Low-cost Bifunctional Catalysts Research Articles

Construction of atom-scale Co/Fe/Ni M-N-C active sites within nitrogen-doped carbon spheres for high-performance bifunctional oxygen catalysts in rechargeable zinc-air batteries

Developing carbon-based catalysts with metal‑nitrogen‑carbon (M-N-C) active sites as efficient and low-cost bifunctional catalysts to replace precious metal catalysts for rechargeable zinc-air batteries devices has garnered significant attention. Herein, nitrogen-doped submicron carbon-based spherical bifunctional electrocatalysts (CNPD-CoFeNi) with abundant atom-scale Co/Fe/Ni M-N-C active sites and defects were synthesized. The introduction of massive amounts of Zn species increases the spatial distance of Co/Fe/Ni metal sites to prevent aggregation during pyrolysis, and the evaporation of Zn at high temperatures creates defects synergizing with M-N-C. Consequently, CNPD-CoFeNi possesses a stable carbon-based spherical structure, high electrical conductivity, rich nitrogen content, abundant atom-scale Co/Fe/Ni M-N-C active sites, and defects, displaying outstanding durability, oxygen reduction reaction (ORR) activity with a half-wave potential of 0.87 V, and satisfactory OER performance, surpassing or comparable with the state-of-art Pt/C and RuO2, respectively. Notably, liquid Zn-air batteries with CNPD-CoFeNi catalyst exhibit an exceptional high peak power density of 146.5 mW cm−2 and stable charge-discharge cycling over 270 h, outperforming Pt/C-RuO2 based devices. Additionally, the all-solid-state Zn-air battery also displays satisfactory practicability and stability. The synthesis strategy and its remarkable results provide valuable guidance and inspiration for the future advancement of precious metal substitutes in ORR/OER and rechargeable zinc-air batteries.

Synthesis of Zr2ON2 via a urea-glass route to modulate the bifunctional catalytic activity of NiFe layered double hydroxide in a rechargeable zinc-air battery

The development of a highly efficient, stable, and low-cost bifunctional catalyst is imperative for facilitating the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, significant challenges are involved in extending its applications to rechargeable zinc-air batteries. This study presents a bifunctional catalyst, Zr2ON2@NiFe layered double hydroxide (LDH), that was developed by utilizing a urea-glass route for synthesizing the Zr2ON2 precursor, followed by riveting NiFe LDH nanosheets using a hydrothermal method. Specifically, the vertical distribution of NiFe LDH on the Zr2ON2 surface ensures the maximization of the number of accessible active sites and interfacial catalysis of NiFe LDH. Notably, Zr2ON2@NiFe LDH demonstrates ORR and OER bifunctional electrocatalytic behavior and high stability owing to its heterostructure and composition. Furthermore, a rechargeable zinc-air battery using a Zr2ON2@NiFe LDH electrocatalyst as the air cathode demonstrated a high peak power density (172 mW cm−2) and galvanostatic charge-discharge cycle stability (5 mA cm−2 over 443 h). Thus, this study presents an efficient and cost-effective strategy for the design of bifunctional electrocatalysts.

Non-Noble Metal High-Entropy Alloy-Based Catalytic Electrode for Long-Life Hydrogen Gas Batteries.

The development of efficient, stable, and low-cost bifunctional catalysts for the hydrogen evolution/oxidation reaction (HER/HOR) is critical to promote the application of hydrogen gas batteries in large scale energy storage systems. Here we demonstrate a non-noble metal high-entropy alloy grown on Cu foam (NNM-HEA@CF) as a self-supported catalytic electrode for nickel-hydrogen gas (Ni-H2) batteries. Experimental and theoretical calculation results reveal that the NNM-HEA catalyst greatly facilitates the HER/HOR catalytic process through the optimized electronic structures of the active sites. The assembled Ni-H2 battery with NNM-HEA@CF as the anode shows excellent rate capability and exceptional cycling performance of over 1800 h without capacity decay at an areal capacity of 15 mAh cm-2. Furthermore, a scaled-up Ni-H2 battery fabricated with an extended capacity of 0.45 Ah exhibits a high cell-level energy density of ∼109.3 Wh kg-1. Moreover, its estimated cost reaches as low as ∼107.8 $ kWh-1 based on all key components of electrodes, separator and electrolyte, which is reduced by more than 6 times compared to that of the commercial Pt/C-based Ni-H2 battery. This work provides an approach to develop high-efficiency non-noble metal-based bifunctional catalysts for hydrogen batteries in large-scale energy storage applications.

