Iron-gelatin aerogel derivative as high-performance oxygen reduction reaction electrocatalysts in microbial fuel cells

Currently, precious based materials are known as highly efficient and widely used catalysts for oxygen reduction reaction (ORR). The expensive price and scarce resource of precious metals have stimulated researchers to explore low-cost and high-performance non-precious metal catalysts. Gelatin is a promising precursor to prepare cost-effective and high-performance catalysts because of abundant micropores and nitrogen self-doping sites after pyrolysis. Herein, we developed a new highly active ORR catalyst (G/C–Fe-2) containing Fe–N coordination sites and Fe/Fe₃C nanoparticles. G/C–Fe-2 exhibited excellent ORR electrocatalytic activity (onset potential: 0.21 V, and limiting current density:7.36 mA cm^?2) and high-performance in air-cathode MFCs (Output voltage: 660 mV, and maximum power density: 560 mW m^?2). It is significant for synthesizing low-cost and high-activity ORR electrocatalysts through this strategy.     

Iron chelates as low-cost and effective electrocatalyst for oxygen reduction reaction in microbial fuel cells

Iron-chelated electrocatalysts for the oxygen reduction reaction (ORR) in a microbial fuel cell (MFC) were prepared from sodium ferric ethylenediamine-N,N′-bis(2-hydroxyphenylacetic acid) (FeE), sodium ferric diethylene triamine pentaacetic acid (FeD) supported on carbon Vulcan XC-72R carbon black and multi-walled carbon nanotubes (CNTs). Catalyst morphology was investigated by TEM; and the total surfaces areas as well as the pore volumes of catalysts were examined by nitrogen physisorption characterization. The catalytic activity of the iron based catalysts towards ORR was studied by cyclic voltammetry, showing the higher electrochemical activity of FeE in comparison with FeD and the superior performance of catalysts supported on CNT rather than on Vulcan XC-72R carbon black. FeE/CNT was used as cathodic catalyst in a microbial fuel cell (MFC) using domestic wastewater as fuel. The maximum current density and power density recorded are 110 (mA m⁻²) and 127 ± 0.9 (mW m⁻²), respectively. These values are comparable with those obtained using platinum on carbon Vulcan (0.13 mA m⁻² and 226 ± 0.2 mW m⁻²), demonstrating that these catalysts can be used as substitutes for commercial Pt/C.     

Non-Pt catalyst as oxygen reduction reaction in microbial fuel cells: A review

Oxygen Reduction Reactions (ORR) are one of the main factors of major potential loss in low temperature fuel cells, such as microbial fuel cells and proton exchange membrane fuel cells. Various studies in the past decade have focused on determining a method to reduce the over potential of ORR and to replace the conventional costly Pt catalyst in both types of fuel cells. This review outlines important classes of abiotic catalysts and biocatalysts as electrochemical oxygen reduction reaction catalysts in microbial fuel cells. It was shown that manganese oxide and metal macrocycle compounds are good candidates for Pt catalyst replacements due to their high catalytic activity. Moreover, nitrogen doped nanocarbon material and electroconductive polymers are proven to have electrocatalytic activity, but further optimization is required if they are to replace Pt catalysts. A more interesting alternative is the use of bacteria as a biocatalyst in biocathodes, where the ORR is facilitated by bacterial metabolism within the biofilm formed on the cathode. More fundamental work is needed to understand the factors affecting the performance of the biocathode in order to improve the performance of the microbial fuel cells.     

Duckweed derived nitrogen self-doped porous carbon materials as cost-effective electrocatalysts for oxygen reduction reaction in microbial fuel cells

Cost-effective metal-free electrocatalysts for oxygen reduction reaction were incredible significance of improvement about microbial fuel cells. In this research, a novel nitrogen self-doped porous carbon material is effectively inferred with KOH activation from a natural and renewable biomass, duckweed. Self-doped nitrogen in carbon matrix of nitrogen-doped porous carbon at 800 °C provides abundant active sites for oxygen reduction and improves the oxygen reduction kinetics significantly. Moreover, the porous structure of nitrogen-doped porous carbon at 800 °C encourages the transition of electrolyte and oxygen molecules throughout the oxygen reduction reaction. Oxygen on the three-phase boundary is reduced to water according to a four-electron pathway on nitrogen-doped porous carbon electrocatalyst. The single-chamber microbial fuel cell with nitrogen-doped porous carbon as electrocatalyst achieves comparable power density (625.9 mW m⁻²) and better stability compared to the commercial Pt/C electrocatalyst. This simple and low-cost approach provides a straightforward strategy to prepare excellent nitrogen-doped electrocatalyst derived from natural and renewable biomass directly as a promising alternate to precious platinum-based catalysts in microbial fuel cells.     

