**5. Durability of metal nitride based catalysts**

Published literature provides a fair idea about the durability of nitride based catalysts under electrolysis conditions [80]. We are revisiting several compositions and their lifetime in the following section. The NiMoN nanotube displayed OER efficiency equivalent to that of the IrO2 reference, as the current versus potential curve almost superimposed in both the cases. The catalyst displayed an overpotential of 295 mV at 10 mA/cm2 current density. The stability was checked by performing 1000 cycles of CV scanning within 1.036 to 1.636 V at a scan rate of 50 mV/sec. The CV traces remained unchanged after 1000 cycles supporting the stability of nanocatalyst. The stability curve at 295 mV overpotential also retained the current density up to 20 h [30]. The stability comparison between monometallic and bimetallic metal nitrides revealed that later is more stable under HER condition. For example, the CoN under constant overpotential of 100 mV retained current density up to 24 h. The NiCo2N exhibited current retention up to 48 h under HER condition at lower overpotential (50 mV). The overpotential values of the bimetallic system was also superior compared to that of the single metal system. The electrolysis was carried out under basic conditions (1 M KOH). These enhanced activity is attributed to the synergistic effect of both the metals that increased the conductivity and facilitated the charge transfer necessary for efficient HER [81]. The iron nickel alloy nitride systems have shown most comfortable overpotentials towards OER as described in the previous section. The stability of these class of catalysts becomes important as the plan is to pursue similar composition for our approach. Successive OER and HER was carried out with the same catalyst for 30 h each at an overpotential of 100 and − 100 mV respectively. The sample exhibited adequate stability and the crystallinity also remained intact after the reactions as displayed from the XRD.

The durability test also revealed that the current was maintained up to 400 h under basic conditions at 10 mA/cm<sup>2</sup> [21]. A Ru nitride (non-stoichiometric) coated on carbon black catalyst retained current density (10 mA/cm2) up to 50 h, even though the electrolyte was changed two times in between the period. At the same time, the reference could sustain the current only for 10 h and the current density started decreasing to 1 mA/cm2 after 2 h. Importantly, the chronoamperometry trace was identical to the original one after 50 h of catalysis supporting stability. Similarly, the overpotential value remained intact after 1000 cycles of LSV, whereas the catalyst without nitride coating degraded with a much higher overpotential [82]. An electrolysis study with Co2N, Co3N and Co4N showed that all three catalyst systems exhibited current retention up to 3.3 h at 437 mV overpotential in 0.1 M KOH. The oxygen production value was similar to that of the theoretically calculated value for 1 h. The stability of Co based system was lower compared to that of the Ni/Fe based system. The chronoamperometry trace after 1000 cycle matched with that of the initial one. Overall, Ni in conjugation with other transition metals in the nitride form have exhibited adequate stability with a low overpotential [83]. The overpotential of Ni3Fe was marginally lower than that of the Ni3Co nitride and much lower than that of the Ni3Mn nitride. Both Ni3FeN and Ni3CoN exhibited retention of potential (1.55 V) under 100 A/g current density. However, the Ni3MnN could sustain the activity up to 16 h under OER conditions. The Ni3FeN exhibited stability under HER conditions with efficiency. The LSV curves were


#### **Table 3.**

*Ni based nitride catalyst efficiency and stability under OER conditions (reproduced with permission from ref. [5] copyright 2019 WILEY-VCH Verlag GmbH).*

repeatable after 1000 cycles [84]. Finally, a summary of the Ni based nitride catalyst efficiency under OER condition is presented to obtain a generic understanding about the capability of these class of materials. The overpotential value is distributed within 50 to 360 mV for OER under basic conditions with stability is termed as good to excellent suggesting the chances of developing a Ni nitride based catalyst for commercial grade electrolysis is fairly high (**Table 3**).

#### **6. Effect of salt on electrolysis efficiency**

Though most of the articles have studied pure water electrolysis using metal nitrides as the catalyst, a few reports suggests that metal nitrides are promising as the catalyst for sea water electrolysis due to their corrosion resistant and electrically conductive properties [85]. The overpotential values marginally increased, when NaCl was added to the water or seawater was directly used as the electrolyte. In case of NiMoN, the overpotential values increased from 130 to 160 mV, when seawater replaced the pure water as the electrolyte. Interestingly, addition of NaCl to the solution, didn't change the HER overpotential value significantly at 500 mA/ cm2 current density. Similarly, the OER overpotential value increased from 340 to 355 mV at 500 mA/cm<sup>2</sup> current density in presence of salt and the value further increased to 365 mV in presence of sea water suggesting, though the salt water affects the overpotential, the increase is not that significant. The durability tests were also conducted in presence of NaCl and sea water. The overpotential value exhibited minor increase after 100 h at 500 mA/cm<sup>2</sup> current density suggesting the catalyst may be a viable option for exploring possibility as a commercial catalyst towards further research and development [63]. Recently, a Mo5N6 nanosheet along *Recent Trends in Development of Metal Nitride Nanocatalysts for Water Electrolysis Application DOI: http://dx.doi.org/10.5772/intechopen.95748*

**Figure 1.**

*The overpotential versus pH for the selective OER and the form of Cl− conversion in presence different pH conditions (reproduced with permission from ref. [3] copyright 2016 WILEY-VCH Verlag GmbH).*

with several metal nitrides were studied for their efficiency towards sea water electrolysis. As per the report, the HER overpotential was least affected by the presence of sea water. Similarly, the current retention was also 100% after a 100 h cycle at an applied potential of 310 mV [86]. Similarly, a NiNS based bifunctional catalyst system was also studied for sea water electrolysis. A current density value of 48.3 mAcm−2 at 1.8 V was achieved for overall sea water electrolysis. The current density value marginally decreased from 15 to 13 mA/cm<sup>2</sup> over 12 h period of electrolysis. To conclude, the overpotential values associated with these metal nitrides based catalyst systems are adequate to carry out electrolysis without affecting Cl− ion. In case of basic pH electrolysis, the allowed overpotential is 450 mV, whereas under acidic conditions, the overpotential is limited to ~250 mV. Moreover, the Cl<sup>−</sup> is expected to release as Cl2 gas under acidic conditions, whereas under basic conditions, the hypochlorate ion is going to precipitate as salt and may not affect the efficiency to a certain extent (**Figure 1**). Therefore, considering the overall scenario, basic electrolysis system may be one of the safer option when sea water is a part of the electrolyte.

#### **7. Conclusions**

In conclusion, the chapter enlightens the necessities of nitride based catalysts for sea and ground water electrolysis in the preliminary section. Subsequently, the synthetic strategy utilized for these metal nitride nanocatalysts, their efficiency and efficacy in presence of sea water as electrolyte is summarized. To ensure H2 energy becomes one of the commercially viable energy source for consumption, a sustainable generation mode is highly desirable. Metal nitride based catalysts have shown promise to fulfill the same with numerous research publications providing scientific data in support of the above. Though, still commercial implementation is yet to be achieved with these class of catalyst materials, we can safely assume that in near future a realistic design of electrolyzers possessing these nitride nanocatalyst modified electrodes may be available for commercial production.

*Electrocatalysis and Electrocatalysts for a Cleaner Environment - Fundamentals...*
