Novel earth-abundant and highly efficient electrocatalysts for hydrogen and oxygen production

Hydrocarbon fuels are a primary source of energy worldwide, but this resource is quickly becoming depleted, and carbon dioxide emissions from burning said fuel contribute greatly to global warming. Alternative clean energy sources are thus required for the development of a sustainable environment and society. The so-called ‘hydrogen economy’ is a promising candidate to provide green and sustainable energy due to its high energy density and carbon-neutral fuel. Water electrolysis has recently gained much attention as an environmentally friendly technology for hydrogen production. However, the occurrence of oxygen evolution reaction (OER) at the anodic electrode of water electrolyzer, a thermodynamically uphill reaction exhibiting sluggish kinetics, severely limits the overall efficiency of water splitting. OER is a critical challenge to be considered, not only for water electrolysis but also for other energy storage and conversion technologies. Currently, the state-of-the-art catalysts for both hydrogen evolution reaction (HER) and OER are noble metal‐based materials. However, their high cost, poor long-term durability, and scarcity hinder application at an industrial scale. Thus, the development of economically viable materials for HER and OER is crucial and relevant to their easier deployment for large-scale applications in electrochemical energy storage and conversion technologies. In the first part of this dissertation, an earth-abundant and efficient oxygen-evolving electrocatalyst based on metal borides is presented. Amorphous nanoparticles were prepared by chemical reduction of metal ions with sodium borohydride in aqueous solution. This method allowed homogeneous incorporation of transition metals in nickel boride nanostructure which improved the electrochemical activity of the catalyst. The as-prepared metal boride materials outperformed the existing commercial precious metal-based catalyst (Ir/C) for OER and for overall water splitting. In addition, multimetallic borides outperformed monometallic borides due to the synergistic effect from well-distributed transition metal constituents. In the second part of this dissertation, an earth-abundant tungsten–nickel alloy electrocatalyst for superior hydrogen evolution was developed and deployed as an electrode without an organic binder. The electrode was prepared by a hydrothermal process followed by annealing in the presence of H2 environment. The as-prepared electrode displayed bifunctional application both for HER and OER. Owing to the excellent electrocatalytic performance arising from the synergistic effect of tungsten–nickel interaction through d-orbital electron transfer, the as-prepared material was comparable to some of the best tungsten-based HER electrocatalysts. The lower adsorption energy of water molecules and a small Gibbs free energy of hydrogen adsorption on tungsten atoms, as measured from DFT calculations, revealed favorable water electrolysis kinetics. Thirdly, in order to boost the electrochemical performance of unsupported metal borides studied in the first piece of work, a practical approach for incorporating metal boride nanosheets onto a highly conductive substrate is presented. The developed material displayed better oxygen evolution activity compared to the unsupported metal borides and advanced stability in harsh alkaline electrolytes. A synergistic effect between highly abundant catalytically active sites and the 3D porous substrate improved the electron transport arising from the presence of highly negative boron and the high conductivity of the substrate, resulting in an outstanding electrocatalytic activity. The results of this work showed an effective method to boost the electrochemical performance of metal borides by supporting them on a highly conductive substrate. Inspired by (1) the facile preparation of metal borides by reducing metal ions in aqueous solution as presented in the first and third parts, and (2) the possibility of reducing graphene oxide by the same reducing agent used in these experiments, a fast and simple method of synthesizing amorphous ternary metal borides while simultaneously reducing the graphene oxide (GO) sheets was developed in the fourth part of this thesis. The as-prepared hybrid material exhibited outstanding OER performance and stability as compared to the pristine catalyst of the same composition under prolonged OER operation. In 1.0 M KOH, only 230 mV was required to afford a current density of 15 mA cm–2 with a small Tafel slope of 50 mV dec–1. This electrocatalytic performance was also much better compared to the commercial RuO2 catalyst. DFT calculations suggested that the in situ formation of MOxHy during electrochemical activation acted as active sites for water oxidation. The superior OER activity of the as-prepared catalyst was attributed to (i) its unique amorphous structure to allow abundant active sites, (ii) synergistic effect of constituents, and (iii) strong coupling of active material and highly conductive rGO. Finally, the last chapter summarizes the results of these projects and proposes an outlook for future works based on them.Doctor of Philosoph

