Carbon-supported Copper Phthalocyanine Tetrasulfonic Acid Tetrasodium Salt And Its Composites As Cathode Catalysts For Alkaline Fuel Cell-electrocatalytic Activity And Oxygen Reduction Reaction(ORR) Mechanisms

With increasing energy demanding and environmental pollution, there is an urgency toexplore new, efficient and sustainable energy sources to compensate and even replace traditionalones. Among different kinds of energy technologies explored, electrochemical energytechnologies such as fuel cells, batteries, and supercapacitors have been considered the mostfeasible options. Particularly, fuel cells are recognized as the most promising energy conversiondevices among different electrochemical energy technologies due to their several advantages suchas high energy efficiency, low/zero emissions, as well as the wide range of applications. In recentyears, there are still two major challenges hindering the commercialization of fuel cells, high costand insufficient durability of Pt-based electrocatalysts. Therefore, the major effort in fuel cellresearch and development has been put on exploring non-noble metal catalysts to replace Pt/C.Among these candidates, the pyrolyzed Co-and Fe-centered phthalocyanines have been proved tobe the most promising catalysts, which are reported to have a close ORR activity to that ofcommercially available Pt/C catalyst. Actually, the ORR activity of metallophthalocyanine (MPc)strongly depends on the pH value of electrolyte and also the composition of the catalysts as wellas the preparation parameters. Recent studies indicate that the other few concerned MPc such asCu-, Ni-and Zn-centered phthalocyanine can give an excellent ORR activity in alkaline media.However, compared to Pt-based catalysts for practical application in fuel cells, these kind ofnon-precious metal catalysts are still facing challenges of both insufficient ORR activity andstability.The carbon-support copper phthalocyanine tetrasulfonic acid tetrasodium salt (CuTSPc/C)catalyst for oxygen reduction reaction (ORR) are synthesized via a combinedsolvent-impregnation along with the one-step pyrolysis.The electrocatalytic activities of the CuTSPc/C catalysts obtained at different pyrolysis temperatures and CuTSPc-FePc/C (FePc act asthe second active component, the carbon support also can be BP2000and MWCNTs) catalysts areintensively evaluated by linear sweep voltammetry (LSV) using both rotating disk electrode (RDE)and rotating ring-disk electrode (RRDE) techniques in alkaline media. The ORR kineticparameters and possible reaction mechanisms are also discussed. XRD, TEM, EDX, TG-DTG andXPS analyses are also performed to identify the possible active sites of these catalysts.The main points are summarized as follows:(1) The heat-treatment has significant effect on ORR activity of the CuTSPc/C catalyst, andthe optimum pyrolysis temperature is around700oC, with the onset potential of0.11V and ahalf-wave potential of-0.02V. Meantime, the electron transfer number (n) of the ORR processcatalyzed by the unpyrolyzed catalyst is about2.5, after the pyrolysis, this number is increased to3.5, indicating that the pyrolysis process can change the ORR pathway from a2-electron transferdominated process to a4-electron transfer dominated one. Suggesting that the CuTSPc/C catalystafter heat-treated at700oC can directly reduce most O2to H2O, validly decrease the generation ofH2O2. Besides the pyrolysis temperature, increasing the catalyst loading also can efficientlyimprove the ORR performance, and the optimal loading is around505μg/cm2.(2) XRD and TEM analysis show that the macrocyclic structure of CuTSPc decomposes andforms the active sites. After pyrolysis at700oC, some bright and uniform distribution particleswith the size about20-30nm can be clearly observed. With further increasing the temperature, theactive sites are destroyed, resulting in reduced catalytic activity. TG results demonstrate that thetotal mass loss of CuTSPc/C is smaller than that of H2Pc/C, and also smaller than that of CuPc/C,indicating that both Cu species and sulfonic acid group prevent phthalocyanine from thermaldecomposition and contribute to higher nitrogen content which can be benefit for ORR activitysites. XPS results indicate that both active N species and S may be the most important factorsaffecting the ORR activity. In addition, the Cu ion can bond on pyridinic-N, graphite-N, andC-Sn-C to form the Cu-N-S/C active structures.(3) The acid-leaching procedures are used to investigate the role of transition metal for ORRactivity. The results show that the ORR activity of the catalyst after acid-leaching((CuTSPc/C)700AL) decreased greatly. The half-wave potential for (CuTSPc/C)700AL wasnegatively shifted more than30mV compared to the catalyst before acid-leaching ((CuTSPc/C)700).The H2O2produced is increased from7.8%for (CuTSPc/C)700to15.0%for (CuTSPc/C)700AL atthe potential of-0.35V. The value of n is decreased accordingly from3.84to3.70. XRD、TEM andXPS analysis revealed that the metal Cu has been removed basically after acid-leaching. Theabsence of metal Cu has negative effect on the ORR activity. Therefore, the metal Cu is essentialfor ORR active sites, also play a part in the stabilization of the active N and S species. (4) The CuTSPc-FePc/C catalysts are prepared by FePc-doped with different quantity. Theoptimal catalyst ORR activity obtains at the composition ratio of CuTSPc and FePc reached to1:1,under which, an onset potential of0.18V and a half-wave potential of0.04V. Moreover, using theBP2000act as the catalyst carbon support can further improve the catalytic activity. This catalystowns an onset potential of0.20V and a half-wave potential of0.11V, it is worth noting that thishalf-wave potential is positive shifted about30mV compared with the commercial Pt/C.