Dense r-Ru/FeCoP heterointerfaces induced by a defect-assisted strategy for ultrastable alkaline overall water splitting.

Simplifying the anchoring process of Ru nanoparticles (NPs) and activating their bifunctional activity are challenges in the construction of Ru-based catalysts. In this work, FeCoP nanosheets anchored by Ru NPs (r-Ru/FeCoP) were innovatively synthesized using an oxygen defect-assisted-gas-phase phosphorization strategy. Surprisingly, the η100 values of r-Ru/FeCoP in the alkaline hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) processes were 0.8 and 0.58 times those of Pt/C and RuO2, respectively. Notably, the rich Ru NPs as HER active sites on r-Ru/FeCoP enhanced the conductivity of the catalyst and promoted the reaction kinetics for HER, while the metal atoms on the FeCoP nanosheets served as OER active sites to accelerate the desorption of O2. The synergy between the two promoted the improvement of overall water splitting efficiency. Overall, this work provides a new approach for the development of low-cost bifunctional catalysts with high Ru utilization.

Facile hydrothermal synthesis of NiMoO4·xH2O nanorods-like structures as bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries

Developing highly active and low-cost bifunctional catalyst materials to replace precious metal species is of utmost importance for the commercialization of clean and sustainable energy systems. In this report, homogenous NiMoO4·xH2O nanorods (NRs)-like structured materials were synthesized by an annealing-free hydrothermal method. The synthesized electrode electrochemical properties were determined by linear sweep voltammetry using a rotating ring disk electrode to determine the characteristics of oxygen reduction reaction in an O2-saturated electrolyte solution. For the oxygen evolution reaction, the NiMoO4·xH2O NRs electrocatalyst showed lower overpotential of 185 mV to obtain 10 mA cm−2. Furthermore, the NiMoO4·xH2O NRs electrocatalyst-based zinc (Zn)-air battery exhibited lower voltage gaps and smaller electrochemical impedance spectroscopy values than the Pt/C catalyst. Importantly, the NiMoO4·xH2O NRs-based battery (60 cycles (∼1100 min)) revealed excellent cycling stability results when compared with the Pt/C-based battery (30 cycles (∼550 min)). The NR structured electrode provided good steady potential after being subjected to a fixed current density of 10 mA cm−2 for 100 h. Therefore, these results demonstrate that the synthesized NiMoO4·xH2O NRs material is promising as a bifunctional electrocatalyst for rechargeable Zn-air batteries.

Interface engineering of heterostructural Ni-Ni0.2Mo0.8N boosts alkaline hydrogen electrocatalysis.

Exploring efficient and low-cost bifunctional catalysts for hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) is highly desirable for the achievement of unitized regenerative fuel cells. Herein, a facile method to prepare hetero-interfacial Ni-Ni0.2Mo0.8N nanosheets with tailored d-center center for efficient alkaline hydrogen electrocatalysis is presented. Mechanism studies indicate that interface engineering can shift down the d-band center of Ni-Ni0.2Mo0.8N nanosheets due to the electron transfer from Ni to Ni0.2Mo0.8N, which weakens the binding strength of reaction intermediates, thereby boosting the catalytic performance. Relative to pure Ni, Ni-Ni0.2Mo0.8N nanosheets show a lower overpotential of 83 mV at -10 mA cm-2 and good stability during 2,000 cycles for HER. Meanwhile, Ni-Ni0.2Mo0.8N nanosheets exhibit an improved exchange current density for HOR with a 10.2-fold enhancement in comparison with that of pure Ni. This work provides valuable insight into the reasonable design of efficient energy-related electrocatalysts based on the tailoring of d-band center by interface engineering.

