Electrochemical water splitting is a pivotal approach for green hydrogen production with sustainable energy, which contains an anodic reaction (oxygen evolution reaction, OER) and a cathodic reaction (hydrogen evolution reaction, HER) in the whole electrolyztic process. Until now, the high-performance electrocatalysts for HER and OER have mainly been inorganic materials, especially for precious metal alloys ^[5,32,33]. In this section, HER and OER are introduced with typical examples, equipped with their mechanisms.

HER process involves 2-electron transfer, which is easier than the 4-electron transfer of the OER process. In the first step, hydrogen adsorption occurs on the electrode surface to form H^* (^* is the active site). In an alkaline solution, the reaction mechanism follows the following steps: H₂O + e⁻ → H^* + OH⁻ (Volmer step); H₂O + e⁻ + H^* → H₂ + OH⁻ (Heyrovsky step). Excellent HER electrocatalysts require moderate hydrogen binding energy. Metalophthalocyanines (MPcs), such as Ni, Fe, and Co, have been extensively studied for their potential in electrochemical hydrogen production when combined with composites like glass carbon electrodes and carbon nanotubes^[13,14]. However, there has been limited research on the use of phthalocyanine-based c-MOFs for investigating HER ^[4]. For example, these phthalocyanine-based c-MOFs (MPc-pz), which incorporate MPc molecules and pyrene units connected by pyrazine (pz) linkages, was synthesized with different metal sources within MPc (M= Fe, Co, Ni, Mn, Zn, and Cu). The as-prepared samples were intended to investigate the electrochemical nitrogen reduction reaction (NRR), and presented improved activity and selectivity towards electrocatalytic NRR^[34]. Interestingly, FePc-pz exhibited outstanding catalytic activity with a high NH₃ yield rate compared to the various synthesized MPc-pz (Fig. 4a). Owing to the competing reaction between NRR and HER in the cathodic area, the higher NRR activity means the lower HER. In fact, the energy barrier for forming ^*H on CoPc-pz was found to be smaller than that for FePc-pz, indicating faster HER kinetics (Fig. 4b). However, due to the limited reports on phthalocyanine-based c-MOFs in the HER process, it is difficult to seek enough cases to demonstrate the HER activity of phthalocyanine-based c-MOFs positively. Several alternative reactions, such as NRR, OER, and other small molecule oxidation reactions, are regarded as competing reactions. Thereby, the activity of the opposite reaction can also be used to evaluate the HER activity to some extent. In addition, density functional theory (DFT) calculations could offer valuable insights into the electrocatalytic pathways of these macromolecular complexes in this example.

The OER process is an important half-process in water splitting electrolyzers, but its sluggish kinetics require a high overpotential to promote OER activity. Although benchmark electrocatalysts like IrO₂ and RuO₂ have been extensively studied, phthalocyanine- based c-MOFs could serve as efficient and abundant alternative electrocatalysts to reduce costs and improve performance. Jia et al. ^[18] developed a NiPc-based c-MOF through a bottom-up approach. Then, the prepared c-MOFs were deposited on FTO (fluorine- doped tin oxide) for electrocatalytic OER, which demonstrates excellent activity with a low overpotential and onset potential. This study firstly reported such performance in a NiPc-based c-MOF.

In a separate study, Song et al.^[35] explored a method to modulate the electronic configurations of phthalocyanine-based c-MOFs by introducing dual metal clusters (Fig. 5a). High-resolution TEM (HRTEM) image presentes a square lattice with a spacing corresponding to the (100) crystallographic planes of the NiPc-Ni c-MOF (Fig. 5b). The dual-metal c-MOFs with optimized structure showe a low onset overpotential and Tafel slope. The authors also explored the different modulations between Ni and Zn clusters within the matrix and their electrocatalytic OER activity (Fig. 5c). Such a relationship study could also be designed and explored for other electrocatalysis. The densities of states (DOS) and PDOS (projected-DOS) could help us understand the electronic structure of the material or individual orbitals through DFT calculations (Fig. 5d). PDOS patterns of dual-metal c-MOFs demonstrate that the electronic structure interaction between Ni-O₄ and Ni-N₄ clusters is better than other configurations, which could enhance their OER activity (Fig. 5e). Unlike the HER study of phthalocyanine-based c-MOFs, OER and their alternative reactions were investigated directly on the phthalocyanine-based c-MOFs. More work was focused on the rational design of MPc complexes and linkages and microenvironmental modulations, to optimize their electronic structure and improve their conductivity, meanwhile exposing more active sites for efficient electrocatalytic reactions.

