Critical Review of Carbon Conversion in “Carbon Fuel Cells”

Synergistic Impact of Alloying with Ni on Cu Cathode Interfaces for Fluoride Batteries

All-solid-state fluoride batteries have the potential to achieve energy densities significantly higher than those of lithium-ion batteries. A common cathode material for fluoride batteries is Cu. Cu has a low polarization, but its rapid capacity degradation due to grain growth and subsequent delamination from the solid-state electrolyte are critical issues. To enhance the performance of Cu-based cathodes in all-solid-state fluoride batteries, we explore alloying of Cu with Ni to create metastable solid solution phases (CuxNi1-x with x = 0, 0.32, 0.52, 0.72, 0.89, and 1.0). Compared to Cu, Ni has a higher polarization but exhibits superior capacity retention. The Cu0.72Ni0.28 alloy demonstrates a polarization as low as Cu, but it has a significantly improved capacity retention, which is comparable to Ni. Transmission electron microscopy observations demonstrate that the thin Ni-rich region formed near the interface inhibits Cu grain growth and delamination from the LaF3 electrolyte. By incorporating an appropriate amount of Ni into Cu, Cu-Ni alloy films combine the advantages of both metals, improving the performance of fluoride batteries.

Energy from waste biomass: an LCA study on a biofuel cell at early design stage

Diversifying energy sources and managing waste biomass are two pressing contemporary issues. The new technology proposed in this study aims to address both by converting waste biomass into energy and fertilizer through the use of a biofuel cell (BFC). The purpose of this study is to assess the environmental impacts associated with this innovative technology through a Life Cycle Assessment (LCA). To achieve the goal, the production and use of the cell were modelled, considering both laboratory-scale operations and industrial-scale approximations. The study explored alternative scenarios, such as sensitivity analyses involving different acids and bases, renewable energy sources, and heat recovery. Comparisons with conventional biomass waste treatments (anaerobic digestion and composting) demonstrated that the BFC technology remains competitive. To further improve the BFC's environmental footprint, efforts should focus on reducing energy requirements and enhancing nutrient recovery during scale-up. These insights are crucial for advancing sustainable waste treatment technologies and maximizing the potential of discarded biomass in an environmentally friendly manner.

Effect of Ionic Mass Transport on the Performance of a Novel Tubular Direct Carbon Fuel Cell for the Maximal Use of a Carbon-Filled Porous Anode

Despite a large number of existing studies about direct carbon fuel cells (DCFCs), sufficient power generation has remained a major technical challenge for the commercialization of DCFCs. This study was designed to implement the benefits of a carbon-filled porous anode developed in our recent studies in a unit cell. First, we developed a new tubular cell assembly comprising an anode, a thin matrix, and a tubular cathode with a certain number of holes in its surface. By employing a reference electrode, we measured the resistance and I-V-P characteristics of the anode, a cathode with a single hole, and the entire cell. As a result, we found that the cathode performance was degraded by resistance to ionic mass transfer, while the anode resistance was invariant (∼0.4 Ω cm²). By developing a semi-empirical current-potential model including an ion mass transport effect, we proved that the number of holes in the cathode surface is the key to the maximal utilization of the present anode. This eventually led to notable gains in the maximum power density to 205 mW cm^-2 at 700 °C in experiments. Lastly, a durability test was conducted to reconfirm the effect of ionic mass transfer on the power generation over time.

Honeycombed Porous, Size-Matching Architecture for High-Performance Hybrid Direct Carbon Fuel Cell Anode

Direct carbon fuel cells (DCFCs) demonstrate both superior electrical efficiency and fuel utilization compared to all other types of fuel cells, and it will be the most promising carbon utilization technology if the sluggish anode reaction kinetics that derives from the use of solid fuel can be addressed. Herein, the electrode morphology and fuel particle size are comprehensively considered to fabricate an efficient DCFC anode skeleton. A honeycombed and size-matching anode architecture with dual-scale porous structure is developed by water droplet templating, which demonstrates an efficient strategy to address the challenge of poor carbon reactivity and improve the electrochemical performance of DCFCs. Single cell with this designed anode framework demonstrates excellent performance, and the maximum power density is as high as 765 mW cm^-2 at 800 °C when using the matching carbon fuel. The size-matching between carbon fuel and anode framework shows a remarkable effect on the improvement of mass-transfer processes at the anodes. The significant contribution of the difficult electrochemical oxidation of carbon to the output performance is also demonstrated. These results represent a promising structural design strategy in developing high-performing fuel cells.