Mo0.2Ni0.8N/CeO2 heterojunction as bifunctional electrocatalysts for overall urea-water electrolysis

Pollution-free and low-cost bifunctional catalysts are important part for hydrogen production. In this paper, composite catalysts Ni0.2Mo0.8N/CeO2-x/NF (x = n(Ce)/n(Ni)- 0.1, 0.3, 0.5) were synthesized via hydrothermal and calcination methods on foam nickel (NF). Experimental results show that the composites have better properties than CeO2/NF, Ni0.2Mo0.8N/NF and MoN/CeO2/NF. The electrochemical properties of the composites depend on the ratio of nickel and cerium. A proper proportion of nickel leads to a larger specific surface area, increased electron capacity, more active sites, and better overall performance. Among the tested composites, Ni0.2Mo0.8N/CeO2-0.3/NF exhibits the best electrochemical performance and is well-suited for catalyzing the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). For HER, the potential is −120 mV @ 100 mA cm−2. And for UOR, the UOR potential is only 1.462 V @ 100 mA cm−2. A two-electrode electrolyzer of Ni0.2Mo0.8N/CeO2-0.3/NF|| Ni0.2Mo0.8N/CeO2-0.3/NF was also built. Overall urea splitting (OUS) is 1.672 V @ 100 mA cm−2, which is 187 mV smaller than overall water splitting (OWS, 1.859 V). Therefore, Ni0.2Mo0.8N/CeO2 has potential application in hydrogen production from urea-rich wastewater electrolysis.

Mo/P doped NiFeSe as bifunctional electrocatalysts for overall water splitting

Designing high-efficiency catalysts for overall water splitting is critical to reduce the cost of hydrogen fuel as a clean and renewable energy source in future society. In this work, a Mo-, P-codoped NiFeSe was successfully synthesized on nickel foam (NF) by one-step electrodeposition. Through the doping strategy, the conductivity can be well promoted, and the production of nanosheets on the catalyst surface and active phases during hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) provided much more active sites, which leaded to efficient HER/OER performances of as-synthesized Mo-, P-codoped NiFeSe catalysts, i.e., a low overpotential of 100 mV/200 mV at current density of 10 mA cm−2 in 1.0 M KOH with stability of 95 h/60 h, respectively. It only required 1.53 V to deliver a current density of 10 mA cm−2 in overall water splitting and maintained outstanding durability for 100 h. This work is beneficial to future design of high efficient and low-cost bifunctional catalysts for overall water splitting.

Valorization of hazardous waste cooking oil for the production of eco-friendly biodiesel using a low-cost bifunctional catalyst

Biodiesel is considered the prospective substitute for non-renewable fossil fuel-derived sources of energy. However, the high costs of feedstocks and catalysts inhibit its large-scale industrial implementation. From this perspective, the utilization of waste as the source for both catalyst synthesis and feedstock for biodiesel is a rare attempt. Waste rice husk was explored as a precursor to prepare rice husk char (RHC). Sulfonated RHC was employed as a bifunctional catalyst for the simultaneous esterification and transesterification of highly acidic waste cooking oil (WCO) to produce biodiesel. The sulfonation process coupled with ultrasonic irradiation proved to be an efficient technique to induce high acid density in the sulfonated catalyst. The prepared catalyst possessed a sulfonic density and total acid density of 4.18 and 7.58mmol/g, respectively, and a surface area of 144 m2/g. A parametric optimization was conducted for the conversion of WCO into biodiesel using the response surface methodology. An optimal biodiesel yield of 96% was obtained under the conditions of methanol to oil ratio (13:1), reaction time (50min), catalyst loading (3.5 wt%), and ultrasonic amplitude (56%). The prepared catalyst showed higher stability up to five cycles with biodiesel yield greater than 80%.

Activation of basal-plane sulfur sites on MoS2 @Ni3S2 nanorods by Zr plasma ion implantation for bifunctional electrocatalysts