The oxygen reduction reaction (ORR) plays a critical role in high-efficiency proton exchange membrane fuel cells (PEMFCs), metal-air batteries (such as Li-air, Zn-air, and Mg-air), and other electrochemical energy conversion devices^[36]. The ORR mechanism is complex, which involves various steps such as adsorption, dissociation, recombination, and desorption of oxygenated intermediates. It also involves interfacial transfer of 4-electron and 2-electron processes^[37]. Overcoming the sluggish kinetics of the ORR requires the discovery of high-quality catalysts.

Zhong et al.^[38] demonstrated a CuPc-based conjugated c-MOF with square-planar cobalt bis(dihydroxy) complexes (Co-O₄) as linkages (PcCu-O₈-Co), which was synthesized through the solvothermal method, resulting in layer-stacked structures (Fig. 6a). The half-wave potential (E_1/2) of PcCu-O₈-Co c-MOF was found to be 0.83 V vs RHE, which is comparable to that of commercial Pt/C (Fig. 6b). When PcCu-O₈-Co c-MOF was combined with CNTs, it exhibites excellent ORR activity in alkaline media. This is due to the extensive coverage of electrochemically Co active sites and porous structure. Based on the in situ Raman spectroscopy, an advanced technique for in situ characterization, they allow the detection of reaction intermediates attached to catalyst surfaces, which provides a basis for proposing reaction pathways leading to specific products through theoretical calculations^[39]. Herein, in situ Raman spectra with various applied potentials and theoretical modeling confirmed that the cobalt nodes in the c-MOF were active sites for the ORR (Fig. 6c). Based on the obtained results, a proposed ORR mechanism was presented (Fig. 6d).

Extensive research has been conducted on the electrochemical reduction of CO₂ (CO₂RR) due to its potential for sustainable CO₂ conversion, carbon recycling, and renewable energy storage, making it a highly valuable process^[40,41]. As for electrochemical thermodynamics, proton-coupled pathways such as 4-electron, 6-electron, and 8-electron processes are preferred^[42,43]. Cu is widely recognized as the primary metal capable of efficiently converting CO₂ into significant quantities of value-added products through electrochemical reduction ^[44]. The diverse products of CO₂RR through electrochemical reduction contribute to the complexity of their reaction mechanisms. However, regular MOFs exhibit poor electrical conductivity due to the inherent structure, and they need other conducting substrates (such as Nafion membrane, nickel foam, and carbon-based materials) to improve the current density in CO₂RR. Metallophthalocyanine complexes (MPcs) have displayed potential in catalyzing CO₂ reduction. However, individual metal complex-based molecular electrocatalysts demonstrate lower efficiency and specificity in comparison with Au, Ag, and Cu-based materials. A successful approach for achieving high current density in CO₂RR involves integrating a substantial amount of electroactive metal complex-based catalysts into porous conductive materials. Porous coordination polymers based on MPc with periodically arranged metal ions/clusters and organic ligands are considered as promising candidates for CO₂RR due to their extensive surface areas, significant CO₂ adsorption capabilities, and a high concentration of adjustable active sites.

