Ruizhi Yang

Impact of Loading in RRDE Experiments on Fe–N–C Catalysts: Two- or Four-Electron Oxygen Reduction?

We have investigated the impact of electrocatalyst loading on rotating ring-disk electrode (RRDE) experiments for the oxygen reduction reaction on Fe-N-C catalysts (ORR) in acid medium. In particular, the fraction of H 2 O 2 produced as a function of catalyst loading was studied. A dramatic increase in H 2 O 2 release was observed as the catalyst loading was decreased. For the same non-noble metal catalyst (NNMC), the fraction of produced H 2 O 2 varied between less than 5% and greater than 95%, depending on the catalyst loading. These observations suggest that oxygen reduction occurs stepwise, via H 2 O 2 , and if the catalyst is sparsely loaded, the produced H 2 O 2 cannot be efficiently reduced to H 2 O before it escapes. These studies have important implications for fundamental studies of ORR on NNMCs.

Facile synthesis and excellent electrochemical properties of NiCo2O4 spinel nanowire arrays as a bifunctional catalyst for the oxygen reduction and evolution reaction

Developing catalysts with high electrocatalytic activity for an oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has recently attracted much attention because the sluggish kinetics of these two reactions limits the performance and commercialization of fuel cells and metal–air batteries. Herein, a facile template-free co-precipitation route was reported for the design and fabrication of well-ordered NiCo2O4 (NCO) spinel nanowire arrays. The as-prepared NCO spinel nanowire arrays are characterized by XRD, SEM, TEM, BET and XPS. BET results show that NCO spinel nanowire arrays have a mesoporous (ca. 8 nm) structure and a high specific surface area of 124 m2 g−1. The catalytic activity of NCO spinel nanowire arrays for the ORR and the OER in 0.1 M KOH solution has been studied by using a rotating ring-disk electrode (RRDE) technique. RRDE results show that the NCO spinel nanowire array catalyst exhibits excellent catalytic activity for the ORR. The ORR mainly favors a direct four electron pathway, which is close to the behavior of the Pt/C (20 wt% Pt on carbon) electrocatalyst under the same testing conditions. Anodic linear scanning voltammogram results show that the NCO spinel nanowire array catalyst is more active for the OER. The chronoamperometric and cyclic voltammogram tests show that the NCO spinel nanowire array catalyst exhibits excellent stability and reversibility for the ORR and the OER.

Phosphorus-doped porous carbons as efficient electrocatalysts for oxygen reduction

Efficient electrocatalysts for the oxygen reduction reaction (ORR) play a critical role in the performance of fuel cells and metal–air batteries. In this study, we report a facile synthesis of phosphorus (P)-doped porous carbon as a highly active electrocatalyst for the ORR. Phosphorus-doped porous carbon was prepared by simultaneous doping and activation of carbon with phosphoric acid (H3PO4) in the presence of Co. Both phosphorus and cobalt were found to play significant roles in improving the catalytic activity of carbon for the ORR. The as-prepared phosphorus-doped porous carbon exhibited considerable catalytic activity for the ORR as evidenced by rotating ring-disk electrode studies. At the same mass loading, the Tafel slope of phosphorus-doped porous carbon electrocatalysts is comparable to that of the commercial Pt/C catalysts (20 wt% Pt on Vulcan XC-72, Johnson Matthey) with stability superior to Pt/C in alkaline solutions.