Water electrolysis is one of the most promising technologies for sustainable and clean energy production, yet the sluggish kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) hampers the commercialization. In this work, Zr-doped MoS2 @Ni3S2 (MSNF) heterogeneous nanorods as bifunctional electrocatalysts were successfully explored to boost the catalytic activity of water electrolysis. The structural characterizations demonstrate the uniform modification of Zr atoms on the basal plane of MoS2 at the heterointerface of MSNF. MSNF implanted with a Zr plasma ion fluence of 5 × 1016 ions-cm−2 (Zr500-MSNF) shows the most remarkable bifunctional electrocatalytic characteristics such as Tafel slope of 60.9 mV dec−1 and overpotential of 98 mV at 10 mA cm−2 for HER, as well as Tafel slope of 78.5 mV dec−1 and overpotential of 275 mV at 20 mA cm−2 for OER. Consistent with the structural and electrocatalytic characterizations, first-principles calculation confirms that the activated S sites in the basal plane of MoS2 upon Zr doping have more empty states in its valence orbital to favor the adsorption of hydrogen and hydroxide, thus contributing to enhanced HER and OER activity. The findings in this work reveal a facile strategy to design high-performance and low-cost bifunctional catalysts based on two-dimensional materials for overall water splitting.

Intriguing 3D micro-flower structure of Co1.11Te2 deposited on Te nanosheets showing an efficient bifunctional electrocatalytic property for overall water splitting

The development of efficient and low-cost bifunctional catalysts for electrochemical water splitting is important for future sustainable hydrogen energy deployment. In this paper, a series of composition-controlled hybrid catalysts (Co1.11Te2/Te) were successfully constructed by introducing different proportions of Co source into the precursor solutions during the hydrothermal synthesis of Te nanosheets(Te NSs). The experimental results show that the content of Co3+active species in the hybrid materials can be adjusted by controlling the molar ratio of Co to Te, thereby significantly improving the electrocatalytic performance of the samples. Benefiting from the highest content of Co3+, the large specific surface area, and the great charge transfer ability, the Te-Co-3 electrode exhibited the best catalytic activity and durability among all samples. The hydrogen evolution reaction(HER) and oxygen evolution reaction(OER) overpotentials of the electrode were only 135 and 261 mV, respectively, to drive a current density of 10 mA cm−2 in an alkaline environment (1 M KOH). Furthermore, it required only a cell potential of 1.56 V to achieve overall water splitting at 10 mA cm−2. This work should be useful for the design and preparation of non-noble metal bifunctional catalysts.

Flexible Zinc-Air Batteries with Ampere-Hour Capacities and Wide-Temperature Adaptabilities.

Flexible Zn-air batteries (FZABs) have significant potentials as efficient energy storage devices for wearable electronics because of their safeties and high energy-to-cost ratios. However, their application is limited by their short cycle lives, low discharge capacities per cycle, and high charge/discharge polarizations. Accordingly, herein, a poly(sodium acrylate)-polyvinyl alcohol (PANa-PVA)-ionic liquid (IL) hydrogel (PANa-PVA-IL) is prepared using a hygroscopic IL, 1-ethyl-3-methylimidazolium chloride, as an additive for twin-chain PANa-PVA. PANa-PVA-IL exhibits a high conductivity of 306.9mScm-1 and a water uptake of 2515 wt% at room temperature. Moreover, a low-cost bifunctional catalyst, namely, Co9 S8 nanoparticles anchored on N- and S-co-doped activated carbon black pearls 2000 (Co9 S8 -NSABP), is synthesized, which demonstrates a low O2 reversibility potential gap of 0.629V. FZABs based on PANa-PVA-IL and Co9 S8 -NSABP demonstrate high discharge capacities of 1.67mAhcm-2 per cycle and long cycle lives of 330h.Large-scale flexible rechargeable Zn-air pouch cells exhibit total capacities of 1.03Ah and energy densities of 246Whkgcell -1 . This study provides new information about hydrogels with high ionic conductivities and water uptakes and should facilitate the application of FZABs in wearable electronics.

One-step construction of NiCo alloy particles encapsulated in N-doped carbon frameworks application for overall water splitting

The development of electrolyzed water catalyst with high activity, low cost, and long-term stability is still a challenge. This work reports a convenient strategy for synthesizing N-doped carbon-coated nickel-cobalt alloy particles (NiCo@NC) composites via a one-step high-temperature carbon reduction method. Among all the catalysts, Ni2Co1@NC shows the best OER and HER activities. In 1 M KOH system, the electrocatalyst Ni2Co1@NC requires only 346 mV to reach 50 mA cm−2 for OER and 189 mV to reach 10 mA cm−2 for HER. Tafel slope values are 50.1 and 83.9 mv dec−1 for OER and HER, respectively. In the 25 h HER and OER stability test for Ni2Co1 @NC catalyst, the current density hardly decays, and even slightly increases. In addition, the Ni2Co1@NC used as the dual functional catalyst achieves the voltage of 1.9 V at 50 mA cm−2. The enhanced catalytic performance is closely related to the microstructure and composition of nitrogen-doped carbon coated NiCo alloy catalysts. This approach provides a facile strategy for designing highly active and low-cost bifunctional catalysts.

