, amongst others. Amidst these fuels, hydrogen outstands for its high energetic efficiency, however, there are some difficulties related to its generation and storage [1-4] The Brazilian government published a technical document in 2010, through the Science, Technology and Innovation Ministry, called “Energetic hydrogen in Brazil – subsides for competitiveness policies: 2010-2025”. In this document, the technological bottlenecks are outlined, to be overcome, as the use of alloys with relatively high cost, use of high temperatures, use of a complex production system, long reaction time, and the catalysts [4]. Benzotriazole (BTAH) is cited in the literature as an efficient corrosion inhibitor, that is, it minimizes the speed of metal oxidation. As reported by Rodrigues (1997) the use of BTAH in low concentrations, lower than 10-3 mol L-1

catalyzes the metal anodic and cathodic reactions [5-6]. Thus, the BTAH can be used as a catalyst in the hydrogen release reaction. Another catalyst reported in the literature is the molybdenum (Mo), which according to its conformation in the metallic alloy, creates active sites where the reaction of hydrogen production is catalyzed [1]. The aim of this study is to produce hydrogen using stainless steel 254 electrodes, with high Mo content, and the use of benzotriazole as a catalyst, seeking to minimize the energy employed in the process and increase the reaction efficiency.

2. Materials and Methods

2.1 Solutions employed

Formic acid solutions (CH2O2): These solutions were prepared with bi-distilled water and reagents of analytical purity (p.a.). The CH2O2 solution concentration was 3 mol L-1. Formic acid + benzotriazole (BTAH) solutions: These solutions were prepared with bi-distilled water and reagents of analytical purity (p.a.). The BTAH concentrations were 1x10-6, 1x10-8, 1x10-10 mol L-1, it is relevant to emphasize that their preparation was carried out with 3 mol L-1 formic acid.

2.2 Electrodes employed

Auxiliary: A platinum electrode was used with approximately 25 cm2 area. Reference: Silver/Silver chloride electrode (Ag(s)/AgCl(s)), coupled to a Luggin capillary. Work: Stainless steel 254 electrode, with 3.8 cm2 average area.

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2.3 Techniques employed

Electrochemical tests were carried out in equipment Gamry®, model PC4-300. For the CPP tests, the potential band studied was from 0 to -1.5 V vs. Ag(s)/AgCl(s), with scanning velocity (s.v.) 1 mV s-1. Tests were carried out at the -1.5 V potential for 600 seconds. The EIS was performed at the corrosion potential, with potential variation of 10 mV and disturbance frequency 10 kHz to 0,01 Hz. Optical micrographs were carried out employing a Media Cybernetics® camera, model Evolution LC Color, coupled to an Olympus microscope, model BX41M. Images were obtained with 100x increase.

3. Results and Discussions

There are two main factors to be evaluated in the hydrogen release reaction, the hydrogen release potential and the current density generated during this process, thus, to check the former, a CPP was performed and to the latter a CR was carried out. In Figure 1, the CPP results for stainless steel 254 with and without the presence of benzotriazole (BTAH) in comparison with platinum are presented.

Platinum Stainless Steel 254

Stainless Steel 254 + [BTAH] =10-6

Stainless Steel 254 + [BTAH] =10-8

Stainless Steel 254 + [BTAH] =10-10

0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

j / A

cm

-2

E / V

Fig. 1 CPP curves for the stainless steel 254 in the presence and absence of BTAH compared to platinum, with s.v. 1 mV s-1. In Fig. 1, it can be seen that in [BTAH] = 10-6 mol L-1 there is an anticipation of the hydrogen release potential and increase in the current density (j) in relation to the system without BTAH. This phenomenon is associated to the BTAH concentration, which allows low adsorption on the metallic surface, generating micro piles, that is, anodic and cathodic micro regions, favoring the hydrogen release. The BTAH adsorption phenomenon is reported by Rodrigues (1997), who visualized the BTAH corrosion inhibitor action on the stainless steel in acid medium, however, at low concentrations, the oxidation process catalysis was registered. In Figure 1, it is also observed that the steel 254, even in the presence of [BTAH] = 1x10-6 mol L-1 presents higher hydrogen release potential and lower current density when compared to platinum [5-6]. In Figures 2 A and B, the diagrams of electrochemical impedance spectroscopy (EIS) in relation to the dynamic stabilization potential (Ees) are presented.

