Comparison of Pt–Cu/C with Benchmark Pt–Co/C: Metal Dissolution and Their Surface Interactions

Today, the development of active and stable catalysts still represents a challenge to be overcome in the research field of low-temperature fuel cells.[...]

A mini review on carbon-based metal-free electrocatalysts for oxygen reduction reaction

Bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are the heart of metal-air batteries, fuel cells, and other energy storage systems. Here, we report a series of a novel class of redox-active viologen-based ionic covalent organic networks (vCONs) which are directly used as metal-free bifunctional electrocatalysts towards ORR and OER applications. These vCONs (named vGC, vGAC, vMEL and vBPDP) were synthesized by the well-known Zincke reaction. The installation of redox-active viologen moieties among the extended covalent organic architectures played a crucial role for exceptional acid/base stability, as well as bifunctional ORR and OER activities, confirmed by the cyclic voltammetry (CV) curves. Among all of them, vBPDP showed high ORR efficiency with a half-wave potential of 0.72 V against a reversible hydrogen electrode (RHE) in 1 M KOH electrolyte. In contrast, vMEL demonstrated high OER activity with an overpotential of 320 mV at a current density of 10 mAcm-2 and a Tafel slope of 109.4 mV dec-1 in 1 M KOH electrolyte solution. This work is exceptional and unique in terms of directly used pristine ionic covalent organic networks that are used as bifunctional (ORR and OER) electrocatalysts without adding any metals or conductive materials.

An enhanced non-noble perovskite-based oxygen electrocatalyst for efficient oxygen reduction and evolution reactions

Here, the hybrid of NiCo2S4 nanoparticles grown on graphene in situ is first described as an effective bifunctional nonprecious electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the alkaline medium. NiCo2S4@N/S-rGO was synthesized by a one-pot solvothermal strategy using Co(OAc)2, Ni(OAc)2, thiourea, and graphene oxide as precursors and ethylene glycol as the dispersing agent; simultaneously, traces of nitrogen and sulfur were double-doped into the reduced graphene oxide (rGO) in the forms of pyrrolic-N, pyridinic-N, and thiophenic-S, which are often desirable for metal-free ORR catalysts. In comparison with commercial Pt/C catalyst, NiCo2S4@N/S-rGO shows less reduction activity, much better durability, and superior methanol tolerance toward ORR in 0.1 M KOH; it reveals higher activity toward OER in both KOH electrolyte and phosphate buffer at pH 7.0. NiCo2S4@graphene demonstrated excellent overall bicatalytic performance, and importantly, it suggests a novel kind of promising nonprecious bifunctional catalyst in the related renewable energy devices.

Pd-based bimetallic or multimetallic nanocrystals are considered to be potential electrocatalysts for cathodic oxygen reduction reaction (ORR) in fuel cells. Although much advance has been made, the synthesis of component-controlled Pd-Sn alloy nanocrystals or corresponding nanohybrids is still challenging, and the electrocatalytic ORR properties are not fully explored. Herein, component-controlled synthesis of PdxSny nanocrystals (including Pd3Sn, Pd2Sn, Pd3Sn2, and PdSn) has been realized, which are in situ grown or deposited on pre-treated multi-walled carbon nanotubes (CNTs) to form well-coupled nanohybrids (NHs) by a facile one-pot non-hydrolytic system thermolysis method. In alkaline media, all the resultant PdxSny/CNTs NHs are effective at catalyzing ORR. Among them, the Pd3Sn/CNTs NHs exhibit the best catalytic activity with the half-wave potential of 0.85 V (vs. RHE), good cyclic stability, and excellent methanol-tolerant capability due to the suited Pd-Sn alloy component and its strong interaction or efficient electronic coupling with CNTs. This work is conducive to the advancement of Pd-based nanoalloy catalysts by combining component engineering and a hybridization strategy and promoting their application in clean energy devices.