Conductive Carbon Materials from the Hydrothermal Carbonization of Vineyard Residues for the Application in Electrochemical Double-Layer Capacitors (EDLCs) and Direct Carbon Fuel Cells (DCFCs)

This study investigates the production of bio-based carbon materials for energy storage and conversion devices based on two different vineyard residues (pruning, pomace) and cellulose as a model biomass. Three different char categories were produced via pyrolysis at 900 °C for 2 h (biochars, BC), hydrothermal carbonization (HTC) (at 220, 240 or 260 °C) with different reaction times (60, 120 or 300 min) (hydrochars, HC), or HTC plus pyrolysis (pyrolyzed hydrochars, PHC). Physicochemical, structural, and electrical properties of the chars were assessed by elemental and proximate analysis, gas adsorption surface analysis with N₂ and CO₂, compression ratio, bulk density, and electrical conductivity (EC) measurements. Thermogravimetric analysis allowed conclusions to be made about the thermochemical conversion processes. Taking into consideration the required material properties for the application in electrochemical double-layer capacitors (EDLC) or in a direct carbon fuel cell (DCFC), the suitability of the obtained materials for each application is discussed. Promising materials with surface areas up to 711 m² g^-1 and presence of microporosity have been produced. It is shown that HTC plus pyrolysis from cellulose and pruning leads to better properties regarding aromatic carbon structures, carbon content (>90 wt.%), EC (up to 179 S m^-1), and porosity compared to one-step treatments, resulting in suitable materials for an EDLC application. The one-step pyrolysis process and the resulting chars with lower carbon contents and low EC values between 51 and 56 S m^-1 are preferred for DCFC applications. To conclude, biomass potentials can be exploited by producing tailored biomass-derived carbon materials via different carbonization processes for a wide range of applications in the field of energy storage and conversion.

A High-Performing Direct Carbon Fuel Cell with a 3D Architectured Anode Operated Below 600 °C

Direct carbon fuel cells (DCFCs) are highly efficient power generators fueled by abundant and cheap solid carbons. However, the limited triple-phase boundaries (TPBs) in the fuel electrode, due to the lack of direct contact among carbon, electrode, and electrolyte, inhibit the performance and result in poor fuel utilization. To address the challenges of low carbon oxidation activity and low carbon utilization, a highly efficient, 3D solid-state architected anode is developed to enhance the performance of DCFCs below 600 °C. The cell with the 3D textile anode framework, Gd:CeO₂ -Li/Na₂ CO₃ composite electrolyte, and Sm_0.5 Sr_0.5 CoO₃ cathode demonstrates excellent performance with maximum power densities of 143, 196, and 325 mW cm^-2 at 500, 550, and 600 °C, respectively. At 500 °C, the cells can be operated steadily with a rated power density of ≈0.13 W cm^-2 at a constant current density of 0.15 A cm^-2 with a carbon utilization over 85.5%. These results, for the first time, demonstrate the feasibility of directly electrochemical oxidation of solid carbon at 500-600 °C, representing a promising strategy in developing high-performing fuel cells and other electrochemical systems via the integration of 3D architected electrodes.

Challenges in developing direct carbon fuel cells

A direct carbon fuel cell (DCFC) can produce electricity with both superior electrical efficiency and fuel utilisation compared to all other types of fuel cells.

A simple high-performance matrix-free biomass molten carbonate fuel cell without CO2 recirculation

In previous reports, flowing CO2 at the cathode is essential for either conventional molten carbonate fuel cells (MCFCs) based on molten carbonate/LiAlO2 electrolytes or matrix-free MCFCs. For the first time, we demonstrate a high-performance matrix-free MCFC without CO2 recirculation. At 800°C, power densities of 430 and 410 mW/cm(2) are achieved when biomass-bamboo charcoal and wood, respectively-is used as fuel. At 600°C, a stable performance is observed during the measured 90 hours after the initial degradation. In this MCFC, CO2 is produced at the anode when carbon-containing fuels are used. The produced CO2 then dissolves and diffuses to the cathode to react with oxygen in open air, forming the required [Formula: see text] or [Formula: see text] ions for continuous operation. The dissolved [Formula: see text] ions may also take part in the cell reactions. This provides a simple new fuel cell technology to directly convert carbon-containing fuels such as carbon and biomass into electricity with high efficiency.

A direct carbon fuel cell with a CuO–ZnO–SDC composite anode

A direct carbon fuel cell with a CuO–ZnO–SDC composite anode was demonstrated. The maximum power density was 130 mW cm⁻² at 700 °C. The results indicate that CuO–ZnO can be used as a nickel-free anode material for direct carbon fuel cells.

Towards high-efficiency nanoelectrocatalysts for oxygen reduction through engineering advanced carbon nanomaterials

We summarize and discuss recent developments of different-dimensional advanced carbon nanomaterial-based noble-metal-free high-efficiency oxygen reduction electrocatalysts, including heteroatom-doped, transition metal-based nanoparticle-based, and especially iron carbide (Fe₃C)-based carbon nanomaterial composites.

Interlocked multi-armed carbon for stable oxygen reduction

N-Doped multi-armed carbon with an interlocked structure shows high oxygen reduction activity and resistance to methanol crossover effects.

Influence of the mediating behaviour of Sn according to its particle size on a Ni/yttria-stabilised zirconia porous anode structure in a direct carbon fuel cell

Comparison of the Sn mediating behaviour according to the particle size and consequent changes in permeation: microparticles tend to accumulate, whereas nanoparticles favour permeation and oxidation due to their smaller dimensions.

