In recent years, the demand on drinking water escalates the research on desalination technology. Existing desalination technologies utilize fossil fuel driven electricity which gains the researchers attention to develop the use of renewable resources. To address this, we have developed a novel device named desalination fuel cell (DFC), [1] that simultaneously desalinate the feed water and generate electricity by the anodic hydrogen oxidation and cathodic oxygen reduction. The bottleneck of DFC performance largely depends on oxygen reduction reaction (ORR) [2] and requires highly active catalysts. Herein, platinum nanoparticles deposited on activated carbon (Pt/C) is recognized as the state-of-the-art catalysts, however, the scarcity, cost and stability limited the usage in the real application. In addition, the catalyst surface poisoning due to chloride ions (Cl⁻) from the feed water deteriorates Pt/C catalytic activity in DFC. Therefore, the seek for chloride tolerant and highly active ORR catalysts for DFC are of interest. Several electro-catalysts have been investigated for chloride tolerance ORR activity [3-5]. Very recently, we have reported Fe/N/C as Cl- tolerant ORR catalyst with an on-set potential of 0.84 V vs. RHE in the presence of Cl- ions was on par with that of the state-of-the-art Pt/C in acidic medium [6]. The optimized Fe/N/C catalyst was utilized in DFC cathodes and delivered the highest open circuit voltage of 1.6 V on comparison with Pt/C cathode (1.46 V).

The present work describes the synthesis and ORR evaluation of atomically dispersed Co nanoparticles on to boron-nitrogen co-doped carbon catalyst. The catalyst was derived using high temperature pyrolysis of Co and B incorporated zeolitic imidazole framework (ZIF) and systematically evaluated with respect to oxygen reduction activities. The effect of B and Co concentration, and pyrolysis temperature were analyzed using XRD, BET sorption studies, XPS and Raman spectroscopic techniques. Fig. 1a shows transmission electron microscopic image of Co/B/N/C catalyst. It is noteworthy that the black spots indicate the presence of atomically dispersed Co nanoparticles on to carbon support material. Flake-like surface morphology is observed for the catalyst synthesized in the present study (Fig. 1a inset). Fig. 1b shows the comparative ORR linear sweep voltammograms (LSVs) of Co/B/N/C and Pt/C catalysts with and without chloride ions. Notably, almost 140 mV difference in onset potential is observed for Pt/C in the presence of Cl^– ions. However, Co/B/N/C catalyst exhibits nearly 10 mV difference in onset potential after the addition of Cl^– ions in both acidic and alkaline media. The results show that Co/B/N/C catalyst is tolerance towards Cl^– ions in acidic as well as alkaline media.

The optimized Co/B/N/C catalyst was utilized as cathode in an indigenous DFC device and the performance was evaluated at room temperature. The DFC device was constructed as three compartments viz., anode, desalination and cathode as shown in the Fig. 2a [6]. In an identical operating condition, the highest OCV of 1.58 V was observed for Co/B/N/C cathode compared with that of Pt/C cathode (1.49 V). Notably, the Pt/C cathode delivered a peak power density of 15.7 mW cm^-2 at a load current density of 19.0 mA cm^-2. However, Co/B/N/C cathode exhibit a peak power density of 12.1 mW cm^-2 at a load current density of 16.5 mA cm^-2 (Fig. 2b) which is on-par with the state-of-the-art Pt/C catalyst. Simultaneously, the desalted water concentration was measured using conductivity meter and the results are presented in Fig. 2c. At OCV, the desalted water concentrations were measured to be 0.39 and 0.37 M for Co/B/N/C and Pt/C catalysts, respectively. At 16.5 mA cm^-2 load current density, concentration of the desalted water was 0.2 M for Co/B/N/C catalyst and for Pt/C, the concentration at 16.5 mA cm^-2 current density was 0.15 M. From the above polarization and concentration profiles, Pt/C shows enhanced performance over in-house synthesized, however, considering non-precious catalyst, the observed results for Co/B/N/C are much appreciable. We are hopeful that the results discussed in this study will provide insights in designing anion tolerant catalyst for the improved DFC performance.

Reference

1. Atlas, S. Abu Khalla, M. E. Suss, J. Electrochem. Soc. 2020, 167, 134517.
2. Abdalla, S. Abu Khalla, M. E. Suss, Electrochem. commun. 2021, 132, 107136.
3. Malko, T. Lopes, E. Symianakis, A. R. Kucernak, J. Mater. Chem. A 2016, 4, 142.
4. Mamtani, D. Jain, A. C. Co, U. S. Ozkan, Catal. Lett. 2017, 147, 2903.
5. Tylus, Q. Jia, H. Hafiz, R. J. Allen, B. Barbiellini, A. Bansil, S. Mukerjee, Appl. Catal. B Environ. 2016, 198, 318.
6. A. Asokan, S. Abu-Khalla, S. Abdalla, M. E. Suss, ACS Appl. Energy Mater. 2022, 5, 1743.