**2.2 Electrocatalytic cycle for H2 evolution**

In the electrocatalytic cycle, V. Artero and coworker observed that in acetonitrile medium, halide ligands are banished with reduced oxidation state from CoII to Co<sup>I</sup> . Upon reduction, the coordination in number decreases from six in CoII state to five in CoI state; this characteristic was supported by DFT calculations [71]. In the catalytic cycle (**Figure 6**), the first step is the transfer of electron and proton by proton-coupled electron transfer (PCET) process. PCET is a chemical reaction that

*Cyclic voltammograms of (6) Br2 and (7) Br2 (1 mM, black traces) recorded in CH3CN at a glassy carbon*

*<sup>1</sup> (the figure is reproduced from Ref. [69], with permission from the*

*) in the presence of* p*-cyanoanilinium tetrafluoroborate. c = the controlled potential*

**Figure 5.**

*publisher).*

**105**

*electrode at a speed of 100 mVs*

*carbon electrode (100 mVs<sup>1</sup>*

*Ar-saturated CH3CN/H2O.*

Table 2.

**Catalysts Redox-active organic ligands**

*DOI: http://dx.doi.org/10.5772/intechopen.92854*

**Catalytic potential**

*Recent Progress of Electrocatalysts and Photocatalysts Bearing First Row Transition Metal…*

[1]a *o*-Phenylenediamine —— THF 5.5 [65] [2] *o*-Phenylenediamine —— THF 2.9 [67] [3] *o*-Phenylenediamine —— THF 0.99 [65] [4] *o*-Phenylenediamine —— THF 0.51 [65] [5] *o*-Phenylenediamine —— THF 0.73 [65] [6]<sup>b</sup> Diimine-dioxime 0.68 V vs. Fc+/0 H2O/CH3CN 300 [70] [7] Diimine-dioxime 0.96 V vs. Fc+/0 H2O/CH3CN 50 [70] [8] Bis(thiosemicarbazone) 1.7 V vs. Fc/Fc<sup>+</sup> CH3CN 37 [74] [9] Bis(thiosemicarbazone) 1.7 V vs. Fc/Fc<sup>+</sup> CH3CN 73 [75] [10]<sup>c</sup> Diamine-tripyridine 0.90 V vs. Fc+/0 acidic-H2O 1.4x 104 [70] [11]<sup>d</sup> TMPA 1.81 V vs. SCE CH3CN/H2O 6180 [76] [12] Cl-TMPA 1.72 V vs*.* SCE CH3CN/H2O 10,014 [76] [13] Bis(benzenedithiolate) 2.25 V vs. SCE CH3CN 0 [77] [14] *o*-Aminobenzenethiolate 1.64 V vs. SCE CH3CN 6190 [77] [15] *o*-Aminobenzene 2.03 V vs. SCE CH3CN 900 [77] [16] 2-Mercaptophenolate 1.62 V vs. SCE CH3CN 5600 [77] *a = photochemical H2 production from [M-opda 1–5] (7.98 <sup>10</sup><sup>2</sup> mmol) with HQ (7.98 <sup>10</sup><sup>1</sup> mmol) in (4 mL) under an N2 atmosphere at* 20°C *for 190 h in the presence of 4 AMS (irradiation with 300 W Xe lamp, 250–385 nm). b = in light-driven condition of TEA and a cyclometalated iridium-based photosensitizer; CV was done in glassy*

**Solvent TON(H2 mol cat<sup>1</sup> )**

**Ref.**

*in phosphate*

**(Ep)**

*electrolysis of complex 11 was measured at pH 2.5 under N2 atmosphere at a scan rate of 50 mVs<sup>1</sup>*

*buffer, over 2 hours using a glassy carbon electrode. d = conditions, 0.1 M n-Bu4NPF6, scan rate 100 mV/s, in*

*Electrochemical data and catalytic efficiency of metal complexes for water-splitting hydrogen evolution reaction.*


*Recent Progress of Electrocatalysts and Photocatalysts Bearing First Row Transition Metal… DOI: http://dx.doi.org/10.5772/intechopen.92854*

*a = photochemical H2 production from [M-opda 1–5] (7.98 <sup>10</sup><sup>2</sup> mmol) with HQ (7.98 <sup>10</sup><sup>1</sup> mmol) in (4 mL) under an N2 atmosphere at* 20°C *for 190 h in the presence of 4 AMS (irradiation with 300 W Xe lamp, 250–385 nm). b = in light-driven condition of TEA and a cyclometalated iridium-based photosensitizer; CV was done in glassy carbon electrode (100 mVs<sup>1</sup> ) in the presence of* p*-cyanoanilinium tetrafluoroborate. c = the controlled potential electrolysis of complex 11 was measured at pH 2.5 under N2 atmosphere at a scan rate of 50 mVs<sup>1</sup> in phosphate buffer, over 2 hours using a glassy carbon electrode. d = conditions, 0.1 M n-Bu4NPF6, scan rate 100 mV/s, in Ar-saturated CH3CN/H2O.*

#### Table 2.

*Electrochemical data and catalytic efficiency of metal complexes for water-splitting hydrogen evolution reaction.*

#### **Figure 5.**

*Cyclic voltammograms of (6) Br2 and (7) Br2 (1 mM, black traces) recorded in CH3CN at a glassy carbon electrode at a speed of 100 mVs <sup>1</sup> (the figure is reproduced from Ref. [69], with permission from the publisher).*

five in CoI state; this characteristic was supported by DFT calculations [71]. In the catalytic cycle (**Figure 6**), the first step is the transfer of electron and proton by proton-coupled electron transfer (PCET) process. PCET is a chemical reaction that