Application of infiltrated LSCM–GDC oxide anode in direct carbon/coal fuel cells

Hybrid direct carbon/coal fuel cells (HDCFCs) utilise an anode based upon a molten carbonate salt with an oxide conducting solid electrolyte for direct carbon/coal conversion. They can be fuelled by a wide range of carbon sources, and offer higher potential chemical to electrical energy conversion efficiency and have the potential to decrease CO₂ emissions compared to coal-fired power plants. In this study, the application of (La, Sr)(Cr, Mn)O₃ (LSCM) and (Gd, Ce)O₂ (GDC) oxide anodes was explored in a HDCFC system running with two different carbon fuels, an organic xerogel and a raw bituminous coal. The electrochemical performance of the HDCFC based on a 1–2 mm thick 8 mol% yttria stabilised zirconia (YSZ) electrolyte and the GDC–LSCM anode fabricated by wet impregnation procedures was characterized and discussed. The infiltrated oxide anode showed a significantly higher performance than the conventional Ni–YSZ anode, without suffering from impurity formation under HDCFC operation conditions. Total polarisation resistance (R_p) reached 0.8–0.9 Ω cm² from DCFC with an oxide anode on xerogel and bituminous coal at 750 °C, with open circuit voltage (OCV) values in the range 1.1–1.2 V on both carbon forms. These indicated the potential application of LSCM–GDC oxide anode in HDCFCs. The chemical compatibility of LSCM/GDC with carbon/carbonate investigation revealed the emergence of an A₂BO₄ type oxide in place of an ABO₃ perovskite structure in the LSCM in a reducing environment, due to Li attack as a result of intimate contact between the LSCM and Li₂CO₃, with GDC being stable under identical conditions. Such reaction between LSCM and Li₂CO₃ was not observed on a LSCM–YSZ pellet treated with Li–K carbonate in 5% H₂/Ar at 700 °C, nor on a GDC–LSCM anode after HDCFC operation. The HDCFC durability tests of GDC–LSCM oxide on a xerogel and on raw bituminous coal were performed under potentiostatic operation at 0.7 V at 750 °C. The degradation mechanisms were addressed, especially on raw coal.

Graphene oxide and H2 production from bioelectrochemical graphite oxidation

Graphene oxide (GO) is an emerging material for energy and environmental applications, but it has been primarily produced using chemical processes involving high energy consumption and hazardous chemicals. In this study, we reported a new bioelectrochemical method to produce GO from graphite under ambient conditions without chemical amendments, value-added organic compounds and high rate H₂ were also produced. Compared with abiotic electrochemical electrolysis control, the microbial assisted graphite oxidation produced high rate of graphite oxide and graphene oxide (BEGO) sheets, CO₂, and current at lower applied voltage. The resultant electrons are transferred to a biocathode, where H₂ and organic compounds are produced by microbial reduction of protons and CO_2, respectively, a process known as microbial electrosynthesis (MES). Pseudomonas is the dominant population on the anode, while abundant anaerobic solvent-producing bacteria Clostridium carboxidivorans is likely responsible for electrosynthesis on the cathode. Oxygen production through water electrolysis was not detected on the anode due to the presence of facultative and aerobic bacteria as O₂ sinkers. This new method provides a sustainable route for producing graphene materials and renewable H₂ at low cost, and it may stimulate a new area of research in MES.

Oxygen- and Nitrogen-Enriched 3D Porous Carbon for Supercapacitors of High Volumetric Capacity

Efficient utilization and broader commercialization of alternative energies (e.g., solar, wind, and geothermal) hinges on the performance and cost of energy storage and conversion systems. For now and in the foreseeable future, the combination of rechargeable batteries and electrochemical capacitors remains the most promising option for many energy storage applications. Porous carbonaceous materials have been widely used as an electrode for batteries and supercapacitors. To date, however, the highest specific capacitance of an electrochemical double layer capacitor is only ∼200 F/g, although a wide variety of synthetic approaches have been explored in creating optimized porous structures. Here, we report our findings in the synthesis of porous carbon through a simple, one-step process: direct carbonization of kelp in an NH3 atmosphere at 700 °C. The resulting oxygen- and nitrogen-enriched carbon has a three-dimensional structure with specific surface area greater than 1000 m(2)/g. When evaluated as an electrode for electrochemical double layer capacitors, the porous carbon structure demonstrated excellent volumetric capacitance (>360 F/cm(3)) with excellent cycling stability. This simple approach to low-cost carbonaceous materials with unique architecture and functionality could be a promising alternative to fabrication of porous carbon structures for many practical applications, including batteries and fuel cells.

Assessment of biochar as feedstock in a direct carbon solid oxide fuel cell

The feasibility of directly converting biochar to electricity in a novel Direct Biomass Fuel Cell (DBFC) was investigated. Three different types of biochars, i.e., pistachio shells (PI), pecan shells (PE) and sawdust (SD) were employed as feedstock.

Effect of carbon type on the performance of a direct or hybrid carbon solid oxide fuel cell

The impact of carbon type on DCFC performance is explored; the optimum performance is obtained with biomass-derived pine charcoal.