Catalysts Containing Transition Metals on Nitrogen and Carbon Structures for Hydrogen Production through Salt Water Electrolysis

Abstract

Hydrogen as an energy carrier is promising as it does not produce green-house gas (GHG) emissions at points of use and can be obtained from sources of low, or even neutral or negative GHG intensities [1,2]. One pathway to produce low emission hydrogen is water electrolysis when using a renewable power source [3]. However, freshwater, commonly used in this process, is relatively scarce, demand is high and expected to rise, and gets scarcer every day because of pollution [4,5,6]. An alternative is using saltwater, either through desalination or directly via adequate electrolysers [4]. Regarding direct use, a major challenge concerns catalyst, typically made of precious metals like Pt in the cathode, which are expensive, limited and susceptible to poisoning, e.g., by chlorine [7]. Likewise, in the anode chlorine is corrosive and reduces faradaic efficiency [4]. In this context, metal-coordinated nitrogen-carbon (M-N-C) nanostructures have proven to be attractive vis-à-vis these challenges [8]. To study their potential for saltwater electrolysis, five materials were synthesized following Malko et al. [8] using 1,5-diaminonaphthalene and 1% wt metallic sulphate salts of Co, Ni, Fe, Ni with Fe, and a blank without metal. The resulting material were suspended in 2-propanol and cast onto a vitreous carbon rotating ring disk electrode in both acidic solution 0,5M H2SO4 and saline medium 0,5M NaCl. Following electrode conditioning with cyclic voltammetries and electrochemical impedance characterization, linear sweep voltammetries (LSVs) were carried out in order to assess activity towards hydrogen evolution reaction (HER), coupled to hydrogen oxidation (HOR) at the Pt electrode in a ring-RDE (RRDE) fashion at constant potential of 0,3 V.The image presents HER and HOR plots in RRDE experiments at 1800 rpm, using either N-C or NiFe-N-C at a load of 0,27 mg cm-2 of catalyst at the disk and a Pt ring: (top) LSV at the disk (current normalized by geometric area, v = 2 mV/s), and (bottom) polarization at the Pt ring (E=0.3V vs NHE). Shown are both acidic medium (0.5 M H2SO4, N-C in 1 blue and NiFe-N-C in 2 red) and saline medium (0,5 M NaCl, N-C in 3 cyan and NiFe-N-C in 4 magenta). It was found that NiFe-N-C catalyst is active towards HER in saline medium at near neutral pH. As anticipated, the activity in acidic media is significantly higher than on saline medium, attributing this difference to several aspects such as conductivity limitations, chlorine presence and absence of free H+ ions and the need of additional steps for water dissociation [9]. In acidic media the metal-free catalyst (N-C) shows larger currents for most of the potential widow. On the other hand, the ring electrode does not show activity at all for N-C, thus it may be concluded that there is no H2 evolution and therefore the current is mostly capacitive, this capacitive contribution will be subtracted from the final results. In contrast, in saline medium N-C shows no activity at all towards HER, even showing smaller capacitive currents, while NiFe-C shows smaller albeit significant currents given the large differences in pH between media. The metal catalysts, the blank and a Pt/C in acidic and saline medium, will be compared using onset potentials, overpotentials at 10 mA/cm2 and Tafel slopes. References Blagojević , V. Minić, D. Minić D. & Grbović Novaković J. Hydrogen Economy: Modern Concepts, Challenges and Perspectives, Hydrogen Energy, IntechOpen (2012).Zhang,Y. & Zou, X. Noble metal-free hydrogen evolution catalysts for water splitting, Chemical Society Reviews, 15, (2015).IEA webstore, The Future of Hydrogen Seizing today´s opportunities, (2019).Dresp, S. Dionigi, F. Klingenhof, M. & Strasser, P. Direct Electrolytic Splitting of Seawater: Opportunities and Challenges, ACS Energy Letters (2019).Boretti, A. y Rosa L. Reassessing the projections of the World Water Development Report, npj Clean Water, 2, 15, (2019).Vos, J. & Koper, M. Measurement of competition between oxygen evolution and chlorine evolution using rotating ring-disk electrode voltammetry, Journal of Electroanalytical Chemistry, 819, (2018).Sethuraman, V. A. & .Weidner, J. W. Analysis of Sulfur Poisoning on a PEM Fuel Cell Electrode, Electrochimical acta, 55, 20, (2010).Malko, D. Lopes, T. Symianakis§a E. & Kucernak, A. The intriguing poison tolerance of non-precious metal oxygen reduction reaction (ORR) catalysts, Mater. Chem. A, 4, (2016).Zhou, Z. Pei, Z. Wei, L. Zhao, S. Jian, X. & Chen, Y. Electrocatalytic Hydrogen Evolution under Neutral pH Conditions: Current Understandings, Recent Advances, and Future Prospects, Energy Environ. Sci., 10, (2020) Figure 1