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0 5000 10000 15000 20000 25000 30000 35000 400000

5000

10000

15000

20000

25000

30000

35000

40000 Platinum Stainless Steel 254 Stainless Steel 254 + BTAH

Zim

g / o

hm.c

m2

Zreal / ohm.cm2

A

0.01 0.1 1 10 100 1000 10000

-70

-60

-50

-40

-30

-20

-10

0

10 Platina Aço Inoxidável 254 Aço Inoxidável 254 + BTAH

θ / °

Frequency / Hz

B

Fig. 2 Nyquist (A) and Bode (B) EIS diagrams for platinum and stainless steel 254 with and without [BTAH]=1x10-6, in HCOOH 3 mol L-1. In the diagram in Figure 2A, it can be seen that for the steel 254, both in the absence and in the presence of BTAH, high resistance was registered, while for the platinum, despite the resistance being high it is lower than that registered for the steel 254. Diagrams in Figure 2B might justify the steel 254 high resistance, where a wide time constant in the region of frequency from 0.01 to 100 Hz was registered for all systems under study, and for the steel 254 the band 0.01 to 0.1 Hz is related to the generation of oxides on the electrode surface and from 1 to 10 Hz, to the transference of charge, and for platinum from 1 to 10 Hz, to the transference of charge. I can also be seen that in this band (0.01 to 100 Hz) the phenomena occur at smaller angle for the platinum in relation to the steel 254, which confirms the high resistance registered for the systems with steel 254. In the polarization used, as it is potentiodynamic, there is no stabilization of current, due to this fact, the results obtained in CPP illustrate an average behavior of the current density according to the potential. Thus, potentiostatic studies (chronoamperometry) are necessary, in order to evaluate the actual current density that each system presents and consequently use it to calculate the hydrogen production.

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In Table 1, the over potential values (ηDH2), are presented, stabilization potential (Ees), hydrogen release potential (EDH2) and the amount of hydrogen (nH2) produced (mols), obtained through the CPP. Table 1 ηDH2, Ees, EDH2 average values and H2 production efficiency, for the platinum and stainless steel 254 samples with and without the presence of [BTAH] =1x10-6 mol L-1, in relation to platinum.

Metals Ees / V EDH2 / V η DH2 / V Efficiency H2 / %

Platinum 0.150±0.025 -0.180 0.330 100 Steel 254 0.010±0.005 -0.360 0.361 48.1 Steel 254 + BTAH 0.033±0.007 -0.300 0.333 59.6

Results presented in Table 1, indicate that the over potential for the production of hydrogen is similar, considering the standard deviation, values do not present significant differences, that is, the energy consumption can be considered the same. The production efficiency was calculated with the CPP curves, for the systems under study in relation to the platinum current density, at the -1 V vs. Ag(s)/AgCl(s) potential, thus it was possible to observe that the presence of BTAH increases the H2 production efficiency, however, these results are not conclusive. In Figure 3, the chronoamperometric tests of the samples under study are presented, at the -1.5 V vs. Ag(s)/AgCl(s) potential.