Highly efficient oxygen reduction reaction activity of N-doped carbon–cobalt boride heterointerfaces

Compositional and structural engineering of metal-metalloid materials can boost their electrocatalytic performance. Herein, a highly efficient and stable electrocatalytic system for the oxygen reduction reaction is obtained by creating heterointerfaces between N-doped carbon and cobalt boride nanosheets. Furthermore, a detailed investigation on the effect of annealing temperature as well as the amount of carbon and nitrogen sources is conducted to tune their performance. The best electrocatalyst among the prepared materials is found to have an onset potential of 1.05 V and half-wave potential of 0.94 V, which are 40 and 72 mV positive in comparison to commercial Pt/C, respectively. Finally, a zinc–air battery is also assembled using the catalyst.Ministry of Education (MOE)This work was supported by the AcRF Tier 1 grant (RG118/18 and RG105/19), provided by Ministry of Education in Singapore

Chemical and structural origin of lattice oxygen oxidation in Co–Zn oxyhydroxide oxygen evolution electrocatalysts

The oxygen evolution reaction (OER) is a key process in electrochemical energy conversion devices. Understanding the origins of the lattice oxygen oxidation mechanism is crucial because OER catalysts operating via this mechanism could bypass certain limitations associated with those operating by the conventional adsorbate evolution mechanism. Transition metal oxyhydroxides are often considered to be the real catalytic species in a variety of OER catalysts and their low-dimensional layered structures readily allow direct formation of the O–O bond. Here, we incorporate catalytically inactive Zn2+ into CoOOH and suggest that the OER mechanism is dependent on the amount of Zn2+ in the catalyst. The inclusion of the Zn2+ ions gives rise to oxygen non-bonding states with different local configurations that depend on the quantity of Zn2+. We propose that the OER proceeds via the lattice oxygen oxidation mechanism pathway on the metal oxyhydroxides only if two neighbouring oxidized oxygens can hybridize their oxygen holes without sacrificing metal–oxygen hybridization significantly, finding that Zn0.2Co0.8OOH has the optimum activity.Ministry of Education (MOE)National Research Foundation (NRF)The authors appreciate the support from the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. We also acknowledge financial support from the academic research fund AcRF Tier 2 (M4020246, ARC10/15), Ministry of Education, Singapore

Facile synthesis of amorphous ternary metal borides-reduced graphene oxide hybrid with superior oxygen evolution activity

Metal borides represent an emerging family of advanced electrocatalyst for oxygen evolution reaction (OER). Herein, we present a fast and simple method of synthesizing iron-doped amorphous nickel boride on reduced graphene oxide (rGO) sheets. The hybrid exhibits outstanding OER performance and stability in prolonged OER operation. In 1.0 M KOH, only 230 mV is required to afford a current density of 15 mA cm⁻² with a small Tafel slope of 50 mV dec⁻¹. DFT calculations lead to a suggestion that the in situ formation of MOₓHᵧ during electrochemical activation acts as active sites for water oxidation. The superior OER activity of the as-prepared catalyst is attributed to (i) its unique amorphous structure to allow abundant active sites, (ii) synergistic effect of constituents, and (iii) strong coupling of active material and highly conductive rGO. This work not only provides new perspectives to design a highly effective material for OER but also opens a promising avenue to tailor the electrochemical properties of metal borides, which could be extended to other materials for energy storage and conversion technologies.Ministry of Education (MOE)National Research Foundation (NRF)This project is funded by the National Research Foundation, Prime Minister's Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program. We also acknowledge financial support from the academic research fund AcRF tier 1 (M4011784, RG6/17) and tier 2 (M4020246, ARC10/15), Ministry of Education, Singapore

Moisture‐Induced Non‐Equilibrium Phase Segregation in Triple Cation Mixed Halide Perovskite Monitored by In Situ Characterization Techniques and Solid‐State NMR