A novel sulfur-nitrogen dual doped ordered mesoporous carbon electrocatalyst for efficient oxygen reduction reaction

Zinc–air batteries (ZABs) have attracted extensive attention due to their remarkable high theoretical energy output. They represent one of the most promising future power sources. However, many barriers restrict their application on a large scale. One of the main challenges is the sluggish rates of the oxygen‐reduction reaction (ORR) and oxygen‐evolution reaction (OER), which govern the discharging and charging processes of the battery, respectively. Here, recent advances related to oxygen electrocatalyst materials for ZABs are discussed. Detailed discussions will focus on unifunctional ORR electrocatalysts and bifunctional ORR and OER electrocatalysts. Pt‐based nanomaterials, as the best ORR electrocatalysts, possess the virtue of high activity, but have the disadvantages of high cost, scarcity, and poor stability. Thus, materials based on transition metals (alloys, metal oxides, metal nitrides, and spinel oxides) and metal‐free materials are widely investigated as nonprecious ORR catalysts owing to their promising catalytic activities. As for bifunctional ORR and OER electrocatalysts, the following two categories are introduced: (i) metal‐based materials, including single metal/metal‐oxides‐based materials and mixed‐metal/metal‐oxides‐based materials; and (ii) metal‐free materials. Finally, perspectives on the continuous research and limitation of the current ZAB technology are provided.

Alkaline fuel cells and metal–air batteries are emerging as an alternative energy conversion devices which can use non-precious and earth-abundant materials as electrocatalysts for oxygen reduction reaction (ORR) [1-3]. Perovskite-type oxides have been widely investigated due to their high activity, long durability and low cost [4]. Mn and Co-based perovskites have shown stable and high ORR performance and their electrochemical properties was explained using the molecular orbital theory. It was shown that the transition-metal-oxide electrocatalysts exhibit maximum catalytic activity when the occupancy of the eg orbital is ~1 which is the primary activity descriptor, whereas spin-state and electronegativity are secondary activity descriptors [5]. However, Co-based oxides have several disadvantages such as high volatility, thermal expansion coefficient, and cost [6-8]. Thus, there is need for alternative Co-free materials for next generation fuel cells. Here, we report Co-free perovskite-type Ba0.5Sr0.5Fe0.8Cu0.2O3-δ as ORR electrode for fuel cells. Effect of Cu-doping in Ba0.5Sr0.5Fe1-xCuxO3-δ (x = 0-0.2) towards the ORR activity in alkaline media has been studied. Partial substitution of Cu in the Fe-site in (Sr,Ba)FeO3 could influence the transition-metal oxygen covalency which replaces and regenerates OH— ions in alkaline ORR process. Thin film rotating-disk electrode (RDE) method with well-defined oxygen transport was previously developed to study the ORR activity of perovskite catalysts [4, 9]. Evaluating the effects of such molecular level engineering on the desired outcome is challenging, since the ORR process consists of multiple adsorption/desorption step involving oxygen containing species. There are fewer literature on understanding the ORR process on transition metal oxides involving the transfer of four electrons and four OH— ions to O2 with subsequent cleavage of double bond O2. Ba0.5Sr0.5Fe1-xCuxO3-δ (x = 0-0.2) powders were prepared through sol gel synthesis method. Powder X-ray diffraction was employed for structural characterization and the activity towards ORR was studied using RDE method. RDE experiments were performed in oxygen saturated electrolyte at 100-1600 rpm. The voltage scan was performed from 0.4-0.9 V vs. RHE. The relation between current and rotational speed was evaluated through a Koutechy-Levich (K-L) plot to deduce the number of electrons (n) involved in the ORR process, i.e. n = (0.62∙F∙D0 2/3∙ v-1/6∙Co∙slope)-1 Where, F is Faraday’s constant, F = 96485 C.mol-1, (Do) diffusion coefficient of oxygen in 0.1M KOH = 1.93x10-5 cm2.s-1, (ν) Kinematic viscosity of 0.1 M KOH = 1.09x10-2 cm2.s-1, (Co) saturation concentration of O2 in 0.1 M KOH at 1 atm O2 pressure = 1.26x10-6 mol.cm-3, and slope – slope from K-L plot. Preliminary results showed that Cu doping in the lattice is affecting the ORR properties of perovskite in alkaline media. The slope of K-L plot gradually increases with increase in Cu content and decreases beyond 0.05 Cu until 0.2 Cu. The number of electron involved in the ORR process is mostly restricted to two. Acknowledgements This research was supported by the Canada First Research Excellence Fund (CFREF) at the University of Calgary. References Lu, J. Pan, A. Huang, L. Zhuang, J. Lu, Proc. Natl. Acad. Sci. U. S. A. 105, 20611 (2008).Pan, C. Chen, L. Zhuang, J. Lu, Acc. Chem. Res. 45, 473 (2012).Oz, K. Singh, D. Gelman, V. Thangadurai, Y. Tsur, J. Phys. Chem. C. 122 (27) 15097 (2018).Ge, A. Sumboja, D. Wuu, T. An, B. Li, F. W.T. Goh, T.S.A. Hor, Y. Zong, Z. Liu, ACS Catal. 5 , 4643 (2015).Suntivich, H. A. Gasteiger, N.Yabuuchi, H. Nakanishi, J.B. Goodenough, Y. Shao-Horn, Nat. Chem. 3, 546 (2011).McIntosh, J.F.Vente, W.G. Haije, D.H.A. Blank, H.J.M. Bouwmeester, Chem. Mater. 18, 2187 (2006).Teraoka, H. Shimokawa, C. Y. Kang, H. Kusaba, K. Sasaki, Solid State Ionics 177 , 2245 (2006).Wang, C. Tablet, A. Feldhoff, J. Caro, Adv. Mater. 17, 1785 (2005).Ge, A. Sumboja, D. Wuu, T. An, B. Li, F. W. T. Goh, et al., ACS Catal. 5, 4643 (2015). Figure 1

