Drew Higgins

BACKGROUND As the world continues to transition toward carbon emissions–free energy technologies, there remains a need to also reduce the carbon emissions of the chemical production industry. Today many of the world’s chemicals are produced from fossil fuel–derived feedstocks. Electrochemical conversion of carbon dioxide (CO 2 ) into chemical feedstocks offers a way to turn waste emissions into valuable products, closing the carbon loop. When coupled to renewable sources of electricity, these products can be made with a net negative carbon emissions footprint, helping to sequester CO 2 into usable goods. Research and development into electrocatalytic materials for CO 2 reduction has intensified in recent years, with advances in selectivity, efficiency, and reaction rate progressing toward practical implementation. A variety of chemical products can be made from CO 2 , such as alcohols, oxygenates, synthesis gas (syngas), and olefins—staples in the global chemical industry. Because these products are produced at substantial scale, a switch to renewably powered production could result in a substantial carbon emissions reduction impact. The advancement of electrochemical technology to convert electrons generated from renewable power into stable chemical form also represents one avenue to long-term (e.g., seasonal) storage of energy. ADVANCES The science of electrocatalytic CO 2 reduction continues to progress, with priority given to the need to pinpoint more accurately the targets for practical application, the economics of chemical products, and barriers to market entry. It will be important to scale CO 2 electrolyzers and increase the stability of these catalysts to thousands of hours of continuous operation. Product separation and efficient recycling of CO 2 and electrolyte also need to be managed. The petrochemical industry operates at a massive scale with a complicated global supply chain and heavy capital costs. Commodity chemical markets are difficult to penetrate and are priced on feedstock, which is currently inexpensive as a result of the shale gas boom. CO 2 capture costs from the flue or direct air and product separation from unreacted CO 2 are also important to consider. Assuming that the advancement of electrocatalytic technologies continues apace, what will it take to disrupt the chemical production sector, and what will society gain by doing so? This review presents a technoeconomic and carbon emissions assessment of CO 2 products such as ethylene, ethanol, and carbon monoxide, offering target figures of merit for practical application. The price of electricity is by far the largest cost driver. Electrochemical production costs begin to match those of traditional fossil fuel–derived processes when electricity prices fall below 4 cents per kWh and energy conversion efficiencies reach at least 60%. When powered by renewable electricity, these products can be made with a net negative carbon emissions footprint. A comparative analysis of electrocatalytic, biocatalytic, and fossil fuel–derived chemical production shows that electrocatalytic production has the potential to yield the greatest reduction in carbon emissions, provided that a steady supply of clean electricity is available. Additionally, opportunities exist to combine electrochemical conversion of CO 2 with a range of other thermo- and biocatalytic processes to slowly electrify the existing petrochemical supply chain and further upgrade CO 2 into more useful chemicals. Technical challenges such as operating lifetime, energy efficiency, and product separation are discussed. Supply chain management of products and entrenched industrial petrochemical competition are also considered. OUTLOOK There exists increasingly widespread recognition of the need to transition to carbon emissions–free means of chemical production. CO 2 pricing mechanisms are being developed and are seeing increased governmental support. The nascent carbon utilization economy is gaining traction, with startup companies, global prizes, and industrial research efforts all pursuing new carbon conversion technologies. Recent advances in electrochemical CO 2 reduction through the use of gas diffusion electrodes are pushing current densities and selectivities into a realm of industrial use. Despite this progress, there remain technical challenges that must be overcome for commercial application. Additionally, market barriers and cost economics will ultimately decide whether this technology experiences widespread implementation. Electrochemical CO 2 conversion. Reduction of CO 2 using renewably sourced electricity could transform waste CO 2 emissions into commodity chemical feedstocks or fuels.

With the approaching commercialization of PEM fuel cell technology, developing active, inexpensive non-precious metal ORR catalyst materials to replace currently used Pt-based catalysts is a necessary and essential requirement in order to reduce the overall system cost. This review paper highlights the progress made over the past 40 years with a detailed discussion of recent works in the area of non-precious metal electrocatalysts for oxygen reduction reaction, a necessary reaction at the PEM fuel cell cathode. Several important kinds of unsupported or carbon supported non-precious metal electrocatalysts for ORR are reviewed, including non-pyrolyzed and pyrolyzed transition metal nitrogen-containing complexes, conductive polymer-based catalysts, transition metal chalcogenides, metal oxides/carbides/nitrides/oxynitrides/carbonitrides, and enzymatic compounds. Among these candidates, pyrolyzed transition metal nitrogen-containing complexes supported on carbon materials (M–Nx/C) are considered the most promising ORR catalysts because they have demonstrated some ORR activity and stability close to that of commercially available Pt/C catalysts. Although great progress has been achieved in this area of research and development, there are still some challenges in both their ORR activity and stability. Regarding the ORR activity, the actual volumetric activity of the most active non-precious metal catalyst is still well below the DOE 2015 target. Regarding the ORR stability, stability tests are generally run at low current densities or low power levels, and the lifetime is far shorter than targets set by DOE. Therefore, improving both the ORR activity and stability are the major short and long term focuses of non-precious metal catalyst research and development. Based on the results achieved in this area, several future research directions are also proposed and discussed in this paper.

Replacing platinum in air-fed fuel cells Replacing expensive and scarce platinum catalysts in polymer electrolyte membrane fuel cells for the oxygen reduction reaction (ORR) with ones based on non-noble metals would speed up the adoption of hydrogen fuel vehicles. Most of the candidate replacement catalysts that have shown high performance do so only when running on pure oxygen. Chung et al. developed an iron-nitrogen-carbon catalyst from two nitrogen precursors that forms a high-porosity structure and exhibits high ORR performance when running on air. The proposed catalytically active site is FeN 4 . Science , this issue p. 479

This paper gives a comprehensive review about the most recent progress in synthesis, characterization, fundamental understanding, and the performance of graphene and graphene oxide sponges. Practical applications are considered including use in composite materials, as the electrode materials for electrochemical sensors, as absorbers for both gases and liquids, and as electrode materials for devices involved in electrochemical energy storage and conversion. Several advantages of both graphene and graphene oxide sponges such as three dimensional graphene networks, high surface area, high electro/thermo conductivities, high chemical/electrochemical stability, high flexibility and elasticity, and extremely high surface hydrophobicity are emphasized. To facilitate further research and development, the technical challenges are discussed, and several future research directions are also suggested in this paper.

Significant advances have been made in recent years discovering new electrocatalysts and developing a fundamental understanding of electrochemical CO2 reduction processes. This field has progressed to the point that efforts can now focus on translating this knowledge toward the development of practical CO2 electrolyzers, which have the potential to replace conventional petrochemical processes as a sustainable route to produce fuels and chemicals. In this Perspective, we take a critical look at the progress in incorporating electrochemical CO2 reduction catalysts into practical device architectures that operate using vapor-phase CO2 reactants, thereby overcoming intrinsic limitations of aqueous-based systems. Performance comparison is made between state-of-the-art CO2 electrolyzers and commercial H2O electrolyzers—a well-established technology that provides realistic performance targets. Beyond just higher rates, vapor-fed reactors represent new paradigms for unprecedented control of local reaction conditions, and we provide a perspective on the challenges and opportunities for generating fundamental knowledge and achieving technological progress toward the development of practical CO2 electrolyzers.