Bimetallic zeolitic imidazole framework derived Co@NC materials as oxygen reduction reaction catalysts application for microbial fuel cells

Microbial fuel cells (MFCs), a promising future energy conversion technology, play a significant role in the area of sustainable and renewable energy. In air-cathode MFCs, the catalytic activity for oxygen reduction reaction (ORR) of cathode electrocatalyst is the key factor to the performance of MFCs. Development of efficient and economical ORR electrocatalysts is an important step for the wide application of MFCs. Herein, Co wrapped carbon nanotubes (CNTs) N-doped nanoporous carbon materials (Co@NC-Co_xZn_y) are constructed via a facile zinc-assisted growth pyrolytic approach of bimetallic zeolitic imidazole frameworks (BMZIFs)-derived strategy. They are directly prepared via carbonization of the precursor Co_xZn_y-BMZIFs. During the pyrolysis process, the evaporation of zinc plays critical role in the in-situ growth of CNTs. For instance, the optimal catalyst, Co@NC-Co₁Zn₃, exhibits excellent ORR performance activity and stability with on-set potential (E_on-set) of 0.830 V (vs. RHE) and diffusion-limited current density (j_L) of 6.706 mA cm^?2, which is superior to the benchmark catalyst of commercial 20 wt% Pt/C. Additionally, Co@NC-Co₁Zn₃ displays four-electron pathway, long-term stability and better resistance to methanol tolerance. The MFC with Co@NC-Co₁Zn₃ cathode shows a maximum power density of 1039 mW m^?2, and outperforms the MFC with commercial 20 wt% Pt/C catalyst (678 mW m^?2). This work paved the way for exploring cost-effective, superior performance non-precious metal-based catalysts for air-cathode MFCs.     

Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells

Cathode oxygen reduction reaction (ORR) performance is crucial for power generation of microbial fuel cells (MFCs). The current study provides a novel strategy to prepare Fe/N-doped carbon (Fe/N/C) catalyst for MFCs cathode through high temperature pyrolyzing of biomass capsuling melamine resin polymer. The obtained Fe/N/C can effectively enhance activity, selectivity and stability toward 4 e^– ORR in pH neutral solution. Single chamber MFC with Fe/N/C air cathode produces maximum power density of 1166 mW m⁻², which is 140% higher than AC cathode. The improved performance of Fe/N/C can be attributed to the involvement of nitrogen and iron species. The excellent stability can be attributed to the preferential structure of the catalyst. The moderate porosity of the catalyst facilitates mass transfer of oxygen and protons and prevents water flooding of triple-phase boundary where ORR occurs. The biomass particles encapsulated in the catalyst act as skeletons, which prevents catalyst collapse and agglomeration.     

Templating synthesis of hierarchically meso/macroporous N-doped microalgae derived biocarbon as oxygen reduction reaction catalyst for microbial fuel cells

N-doped carbons have been hailed as cost effective catalysts for the large-scale commercialization of microbial fuel cells (MFCs). In this paper, we developed a hierarchically meso/macroporous N-doped biocarbon by templating approach using Chlorella pyrenoidosa as precursor. The results showed that graphitic-N was the dominating functional group contributing to oxygen reduction reaction (ORR) performance. In addition, the role of pore structure was identified and the results suggested that mesopores exhibited a nearly linear correlation with limiting current density and half-wave potential, while electrochemical surface area almost linearly varied with macropores in the carbon materials. These results implied that mesopores play a dominating role in facilitating ion and oxygen supply and creating accessible active sites for ORR, while macropores mainly served as an electrolyte buffering reservoir shortening the electrolyte diffusion distances in the prepared catalysts. The optimized meso/macro pore structure enhanced the accessibility of the active sites and facilitated the mass transport of ion and oxygen, and consequently improved ORR performance of catalyst. The as-prepared catalyst exhibited a remarkably higher power generation than that of the commercial Pt/C in MFCs. This paper offered an insight into the effect of pore structure on the ORR performance of catalysts, and also provided an alternative avenue for synthesizing meso/macroporous carbon catalysts for the applications of MFCs.     