A facile synthesis of CoFe2O4/biocarbon nanocomposites as efficient bi-functional electrocatalysts for the oxygen reduction and oxygen evolution reaction

Efficient electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial for improving the performance of metal–air batteries. In this study, CoFe2O4/biocarbon (CFO/BC) nanocomposites have been synthesized via a facile biosynthesis method by using yeast cells as carbon sources and structural templates. The as-prepared CFO/BC nanocomposites possess a hierarchical structure with a high surface area (79.84 m2 g−1). The rotating ring-disk electrode (RRDE) and rotating disk electrode (RDE) measurements revealed that CFO/BC nanocomposites exhibit excellent catalytic activity for both the ORR and OER. The onset potential of CFO/BC for the ORR is −0.14 V (vs. Ag/AgCl), which is higher than that of CoFe2O4 (−0.29 V) and that of biocarbon (−0.25 V), respectively. Meanwhile, the CFO/BC nanocomposites show much higher activity for the OER as compared to CoFe2O4 and biocarbon. The chronoamperometric tests show that the CFO/BC catalyst shows high durability for both the ORR and OER, outperforming the commercial Pt/C (20 wt% Pt on Vulcan XC-72, Johnson Matthey). The high electrocatalytic activity and durability of the CFO/BC nanocomposite are mainly attributed to the strong coupling between CoFe2O4 nanoparticles and biocarbon as well as the hierarchical structure of CFO/BC.

Co – C – N Oxygen Reduction Catalysts Prepared by Combinatorial Magnetron Sputter Deposition

Thin-film libraries of CO x C 1-x-y N y . (0 < x < 0.107, 0.003 < y < 0.389) were prepared by combinatorial magnetron sputter deposition in an Ar/N 2 gas mixture followed by subsequent heat-treatment at 700, 800, or 1000°C in N 2 atmosphere. By increasing the nitrogen partial pressure during sputtering, the nitrogen content was increased significantly in the as-sputtered libraries and more nitrogen remained in the libraries after heat-treatment. The catalytic activities of the libraries towards the oxygen reduction reaction (ORR) were studied using the rotating ring-disk electrode (RRDE) technique. CO x C 1-x-y N y libraries heat-treated at 800°C with 0.004 < x < 0.105 and 0.026 < y < 0.097 showed good catalytic activities towards ORR in 0.1 M HClO 4 solution at room temperature. The heating temperature that induces the onset of catalytic activity coincides with the temperature at which both substantial nitrogen release from the originally amorphous films and the formation of graphitic carbon and β-Co occurs. This temperature varies significantly with both the Co and N content in the films. The production of H 2 O 2 and the corrosion stability of the libraries are also discussed.

Nano Co3O4 particles embedded in porous hard carbon spherules as anode material for Li-ion batteries

Co3O4 particles were embedded in the nanopores of hard carbon spherules (HCS) by thermal decomposition of cobalt compound in long carbon-chain alcohols at low temperature. The electrochemical performance of the composite was evaluated by galvanostatic cycling and cyclic voltammetry. The significantly improved electrochemical performance of the composite material as anode for lithium-ion batteries is attributed to the high Li-storage capacity of Co3O4 and the prevention of aggregation of the Co3O4 nanoparticles embedded in stable HCS

Thermal Evolution of the Structure and Activity of Magnetron-Sputtered TM–C–N ( TM = Fe , Co ) Oxygen Reduction Catalysts

Thin-film libraries of TM x C 1-x-y N x (TM = Fe, Co; 0 < x < 0.09; 0 < y < 0.5) have been prepared by combinatorial sputter deposition. The libraries were subsequently annealed at 700-1000°C to induce structural and compositional changes. Using grazing-incidence X-ray diffraction and scanning electron microscopy, structural changes were followed as a function of annealing temperature. At temperatures above 700°C, the previously homogeneous and amorphous thin films became a heterogeneous mixture of (partially) graphitized nitrogen-containing carbon and either Fe 3 C or β-Co. The onset of this transformation is accompanied by a rapid decrease in N content and occurs as a function of both transition metal content and temperature. Catalytic activity for oxygen reduction is at its maximum partway through this transformation.

