Using Electrocatalysts To Find New Uses For Captured CO2
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Earth’s ecology has been transformed by humans at an alarming rate and will worsen as the world population continues to expand and as limited natural resources are depleted. These realities are guaranteed — only the degree is debated.
The transition to sustainable (non-petroleum) chemical feedstocks, fuels, and manufacturing is an essential part of restoring the natural balance to Earth’s ecology. However, this is a daunting task technologically, economically, and in terms of public perception. One indication of this imbalance is the level of carbon dioxide (CO2) in the atmosphere. It’s reached a record high of 410 ppm, having increased by 30% since 1960. As a result, the global temperature has risen and volatile climate events have endangered cities with floods and extreme storms. Hence, it is both urgent and important that we find ways to lower CO2 in the atmosphere.
To address this issue, governments have pledged to reduce CO2 and other greenhouse gas emissions (for the US, this goal was 17% by 2020 and 80% by 2050); however, according to Stanford scientists, “reducing CO2 emissions may not be enough to curb global warming”. This means that, in addition to curbing emissions, we must actively remove CO2 from the atmosphere.
Recent advances in polymers that reversibly bind CO2 have brought the cost of its direct removal from the air down to below $100/metric ton. For concentrated point sources, such as refineries, power plants, and cement factories, that cost can be as low as $8/metric ton. But what can be done with the CO2 once it is captured? Currently, underground storage of gaseous CO2 is being tested on small scales but has to overcome public perceptions (Not In My Backyard). Direct recycling of CO2 is far more exciting because it:
- Provides a renewable alternative for producing traditionally fossil-derived gasoline, natural gas, chemicals, and polymers;
- Generates revenue from the commercialization of the products, covering for the cost of capture; and
- Can be used to store energy in chemical bonds from intermittent renewable energy sources such as solar and wind.
All of these benefits can be achieved through a process called electrochemical CO2 reduction. Using a catalyst and electricity, ideally from renewable sources, the bonds in water and CO2 can be rearranged to form new organic molecules. Although electrochemical CO2 reduction has been known for decades, its power efficiency and product selectivity have been limited.
Researchers have demonstrated that it is possible to use this process to produce carbon monoxide, methanol, ethanol, methane, and ethylene with relatively high yields. Unfortunately, these products have a low commercial value and are energetically inefficient to produce, resulting in a high production cost that is not feasible at the commercial scale. However, a newly published research from Rutgers University describes the discovery of a new family of electrocatalysts — materials that are both electrically conducting and can rearrange chemical bonds of adsorbates — that can generate higher molecular weight products of intrinsically higher value, at high energy conversion efficiencies, opening the possibility of electrochemical CO2 reduction for profit.
In the world of organic compounds, the longer the carbon chain, the more valuable the product. Nickel phosphide catalysts, previously known for their hydrogen evolution ability, can selectively produce molecules with 3 and 4 carbons or can be used as intermediate to generate longer polymers. These electro-catalysts are the first class of materials, other than enzymes, that are able to convert CO2 to C3 and C4 products in aqueous media at > 99% electrical efficiency. The two products, methylglyoxal and 2,3-furandiol, can be utilized as precursors for biodegradable plastics, adhesives, and pharmaceuticals. In addition to being highly efficient, nickel phosphides are also cheap, abundant, and have a long catalyst life (which is key for successful use in industry). The discovery of these highly active electrocatalysts represents a breakthrough in CO2 reduction research and could pave the way for industrial-scale CO2 conversion into high-value chemical feedstocks and products.
The adverse effects of rising atmospheric CO2 levels already observed in rising ocean temperatures and extreme weather events have stressed our growing need to remove CO2 from the atmosphere. This study provides new optimism that global carbon dioxide levels can be reversed sustainably by displacing fossil carbon resources while using renewable energy sources and making valuable products — all at a net profit. While several engineering challenges need to be addressed to achieve scalable electrochemical CO2 reduction processes, the future is looking a whole lot brighter.
These findings are described in the article entitled Selective CO2 reduction to C3 and C4 oxyhydrocarbons on nickel phosphides at overpotentials as low as 10 mV, recently published in the journal Energy & Environmental Science.
This work was conducted by