Three-dimensional ZIF-67 attached lamellar Ti3AlC2 combined with ZnAl-LDH as cathode catalyst for enhancing oxygen reduction reaction of microbial fuel cells

In this study, a simple hydrothermal method was used to prepare the cathode catalyst of microbial fuel cells (MFCs). The three-dimensional structure of ZIF-67 attached to the lamellar Ti₃AlC₂/ZnAl-layered double hydroxide (LDH) was observed. (010), (012), (015) were the obvious peaks of the composite ZIF-67@Ti₃AlC₂/ZnAl-LDH. Ti, N, C, Al, O, F were relatively uniformly distributed on the surface of the composite material. The maximum voltage of ZIF-67@Ti₃AlC₂/ZnAl-LDH-MFC was 576 mV and the stabilization time was 8 d. The maximum power density of ZIF-67@Ti₃AlC₂/ZnAl-LDH-MFC was 587 mW/m², which was 1.32 times of Ti₃AlC₂/ZnAl-LDH-MFC (446 mW/m²) and 2.69 times of Ti₃AlC₂-MFC (218 mW/m²). Ti₃AlC₂ with large interlayer spacing and high specific surface area were perfectly composited with multi-layer nanosheets of ZnAl-LDH, and ZIF-67 attached to the surface enhanced the reaction center and activity of the composite material, which promoted oxygen reduction reaction and improved MFC performances.     

Cobalt/nitrogen-Co-doped nanoscale hierarchically porous composites derived from octahedral metal-organic framework for efficient oxygen reduction in microbial fuel cells

Microbial fuel cell, a promising energy conversion technology, plays a crucial role in the field of renewable and sustainable energy. In an air-cathode microbial fuel cell, the oxygen reduction reaction catalytic activity of cathode catalyst is a critical factor that determines the performance of microbial fuel cell. This work reports a facile route for the synthesis of Co/N incorporated carbonaceous electrocatalyst using a Zr-based metal-organic framework UiO66-NH₂ as a template. This electrocatalyst exhibits outstanding activity and stability toward four-electron mechanism. In the microbial fuel cell application, Co/UiO66-900 shows superb electrochemical performance with a stable output voltage of 395 mV and maximum power density of 299.62 mW/m², which is 95.8% of the power density achieved in microbial fuel cell catalyzed by Pt/C catalyst (312.59 mW/m²). Co/UiO66-900 possesses high-performance catalytic activity because of its 3D-structured micropores, nitrogen-coordinated cobalt species and the synergistic effects between carbon and metal ion center. These unique properties can facilitate the oxygen reduction reaction by exposing abundant efficient active sites and accelerating mass transfer at oxygen reduction reaction interfaces. This work suggests that Co/UiO66-900 catalyst with superb electrocatalytic ORR activity is a promising alternative which can replace the expensive Pt/C in air-cathode MFC.     

Comparative study of Pd and PdO as cathodes for oxygen reduction reaction in intermediate temperature solid oxide fuel cells

Metallic Pd and PdO electrodes were prepared by using Pd and PdCl₂ slurries, respectively, and their electrochemical performance as a cathode for oxygen reduction reaction in intermediate temperature solid oxide fuel cells was evaluated by electrochemical impedance spectroscopy (EIS) and direct current polarization (DC polarization). The electrochemical activity of metallic Pd was much higher than that of PdO for the reaction of oxygen reduction; below the decomposition temperature, a thin layer of PdO formed on the surface of metallic Pd electrode, which increased its polarization resistance. The decomposition temperature of PdO decreased from 810 to 750 °C as oxygen partial pressure decreased from 20 to 5 kPa, and was further lowered under the influence of the applied current during DC polarization test. The charge transfer resistance of PdO increased by decreasing oxygen partial pressure, while that of metallic Pd was less sensitive to it.     