Investigation of Activity of Sputtered Transition-Metal (TM)–C–N (TM = V, Cr, Mn, Co, Ni) Catalysts for Oxygen Reduction Reaction

Variations with heat-treatment temperature of the structure and the electrocatalytic activity of sputtered transition-metal (TM)-C-N (TM = V, Cr, Mn, Co, and Ni) films for the oxygen reduction reaction (ORR) in acid and alkaline electrolytes were studied. The films prepared were all originally amorphous. At a critical heat-treatment temperature (T c , different for each composition) substantial nitrogen release occurred and the films transformed to a heterogeneous N-containing carbon structure with either Cr 3 C 2 , Co, Ni, V 8 C 7 , or Mn 7 C 3 , depending on the TM used. In acid electrolyte, heat-treated Cr x C 1-x-y N y , Co x C 1-x-y N y , and Ni x C 1-x-y N y films began to show oxygen reduction activity at T c , while V x C 1-x-y N y and Mn x C 1-x-y N y films showed little or no activity for ORR in acid electrolyte for all temperatures studied. However, all of the heat-treated TM x C 1-x-y N y (TM = V, Cr, Mn, Co, and Ni) films showed oxygen reduction activity in alkaline electrolyte even when heated to temperatures below T c . Because all the TM x C 1-x-y N y films heated above T c contained N-containing carbon, the activity of heat-treated sputtered C 1-x N x and C films, as well as graphite powder, were studied for comparison. Their activities were much lower in acid electrolyte than those of the Cr x C 1-x-y N y , CO x C 1-x-y N y , and Ni x C 1-x-y N y films heat-treated at the same temperatures. C 1-x N x films, C films, and graphite powder all showed activity in alkaline electrolyte but were less active than some heated Co x C 1-x-y N y and Ni x C 1-x-y N y films. The choice of TM and the heat-treatment temperature played important roles in determining the activity of the sputtered TM x C 1-x-y N y films for ORR in acid and alkaline electrolytes. The corrosion stability of the heat-treated TM x C 1-x-y N y libraries was studied as well. Heat-treated Cr x C 1-x-y N y libraries showed good passivation against corrosion.

Magnetron Sputtered Fe – C – N , Fe – C , and C – N Based Oxygen Reduction Electrocatalysts

Thin-film libraries of Fe x C 1-x-y N y (0 < x < 0.06, 0 < y < 0.5) have been prepared by combinatorial sputter deposition in a 40/60 N 2 /Ar process gas. The libraries were subsequently annealed between 800 and 1000°C to induce structural and compositional changes. These libraries contained/retained more nitrogen than those sputtered in lower N 2 partial pressures. Physical and electrochemical properties of these combinatorial libraries were studied using scanning electron microscopy, X-ray photoelectron spectroscopy, and rotating ring-disk electrode cells. Maximum catalytic activity for oxygen reduction reaction in acid was achieved for the libraries annealed at 800°C and approaches the best activity of Fe-C-N-based electrocatalysts reported in the literature to date.

Dependence of the Activity of Sputtered Co-C-N Oxygen Reduction Electrocatalysts on Heat-Treatment Temperature

The variations of the structure and the activity of a sputtered Co-C-N sample for the oxygen reduction reaction (ORR) with heat-treatment temperature are studied. The sample begins to show activity for ORR in acidic electrolyte at a critical heat-treatment temperature (T c ) of 725°C, at which both substantial nitrogen release from the originally amorphous films and the transformation to a heterogeneous structure (β-Co + N-containing graphitic carbon) occurs. The sample shows activity in alkaline electrolyte when heated above 500°C, the activity increases substantially near T c and the highest activity occurs at 850°C. This suggests that active sites, apparently created below T c , which are active below T c , are blocked in acidic electrolyte. When N loss, graphitization and formation of β-Co occurs at T c , sites apparently become available in acidic electrolyte. The variation of the transformation temperature, T c , with x and y in sputtered Co x C 1-x-y N y combinatorial libraries was mapped using electron microprobe and X-ray diffraction studies. T c decreases strongly as the Co content increases since Co is a graphitization catalyst and T c increases weakly as the N content increases.