**2.2 Electrocatalytic cycle for H2 evolution**

*Plausible mechanism for photochemical HER with [M-opda] complexes.*

*Structure of redox-active ligands, electrocatalysts, and photocatalysts.*

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

Co<sup>I</sup>

**104**

**Figure 4.**

**Table 1.**

In the electrocatalytic cycle, V. Artero and coworker observed that in acetonitrile medium, halide ligands are banished with reduced oxidation state from CoII to

. Upon reduction, the coordination in number decreases from six in CoII state to

involves the transfer of electron and proton in which the oxidation number changes by CoII to Co<sup>I</sup> . In the second steps, further electron and proton transfer takes place by PCET process, and the oxidation number changes from CoI to CoII. In the last step, H2 is produced in dihydrogen bond through an intramolecular mechanism. The authors also confirmed cobalt diimine-dioxime catalysts **6** and **7** active for H2 evolution under light-driven conditions in the presence of photosensitizers, associated with Ru, Ir, or Re derivatives. The photocatalytic activity of **6**-Br2 was observed in mixed H2O/CH3CN solvent in the presence of TEA and cyclometalated iridium-based photosensitizer [72]. Turnover numbers (TON) determined after continuous 4 h UV–visible light irradiation **6**-Br2 showed 300 (H2 mol cat<sup>1</sup> ), whereas **7**-Br2 complex shows only 50 (H2 mol cat<sup>1</sup> ) as shown in **Table 2**.

moiety diacetyl-bis (N-4 methyl-3-thiosemicarbazide) exhibits the homogeneous catalysis of electro-driven H2 evolution through proton reduction with a maximum of TOF 1170 s<sup>1</sup> in CH3OH and 11,700 s<sup>1</sup> in CH3CN at an overpotential of 0.756 V and 1.074 V, CH3COOH used as proton source. Bulk electrolysis showed that the TON of H2 evolution of ZnII is 37 in over 2.5 h experiments. To make this more comparative, Grapperhaus and coworker synthesized another CuII complex with similar ligand diacetyl-bis(N-4-methyl-3-thiosemicarbazide) and examined H2 evolution reaction. CuII complex **10** exhibits a maximum TOF of 10,000 s<sup>1</sup> in CH3CN and 5100 s<sup>1</sup> in DMF at an overpotential of 0.80 and 0.76 V, respectively. Controlled potential electrolysis confirmed CuII complex act as an excellent

*Recent Progress of Electrocatalysts and Photocatalysts Bearing First Row Transition Metal…*

*DOI: http://dx.doi.org/10.5772/intechopen.92854*

electrocatalyst to produce H2 with a minimum faradic efficiency of 81% and TON as high as 73 during experiment over 23 h. They examined HER mechanism of complex **10** through DFT computational studies. In the proposed mechanism, initially the protonation occurs at the hydrazino nitrogen ligand. This was followed by an

protonolysis occurs at the opposite hydrazine nitrogen of the ligand to yield

the identity of metal ions at the active site affects the HER mechanism.

and path(B), protonation occurs at one of the nitrogen atoms of the ligand (Eqs. (6), (7), and (8)). H2 generation reaction in path B takes place by two successive proton-coupled reduction processes. Protonation occurs at the ligand

This provides [(bztpenH)CuII(H)]2+copper hydride species, which release H2 and

centered in the first reduction step, followed by the Cu<sup>I</sup>

*Plausible mechanism for proton reduction in complex [CuL].*

of the H2 evolution illustrated in **Figure 7**. Hence, here it is worth to mention that

Professor Wang and group proposed [75] a significant homogeneous mononuclear copper electrocatalyst for H2 production attributed to diamine-tripyridine ligand; complex **11** attains trigonal bipyramidal geometry. According to the author, this ionic copper complex [Cu(bztpen)]2+ with a five coordinating nitrogen atom shows a Jahn-Teller effect. Electrochemical and spectroscopic studies supported that the H2 generation reaction takes place by two successive proton-coupled reduction processes. On the experimental observations of DPV, CV, UV–vis, and <sup>1</sup> H-NMR spectroscopic study, the authors proposed two possible pathways: path (A), protonation takes place at the CuI centered in the first step (Eqs. (4) and (5)),

(HL)]. The second

(H2L)]<sup>+</sup> leads to the formation

centered in the second step.

electrochemical step as a formation of reduced Cu<sup>I</sup> species [CuI

. Further one-electron reduction of [Cu<sup>I</sup>

[CuI

(H2L)]<sup>+</sup>

regenerate CuII catalyst.

**Figure 7.**

**107**

Similarly, cobalt bis(iminopyridine) complex **8** was prepared for electrocatalytic water-splitting reaction [73]. The ligand-centered redox activity was observed during the cyclic voltammetry, suggesting the considerable role of redox-active ligand which is completely involved to stabilize the cobalt metal in higher oxidation state. The two reduction potentials were observed for the CoIII/CoII (quasi reversible) and CoII/CoI (reversible) couples at 0.34 V and 0.86 V (vs. Ag/AgC), respectively. The improved water reduction was attributed to the assimilation of a redox-activeligand to cobalt which facile reduction of [Co-H]n+ species.