Carbon supported nickel-phthalocyanine/MnOx as novel cathode catalyst for microbial fuel cell application

Performance of microbial fuel cells (MFCs) with carbon supported nickel phthalocyanine (NiPc)MnO_x composite (MFC-1) and nickel phthalocyanine (MFC-2) incorporated cathode was compared with a control MFC with non-catalysed carbon felt as cathode (MFC-3) and MFC-4 having Pt on cathode (as benchmark reference control). MFC-1 exhibited power density of 8.02 Wm^?3, which was four folds higher than control MFC-3 (2.08 Wm^?3) and 1.14 times higher than MFC-2 (6.97 Wm^?3). Coulombic efficiency of 30.3% obtained in MFC-1 was almost double of that obtained for control MFC-3 and it was 5.4% lesser as compared to MFC-4 (35.7%). Linear sweep voltammetry study of cathodes revealed that NiPc-MnO_x could enhance the electrocatalytic activity of oxygen reduction reaction (ORR) in comparison to control cathode. However, the power recovery from MFC-1 was noted little lower than what obtained from MFC-4 (10.58 Wm^?3), however the cost normalized power was two times higher than Pt catalyst on cathode. Thus, NiPc-MnO_x based catalyst developed in this study has potential to enhance ORR in cathodes of MFCs in order to harvest more power.     

Manganese dioxide-coated carbon nanotubes as an improved cathodic catalyst for oxygen reduction in a microbial fuel cell

To develop an efficient and cost-effective cathodic electrocatalyst for microbial fuel cells (MFCs), carbon nanotubes (CNTs) coated with manganese dioxide using an in situ hydrothermal method (in situ MnO₂/CNTs) have been investigated for electrochemical oxygen reduction reaction (ORR). Examination by transmission electron microscopy shows that MnO₂ is sufficiently and uniformly dispersed over the surfaces of the CNTs. Using linear sweep voltammetry, we determine that the in situ MnO₂/CNTs are a better catalyst for the ORR than CNTs that are simply mechanically mixed with MnO₂ powder, suggesting that the surface coating of MnO₂ onto CNTs enhances their catalytic activity. Additionally, a maximum power density of 210 mW m⁻² produced from the MFC with in situ MnO₂/CNTs cathode is 2.3 times of that produced from the MFC using mechanically mixed MnO₂/CNTs (93 mW m⁻²), and comparable to that of the MFC with a conventional Pt/C cathode (229 mW m⁻²). Electrochemical impedance spectroscopy analysis indicates that the uniform surface dispersion of MnO₂ on the CNTs enhanced electron transfer of the ORR, resulting in higher MFC power output. The results of this study demonstrate that CNTs are an ideal catalyst support for MnO₂ and that in situ MnO₂/CNTs offer a good alternative to Pt/C for practical MFC applications.     

FeNx nanoparticles embedded in three-dimensional N-doped carbon nanofibers as efficient oxygen reduction reaction electrocatalyst for microbial fuel cells in alkaline and neutral media

Herein, an approach is reported for the fabrication of 3D carbon nanofibers (CNFs) wrapped by carbon nanotubes (CNT) with graphitic carbon-encased FeNx nanoparticles originated from metal–organic frameworks (MOFs). It is found that Fe-FeNx@N-CNT/CNFs exhibits outstanding catalytic activity towards ORR, whose half-wave potential are 0.89 V and 0.87 V in alkaline and neutral environments, respectively, much higher than MOF-based catalysts reported so far and commercial Pt/C. When the obtained cathode catalysts are loaded in MFCs for power generation test, the experimental consequences show that the Fe-FeNx@N-CNT/CNFs cathode exhibits a supernal power density of 742.26 mW·m^?2 and output current density of 3241 mA·m^?2 which are comparable to Pt/C. The splendid ORR catalytic performance is mainly attributable to the three-dimensional structure of carbon nanofibers and the active sites of Fe-Nx. These result in a higher graphitization degree beneficial for electronic mobility, high specific surface area, benign mesoporous nanostructure and excellent mass transfer capability. The strategy provides a new scheme to devise and research Fe-Nx electrocatalysts with MOF-based for the conversion of clean and environment-friendly energy.     

