**4. Catalytic efficiency of metal nitride based catalysts**

Overall, the literature supported that nitrides are more conductive compared to that of the oxides. Owing to the above, metal nitrides exhibit lower overpotential compared to that of the oxides and may be used as bifunctional catalysts for both HER and OER. Doping improves the stability and in some cases the efficiency of the catalyst. Metal alloy can be advantageous as base material. Ni, Fe, Co, Mo are some of the possible low cost metal precursors for utilization as catalyst. Shape of the nanocatalyst also offer possibility of further efficiency improvement. Thin coating of metal nitrides is advantageous compared to the bulk coating. For example, Ni3N nanosheets exhibited an overpotential value of ~380 mV at 100 mA/cm<sup>2</sup> current density, whereas the bulk sample and the corresponding oxide exhibited

overpotential value of 600 mV to achieve the above current density. The Tafel slope value for the nanosheet was 45 mV/dec, whereas the bulk catalyst exhibited a Tafel slope value of 85 mV/dec. [25] Recently, a CuNi2N fabricated on carbon cloth, exhibited 71.4 mV overpotential during HER with a Tafel slope value of 106.5 mV/ dec. Long time stability test showed the retention of voltage at 10 mV/cm<sup>2</sup> current density for 60 h [26]. A plasma transformed Ni3N porous nanosheet exhibited 46 mV/dec of Tafel slope value during HER with overpotential marginally higher than that of the Pt/C [27].

Among the metal nitrides studied, ternary metal nitride systems are the most promising, since the coordination number is close to one and the adsorption free energy is close to zero that is most favorable for HER as per the Sabatier's principle [28]. In fact, a large number of ternary metal nitrides have shown high activity towards HER such as NiMo-N [29, 30], CuNi-N [31], CoMo-N [32], Fe2Ni2N [33], ZCo3FeN [34], NiCo2N [35] and Ni3FeN [36] Among the non-noble metal systems, Ni-Fe based nitride systems with metallic characteristics, strong absorption of water molecule and unique electronic structure have displayed efficiency towards both HER and OER reaction. Especially, Ni rich compositions have shown promise towards full water splitting. For example, Ni3Fe-N and Ni2Fe2-N have shown efficiency towards HER and OER [37–40]. Morphology of the catalyst and contact with the GC electrode can significantly affect its performance and long term stability. Therefore, in-situ growth is given importance in the later stage of catalyst development. Especially with the Ni based system, Ni foam could serve as one of the most appropriate base electrode since the contact becomes more efficient. Stacking of multiple electrodes can also be utilized to further aid the efficiency. Overall, the literature has shown that effective catalyst systems based on Ni-Fe/ Co-N based systems can be formulated and fabricated for overall electrolysis at low overpotential. The Ni based system is able to yield current density value of 100 mA/ cm2 at a low overpotential of 100 mV along with superior durability. The already formidable HER activity of Ni based system may be further augured by decorating the catalyst with Pt and further improve the current density to 200 mA/cm<sup>2</sup> at 160 mV overpotential [41]. The Co based system are reported to exhibit low activity towards HER since the d band is far from the centre of HER energy level. Though this can be circumvented to some extent by doping with vanadium [42]. Bimetallic systems have invariably demonstrated superior catalytic activities compared to that of their monometallic analog. For example, Ni-Fe [43–45], Ni-Co [46, 47], and Co-Fe nitrides [48] have all exhibited improved catalytic activities compared to that of their monometallic counterparts. Ni3FeN catalyst materials are one of the leading candidates for use as HER electrocatalysts. Though the mechanism for HER on these nitride surfaces are a matter of intense research, several studies have supported the metallic nature of the catalyst for swift electron transfer necessary for HER. The surface of metal or the "N" that acts as an active centre is still under investigation. OER being a more energy intensive process compared to that of the HER, is more facilitated, when conductive metal nitrides are used instead of oxides. The ratio of metal and "N" influences the electrical conductivity, which subsequently affects the OER efficiency. It was proved in a Co based system (Co2N, Co3N and Co4N) that, increase in Co amount increases the intrinsic conductivity [49].

Nitrides based on other metals such as Mo and Co have shown activity towards either HER or OER. For example, the binary nitride based on Mo2N showed adequate HER catalytic activity. A composite of Mo2N-Mo2C showed enhanced HER activity compared to that of the Mo2N alone [50]. Similarly, layered conjugation of MoS2 with MoN2 also improved the electrocatalytic activity [51]. Direct growth of Mo2N on CNT and N doped carbon matrix can be utilized to improve the electrocatalytic activity [52, 53], Recently, Mo2N–Mo2C heterojunction on the

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

reduced graphene oxide displayed superior HER activity with low onset potentials of 18 mV under basic medium, that is superior to Pt/C electrode in alkaline media at large current densities [54]. In general, it has been observed that Metal-NC system exhibits superior activity compared to that of the metal nitrides only. Just recently, nitride MXene (V-Ti4N3Tx) based systems have shown superior HER activity [55]. Several specific examples are discussed below to obtain a fair idea about the catalytic activity of such systems.