Proton exchange membrane hydrogen fuel cell (PEMFC) deployment is presently limited by the high material cost of oxygen reduction reaction (ORR) electrocatalysts at the cathode. Developing low-cost, earth-abundant alternatives for use in acidic environments presents a material stability challenge, especially for long-term device operation. Platinum (Pt)-based materials are used in typical commercial PEMFC cathode formulations due to their simultaneous high performance and long-term stability despite the prohibitively high material cost. Developing lower-cost, longer-life catalyst materials based on earth abundant transition metals can accelerate the adoption of PEMFC technology. The impact of potential, current density, and local microenvironment has been shown to cause degradation of non-Pt materials but standard ex situ measurements can only probe surface restructuring and cumulative dissolution after operation. Material-specific degradation mechanisms that are strong functions of the operating conditions cannot be ascertained without in situ quantification of material degradation under PEMFC-relevant conditions. Information about the degradation pathways of these non-precious catalysts can assist with material design for devices; however, most efforts towards developing an in-situ understanding of PEMFC cathode catalyst degradation under operating conditions are generally limited to Pt-based materials. Extensive in situ characterization has revealed Pt-specific mechanistic information about corrosion pathways as well as conditions that exacerbate Pt alloy leaching. It has been hypothesized that the Pt degradation processes are related to oxide formation under potential cycling. With this critical in-situ information, Pt conditioning protocols have been developed to enhance material lifetime; however, investigations on non-precious materials are less common. Insights into non-precious catalyst degradation in PEMFC operating conditions are not as well-established but could be essential in enabling low-cost materials for commercial use. There is a need for more insights into a vast array of materials which necessitates the development of faster, preferably benchtop techniques for degradation analysis.In this work, we use an in situ electrochemical flow cell assembly and coupled it with an inductively coupled plasma-mass spectrometer (ICP-MS) to quantify the loss of material in real time for several non-Pt materials under reaction conditions. The flow cell is fed by a peristaltic pump at 2.5 mL/min of gas-saturated acidic electrolyte for optimal mass transport. The cell has a three-electrode configuration with a compression-mounted metal foil working electrode, silver/silver chloride reference electrode, and Pt counter electrode downstream of other components. We evaluate a variety of transition metals across the spectrum of ORR activity: palladium, silver, nickel, copper, manganese, and cobalt. Each metal is tested under oxygen and nitrogen saturation in five electrolytes of identical pH: perchloric acid, sulfuric acid, nitric acid, hydrochloric acid, and hydrobromic acid. This setup allows for nearly real time measurement of ORR kinetics and in situ element-specific dissolution rates as a function of time. With careful design of experimental conditions and controlled variables, we are able to suggest mechanisms of degradation with specific reference to conditions under which material loss occurred such as high/low potential, oxygen-saturated/nitrogen-saturated electrolyte, or faradaic/non-faradaic current. We find some metals are stable under nitrogen-saturated electrolyte but corrode more under oxygen-saturated electrolyte only while ORR active. Furthermore, other metals are stable in nitrogen saturated electrolyte at all tested electrochemical conditions but unstable under oxygen saturated electrolyte but only outside the ORR range, implying that cathodic faradaic processes might be stabilizing the surface at low potentials. This stabilization where a thermodynamically unstable material (based on its Pourbaix diagram) exhibits greater immunity while performing ORR in certain electrolytes could inform the development stability mechanisms that direct the design of next generation, non-precious PEMFC cathode catalyst materials. With combined in situ catalyst stability analysis in five pH 1 electrolytes, we can recommend specific conditions for optimizing activity and stability based on the potential range of operation and the electrolyte composition/saturation. We can also distill mechanistic insights from these experiments by creating models that validate causes of catalyst degradation. These models are built using “graphical causal modeling”, which is a mathematical field that goes beyond correlative predictions by venturing into causal inference. We hope to expand this framework into other electrocatalytic reactions of interest. The in-situ ICP-MS electrochemical flow cell setup exhibits promise for accelerating materials stability analysis for enabling next generation Pt-free ORR electrocatalysts through practical experiments and direct comparability to established electrochemical techniques.