Highly active Pt decorated Pd/C nanocatalysts for oxygen reduction reaction

This article describes findings of the correlation between the atomic scale surface structure and the electrocatalytic performance of nanoengineered Pt-Pd/C catalysts for oxygen reduction reaction (ORR), aiming at providing a new fundamental insight into the role of the detailed atomic decorated structure of the catalysts in fuel cell reactions. Carbon-supported Pt decorated Pd nanoparticles (donated as Pt-Pd/C), with Pt coverage close to a monolayer, were prepared from a simple galvanic replacement reaction between Pd/C particles and PtCl₄^2? at room temperature. The decorated architecture was confirmed by extensive microstructural characterization techniques, including TEM, XRD, XPS, HAADF-STEM, ICP and HS-LEIS. The catalysts were also examined for their intrinsic kinetic activities towards oxygen reduction reaction. The results have shown that the Pt-Pd/C catalysts are highly active towards molecular oxygen electrocatalytic reduction. These findings have profound implications to the design and nanoengineering of decorated surfaces of catalysts for oxygen reduction reaction.     

Zeolitic imidazolate framework ZIF-67 grown on NiAl-layered double hydroxide/graphene oxide as cathode catalysts for oxygen reduction reaction in microbial fuel cells

In this study, the cathode catalysts for microbial fuel cells (MFCs) were successfully synthesized by two-step feeding method. NiAl-layered double hydroxide (LDH) nanoparticle was attached efficiently on the surface of graphene oxide (GO) in situ, zeolitic imidazolate framework (ZIF-67) was modified on LDH surface layer; Highly crystalline NiAl-LDH/GO@ZIF-67 composite was flawlessly prepared, nano-hybrid had (003), (006), (011), (112) and (222) characteristic crystal planes attribute to NiAl-LDH/GO and ZIF-67 by X-ray diffraction (XRD), and the sheet-like structure and polyhedral structure were observed. The NiAl-LDH/GO@ZIF-67 nano-hybrid was rich in metal elements and provides a wealth of electrochemical active sites by EDS test. The study displayed that the maximum output voltage of NiAl-LDH/GO@ZIF-67-MFC was 516 mV and the stabilization time could last for about 8 d. The maximum output power was 501.26 mW/m², which was 1.31 times of NiAl-LDH/GO-MFC (381.92 mW/m²) and 2.76 times of ZIF-67-MFC (181.48 mW/m²). The GO with high conductivity was used as the substrate to ensure the stability of electrode cycle and the efficiency of power generation, the laminar structure of NiAl-LDH provided the specific surface area for the reaction, which facilitated the transport of electrons. The good structure of ZIF-67 increased the active sites of the composite. The excellent properties of the composites promoted the electrochemical stability and electricity production output of MFC. This study provided a method for the further application of MFC in the wastewater field.     

Pt-Pd nanodendrites as oxygen reduction catalyst in polymer-electrolyte-membrane fuel cell

Platinum-palladium (Pt-Pd) bimetallic alloys have shown prospect as electrocatalyst for the oxygen reduction reaction (ORR) in the cathode of polymer-electrolyte-membrane (PEM) fuel cells. This article reports a facile solvothermal synthesis of Pt-Pd bimetallic nanodendrites (Pt-Pd NDs). The characterization with a variety of spectroscopic techniques indicates that the Pt-Pd NDs possess a three-dimensional (3-D) porous structure consisting of interconnected branches of highly alloyed Pt-Pd nanorods (NR). The measurements using rotating disk electrode in electrolyte solution show that the catalyst of Pt-Pd NDs supported on carbon (Pt-Pd NDs/C) possesses a Pt mass activity for ORR that is more than 3 times higher than that of the state-of-the-art Pt/C catalyst, as well as the significantly improved stability due to the branched porous structure. The measurements using membrane-electrode-assembly (MEA) in a single PEM fuel cell indicate the 3-D interconnected dendrite structures make the Pt-Pd NDs/C catalyst significantly advantageous over the nanoparticle Pt/C catalyst in reducing the mass transport and ohmic polarization which would become significant at high current density in MEA.     

Enhancing the oxygen reduction reaction activity and durability of a solid oxide fuel cell cathode by surface modification of a hybrid coating

While (La_0.6Sr_0.4)_0.95Co_0.2Fe_0.8O_3-δ (LSCF) has been one of the most investigated materials for a long time, its relatively insufficient oxygen reduction reaction (ORR) activity and inherent performance degradation are still two main obstacles to its massive application on oxygen-ion conducting solid oxide fuel cell (SOFC). To solve those issues, a composite of Pr₆O₁₁ and NiO has been deposited on LSCF successfully via a facile infiltration method in this study. The modified LSCF cathode exhibits ∼30% lower polarization resistance than LSCF. The excellent performance promotion may be due to the synergistic effect of Pr₆O₁₁ and NiO on the LSCF surface. The distribution of relaxation time (DRT) analyses of electrochemical impedance spectra (EIS) in different oxygen partial pressure and long-term operation indicate that the performance enhancement is caused by the facilitated oxygen surface adsorption-dissociation process and suppression of Sr segregation on modified LSCF cathode, thus achieving a higher peak power density of 1.40 W cm⁻² at 800 °C and better long-term operation stability of only 3% voltage decline rate after 80 h operation. These results indicate that Pr₆O₁₁ and NiO composite modification is a promising method for improving the electrochemical performance of LSCF.     

