Loosely spread 10 nm silver particles give the best CO yield from CO2 in HZB tests

"We already knew that too tiny nanoparticles promote hydrogen evolution, which reduces the carbon monoxide yield. Conversely, nanoparticles that are too large are catalytically less active. We wanted to identify the exact optimum," said Niklas Hausmann, a co-author of the study ^[4].

The yield depends on both particle size and how closely the particles sit on the support. The paper's title, "Decoupling the Size and Loading Effects in Silver Nanoparticles for Efficient Paired Carbon Dioxide and Formaldehyde Electrolysis," states the aim of separating the two ^[2]. The team changed size and density as separate variables on the carbon powder ^[1], so a change in CO yield can be pinned to one of them.

The sparse result bears directly on materials. At 0.2 mg per square centimetre, the optimum layer holds about 2 g of silver per square metre of electrode ^[12]. The loosely distributed layer gave the best yield ^[3], so in these tests more silver beyond that loading did not increase CO yield.

A Faradaic efficiency near 100% over 100 hours ^[7] is a selectivity figure. It says the charge at the cathode went into CO and not into hydrogen. The article does not report current density or cell voltage, and without them the result cannot be placed against an industrial electrolyser.

The anode result has a clear baseline. It is the same process running oxygen evolution, a reaction HZB describes as consuming a great deal of energy while yielding only oxygen, a gas with no economic value ^[6]. Replacing it with aldehyde oxidation cut the whole process's energy use by more than 30% ^[5]. Put the other way, the paired cell needs less than 70% of the energy of the conventional one ^[13]. The saving adds an input. The aldehyde goes into the electrolyte, and its oxidation yields hydrogen and carboxylic acids such as formic acid in place of oxygen ^[5]. The paper's title names formaldehyde as the partner reaction ^[2].

The hydrogen matters further down the line. In the GreenQUEST scheme, CO and hydrogen form syngas, which is converted to DME and then to a green liquefied fuel made mostly of propane and butane ^[8]. This paper covers the electrolysis at the start of that chain ^[2]. "If we combine the production of CO with hydrogen generation and the simultaneous formation of other value-added chemicals such as formic acid, we can improve the overall value of the electrochemical process. The CO and hydrogen can serve as building blocks for the subsequent production of sustainable fuels and chemicals," Menezes said ^[11]. The fuel counts as carbon neutral only if the electricity for the electrolysis comes from solar or wind ^[10].