Synthesis and characterization of Pd-Ni nanoalloy electrocatalysts for oxygen reduction reaction in fuel cells

One of the most severe bottlenecks in the operation of proton exchange membrane fuel cells (PEMFCs) is the sluggish kinetics of the Oxygen Reduction Reaction (ORR). Thus, suitable ORR electrocatalysts are required in order to obtain PEMFCs able to yield a sufficiently high performance [1]. State-of-the-art ORR electrocatalysts for application in PEMFCs are obtained by supporting Pt nanocrystals on active carbon supports characterized by a very large surface area [2]. However, despite extensive investigations the performance of these electrocatalysts does not meet yet the strict requirements necessary for a large-scale application of PEMFC technology. A new preparation protocol has been proposed recently to obtain nano-electrocatalysts with a well-controlled chemical composition, based on the pyrolysis and activation of hybrid inorganic-organic precursors. The resulting carbon nitride nano-electrocatalysts thus prepared show an outstanding ORR performance both in “ex-situ” studies and in a single-cell configuration [3]. In this work, a new family of multimetal carbon nitride nano-electrocatalysts is described. The materials are synthesized by pyrolysis of a hybrid inorganic-organic precursor including Pt and Ni atoms impregnating Cu nanoparticles acting as the support. The final ORR electrocatalysts are obtained after an extensive electrochemical dealloying activation process. The products are fully characterized before and after the activation processes. The chemical composition of the electrocatalysts is determined by Inductively-Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and microanalysis. The morphology is investigated by High-Resolution Scanning Electron Microscopy (HR-SEM) and High-Resolution Transmission Electron Microscopy (HR-TEM). The structure is probed by powder X-Ray Diffraction (XRD). The electrochemical performance of the electrocatalysts in the ORR is determined both by “ex-situ” measurements carried out by cyclic voltammetry with the rotating ring disk electrode (CV-TF-RRDE method) and by “in-situ” fuel cell studies in a PEM single-cell configuration. Preliminary results indicate clearly that the proposed electrocatalysts show a remarkable ORR performance, which is significantly enhanced in comparison with the Pt/C reference both in terms of overpotential and selectivity in the 4-electron mechanism of the ORR (see Figure 1). REFERENCES [1] J. X. Wang, F. A. Uribe, T. E. Springer, J. Zhang, R. R. Adzic, Faraday Discuss. 140, 347 (2008).[2] J. Zhang, Front. Energy 5, 137 (2011).[3] V. Di Noto, E. Negro, Electrochim. Acta 55, 7564 (2010).