Huang and co-workers et al.^[45] developed a conductive NiPc-based complex using 2,3,9,10,16, 17,23,24-octa-aminophthalocyaninato Ni(Ⅱ) and tert-butylpyrene-tetraone for efficient CO₂RR. The prepared complex exhibited high selectivity towards CO production with a large current density due to its high conductivity and more accessible active sites. In the same group, they synthesized NiPc-NiO₄ MOF nanosheets, which differ from the above structure and exhibit a high selectivity (nearly 100%) of CO production with high current density (Fig. 7a) ^[46]. The obtained NiPc-NiO₄ c-MOF exhibits strong conductivity due to the in-plane full p-d conjugation between the NiPc and NiO₄ linkages. The outstanding electrocatalytic CO₂RR activity of the prepared nanosheets can be attributed to their high electrical conductivity, available single active sites, and significant CO₂ adsorption capacity (Fig. 7b). Moreover, DFT calculations showed that the nickel center in NiPc acts as the active site. NiPc-NiO₄ c-MOF demonstrates superior activity compared to MPc molecules, which was attributed to its rapid electron transfer capability and exceptional reducibility (Fig. 7c). It can be concluded that phthalocyanine-based c-MOFs presents exceptional conductivity owing to the structural merit, including the conjugated molecular rings and in-plane full p-d conjugation between MPc and linkages. In addition, to obtain carbon-based products with high selectivity and efficient production after the CO₂RR process, it is necessary to rational design the MPc and linkages. For attention, the regulation mechanism of the structure-activity of phthalocyanine-based c-MOFs towards electrocatalytic CO₂RR still needs further exploration.

Electrocatalytic NRR is a sustainable approach for nitrogen-to-ammonia generation in gas-phase electrocatalysis, but it still suffers from the low ammonia production issue due to the competing H₂ evolution reaction^[47]. Moreover, the inert nature of triple bonds of N₂ makes it difficult to achieve nitrogen conversion under normal conditions. In response to this challenge, the electrochemical NRR combined with renewable energy has emerged as a viable and sustainable alternative to the high temperature/pressure Harber-Bosch process for ammonia production. This approach offers a promising solution to the challenges associated with nitrogen conversion ^[48].

The well-organized MOFs with microporous structures played a vital role in the NRR application. In this context, 2D c-MOFs based on phthalocyanine could serve as promising NRR electrocatalysts due to (i) the layer-stacked structure with a high surface area, which can offer a substantial number of easily accessible active sites and adequate mass transport, and (ii) the inherent electrical conductivity, which can be advantageous for efficient electron transfer behaviors. Phthalocyanine-based c-MOFs with a microporous structure have been proven to be essential in gas-phase catalysis. Feng et al.^[34] reported the initial instance of crystalline 2D phthalocyanine-based c-MOFs integrated with abundant M-N₄-C sites as efficient catalysts, realizing both improved activity and selectivity in nitrogen-to-ammonia conversion. This example was previously mentioned in Section 3.1 as HER application.

Based on their studies, the researchers used MPc and pyrene units to prepare MPc-pz through a solvothermal approach (Fig. 8a, b). Among these catalysts, FePc-pz with Fe-N₄ sites exhibited the highest ammonia yield rate (33.6 μg·h^-1·mg_cat^-1) and Faradaic efficiency (FE, 31.9%) at −0.1 V (versus reversible hydrogen electrode (vs RHE) compared to those with other MPc-pz with M-N₄ sites), making them one of the most effective NRR electrocatalysts. DFT calculations further confirm the effectiveness of FePc-pz, which contains Fe-N₄ active sites for electrocatalytic NRR (Fig. 8c~e). Due to the columnar stacking of porous structure, high in-plane π-conjugation for fast electron transfer, and abundant Fe-N₄ active sites, FePc-pz exhibits exceptional catalytic NRR activity and an impressive ammonia yield rate. The significant properties of the FeN₄ active sites, including optimal substrate adsorption energy and low energy barrier for the rate-determining step, enable efficient N₂ activation and conversion.