International audienceEnvironmental stability is a major bottleneck of perovskite solar cells. Only a handful of studies are investigating the effect of moisture on the structural degradation of the absorber. They mostly rely on ex situ experiments and on completely degraded samples, which restrict the assessment on initial and final stage. By combining in situ X-ray diffraction under controlled 85% relative humidity, and live observations of the water-induced degradation using liquid-cell transmission electron microscopy, we reveal two competitive degradation paths leading on one hand to the decomposition of state-of-the-art mixed cation/anion (Cs0.05(MA0.17FA0.83)0.95Pb(Br0.17I0.83)3 (CsMAFA) into PbI2 through a dissolution/recrystallization mechanism and, on the other hand, to a non-equilibrium phase segregation leading to CsPb2Br5 and a Cesium-poor/iodide-rich Cs0.05-x(MA0.17FA0.83)0.95Pb(Br0.17−2yI0.83+2y)3 perovskite. This degradation mechanism is corroborated at atomic-scale resolution through solid-state 1H and 133Cs NMR analysis. Exposure to moisture leads to a film containing important heterogeneities in terms of morphology, photoluminescence intensities, and lifetimes. Our results provide new insights and consensus that complex perovskite compositions, though very performant as champion devices, are comparatively metastable, a trait that limits the chances to achieve long-term stability

Probing moisture-induced structural and compositional changes in triple cation mixed halide hybrid perovskites

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Modulation of single atomic Co and Fe sites on hollow carbon nanospheres as oxygen electrodes for rechargeable Zn–air batteries

Efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required for metal air batteries, to replace costly metals, such as Pt and Ir/Ru based compounds, which are typically used as benchmarks for ORR and OER, respectively. Isolated single atomic sites coordinated with nitrogen on carbon supports (M-N-C) have promising performance for replacement of precious metal catalysts. However, most of monometallic M-N-C catalysts demonstrate unsatisfactory bifunctional performance. Herein, a facile way of preparing bimetallic Fe and Co sites entrapped in nitrogen-doped hollow carbon nanospheres (Fe,Co-SA/CS) is explored, drawing on the unique structure and pore characteristics of Zeolitic imidazole frameworks and molecular size of Ferrocene, an Fe containing species. Fe,Co-SA/CS showed an ORR onset potential and half wave potential of 0.96 and 0.86 V, respectively. For OER, (Fe,Co)-SA/CS attained its anodic current density of 10 mA cm at an overpotential of 360 mV. Interestingly, the oxygen electrode activity (ΔE) for (Fe,Co)-SA/CS and commercial Pt/C-RuO is calculated to be 0.73 V, exhibiting the bifunctional catalytic activity of (Fe,Co)-SA/CS. (Fe,Co)-SA/CS evidenced desirable specific capacity and cyclic stability than Pt/C-RuO2 mixture when utilized as an air cathode in a homemade Zinc-air battery.Ministry of Education (MOE)This work was supported by the AcRF Tier 1 grant (RG105/19), provided by Ministry of Education in Singapore. H.H. and J.C. like to thank National Natural Science Foundation of China (Grant No. 11874044)

An Earth-Abundant Tungsten–Nickel Alloy Electrocatalyst for Superior Hydrogen Evolution

Hydrogen production with high purity through water splitting has been proved to be a potential energy technology but requires highly efficient, low cost, and robust electrocatalysts. Herein, a tungsten–nickel/nickel foam hybrid is prepared by a facile method and exhibits an outstanding hydrogen evolution reaction activity and remarkable stability in alkaline solution. It only requires an overpotential of 36 mV to afford the current density of 10 mA cm^–2 with a small Tafel slope of 43 mV dec^–1. Owing to the excellent electrocatalytic performance arising from the synergistic effect of binary tungsten–nickel interacting through the d-orbital electron transfer, the as-prepared material is the best among tungsten-based HER electrocatalysts. The lower adsorption energy of water molecules and a small Gibbs free energy of hydrogen adsorption (0.17 eV) on tungsten atoms of WNi (111) from DFT calculations reveal the favorable water electrolysis kinetics. Moreover, the simple preparation strategy can be extended to design of other active materials for clean energy technology applications and beyond