Electrocatalysis of Oxygen Reduction on Pristine and Heteroatom-Doped Graphene Materials

Developing durable and highly active low-Pt electrocatalysts for oxygen reduction reaction (ORR) in acidic media is necessary for highly effective polymer electrolyte membrane fuel cells (PEMFCs). Intermetallic structure is thermodynamically more stable than random alloy metal compounds. In oxygen reduction reaction (ORR), Pt3Ni1 with (111) facet is to be the most effective electrocatalyst for ORR. Therefore, to develop a highly efficienct electrocatalyst for oxygen reduction reaction (ORR) in effective polymer electrolyte membrane fuel cell (PEMFC), an ordered PtNi/C intermetallic alloy on a carbon black support has been designed. To enhance ORR performance and reduce Pt usage, a Pt-transition metal alloy catalyst has been developed. Alloying Pt with a second metal on a carbon support material can improve activity due to the change in lattice parameter and the electronic effect. According to the d-band theory, the downshift effect of the d-band induced by the upshift of the fermi level is predicted to improve ORR activity by weakening the binding energy. In case of alloying Pt and Ni, it can be predicted that the difference in electronegativity between Pt and Ni will induce a fermi level shift, resulting in a d-band center shift effect, which can enhance ORR activity. Compared to random alloy structured electrocatalysts, intermetallic structures of ordered PtNi were more thermodynamically stable. Moreover, intermetallic structure stabilizes the alloy, improving the durability of metal components against chemical corrosion in acidic media, such as HClO4 or H2SO4. Therefore, to improve catalyst’s durability, heat treatment was used to align the structure, making it structurally stable and reducing Pt-Pt distance. Heat treatment screening was carried out between 500℃ and 800℃ using a thermal shock method to minimize particle size growth. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and rotating disc electrode (RDE) were used to investigate the compositional difference and crystallinity of the intermetallic structure, electrochemical ORR activity and durability between disordered PtNi/C (d-PtNi/C) and ordered PtNi/C (o-PtNi/C). The electronic effect of the three Nicore@Ptshell models, namely ordered Nicore@Ptshell (o-PtNi), disordered Nicore@Ptshell (d-PtNi), and pure Pt147 (pure Pt), was estimated through density functional theory (DFT). The d-band theory and Bader charge analysis were used to estimate ORR activity and charge transfer, respectively. Additionally, the durability in acidic media of each model was estimated through dissolution potential calculations (Udiss). This work has supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT in Republic of Korea (MSIT) (NRF-2022R1A2C2093090). This work was supported by the Technology Innovation Program (20019175) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). This research was supported by the Ministry of Trade, Industry, and Energy (MOTIE), Korea, under “Digital manufacturing platform" (No. P0022331) supervised by the Korea Institute for Advancement of Technology (KIAT).

Electrospun bimetallic PtNi nanowires as electrocatalyst for oxygen reduction reaction in PEMFCs