Zinc and fluorine ions dual-modulated NiCoP nanoprism array electrocatalysts for efficient water splitting

Designing efficient, stable, and low-cost bifunctional catalysts for overall water splitting is significant but challenging. In this work, Zn and F ions co-doped NiCoP nanoprism arrays grown directly on nickel foam (Zn/F-NiCoP/NF) was synthesized via hydrothermal method followed by phosphorization treatment. The resultant Zn/F-NiCoP/NF exhibits high electrocatalytic activity towards hydrogen evolution reaction (HER, η10 = 59 mV) and oxygen evolution reaction (OER, η50 = 285 mV). An alkaline electrolyzer using Zn/F-NiCoP/NF as both cathode and anode requires a low cell voltage of 1.568 V at a current density of 10 mA cm−2 with a high long-term stability of up to 40 h, which outperforms many reported Ni,Co-based catalysts. Density functional theory (DFT) calculations proof that simultaneous doping of NiCoP with Zn and F ions provides flexibility to regulate the electronic configuration and downshifts the transition metal d-band center, thereby optimizing adsorption energy between reactants and intermediates, which enhances the HER and OER catalytic activities. This work highlights that cation–anion co-doping strategy is an effective way to develop highly active transition metal phosphides electrocatalyst for water splitting.

High Performance Bifunctional Electrocatalysts Designed Based on Transition-Metal Sulfides for Rechargeable Zn-Air Batteries.

Due to the energy crisis by the excessive consumption of fossil fuels, Zinc-air batteries (ZABs) with high theoretical energy density have attracted people's attention. The overall performance of ZABs is largely determined by the air cathode catalyst. Therefore, it is necessary to develop high-efficiency and low-cost bifunctional catalysts to replace noble metal catalysts to promote the development of ZABs. Among a variety of cathode catalysts, TMS has become a research hotspot in recent years because of its better electrical conductivity than metal phosphides and metal oxides. In this work, we focus on the means of improving the electrocatalytic performance of transition-metal sulfides (TMS) providing ideas for us to rationally design high-performance catalysts. Furthermore, the performance improvement law between catalyst performance and ZABs is also discussed in this work. Finally, some challenges and opportunities faced in the research of TMS electrocatalysis are briefly proposed, and strategies for improving the performance of ZABs are prospected.

3D self-supporting nickel sponge substrate derived CoFeLDH/SN as an efficient bifunctional electrocatalyst for overall water splitting

The design and fabrication of high-efficiency and low-cost bifunctional catalysts in overall water splitting is crucial for the industrial application of green and clean hydrogen energy. Herein, a three-dimensional (3D) self-supporting nickel sponge (SN) with many nickel synapses prepared by hydrothermal method was employed as the substrate. 2D CoFeLDH nanosheets were grown on nano-Ni synapses of SN by electrodeposition method. For comparison, CoFeLDH also was deposited on a nickel foam (NF) substrate using the same deposition process. The as-prepared CoFeLDH/SN exhibited excellent electrocatalytic performances with 208 mV at 10 mA cm−2 for OER and 63 mV at 10 mA cm−2 for HER in 1 M KOH alkaline solution, which is better than that of CoFeLDH/NF (276 mV at 10 mA cm−2 for OER, and 173 mV at 10 mA cm−2 for HER). The electrolytic cell using CoFeLDH/SN as both cathode and anode achieves a voltage of 1.52 V at current density of 10 mA cm−2, which is better than that of most previously reported bifunctional catalysts. Our work provides a facile and inexpensive option for the design of bifunctional catalysts in overall water splitting.