0 100 200 300 400 500 600

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

j / A

cm

-2

E / V

Platinium Stainless Steel 254

Stainless Steel 254 + [BTAH] =10-6

Stainless Steel 254 + [BTAH] =10-8

Stainless Steel 254 + [BTAH] =10-10

Fig. 3 CR curves for the stainless steel 254 in the presence and absence of [BTAH]= 10-6 , 10-8 and 10-10 mol L-1, compared to platinum, at a -1.5 V vs. Ag(s)/AgCl(s), potential for 600 seconds. In Fig. 3, a different behavior from that presented in the polarization curves in Fig. 1 is observed, as in chronoamperometric tests the stabilization current of the stainless steel 254 in the absence of BTAH presented higher j values, when compared to the steel 254 with BTAH at different concentrations, this fact is explained by the availability of active molybdenum sites. When there is only steel 254 the molybdenum sites are available, however, when the BTAH is added, it complexes the iron present in the stainless steel alloy and minimizes the availability of molybdenum sites in the alloy, through steric impediment [5]. It is important to emphasize the tendency observed as the lower the BTAH concentration is, the higher the hydrogen production is. In Table 2, the over potential values (ηDH2), stabilization potential (Ees), hydrogen release potential (EDH2) and the amount of hydrogen produced (mols) obtained through the CR curves, are presented. Table 2 j and nH2 and production efficiency H2 average values, for the platinum and stainless steel 254 samples with and without the presence of [BTAH] =1x10-6 mol L-1, in relation to steel 254.

Metals j / A cm-2 nH2 / mol Efficiency H2 / %

Platinum 0.006 3.11x10-5 54.5 Steel 254 0.011 5.70x10-5 100 Steel 254 + BTAH 0.005 2.59x10-5 45.5

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The results in Table 2 show that the steel 254 has high current density and consequently higher H2 production, when compared to the steel 254 + BTAH and platinum systems. In Figures 4A and 4 E, the optical micrographs (OM) of the systems under study, before and after polarization are shown.

4A

4 B

4 C

4 D

4 E

Fig 4. Optical micrographs before CPP, for platinum (A) and stainless steel 254 (C), and after CPP for platinum (B), stainless steel 254 (D) and stainless steel 254 + [BTAH] = 10-6 mol L-1 (E). 100x increase. Results regarding the OM test presented in Figures 4 A to 4 F, indicate that there was no significant oxidation of substrates in any of the systems under study.

4. Conclusions

(1st ) Hydrogen can be produced by using stainless steel 254 electrodes, and this metal can be an alternative to platinum in the production of H2(g), which minimizes the cost of the process; (2nd) The use of BTAH in the formic acid solution, in the case of the austenitic stainless steel 254, due to its character of inhibitor of the hydrogen release reaction, contributes negatively due to the partial blockage of the metal Mo active sites; (3rd) There was no significant oxidation of stainless steel 254 in the electrolyte used, which highlights the use of this material in the generation of hydrogen; (4th) The efficiency in the production of H2(g) in E= -1 V vs. Ag(s)/AgCl(s), for the steel 254 was 45.5% higher than platinum, using formic acid 2 mol L-1 derived from the biomass, as electrolyte.

Acknowledgments To CAPES and CNPq Finep for the financial support.

References [1] Botton, J. P. Líquidos Iônicos como Eletrólitos para Reações Eletroquímicas. PhD Thesis. Federal University of Rio Grande do

Sul. Porto Alegre; 2007. [2] Guo, W.L., et al. Hydrogen production via electrolysis of aqueous formic acid solutions. International Journal of Hydrogen

Energy, 2011; 36; 9415-9419. [3] Holze, L. R. B. Comportamento eletroquímico do alumínio em misturas etilenoglicol-água. Efeito da adição de agentes quelantes.

PhD Thesis. Federal University of Rio Grande do Sul. Porto Alegre; 2005.

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[4] Medeiros, E. F. et al., Hidrogênio energético no Brasil: subsídios para políticas de competitividade 2010-2025. Centro de Gestão e Estudos Energéticos (CGEE); CGEE: Brasília, 2010

[5] Rodrigues, P. R. P. O benzotriazol como inibidor de corrosão para ferro e ligas ferrosas em meios de ácido sulfúrico. PhD Thesis. University of São Paulo, 1997.

[6] Silva, D. K. et al. Benzotriazole and tolytriazole as corrosion inhibitors of carbon steel 1008 in sulfuric acid. Port. Electrochim. Acta . 2006; 24; 323-335.

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