**2. Metal oxide and hydroxide based catalysts for electrolysis**

Metal oxides were the first generation of catalysts synthesized and studied to facilitate sustained electrolysis of water for H2 generation. Bennet reported anodes based on MnOx that exhibit high selectively towards OER over CER in acidic saline water [1]. Since then, many oxides and (oxy)hydroxides (-OOH) of first-row transition metals have been investigated for OER as low-cost alternatives to the noble metals. Kato and coworkers evaluated Mn-based mixed metal oxides quoted on an IrOx/Ti surface and the catalyst system exhibited nearly 100% selectivity towards OER [2]. Strasser and co-workers used NiFe layered double hydroxide nanoplates as OER selective electrocatalysts in seawater. However, the selectivity was limited within over potential range of <480 mV at current density value of 10 mAcm−2 [3]. Koper and co-workers used the strategy of depositing a thin MnOx film onto IrOx on glassy carbon support that moderately decreased the catalytic activity and strongly shifted the product selectivity from Cl2 towards O2. The MnOx deposit was catalytically inactive and instead seemed to function as a diffusion barrier that prevented Cl− ion from reacting on the IrOx catalyst surface present below, while ensured the transport of water, H+ , and O2 between IrOx and the electrolyte solution required to maintain OER activity [4]. Overall, the issue with oxide and hydroxides was the overpotential value that render the CER as competitive reaction along with OER. As can be seen in the **Table 1** below, the overpotential value with most of the hydroxides were on the higher side.


#### **Table 1.**

*The OER overpotential value of metal hydroxide catalysts reported in recent literature. (reproduced with permission from Ref. [5] Copyright 2019 WILEY-VCH Verlag GmbH).*

#### *Recent Trends in Development of Metal Nitride Nanocatalysts for Water Electrolysis Application DOI: http://dx.doi.org/10.5772/intechopen.95748*

From the above discussions, it is apparent that major technical challenges that hinder the progress of sea-water/ground-water splitting are the Cl2 gas evolution and the deposition of insoluble Mg(OH)2 and Ca(OH)2 precipitates. Suppression of CER at high current density is possible by using an active electrocatalyst which works for OER below the overpotential of CER. Additionally, use of ultrathin coatings based on SiO2 or TiO2 can selectively block Cl− ions and thereby suppress the undesirable CER. Precipitation process can be overcome by maintaining the pH at or near neutral value as pKa for the Mg(OH)2 and Ca(OH)2 precipitation reactions are 10.8 and 12.5, respectively. Therefore, it is imperative to develop more advanced electrolyzer systems that can split such waters efficiently and cost-effectively at or near neutral pH environment. Various strategies were adopted to address the issue of CER through development of suitable catalyst system. For example, Dai and coworkers have recently developed a NiFe/NiSx/Ni anode for active, stable and long term seawater electrolysis [6]. The Ni foam was uniformly electrodeposited with NiFe possessing an underneath NiSx interlayer served as a highly selective OER catalyst for seawater splitting under alkaline condition, the conductive interlayer based on Nickel sulfide provided the stability to the electrode against Cl− corrosion and degradation. This seawater electrolyzer working under a potential of 2.1 V achieved current density value of 400 mA/cm2 in seawater electrolyte under room temperature conditions. The stability test revealed that no loss in activity was noticeable up to 1000 h. In spite of the significant efforts to develop electrocatalysts for HER and OER, to the best of our knowledge, there is no commercially available electrolyzer that can split sea water or high total dissolved solid (TDS) containing water into H2 and O2. Abundance of Ni is ample superior (90 ppm in nature) compared to other transition metals and the cost is ~4000 times lower compared to that of the benchmark Pt. Therefore, the aim is to design low cost affordable sea water electrolyzer using non-noble metal based nitride/phosphide/sulfide/carbide/graphene nanocatalysts. The chapter especially focuses on the development of nitride based affordable catalyst systems to understand the state of the art and the promise associated with such catalyst system towards catalyzing sustainable sea water or low value water in a sustainable manner.