Iron carbide and iron phosphide embedded N-doped porous carbon derived from biomass as oxygen reduction reaction catalyst for microbial fuel cell

The splendid activity of oxygen reduction reaction (ORR) catalyst can greatly promote the power generation of air cathode microbial fuel cell (AC-MFC). Here, benefiting from the rich P element in radish, Fe₃C and Fe₂P incorporated N-doped porous carbon (Fe₃C/Fe₂P@NC-N₄Fe₂) are prepared with the assistance of NH₄Cl through carbothermal reduction method without adding P resources. As expected, Fe₃C/Fe₂P@NC-N₄Fe₂ possesses excellent ORR performance, in which Fe₃C and Fe₂P furnish abundant ORR active sites and the porous structure in N-doped carbon matrix can facilitate mass transfer. Moreover, the AC-MFC assembled with Fe₃C/Fe₂P@NC-N₄Fe₂ as cathodic ORR catalyst exhibits superior power output performance with the maximum power density of 948.9 mW m^?2, which is 1.03 times of that of 20 wt% Pt/C catalyst. Therefore, Fe₃C/Fe₂P@NC-N₄Fe₂ should be a viable ORR catalyst to replace Pt/C catalyst in the application in AC-MFC.     

Cathode catalyst selection for enhancing oxygen reduction reactions of microbial fuel cells: COF-300@NiAl-LDH/GO and Ti3AlC2/NiCoAl-LDH

In this study, a cathode catalyst for microbial fuel cells (MFCs) was successfully prepared by a simple step-by-step hydrothermal method. Graphene oxide (GO) and layered double hydroxide (LDH) composite substrate and three-dimensional covalent organic framework materials (COF-300) grown vertically on the surface were observed. (003), (006), (012), (018), (110) were the obvious crystal plane of the composite COF-300@NiAl-LDH/GO. C, O, N, Ni, Na, Al and other elements existed on the surface of the composite. The maximum power density produced by COF-300@NiAl-LDH/GO-MFC was 481.69 mW/m², which was 1.22 times of Ti₃AlC₂/NiCoAl-LDH-MFC (393.82 mW/m²) and 2.21 times of Ti₃AlC₂-MFC (217.73 mW/m²). The maximum voltage of COF-300@NiAl-LDH/GO-MFC was 518 mV and it could remain stable within 8 days. GO was used as the substrate to improve the conductivity; LDH was used to enhance the catalytic activity and electron transfer rate; The three-dimensional bulk COF-300 attached to the surface enhanced the surface area and catalytic properties; The above jointly promoted oxygen reduction reaction of cathode, so as to improve MFC performances.     

Ruddlesden-Popper-based lanthanum cuprate thin film cathodes for solid oxide fuel cells: Effects of doping and structural transformation on the oxygen reduction reaction

Highly textured, c-axis oriented thin films of undoped, Ba-doped, and Sr-doped La₂CuO₄ are successfully prepared on YSZ (100) substrates by using pulsed laser deposition. Intriguingly, the films undergo a structural transformation from T′(square-planar) to T (octahedral) crystal structure with partial substitution of Ba and Sr ions on La site. Meanwhile, the addition of both Ba and Sr dopants lead to reducing in polarization resistance and alternative kinetics of oxygen reduction reaction (ORR), hereinto La_1·8Ba_0·2CuO₄ film presents superior electrochemical properties because it can accommodate much more oxygen vacancies than the other two films. First-principles calculations reveal that much lower O-defect formation energy originate fundamentally from the increase of Cu–O bond length along c-axis orientation after structural transformation from T′-phase to T-phase. These findings unveil the relationship between the structural transformation and ORR activity, and provide a novel approach to rational design film cathodes for higher electrochemical performance.