FeNi incorporated N doped carbon nanotubes from glucosamine hydrochloride as highly efficient bifunctional catalyst for long term rechargeable zinc-air batteries

The ingenious design and synthesis of low-cost, non-precious metal bifunctional catalysts for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) is a pressing demand to solve energy crisis and to promote the development of zinc-air batteries. Herein, a simple strategy was developed to derive FeNi incorporated N-doped carbon nanotube (denoted as glu-NiFe) from glucosamine. Extensive metal-nitrogen-carbon (M-N-C) and FeNi alloys act as electrocatalytic active sites for ORR and OER, respectively, together with highly graphitized carbon nanotubes for facilitating electron transport. And the combinations endow glu-NiFe with an ORR half-wave potential of 0.85 V and an OER potential of 1.67 V at 10 mA cm−2, which exceed commercial catalysts of Pt/C and RuO2. Furthermore, the rechargeable Zn-air battery based on glu-NiFe delivers a high peak power density of 127 mW cm−2 and superior cycling stability (over 240 h). Moreover, a favorable performance is exhibited by glu-NiFe based all solid-state Zn-air battery. This work will provide some assistance in designing efficient and low-cost bifunctional catalysts for high performance rechargeable Zn-air batteries.

Bifunctional electrocatalyst with CoN3 active sties dispersed on N-doped graphitic carbon nanosheets for ultrastable Zn-air batteries

Realizing large-scale production of low-cost bifunctional catalysts is pivotal to promoting the practical application of Zn-air batteries. Herein, we successfully constructed unique bifunctional CoN3 catalytic sites atomically dispersed on N-doped graphitic carbon nanosheets (CoSA/NCs), which enable Zn-air batteries with ultrahigh durability for over 6000 cycles (~ 2000 h). The 2D carbon construction and single-atom Co formation were achieved simultaneously in salt-assisted process employing CoCl2. CoCl2 serves as a recyclable template, a pore-making agent, and a catalyst for graphitization, which effectively enables the catalyst with abundant active sites catalyze ORR and OER. Our experimental and theoretical modeling results confirm that of CoN3 surpasses CoN4 in term of the ORR and OER catalytic activity. The Zn-air battery based on CoSA/NCs catalyst exhibits a high peak power density of 255 mW cm−2. With unparalleled catalytic performance and low production cost, this catalyst paves the way for the potential large-scale application of Zn-air batteries.

FeS2/CoS2 nanoparticles supported on N, S co-doped carbon nanotubes as advanced bifunctional electrocatalysts for rechargeable zinc-air batteries

Designing a highly active and durable bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of crucial importance to the commercial development of zinc-air batteries (ZABs). Herein, a facile strategy is employed to synthesize FeS2/CoS2 nanoparticles anchored on N, S co-doped carbon nanotubes (NSCNTs). The obtained FeS2/CoS2/NSCNTs displays superior bifunctional electrocatalytic activities for both ORR and OER in an alkaline electrolyte, with a relative low potential difference value of only 0.66 V. Importantly, FeS2/CoS2/NSCNTs based solid-state ZAB endows a high specific capacity of 786.9 mAh g−1 at the current density of 10 mA cm−2, outstanding long-term charge-discharge stability over 70 h, as well as excellent flexibility. This research provides a new pathway for the construction of robust and low-cost bifunctional catalysts for ZABs.

Anchoring Co3S4 nanowires on NiCo2O4 nanosheet arrays as high-performance electrocatalyst for hydrogen and oxygen evolution.

The development of catalysts which can substitute expensive metals to efficiently split water is currently a hot research topic. Here, a multi-layered NF/NiCo2O4/Co3S4 nanocomposite was prepared on 3D porous nickel foam by hydrothermal and annealing processes. The electrode is a multi-layered, highly conductive and high specific surface area network without a binder, which facilitates electron transport and exposure of active sites, enhances full contact of the electrolyte/electrode interface, and accelerates H2 and O2 diffusion during water electrolysis. As expected, NF/NiCo2O4/Co3S4 exhibits surprising electrocatalytic activity, and only required 71 and 170 mV to reach 10 mA cm-2 in the hydrogen and oxygen evolution reactions, respectively, and exhibited stability up to 36 h. This strategy provides an efficient, durable, and low-cost bifunctional catalyst.