The electrochemical explorations of phthalocyanine-based c-MOFs are not limited to the above-mentioned water electrolysis (HER and OER), CO₂RR, NRR and ORR. For example, the urea oxidation reaction (UOR), hydrazine oxidation reaction (HzOR), and biomass oxidation reaction (BmOR) can also contribute to energy-saving hydrogen production and environmental protection in specific situations. Phthalocyanine-based c-MOFs can also be used to understand the common mechanisms of reported redox reactions and to regulate their coordination structure and micro-environmental electronic states for potential electrochemical reactions. Phthalocyanine- based c-MOFs can also refer to the common mechanisms of reported redox reactions and regulate their coordination structure and micro-environmental electronic states for the potential electrochemical reactions. It is important to note that different electrochemical reactions occur under specific reaction conditions, such as UOR preferring alkaline electrolytes and HER being easier in acidic solutions. These conditions should be considered in the development of novel phthalocyanine-based c-MOFs.

Electrocatalytic reactions encounter various issues, such as high overpotential, low current density, unsatisfactory selectivity, and susceptibility to poisoning. To tackle these challenges, understanding the reaction mechanism and assessing the impact of different factors on kinetics is crucial for the design and synthesis of high-performance phthalocyanine- based c-MOF catalysts. Advanced techniques like DFT calculations and in situ characterizations can provide insights into the electrocatalytic reactions, surface state changes, and atomic structure transformations. By utilizing those techniques, metal nodes or organic ligands can be tuned to decrease the free energy of adsorption and desorption, enhancing the activity of phthalocyanine-based c-MOF catalysts. Additionally, optimizing the morphologies and sizes of the catalysts can increase the number of exposed active sites, further amplifying their catalytic activity. By systematically addressing these considerations, it becomes feasible to overcome the existing challenges and develop highly efficient phthalocyanine-based c-MOF catalysts for electrocatalytic reactions.

The limited conductivity of pristine MOFs, attributed to the inherent properties of organic ligands, poses a significant constraint on the development of highly electroactive MOFs. Enhancing the conductivity of phthalocyanine-based c-MOFs can be achieved through various strategies, including morphology control, component manipulation, and structural engineering. For instance, the use of high-quality and highly crystalline phthalocyanine- based c-MOFs, such as single crystals, ensures efficient charge transfer and prevents conductivity loss. Additionally, incorporating heterostructures in phthalocyanine-based c-MOFs opens possibilities for designing highly efficient electrocatalysts. An effective approach involves anchoring MOF films onto conductive substrates, which further enhances conductivity and catalytic performance. These advancements in charge transport pathways and the increase of mobile charge carriers contribute to improved conductivity in phthalocyanine-based c-MOFs, enabling their potential as highly electroactive materials.

An effective electrocatalyst not only demonstrates outstanding catalytic activity but also exhibits exceptional selectivity. Selectivity plays a crucial role in electrocatalytic reactions, encompassing various processes such as ORR (2-electron and 4-electron pathways), CO2RR (C1 and C2 + products), and NRR (ammonia and other nitrogen oxides). For example, the oxygen reduction reaction is a key process in proton exchange membrane fuel cells. The 4-electron pathway on the electrocatalyst is highly desirable for improving the fuel cell’s current density and powder density. However, oxygen reduction may follow the 2-electron pathway to generate hydrogen peroxide, and the hydroxyl radical can be detrimental to the membrane's longevity and the stability of the electrocatalyst. Therefore, precise control of the structure of phthalocyanine-based c-MOFs with sufficient active sites can effectively facilitate ORR with the 4-electron pathway. Additionally, the inter-distance, often overlooked, plays a critical role in controlling the diffusion and adsorption of intermediates, thereby contributing to improved selectivity. By comprehensively addressing these factors, advancements in electrocatalysis can be achieved, taking into account both catalytic activity and selectivity.

Stability, as a crucial practical indicator for long-term operation, can be understood through variations in morphological structure, composition, and size information. Just like catalytic activity, selectivity and stability can be enhanced by adjusting metal centers of MPc or linkages (organic ligands and metal nodes) within the matrix. Additionally, utilizing metal centers with high oxidation states can contribute to enhanced stability. The capacity to modify metal nodes and organic ligands allows for the design and acquisition of materials with specific structures and corresponding applications. By considering stability alongside other factors, it is feasible to develop materials that demonstrate excellent catalytic activity as well as sustain performance over extended periods of operation.