Recently, Cai and coworkers have synthesized a Ru cluster of ~1 nm size anchored on N doped carbon surface using a one pot procedure [56]. The resulting catalyst displayed activity towards both HER and OER in alkaline (1 M KOH) medium. Importantly, the over-potential values were notably less in case of both the reactions (HER: 15 mV vs. RHE and OER: 285 mV vs. RHE). The durability test showed that the activity remained largely unaffected after 5000 cycles. They hypothesized that the presence of "N" in the catalyst matrix improved the stability and promoted the HER and OER activity. Similarly, Wu and coworkers have immobilized Co5.47N nanoparticles on C-N matrix by utilizing Co based Zeolite framework as the starting material [57]. The frameworks were pyrolyzed at 700°C in presence of NH3 to form the CN nanoparticles in-situ. The catalyst system was effective for both HER and OER and the over-potential values (149 mV for HER and 248 mV for OER) were lower compared to that of the IrO2 benchmark. The catalyst retained ~82% of original current density at an overpotential value of 248 mV after 10 h, which is superior compared to that of the benchmark (56%). The efficiency of nitrides in other cost effective metal systems were analyzed. Catalysts based on Ni3N nanosheet exhibited adequate OER performance in an alkaline solution and achieved 52.3 mA cm−2 current density at relatively low over-potential (350 mV) with small Tafel slope [58]. The nanosheets were prepared by coating an activated carbon cloth with Ni salts and heating the salt coated carbon cloth at 380°C in presence of NH3. The catalyst also exhibited Tafel slope value up to 45 mV/dec.

The Fe based catalyst (Fe3N/Fe4N) nanoporous film on a conducting Ni-graphene foam displayed low OER overpotential (238 mV) corresponding to current density of 10 mA/cm2 and a low Tafel slope value of 44.5 mV/dec along with high 96.7% faradaic yield. These numbers were superior compared to that of the benchmark IrO2 and attributed to high electron transfer and surface area of the catalyst [59]. The OER overpotential of the nitrides were far superior than the corresponding oxide as shown below. In fact, a similar trend was noticed with Co based nitride (Co4N) catalyst system. In which a current density value of 10 mA/ cm<sup>2</sup> was achieved at an overpotential of 257 mV with small Tafel slope [60]. Most of the reports on nitride based system revealed the bi-functional nature of the catalyst. These catalysts were effective against both HER and OER under basic conditions. Additionally, most of the catalyst based on the nitrides displayed lower overpotential value compared to that of the oxide based systems. The lower overpotential values of these systems were assigned to their higher conductivity values that resulted from the metallic character arising out of the overlap between Ni-3d and N-2p orbitals in the catalysts. The second promising aspect was their bifunctional behavior, that resulted from the tendency of "N" to donate electron more easily compared to that of the "O" and polarization associated with the shift of "H" from "O" to "N", a key step during HER [61]. The adsorption energy value of "H" on nitride based catalyst system was much lower compared to that of the H2O and similar to that of the Pt-C bench mark that facilitated the rate of HER. The nitride systems based on the metal alloys exhibited lower overpotential values compared to that of their single metal counterparts. For example, the overpotential value of CoFe(3:1)-N was ~150 mV lower compared to that of the Co-N or Fe-N for OER. The Tafel slope value was approximately 3 times lower in case of alloy

nitrides further supporting the efficiency of these systems towards OER. The ratio between the two metals in the alloy played an important role, while determining the catalytic efficiency [62]. The stability of these catalysts were also excellent as only 4.5% decrease in efficiency was noticed after 10 h of OER. The spatial arrangement of these alloy nitride nanostructures on the base electrode also displayed a variation in catalytic efficiency. For example, NiFe-N grown on Z plane from the electrode surface displayed 277 mV overpotential at 100 mV/cm2 current density and 337 mV at 500 mV/cm2 , which was lower compared to that of the IrO2 benchmark (542 mV at 500 mV/cm2 ) [63]. The Tafel slope value for the above catalyst system was 58.6 mV/dec.

Further studies have shown that, the shape of the nanocatalyst controls the efficiency to a substantial extent. Report based on Rh nanocrystals have shown that, the benzoid structures exhibited lower overpotential in both OER and HER compared to that of the tetrahedral structures [64]. However, the deviation was within 100 mV in case of OER. Interestingly, the Tafel slope value displayed a strong improvement on optimization of the shape. The value for the benzoid structure was 87 mV/dec, whereas the value for the tetrahedral structure was 205 mV/dec. Other factors such as doping of metal nanoparticles on the catalyst can also be used as a procedure to further improve the overpotential value and dependency of current on the overpotential. Recently, a cobalt nitride based nanofiber system was doped with Ir nanoparticle. The resulting catalyst exhibited much lower Tafel slope value compared to that of the undoped system. The overpotential value of the doped system was also lower compared to that of the base nitride system [65]. Similarly, a chromium doped Co-N system exhibited a Tafel slope value of 38.1 mV/dec and retained current density up to 200 h [66]. The system displayed an overpotential value of 99 mV at 100 mV/cm<sup>2</sup> . The current versus potential curve was superimposable


#### **Table 2.**

*The table summarizes the values of the overpotential and Tafel slope values for different catalyst compositions. Ref. [67] (supporting information).*

after 1000 cycles. The values were superior compared that of the Pt benchmark. A common study involving various earth abundant metal catalysts revealed that Tafel slope value of the non-stoichiometric nickel nitride catalyst coated on Ni (Ni3Nx) foam is the lowest among various catalyst systems studied (**Table 2**).
